Forums » General Electronics Discussion

Hi all, I need to have a constant current source to feed a thermistor. However the power source should be a battery and, when discharging, it's voltage decreases. The result is that the current in the thermistor decreases then the measure is not precise. Any idea?

You'll need to create a constant voltage reference source and then use a bjt or maybe a bjt with an opamp wrapped around it to create the current sink.

The voltage reference has to be be less than the lowest battery voltage at which you expect the circuit to operate.

Try searching CL for:

learning resources

and

constant current

current source

current sink

Hi, thank you for the reply. I already tried that way using a LM317 but it seems that the output voltage depends on the input, then when the battery is low, even the output current is lower. I'll try again then I let you know

Ah. Yes.

The LM317 and LM137 CL models are, um, interesting ...

See:

I guess you've read:

http://en.wikipedia.org/wiki/Current_source

So this:

is what they are trying to show in the ASCII art between figs 6 and 7.

That's about as simple as it gets.

:)

Thank you! I already saw that page but i didn't try to interpreter the ASCII art. What about replacing it with your schematics? ;-) Now I try to learn it

Hi, I tried the second schematics but I think that there still is a problem

The current has a variation of the order of the variation due to the temperature changes, so it makes noisy measures, I think.. am I wrong?

@HAL9000,

Please make your circuit either public or unlisted.

Without it, your plot conveys little useful information.

The CL blog suggests using

http://imgur.com/

to post images but posting a link to the CL circuit and being able to run it's simulation is far more helpful.

If I get time, I may get myself set up to edit the wiki page ...

:)

I set it public here: https://www.circuitlab.com/circuit/3yz7ye/voltage-source/ Anyway this first part only aims to get a constant voltage source, not current

ok I tried your first schematics. I found out that the output voltage is stable only over 14.5 volts. Is there a way to reach the same behavior with much less voltage? I'd like to use a 9V battery or less

Er, I think you need to go to ti.com, find and read the datasheet for the LM137 family of adjustable regulators to see how to set o/p voltage.

Note that this series of regs has a roughly 3V dropout voltage (needs > 3V Vin-Vout for it to work properly.

Sorry, I meant LM317 (but the sums are pretty much the same for the LM137 ... just put a '-' sign in front).