Hi Cyril.

I can see several good reasons to prefer using voltage over current to represent signals. One reason is that voltages can propagate over relatively long distances compared to current because of lower resistive losses.

For example, early telegraph systems in the US worked on relatively low voltages and the current carried the telegraph pulses. The telegrapher’s switch drove an inductive coil directly, but miles away. As the distances grew longer, increasingly higher voltages were necessary to overcome resistive loss in the copper wires used to carry the signal. To compensate somewhat, the copper transmission lines themselves were increased in diameter to reduce series resistance. The problem couldn’t be resolved until practical voltage to current transducers were developed. At that point, even a small voltage could be sensed through many miles of relatively thin copper wire. This is because (virtually) no current is required to detect a voltage. The detected voltage could be transformed into a current to power the telegraphers switch/coil from a local source, thereby bypassing the resistive loss problem.

A similar example: In the early days of electric power distribution in the US, the (Thomas) Edison power company advocated a direct current (DC) system (Edison patented several DC “dynamo” designs). His vision would have required large power generating plants every few miles and very large diameter copper wires and cables to compensate for resistive losses. Nicola Tesla, the famous Serbian-American inventor (and former employee of Edison) developed a different type of generator and showed that it was more practical to distribute power using an alternating current (ie: sinusoidal voltage) system. Such a system shifts the propagation of power from the electric current to the electric field (AKA: voltage). Edison disagreed with Tesla, who eventually stopped working for Edison over the issue. Later, George Westinghouse, the famous entrepreneur, bought Tesla’s patents and built the very first AC power generation facility a Niagara Falls in New York, US. This established what is essentially the standard pattern of power distribution today. Virtually all long-haul power distribution lines operate at extremely high voltages (often in excess of 100 kilovolts). Paradoxically, the longest transmission lines do not use AC, but DC instead. However, such lines again use voltages of many hundreds of kilovolts. The idea, though, is to put all the energy into the electric field, thereby reducing the necessary current to a minimum to combat resistive losses.

In a further example, let’s look at an EM field propagating through free space (a vacuum). Here’, one may transmit a signal with absolutely no electrical current. (And, not only a signal, but possibly power as well.) Sure, the signal is contained in both the electric and magnetic fields, but neither requires electrical conductivity (ie: a current) to propagate. With a large microwave dish antenna (e.g: the Arecibo observatory), signals can be sent across interstellar distances. First done at Arecibo in 1974, there have been several interstellar messages transmitted over the years.

Let’s look at electronics. A bi-polar junction transistor (BJT, invented in 1947 at Bell Labs in the US) requires a current signal on the base to modify it’s collector-emitter resistance (thus the term “transistor”). However, modulating the transistors resistance (as in an amplifier) or making it switch off and on (as in a logic circuit) requires significant signal power and significant power dissipated in the device itself. When integrated circuits were developed (Jack Kilby at Texas Instruments patented the first practical IC design in 1959.), designers found that maximum transistor density on the surface of the chip was not limited by device size, but rather power dissipation per device and “fan-out”, the ability to split a signal into multiple paths.

These problems were largely overcome with the invention of the field-effect transistor (FET, actually invented in 1925 but not made practical until William Shockley at Bell Labs just after WWII). The FET is a voltage device. The signal, a voltage applied to the gate, modulates the drain-source resistance (Rds). Because of this, the only current that must flow is that necessary to charge/discharge the various (and relatively small) parasitic capacitances within the device. The result: extremely low power is required to make the device switch (though, the power is proportional to the square of the switching frequency). The fan-out problem was also largely resolved with this approach because the signal is a voltage, not a current. Fan-out is still limited, but this time by how much current the originating device can deliver to charge the very small parasitic capacitances. The device itself can also be made much smaller than a typical BJT. The result is the modern CPU, with literally billions of devices on a chip and feature lengths as small as 22 nm (truly nanotechnology).

Now, that’s not to say the BJT is a useless device. There are many applications where the BJT can out-perform a FET. This is especially true when power consumption and dissipation are not an issue. For example voltage regulator ICs (not switching regulators) are typically based on BJTs. Audiophiles know that (next to vacuum tubes which are still popular in that field) BJT power transistor amplifiers can give the lowest total harmonic distortion (THD) levels. There remain many many other uses for BJTs as preferred over FETs.

I hope this helps!

Hi Cyril.

I can see several good reasons to prefer using voltage over current to represent signals. One reason is that voltages can propagate over relatively long distances compared to current because of lower resistive losses.

For example, early telegraph systems in the US worked on relatively low voltages and the current carried the telegraph pulses. The telegrapher’s switch drove an inductive coil directly, but miles away. As the distances grew longer, increasingly higher voltages were necessary to overcome resistive loss in the copper wires used to carry the signal. To compensate somewhat, the copper transmission lines themselves were increased in diameter to reduce series resistance. The problem couldn’t be resolved until practical voltage to current transducers were developed. At that point, even a small voltage could be sensed through many miles of relatively thin copper wire. This is because (virtually) no current is required to detect a voltage. The detected voltage could be transformed into a current to power the telegraphers switch/coil from a local source, thereby bypassing the resistive loss problem.

A similar example: In the early days of electric power distribution in the US, the (Thomas) Edison power company advocated a direct current (DC) system (Edison patented several DC “dynamo” designs). His vision would have required large power generating plants every few miles and very large diameter copper wires and cables to compensate for resistive losses. Nicola Tesla, the famous Serbian-American inventor (and former employee of Edison) developed a different type of generator and showed that it was more practical to distribute power using an alternating current (ie: sinusoidal voltage) system. Such a system shifts the propagation of power from the electric current to the electric field (AKA: voltage). Edison disagreed with Tesla, who eventually stopped working for Edison over the issue. Later, George Westinghouse, the famous entrepreneur, bought Tesla’s patents and built the very first AC power generation facility a Niagara Falls in New York, US. This established what is essentially the standard pattern of power distribution today. Virtually all long-haul power distribution lines operate at extremely high voltages (often in excess of 100 kilovolts). Paradoxically, the longest transmission lines do not use AC, but DC instead. However, such lines again use voltages of many hundreds of kilovolts. The idea, though, is to put all the energy into the electric field, thereby reducing the necessary current to a minimum to combat resistive losses.

In a further example, let’s look at an EM field propagating through free space (a vacuum). Here’, one may transmit a signal with absolutely no electrical current. (And, not only a signal, but possibly power as well.) Sure, the signal is contained in both the electric and magnetic fields, but neither requires electrical conductivity (ie: a current) to propagate. With a large microwave dish antenna (e.g: the Arecibo observatory), signals can be sent across interstellar distances. First done at Arecibo in 1974, there have been several interstellar messages transmitted over the years.

Let’s look at electronics. A bi-polar junction transistor (BJT, invented in 1947 at Bell Labs in the US) requires a current signal on the base to modify it’s collector-emitter resistance (thus the term “transistor”). However, modulating the transistors resistance (as in an amplifier) or making it switch off and on (as in a logic circuit) requires significant signal power and significant power dissipated in the device itself. When integrated circuits were developed (Jack Kilby at Texas Instruments patented the first practical IC design in 1959.), designers found that maximum transistor density on the surface of the chip was not limited by device size, but rather power dissipation per device and “fan-out”, the ability to split a signal into multiple paths.

These problems were largely overcome with the invention of the field-effect transistor (FET, actually invented in 1925 but not made practical until William Shockley at Bell Labs just after WWII). The FET is a voltage device. The signal, a voltage applied to the gate, modulates the drain-source resistance (Rds). Because of this, the only current that must flow is that necessary to charge/discharge the various (and relatively small) parasitic capacitances within the device. The result: extremely low power is required to make the device switch (though, the power is proportional to the square of the switching frequency). The fan-out problem was also largely resolved with this approach because the signal is a voltage, not a current. Fan-out is still limited, but this time by how much current the originating device can deliver to charge the very small parasitic capacitances. The device itself can also be made much smaller than a typical BJT. The result is the modern CPU, with literally billions of devices on a chip and feature lengths as small as 22 nm (truly nanotechnology).

Now, that’s not to say the BJT is a useless device. There are many applications where the BJT can out-perform a FET. This is especially true when power consumption and dissipation are not an issue. For example voltage regulator ICs (not switching regulators) are typically based on BJTs. Audiophiles know that (next to vacuum tubes which are still popular in that field) BJT power transistor amplifiers can give the lowest total harmonic distortion (THD) levels. There remain many many other uses for BJTs as preferred over FETs.

I hope this helps!

Dear Patrick,

I highly appreciate your "historical" approach showing the evolution of this fundamental problem through the years and centuries. Really, when asking this question, I had forgotten these exciting stories well-known from the history of the electricity; I thought only for today's electronic converters. But now I see that, as usual, the truth about great ideas is hidden in the past and it is worth to rummage in the old stories.

Well, let's extract and summarize all the wisdom from the first electricity stories to draw the first and maybe the major conclusion: the reason of using the voltage as an energy and data carrier is to overcome the line resistance (it can be significant because of the thin wire, the long line, the material or a bad contact). If there is no current flowing through the line resistance, there is no voltage drop (losses) across the line and the end voltage is (almost) equal to the input voltage. Figuratively speaking, as though there is no line resistance... and we can draw a little odd and paradoxical conclusion about the nature of the resistance:

"Resistance" means to "resist" something and this is the current. But if there is no current, there is no what to resist and the resistance (resistor) is not a resistance; it is just a piece of wire-:) I use this word game to explain to my students why when the transistor is cut off, the output (collector) voltage is equal to the supply voltage (unloaded circuit), and the collector resistance is of no importance even if we vary it...

About the "practical voltage to current transducers" used in the telegraphy... It turned out the telegraph transmission line consisted of a cascaded current-to-voltage, the line and an end voltage-to-current converter. I suppose they were just relays...

You have arisen my thinking in these lines... but I have to go to bed for an hour went after the midnight here...

Thank you for the nice stories. Cyril

Voltage controlled analog circuitry is easier to control in terms of bandwidth, has a lower power consumption and circuit design is more flexible. While it is easy to configure operational amplifiers as inverting or non-inverting, current controlled operational amplifiers are less flexible.

Current controlled circuitry allows ultimately to design faster electronics as the low output impedance of current devices allows to overcome capacitance and inductance of lines and devices faster than a high impedance voltage output. Overall bipolar transistors are current controlled, FETs are voltage controlled.

Unfortunately in electronics speed comes at the cost of noise, accuracy and linearity. Fast electronics are more noisy than slow electronics, are harder to maintain in stable condition, means not to oscillate. Current driven electronics can suffer from variation in inout impedance much more than high impedance voltage controlled devices. That makes it harder to make circuits that amplify current differences over a wide dynamic range of input current signals with high accuracy. E.g. current amplifiers suffer form a larger input offset variations compared to high impedance voltage controlled devices.

Short version:

Current control is ultimately faster than voltage control.

Use: Microwave frequency amplifiers, fast electrical switches, microprocessor cores

Currents are also used for analog signal transduction in noisy environments as EMI induced currents are tiny compared to the signal currents used (4-20mA loops)

Voltage control is more linear, less noisy and more accurate.

Use: Analog signal processing, amplification, low noise, low power digital and analog applications, sensor systems.

Data carrier are energies neither Current nor voltage, and all are related to each other fundamentally i.e Energy = Power X time = Voltage X current X time.

Circuit theory takes care of current and voltage only and is almost silent about time. When we do the analysis using ohm's law or any traditional network analysis theorems we do not consider time at all. But fact is time involvement can not be negated.

Electromagnetic field theory (EMFT) takes care of time as one of very important variable, hence can explain electrical energy being transported from one place to other or what we call data transmission.

So choice is currents for BJT devices (Current controlled devices) and voltages for FETs. Most of recent devices are FET based and hence voltage becomes a better choice..

And where is the time in the case of a DC current?

To Prof. M. Baller:

(Quote:"Overall bipolar transistors are current controlled")

may I direct your attention to the following discussion

https://www.researchgate.net/post/Voltage_or_current_controlled_device_is_a_BJT_And_what_does_voltage_controlled_and_current_controlled_mean_Is_this_a_property_of_the_BJT

Regards

Lutz vW

Here is what the famous Barrie Gilbert thinks about voltage vs. current mode:

If somebody is interested in this subject - here comes another detailed paper:

Dear Patrick,

Nice introduction, but when you are propagating an EM wave (in vacuum or in air), you are actually propagating electrical AND magnetic fields, not only the electrical component.

Another comment: for long lines is more efficient to distribute electrical power in the form of DC because this way you don't loose power to the parasitic inductances and capacitances of the lines, which start to be quite important above around 600-800km for airlines (or around 150-200 km for submarine lines)

Very interesting but difficult to understand, Lutz... These people are not only circuit geniuses but philosophers as well... What do you think, is it possible to invite B. Gilbert to join our "voltage versus current" discussions-:)?

Thank you for the great materials.

Cyril

Hi Cyril - I don`t know if this would be possible.

However, here comes a short message from B. Gilbert (from end of 2011, but still valid?).

http://www.cypress.com/?rID=55772

... But it would be wonderful... Usually, famous people gradually lose interest in their own achievements in their youthful days... Obviously, B. Gilbert is an exception from this rule...

I would like now to direct the discussion to some unique "current carrier" features (maybe, we should ask a special question about the ubiquitous current loop... but let's begin here to make a connection between the questions).

As the current along the (long) loop is the same, the current source can "sense" (by only one wire!) when "something" new (resistance, voltage or other disturbance) connects in series into the line. So, the main line (the current loop) acts as a feedback as well, and the source can keep up the desired current without the need of an additional wire. It will raise its internal voltage so that to compensate the new losses somewhere; it does know (and does not interest in) where it happened. Thus it will compensate the resistance variations of the cable. If the resistance is constant, it can be compensated by means of negative resistance without using a negative feedback (the negative resistor will "sense" the current variations and will change its voltage to compensate the voltage drop across the "positive" cable resistance).

Now compare this behavior with the "voltage-carrier" arrangement when "something" connects between the line and the ground - the voltage source will not sense this "intervention" and will not react adequately. To do this "magic", it needs a third wire (a feedback)...

It is an interesting question. But be careful how you phrase the question. Because V=IR, all signals have both V and I. What we are really talking about here is the question of constant voltage sources versus constant current sources. Why is one apparently more common than the other?

It's possibly all to do with the impedances we encounter in reality and the question of power efficiency. The nice thing about an (ideal) voltage source it delvers no energy to an open circuit :-)

But if an ideal current source will deliver some energy to a closed circuit-:)?

"But if an ideal current source will deliver some energy to a closed circuit-:)?"

Provided - such a source does exist in reality. Does it?

Cyril, your current loop example would be more illustrative if you would add a short circuit diagram.

Lutz

An ideal current source in parallel with a large internal resistance represents the practical current source so that there will not be any current diverted by the internal resistance from the load so as to supply full current to the load.

An ideal voltage source in series with a very small internal resistance represents the practical voltage source so that the voltage drop across the internal resistance will be very very small so as to supply full voltage to the load.

In practice there is only voltage source for eg. Signal voltage.

In the case of our discussions on biasing of a transistor, with reference to Base Emitter Biasing if the Resistance Rb is very small the signal source acts as a voltage source and if Rb is very high it acts as a current source because Vbe is very small during conduction

Normally the data or Transducer output will only be a voltage and not a current.

Faraday's laws of Electromagnetic induction also deals with generation of induced emf and not induced current.

In the case of current transfer in transformers the current transformation ratio is the reverse of voltage transformation ratio. The transistor is considered to be a current controlled device, but Ib is due to Vbe.

P. S.

Lutz, Pisupati and all the contributors here,

I have not yet got an answer to my questions, "What is a charged inductor when shorted? Is there any voltage inside it?" If not, then it is a true current source...

Analogical questions can be, "What is the pendulum bob when crossing the equilibrium position? Is there any pressure applied to it in this moment? Is there any potential energy in this moment?"

Regards, Cyril

Quote P.S.: "The transistor is considered to be a current controlled device, but Ib is due to Vbe."

My question: What does this mean? (is considered to be...). Is it or is it not?

Cyril: "What is a charged inductor?" I think, there is a magnetic field during charge flow (current) only?

I would like to provide some considerations to the original question by Cyril in this thread.

First of all, I would disagree with the idea that most of electronic circuits process data, and not energy. On the contrary, a great deal of circuits, namely those that are not involved in data input, output or performing some operations upon data (computation, recognition, interpretation or whatever), process not data, but (if there is data at all) process signals at most. The fact that the circuit processes a signal does not mean that it processes data. The aim of the circuit may be just to perform a useful operation with the signal - amplification, modulation, detection, filtering etc. with as little distortion to the data carried as possible. So far as the circuit processes a signal (or just a waveform, as, for example, for heating purposes), it processes some physical processes (excuse me for the tautologism), and may be absolutely data-unaware.

Second, I understand what Cyril means by saying that "voltage is preferred" (although, as described below, I do not find it a good expression), and I would attribute that to the simple fact that, for fundamental physical constructs that serve the basis for modern electronics, such as p-n junction or MOS structure, it is the applied voltage that serves as the control mechanism. Although we may find advantages of "voltage before current" indeed, but ultimately this is not a question of some reasonable engineering preference (like "oh well, we surely should prefer voltage!"), but just a consequence of the physical properties that happen to be utilized.

Third, I would not agree that voltage is always necessarily the "carrier". On the contrary, current is often the "carrier" in analogue circuits. On the input of an amplifier we have the gate voltage as a "carrier", But the voltage being the cause here, it results in the alteration of the drain current, as its physical effect. So it is current that becomes the "carrier" here, with perhaps voltage regaining its role at the input of the next stage of amplification. I would rather suggest that, in the end-to-end information transmission, a variety of physical phenomena may be used, the parameters of which represent data in a "seamless" way due to the fact that physical laws relate different physical quantities to each other. In the communication channel, a variety of physical phenomena may be utilized - for example circuitry at the transmitter and at the receiver with some optical transmission media in between. But the ultimate goal is to provide for the end-to-end information transmission with little distortion thereof.

The "role" of voltage in the digital circuitry is explained that the very foundation of the latter is to have the logic implemented - i.e., to implement two levels - high and low - through some physical principle. So far that the voltage is the physical factor of control (refer to above) in the devices used as logical elements, it is with voltage levels that this logic is practically implemented.

Anton, thank you for the profound answer! You made me think about the structure of transistor amplifiers...

I have not realized so clearly the fact that there is an alternation between the two electrical quantities voltage and current along the whole amplifier. We can think of it as of cascaded and alternating voltage-to-current (transistors) and current-to-voltage (resistors) converters, and we have the alternation voltage -> current -> voltage -> current... -> voltage. Thus, in the ordinary voltage amplifiers, the current is an internal quantity while in the strange current amplifiers (based on current mirrors), the voltage is an internal quantity (inside the current mirrors). It is interesting to explain the need of such an alternation...

Regards, Cyril

I'd like to reproduce a couple of small excerpts from a chapter written by Barrie Gilbert in 'Analogue IC design: the current-mode approach' (edited by C. Toumazou, F.J.Lidgey, & D.G. Haigh).

In paragraph 2.2., he writes 'At the outset, however, it must be stated that the term "current mode circuit" has no rigorous meaning. The behaviour of electrical networks is always the result of an interplay between voltage and current.'.

In paragraph 2.2.1, he writes 'While we have stressed the importance of considering both the current and voltage levels in an optimal view of BJT circuits, there nevertheless exists an extensive class of BJT circuits whose function depends primarily on the use of CURRENTS as signals or functional variables, and which can be designed and understood, certainly at a basic level, using methods in which VOLTAGES ARE NOT CONSIDERED AT ALL. It is circuits of this sort that are often meant by the use of the term "current mode" '.

From these excerpts, I would draw out the following interesting features:

1) Barrie Gilbert's use of the phrase 'an interplay between voltage and current'. Here, he avoids attributing roles of cause and effect to either voltage or current; he merely emphasises RELATIONSHIP between the two.

2) Barrie Gilbert writes of a 'class of BJT circuits whose function depends primarily on the use of currents as signals or functional variables'. To me, the word 'use' is key here: it corresponds with an engineer's decision or intention regarding the physical carrier of his signal. He could choose to use either voltage or current as the 'analogue' of the variable to be processed (or indeed both - for example in a transimpedance amplifier, the analogue at the input is current, at the output is voltage).

3) With regard to the design and understanding of current-mode circuits, Barrie Gilbert points to 'methods in which VOLTAGES ARE NOT CONSIDERED AT ALL'. This does not mean that voltages do not exist, or are not essential to the function of the circuit, but simply that in certain circumstances (particularly those circumstances arising in analogue IC design, where one can rely on temperature and process matching), one can find the transfer function without explicitly introducing voltage variables into the analysis. The voltages exist and are an essential part of the circuit action, even though the circuit is "current-mode"; but for the purpose of finding the transfer function, one does not need to know their magnitudes.

Further, regarding the choice of current or voltage as the analogue of the signal, I believe it has largely been a pragmatic choice. I think Patrick Bisson's information on early telegraphy systems is illuminating. Another reason for the prevalence of voltage-mode circuits could be this: it is easier to measure the voltage between circuit nodes than to measure current in a conductor. For the latter measurement, one has to open the circuit to insert an ammeter, which is simply not so convenient to do....

(Part II)

I also conjecture that another reason for the prevalence of voltage mode is that voltage mode circuits can be realised more simply, or with fewer components. Take any analogue circuit function and consider a) how you would implement it as a voltage-mode circuit, then b) how you would implement it as a current-mode circuit. I think that in the earlier days, when amplifiers were designed with thermionic valves or discrete transistors, voltage-mode circuits arose more naturally (or immediately) from simple circuit topologies. The cost of components made it necessary to use simple circuit topologies. (It will be very interesting to hear from anyone with counter-examples against this conjecture . . . ) It is only with advances in circuit integration that current-mode circuits became a practical alternative. The advent of integrated circuits enabled more complex circuit topologies to be implemented economically, and provided the fertile ground for Barrie Gilbert's ideas. As these ideas developed, the interplay of voltage and current in BJT networks could be exploited to realise more complex analogue circuit functions (such as multipliers and trigonometric function generators) both elegantly and economically.

To sum up my ideas: 1) Both currents and voltages are always present in circuits. 2) In principle, the engineer could use either voltage or current as the analogue for the signal to be processed. 3) Voltage-mode has generally been preferred for pragmatic engineering reasons.

In my discussion, I have focussed on analogue signal processing. There are other application areas, e.g., transmission of power (mentioned by another contributor earlier), and then there is the particular case of the 4~20mA current loop, as typically used for process control. The 4~20mA current-loop was used a) to transmit variables in analogue form without loss of accuracy through wiring resistance, b) because the device at the remote end could be powered very conveniently by the 4~20mA signal itself, and c) because an open-circuit fault could be very easily detected (current falls to 0mA or rather, somewhere below 4mA). Another advantage cited for the current loop is better noise immunity, however, I'd welcome comment from anyone better informed about the underlying reasons for the noise immunity advantage - a topic for another discussion thread perhaps.

A remarkable story about the interplay between voltages and currents... I need time to realize all this wisdom... Thank you, David!

Cyril

Historically, voltage is easier to be measured, without breaking the orignal circuit.

Very valid reason to work with voltages instead currents; thank you, Falin. BTW is it still possible to measure a current without breaking the circuit? I remember experienced TV repairmen frequently used such a trick... Cyril

Yes, it is possible to measure current without breaking the circuit by sensing the magnetic field associated with the current in a conductor. Commercially available current transformers, current clamps, or current probes (for oscilloscopes) do this. Some probes use a combination of a current transformer and hall-effect sensor to provide an extended frequency response from DC up to 100MHz.

After 26 replies, perhaps my answer is useless. A popular transmission protocol makes use of 4 ma meaning LOW LOGIC LEVEL and 20 mA for the HIGH one. What happens if we measure 0 mA. Just the wire is broken! Look for the gap and join the ends!

And what happens when the line is shorted?

David, I mean a simple trick for measuring a current without breaking the circuit (based only on Ohm's law)...

Good answer! Typically, the 4-20 mA protocol is said to detect broken loops and academic texts state so. What happens if there os a short circuit? I suppose that the current is diverted and nothing, or very little, reaches thw load.

Hello Cyril,

I do not know the TV repair man's trick that you referred to.

One way to measure current in a circuit, without breaking the circuit, assuming that the circuit consists of

an unknown voltage source with an unknown but non-zero output resistance, and

an unknown but non-zero load resistance

is

1) measure voltage at the load resistance

2) connect a known resistance across the load resistance and measure the voltage again (so instead of breaking the circuit, we merely connect a resistance across the load).

3) connect another known resistance (different value to that used in step 2) and measure the voltage for the third time.

The data from these measurements allow you to set up three equations having the three unknowns (source voltage, source resistance, and load resistance), which can now be determined. With the unknowns determined, the current in the circuit can be determined.

This method meets two of the criteria - the circuit is not broken, and it is based on Ohm's law. However, I'm sure it would NOT meet the TV repair man's criterion of simplicity! Perhaps you can enlighten me about the TV repair man's technique?

Ooh David, your method is very interesting; it reminds me a similar method for measuring the device's output resistance by connecting loads. But IMO it would be too sophisticated for those good old guys from the good old days when TV and radio receivers were expensive but timeless...and they did not contain embedded microcomputers...

I imagined something simpler - they were trying to find a resistor of known resistance R connected in series somewhere in the current loop (e.g., Rb, Rc or Re); then they measured the voltage drop V across it, and finally, calculated the current I = V/R. But I don't know what they did when there was not such a resistor-:)

Regards, Cyril

Series insertion needs cutting of loop …..

Hi Cyril, Yes, if there is a known resistance in the current path already, and the repair man knew about that, he could measure the voltage drop and use I=V/R.

This discussion brings to my mind a current probe made by Hewlett Packard (many years ago, when the Hewlett-Packard name was still associated with electronic test equipment), the 547A current tracer . It was primarily intended for use in TTL logic circuits. If you had a short circuit to +5V or GND causing the node to be 'stuck at 1' or 'stuck at 0', you could use this current probe to trace the high current flow along the etched tracks of the PCB, in the hope of locating the short circuit. For this purpose, the probe had a small sensing tip that you placed over the etched track in question, and moved it along the etched track until you found the placed where the current flow stopped. The node had to be pulsed - the probe picked up on AC.

Although the probe was quite a sophisticated item of equipment, it was quick and simple to apply in fault-finding situations.

A very interesting story from the romanticTTL times when circuits were still accessible...computers were only computers... and cars - only cars... It provokes me to fabricate another Ohm's law story, now about good old auto mechanics for which electronics was like black magic:-) So, the tricky question is, "How could they measure the big current (hundreds of amperеs) of the car starter without breaking the circuit?" I assume they knew the Ohm's law:-)

Hi Cyril

Could you please tell me how do we basically inject high voltage or low voltage or high current or low current in any circuit and what should be the guiding element to help us decide what we should use where?