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4-20mA Loop powered isolated transmitter

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ele2freak

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Hello All,

I am working on a circuit design for isolated 4-20mA transmitter. I found an example circuit and try to simulate it in spice. Receiver functions very well, transmitter has some problems. The current flowing thorugh loop is always around 3.4mA. I couldnt find out why: Can somebody help me?

Between +ILOOP and -ILOOP there is a 24V voltage source in spice simulations. I measure the current there. Input is 0V to 4V.

Thanks for any help.

Regards
 

Your photodiode circuit doesn't look right. There will be a voltage drop across R5 and this voltage is fed into the inverting input of the opamp through R3. This will take the inverting input below the opamp GND. It will not work with inputs below ground. My guess is your transistor will always be ON, althiough I would have expected a lot more than 3.4mA from a 24V source in that case.

When the photodiode voltage is high enough the transistor would turn off.

Keith.
 

Hi Keith

Thanks for recomm. I quickly built the circuitry on breadboard. There was continuously ~28mA on circuit.

I guess the problem (as you said) the input side of opamp. Thus I swapped the inputs. Vin+ should be above, as the voltage difference is going to be positive and output of LM158 turn on 2N3904. However I am observing oscillation as attached. BJT is turning on and off and thus whole systems is modulated. Do you see why or how can I avoid it?

Regards
 
My guess is that you need a very large capacitor on your supply. When the transistor turns on the power supply collapses so the transistor turns off etc. Try 100uF instead of 100nF for the decoupling across the opamp power supply.

Keith.
 

Hi Keith,

Thanks. I found also another problem. The polarity of loop powered LM158 should be swapped and also capacitor (1nF) should be kept at Vin- for feedback. When I fine tune few resistor values, the system functions perfectly as 4mA-20mA loop transmitter. So it is an fully isolated 4-20mA.

Tomorrow I am going to give a try on breadboard at university's lab. Hope it functions as simulator shows.

Regards
 

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