constant current reversal circuit

Thread Starter

dtak

Joined Jan 11, 2010
10
Hi,

I am pretty new at this but have been given a task to come up with a design for a constant current source that can be reversed probably, I would guess, through an analog switch. The current source would be running a pump for an oxygen sensor which alternately fills and empties a measurement chamber, with the length of time of one cycle being related to oxygen concentration around the sensor. I have been looking around the internet but have been unable to find anything that might give me some guidance (at least that I can understand!) on how to reverse the current to the pump.

Any help would be greatly appreciated!
dtak
 

MikeML

Joined Oct 2, 2009
5,444
How much current? What is the compliance of the current source. Is the entire circuit "floating"?

I would probably just use two current sources and switch one or the other electrode to ground with a two Fets.
 

Thread Starter

dtak

Joined Jan 11, 2010
10
Switching rates would probably be on the order of 0.2 to 20 times a second depending on concentration

40 microamps for the current - at this point I am not sure if the whole thing will be floating or not.
 

SgtWookie

Joined Jul 17, 2007
22,230
Ahh, is that 40 milliamperes (40mA) or 40 microamperes (40uA)?

And what is the voltage requirement?

Just for starters, an H-bridge would be a semiconductor solution. A relay would be too noisy, and rather slow - and it's life would be limited.
 

Thread Starter

dtak

Joined Jan 11, 2010
10
40uA. The pump resistance at the operating temperature is about 1 kohm, so the voltage would be on the order of 0.04 volts. The sensor is a ZrO2 type see attached.
 

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MikeML

Joined Oct 2, 2009
5,444
The current sources are derived off a 5V supply. Since the pump resistance is low (~1K), 5V through 124K is a pretty good current source...

The Fets ground either one side, or the other, or both. The logic control signals A and B are non-overlapping. I'm assuming you need three states, +, -, and OFF.
 

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SgtWookie

Joined Jul 17, 2007
22,230
Variation on a theme...

This is using a 5v supply, a single 4000-series CMOS 4093 IC with a couple resistors, a cap and a log 5 Meg pot as a standalone astable multivibrator driving a low-current H-bridge. You can set the pot to automatically reverse the pump anywhere in the range you specified.

I used a 4093 because it has Schmitt-trigger inputs, which work well for things like that.

If you wanted to control it with an external 0v-5v signal, just remove C1 from the input of U1A, and connect your signal source. You could also remove R1 and R2 as well.

[eta]
There is no "off" function implemented; it's either forward or reverse. If the inputs to U1A are high, the motor runs in a forward direction; if low, in reverse.

I don't know if "off" is a requirement. If it is, you could simply turn off the 5v supply to the 4093 IC after placing a logic low on the input to U1A.

Not shown are the power/ground connections to the IC, nor a 0.1uF (100nF) poly capacitor across the Vdd/Vss (Vcc/GND) power terminals of the 4093 IC. Pin connections are available in manufacturer's datasheets.
Common part numbers for the 4000-series CMOS IC's are CD4093B, HEF4093B, MC14093B, etc.
 

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Thread Starter

dtak

Joined Jan 11, 2010
10
Thanks for the ideas, I really appreciate it! Definitely a long way from what I was originally considering. I will see what I can do.
 
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