# 0-5v voltage clamp and pass thru?

Discussion in 'General Electronics Chat' started by elRey, Jul 6, 2009.

1. ### elRey Thread Starter Active Member

Feb 23, 2009
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0
0-5v voltage clamp and pass thru?

DC regulated 5v.

Hello, I need a simple circuit that takes a 0-5v input from a sensor and has 2 outputs.
1) a straight 0-5v pass thru (same as input). I could just use input
2) an adjustable clamp with range 3v to full 5v (no clamping).

I'm currently using a zener diode, but have no way to get the non-clamped voltage.

Thanks,
Rey

2. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
The secret is to use a "variable" zener where one can set what voltage the zener diode become active(conducting), via a variable resistor or POT.

The part name is TL431. The attached image shows how to use it. Vref is always 2.5V.

• ###### TL431_V.png
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Last edited: Jul 7, 2009
3. ### elRey Thread Starter Active Member

Feb 23, 2009
65
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So, with above circuit can I also get the true input voltage along with the clamped voltage?

4. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
As you have said, you can get the true input voltage using the input as is.

The above circuit only provide a variable level clamped output.

5. ### elRey Thread Starter Active Member

Feb 23, 2009
65
0
Great! Another quick question. With the zener diode I'm using now, I have no resistor between input source (sensor) and diode. And any reading I take between the sensor and diode is clamped. If I add a resistor between sensor and diode, can I get a true (non-clamped) reading before the resistor and a clamped reading after the resistor?

Thanks,
Rey

6. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
I would say you probably can.

Here is a quick simulation. Note that the clamp voltage is lower than the zener rated voltage because the zener current is small.

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7. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
You don't mention the impedance of your sensor.

Placing a voltage clamp across the sensor's output without limiting the current may damage it.

You may need a buffer circuit between the sensor and the clamping circuit to prevent the sensor's signal from being overloaded.

8. ### elRey Thread Starter Active Member

Feb 23, 2009
65
0
I greatly appreciate the help!!!!
Sgt, I and many others have been running 4.3v and 4.7 zener diodes and no resistor on our MAP sensors for a while now without killing the MAP. Is that any indication of the impedance in relation to the sitiuation you mention?

9. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
As far as I know, the power for the MAP comes from the ECU.

If you're clamping the output voltage to a certain level, you might risk engine damage when running near WOT (wide open throttle).

When at lower throttle settings (below perhaps 70%), the ECU will attempt to maintain a stochiometric (14.7/1) air/fuel mix by looking at the O2 sensor output.

But since I'm not sure what you're trying to do, go ahead and 'splain it to us.