Author Topic: eev-esque constant current dummy load.  (Read 1550 times)

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Offline dda

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eev-esque constant current dummy load.
« on: August 18, 2012, 07:38:05 am »

I  decided to have a go at making a constant current dummy load as seen on eevblog #whatever.

I decided I would try to power it from a 3V coin cell, and the op-amp i had laying around was a lm348n. The non-inverting input is 0-1.5V obtained through a pot/voltage divider. Ideally I would have a 0V to 1.something V constant current load.

Now the problems.

Using 3V as Vcc doesnt allow the voltage to change as the non interting input is altered.
Powering from 9V works, but the lowest output is 0.6V(i.e, A), Id want it lower. <~100mA ideally.

Obviously the op-amp I had laying around (lm348) isnt suited for this application.

So... Is there a better op-amp to choose. is a lm324 (as dave used) going to work better? What value on the data-sheet do I need to be looking at to meet my objectives?

Any suggestions would help

Offline Rerouter

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Re: eev-esque constant current dummy load.
« Reply #1 on: August 18, 2012, 08:07:46 am »
what you want is an op amp with a common mode input and output range including the negative rail (generally listed in the fist page of schematics)

as your signal level is 0-1V its very easy, just make sure your input range is better than vcc-2V, generally this means only 1.5V Maximum on the really cheapy ones so it shouldnt be an issue, for the input, 

however for the output level, you run into an issue, if your aiming to make the circuit as simple as possible with a 1 Ohm measuring resistor so the signal levels match, then a vcc-1.5 wont cut it, and you will have to spend a few more cents, as at 1A your resistor is at 1V, when you add on the base-emmittor junction of the resistor this comes to 1.75V which is higher than the cheapy can handle,

short version: an op amp with an input range of 0-greater than 1, and an output range of 0 to greater than 2, those are the specs you want to make it work,

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