# Voltage Clamping Circuit need help

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#### panfilero

##### Member
I need to make a circuit that limits the voltage range in two direction, my range can't fall below 3.9V and can't go over 5.3V... does anyone know how I could make this? I've been thinking about using Zeners, but I see how that would be spiffy for one direction say a 3.9Vz but once I start going the other way I'd be limited by the Zener's forward voltage... around 0.7V... which is not good...

I need to be within 200mV of my target...

thank you

#### Tesla23

##### Member
I need to make a circuit that limits the voltage range in two direction, my range can't fall below 3.9V and can't go over 5.3V... does anyone know how I could make this? I've been thinking about using Zeners, but I see how that would be spiffy for one direction say a 3.9Vz but once I start going the other way I'd be limited by the Zener's forward voltage... around 0.7V... which is not good...

I need to be within 200mV of my target...

thank you
Lots of ways, one is to have two voltage rails at say 4.5V (= 3.9 + 0.6) and 4.7V (= 5.3 - 0.6), two diodes (assuming 600mV forward drop) and say a 4k7 resistor. You can refine it.

#### panfilero

##### Member
I'm sorry you lost me...

Are you saying something like... get two zeners, a 3.9 and a 5.3, put them in series with one another but pointing in opposite directions... and put that 2 zener combo in parallel with my load?

So in one direction it would be 3.9 + 0.6 and in the other direction it would be 5.3 + 0.6... basically Zener Fwd Drop + Zener Voltage in both directions...

Are Zeners ok to use in a fwd drop voltage limiting kinda way?

much thanks

#### eblc1388

##### Active Member
Zener voltage varies a lot and also depends on current. You are better off using a variable zener TL431 which enable you to trim the voltage easily.

I throw up the following circuit to see if it fits your requirement.

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#### MikeMl

##### Well-Known Member
About the best you can do without using an op-amp. V3 has to regulate when back-fed, so R2 provides a minimum load on it. Clipping levels at both ends adjustable by varying V1 and V3.

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