bandpass filter ( values not frequencies)

Hmmmmm so its a sort of bandpass filter you want, but if the frequency is above the high cutoff you want the output to be at the high cut off frequency?? and vice versa for the low cutoff frequency?

Is that right, and why do you need that?

John.

Oh! or are you after a window detector for a dc voltage....?

A lot more info is needed.

i have a dc current between 4-20mA i have to pass the values between 6-15mA

I've done this with a PLC (analog in and then analog out) but I've never heard of a device that can do the same thing. There might be one but I don't know about them.

We used to do that with an Op amp, but with voltage imputs. We would limit the OA output between adjustable upper and lower values. If you used a burden resistor and converted milli amps to volts , clamped the levels, then converted Volts to current again it may work. Essentially you make a high gain voltage comparator on the output of the OA. If the output is below the threshold of the comparator it saturates negative, but the feedback from the comparator to the input of your OA is blocked by a low leakage signal diode. If the OA voltage tries to exceed the voltage comparitor threshold then its output switches polarity and the diode conducts and clamps the OA output. The lower value can be treated with a similar arrangement that limits the lower value.

The circuit was used to limit the current reference to a current minor loop in DC variable speed drives, the old analog style.

I think this is closest to a solution. The power supplies to the circuit might need to be floated so as not to affect the current loop. I also thought there might be a solution along the lines of a current regulator, where a sense resistor controls a transistor shunt when the current goes above the upper limit. The tricky bit was maintaining the minimum current when the input current goes below the lower limit.

Not sure what it is you want but you can use a phase locked loop to track frequency and then perhaps set thresholds to switch in various circuits based on frequency range of your input signal.

It sounds like you just need a simple limiter circuit.

Most limiters (also that of post 8) refer on voltage shaping. I would use a discrete 2-dollar-solution as follows:

1. Convert your current to voltage. This can be done by a simple 1k-resistor (if allowed up to this value; depends on your current transmitter and cable length).
Example: 4mA --> 4V and 20mA --> 20V.

2. Subtract 10V by an OP, then 4mA make -6V and 20mA make 10V.

3. Clip the positive voltage of 10V with a Zener diode to 5V and the negative voltage of -6V to -4V with another zener diode.

4. Add 10V by an OP, then you get 6V at 0...6mA and 15V at 15 to 20mA.

5. Convert your voltage with a programmable current source.

This is a first step-to-step-solution which can be simplified after drawing these single units. You can simplify this par example by connecting a 15mA current source (made by a cheap linear voltage regulator plus 1 resistor), this will do the upper clipping.

There are a lot of ways, hope that helps.
Regards Uwe

This is a good solution uweka. I was trying to think of a simpler solution but it's not easy to do this with current. As you've pointed out, voltage is easy to work with, and converting to current is easy enough.

My solution (using a PLC) was on the assumption that there was a PLC available. Obviously, it you don't have one around, you can't use it . However, making a limiter in a PLC program is very easy.

Good post.