# constant current reversal circuit

Discussion in 'General Electronics Chat' started by dtak, Jan 27, 2010.

1. ### dtak Thread Starter New Member

Jan 11, 2010
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0
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

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
A simple pair MOSFETs would do this nicely, as would a standard relay.

How often do you have to switch?

3. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
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.

4. ### dtak Thread Starter New Member

Jan 11, 2010
10
0
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.

5. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
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.

6. ### dtak Thread Starter New Member

Jan 11, 2010
10
0
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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7. ### MikeML AAC Fanatic!

Oct 2, 2009
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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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8. ### SgtWookie Expert

Jul 17, 2007
22,183
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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.

• ###### 40uA Motor H-Bridge.PNG
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Last edited: Jan 28, 2010
9. ### dtak Thread Starter New Member

Jan 11, 2010
10
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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.