I want to send a constant current of roughly 0-3ma through a load with varying resistance (I think enough resistance that I need to use a 9v power supply switched by a transistor rather than directly powering it with an arduino pin), and to be able to switch it to go in two directions.
I can use an h-bridge and a potentiometer to do it by adjusting the load until it is right, but a)this is a pain, and b)it breaks if things change over time.
So, is there some obvious constant current h-bridge circuit that I don't know about which I can use for this? Or some kind of existing motor control chip that will give constant current at this low current?
This circuit will pass about 3mA (if the voltage across it is greater than about 2V):
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So put that in series with your load but to allow for polarity reversal from your H-bridge, connect it via a full-bridge rectifier.
To allow for the extra voltage drop of the rectifier and the circuit, increase your supply to the H-bridge from 9V to 12V.
I hope that works!
To change the current, change the 220Ω. Current = Vbe / R, where Vbe is about 0.65V. EDIT: I am assuming you are controlling your H-bridge using your Arduino but don't need to control the value of the constant(ish) current by your Arduino.
So put that in series with your load but to allow for polarity reversal from your H-bridge, connect it via a full-bridge rectifier.
Unless I am missing something that isn't going to work because the OP needs the polarity of the constant current source to reverse and the constant current source circuit shown can only be operated with polarity shown. It cannot be reversed.
An H-bridge only works for a bidirectional load (a load that can than operate with the current flowing in either direction)
A DPDT relay could reverse the two wires coming from the constant current source.
raschemmel:
Unless I am missing something that isn't going to work because the OP needs the polarity of the constant current source to reverse and the constant current source circuit shown can only be operated with polarity shown. It cannot be reversed.
That's why I wrote " connect it via a full-bridge rectifier".
Perhaps I should have written "connect it via a diode bridge" but I wanted to imply use of a bridge rectifier component rather than four diodes.
Ok, I get it now. The H-bridge load terminals go to the bridge rectifier AC input terminals and the DC output terminals of the bridge rectifier go to the "+" & "-" terminals of the CCS. Clever. The H-bridge output isn't a sinewave but I guess it would be hard to arguecthe fact that it's alternatinv current. ha ha.
joemarshall:
I want to send a constant current of roughly 0-3ma through a load with varying resistance (I think enough resistance that I need to use a 9v power supply switched by a transistor rather than directly powering it with an arduino pin), and to be able to switch it to go in two directions.
I can use an h-bridge and a potentiometer to do it by adjusting the load until it is right, but a)this is a pain, and b)it breaks if things change over time.
So, is there some obvious constant current h-bridge circuit that I don't know about which I can use for this? Or some kind of existing motor control chip that will give constant current at this low current?
thanks,
Joe
The normal way to do a programmable current source/sink is to use an op-amp as a voltage controlled
current source/sink.
