I need to build a conversion circuit. It needs to convert a voltage ranging from (135VDC to 160VDC) to a current ranging from (16mA to 8mA).
The only way I can think of to do this is to first build a voltage regulator at about 130VDC then uses an op-amp to invert a current output. One of the problems that I have is that I want the keep the higher voltage source isolated from the op-amp and output voltage source.
When Input is 135VDC the output is 16mA and when Input rises to 160VDC output drops to 8mA.
Is there an easier way to do this?

 

You could use a voltage isolator of some type as a starting point. Data for a typical unit attached - not a recommendation to buy this type, rather just to give an idea of what might be used.

The isolator gives you the advantage of a galvanic isolation of the high voltage from the measurement side.

If you need further advice come back.

 

Thanks for the reply. I looked at the link but I only have an 8VDC supply voltage. Do you have nay experiance with these voltage tranducers?
Can the output be inverted?

 

Well - if you only have an 8V supply that will restrict what you can do by way of electronic solutions for your problem.

Re the transducer - With the right additional circuitry you can manipulate the signal to give the required trend.

The only other limitation is that you are looking for a 25 V change on an average base of 147.5V i.e. a 17% change. So you can't use the full dynamic range of the transducer. Depends on the accuracy you require.

I have used several isolation devices of this type for various applications.

Another question - what current are you able to draw off the high voltage supply for measurement purposes - is a 15mA drain (say) going to upset the supply?

It might help if you gave a bit more information about your circuit and overall design concept.

 

The application is for an electric car conversion. I want the existing fuel gauge to show how much battery charge I have left. The existing fuel gauge is run off a buss from the Powertrain Control Module (PCM). The PMC has dampening built into it to account for the sloshing around of the gasoline in the tank (which is no longer needed since it is electric). I found the wires that read the fuel level. The fuel level sensor is a POT 70Ω when Empty and 1kΩ when Full. The voltage supplied to the POT from the PCM is 8VDC. There is also a GND supplied from the PCM available if needed.

I guess another way of looking at it is, as a voltage controlled resistor.
The input resistance is about 1.35kΩ.

In case you’re interested in the project
http://www.evalbum.com/2243
Thank you for helping.