Hi All,
Can power supply be used as a current source? The power supply which I have has CC and CV mode. However, if I want to use as a current source (CC mode) it adjusts per load and goes to CC mode depending on the load and current limit. However, it is unable to give constant say 5 mA or any specified value. For IV measurement of thin film is it good to go with constant power supply, or constant current source, or source meter? I do not have a source meter/constant current source right now but I do have a constant power supply i.e Agilent E3617A which can give 0-60V/0-1A. Would it be good to measure a thin film with a resistance of the order of 1 k-ohm or so?
The CC on most bench power supplies acts more as a current limit than a constant current source. I would suggest using an op amp current source when trying to source currents under 10mA.
Thanks Mr. Reiserer for your reply. Could you tell me which would be better. BJT based current source or Howland current source (OP amp based). BJT based current source looks to be simple but I am not sure about the temperature compensation part. One good current simulator I found in internet http://www.falstad.com/circuit/. If you see in the circuits option of the simulator they discussed many standard circuits, one of them is constant current source in the "transistor" option. One issue is here, I am not sure how to vary the current steadily? Is it by varying the input voltage? But in that case current varies drastically as transistor beta is 100. Let me know your thought how to vary the current of the current source.
I think BJT current sources are a bit more difficult to work with than op-amp current sources. A simple voltage to current converter is quite easily designed with an op amp. By using a 1/2 Vcc reference, you can make a bipolar current source using a unipolar power supply and operate the op-amp within its linear range. This works quite well for a current source up to about 10mA with most op-amps. For measuring unknown impedance, this is typically adequate for most measurements. For higher currents, you may either use a more specialized high current op-amp or add transistors (also adds complication).
Really do you want to design a current source using a java applet ?
In the attached links you can find professional free circuit simulators with models of real OPAMP.
regards
S.
http://www.linear.com/designtools/software/#LTspice
http://www.ti.com/tool/tina-ti
Dear Ankur,
The power supply you have can be used as a current source. But it depends how much accuracy you want. You say you need to set 5mA for example. So the procedure I am going to describe will give you 5mA(+/-)x in CC mode where 'x' will depend on the design of your power supply. Typically it may be .1% of full range to 1% of full range (1mA to 10 mA in this case). Here is the procedure:
Suppose you want 5mA through your load. Connect your load to the power supply output. Set voltage and current potentiometers at the minimum (zero). Put On the the power supply. Try to increase the voltage to say 1 volt. The power supply will go into CC mode (with limit value appx zero). Now turn the current potentiometer and try to increase the current. keep eye on the current and voltage readings of the power supply and whether it is in CC mode or in CV mode. suppose you happen to reach 5mA with power supply still in CC mode. Take the voltage and current readings of the load. But suppose the power supply only goes up to 3 mA and then suddenly goes into CV mode. It means the voltage required is more than the max voltage set at present. It means you should turn the voltage potentiometer now. Since you got 3mA at 1V, you need less than 2 volts for reaching upto 5mA (I assume linear characteristic for the load.) So set voltage knob at around 2 volts. Now increase current until it displays 5mA. It must remain in CC mode.
If you need to go to 7mA now, increase the current again until it reaches 7mA or changes into CV mode. If it goes into CV, you need to increase the voltage setting and proceed as described above.
I do not know the I-V characteristic of the load and at what current, it may get damaged. If it may get damaged at 25 mA, do something so that load current is never able to reach 25mA. For this, you can do two things-
(1) Use a fuse of lesser rating than the damaging current.
(2) Use a resistance in series with the load. For example, your power supply is 60V rating. You will need a resistance of 60V/25mA= 240 ohm or less for this. With the resistor in series, you can freely do your experiment.
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