# Clip signal with very low threshold

Discussion in 'The Projects Forum' started by pauloborges, Oct 26, 2012.

1. ### pauloborges Thread Starter New Member

Oct 26, 2012
3
0
I need to limit a signal to the following range: [-0.25 V, 0.25 V]. This signal is comming from a shunt resistor in series with a load with variable impedance on a 220 V power grid.

The project contemplates loads up to 1 A peak, but I can't guarantee this restriction will be obeyed all the time. But I need to guarantee the resulting voltage don't exceed the specified range.

To give a small guarantee, I'm using a shunt whose equivalent resistence is approximately 0.235 Ω, which generates a voltage slightly below the limit when 1 A is consumed.

I cannot use normal diodes because its forward voltage is too high. I've tried to use two low forward voltage Schottky diodes (BAT46), but didn't worked perfectly in my simulations.

Here's my current schematic:

OBS: the thing that looks like a voltage source on the right side is a voltmeter, you can ignore it.

Where:
• R_IC is the IC's input pin impedance, according to its datasheet.
• R_LOAD is the already mencioned variable load. In most circumstances, it will consume less than 1 A peak, in another words, its "average" impedance will always be above 311 Ω. But I'm trying to avoid the range of the input pin to be exceeded in possible peaks (when the load is connected, for example).

The result:

Where both signals have a scale of 0.1 V/div.

As you can see, the signal is being clipped, but in a threshold above the allowed [-0.25 V, 0.25 V].

How can I modify this circuit to achieve the correct threshold? I'm focusing primarily on solutions involving components easily obtainable (even if the circuit design change completely, I was thinking about use some op amps), because I live in Brazil and here there is a very low supply of components. So if I need to buy something "different", I have to import and it will take weeks to arrive.

2. ### wayneh Expert

Sep 9, 2010
15,551
5,758
You could use a simple resistive voltage divider, maybe using a multi-turn pot which would allow you to dial in whatever safety factor you need.

3. ### pauloborges Thread Starter New Member

Oct 26, 2012
3
0
I can't see how a resistive voltage divider can establish a threshold to a signal. It will only decrease the signal proportionally. Right?

4. ### wayneh Expert

Sep 9, 2010
15,551
5,758
Well yes, but you could also use a zener diode before the divider, or between two dividers, to limit the signal. Just be sure to also limit the max current through the zener so you don't destroy it.

5. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
Is there some reason you haven't mentioned the IC you are applying this signal to? We might be able to provide more help if we knew the application.
Do you have positive and negative power supplies available?
Does the signal wave to be proportional to load current when it is below the clipping threshold? In other words, are you sensing the actual amplitude of the voltage across the 0.235Ω resistance?

6. ### pauloborges Thread Starter New Member

Oct 26, 2012
3
0
No reason, really. The IC is a CS5463 (datasheet).

I don't have negative supply and I think the solution might involve op amps, thus I will need it. How can i make it?

Yes. My intention is to sense the current across the load (voltage across the shunt).