Help with a current loop circuit

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jenifer85

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Hi I am currently designing a current loop circuit based on this Maxim's app note:
**broken link removed**

I attached my circuit. The problem is that the op-amp is heating. I have tried different op-amp and it is always heating. I tried LM2904 and LM7332. When I disconnect the op-amp output from the transistor it is not heating so it is a problem with the circuit not the op-amp.

Could someone help me and/or explain me why the circuit would make heat the op-amp.

Thanks
 

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hi,
The transistor connected to the OPA output is effectively shorting out the OPA, so the OPA will get hot.
EDIT:
Resistor Ry, transistor base resistor shown in the datasheet is missing from your circuit.
 
Last edited:
ok thanks for the quick reply.

thats what I tought. But how to select a good resistor value I don't understand their explaination in the app note?
 
jenifer85 said:
ok thanks for the quick reply.

thats what I tought. But how to select a good resistor value I don't understand their explaination in the app note?
Click to expand...

hi,
Look at this image from the d/s.
 

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Yes I tried a 5k.. but there's no result. The op-amp is still heating, not alot but still. I really need it to not heat because it's a small temperature sensor application.
 
Oh problem seems to be solved! I changed the value of Rx. And it's not heating! now I just need to adjust resistors to get the good voltage.

Thanks
 
I'm not sure you are using the circuit as the Maxim ap note shows. The +24V supply return (V-) must be connected to the output, not GNDV. Is that the way you connected it? If not, that would explain the overheating of the op amp. The 20mA loop is through the supply voltage and thus the supply must be isolated from circuit gnd.
 

hi Carl,
I suspect Jenifer is using just an OPA rather than the IC, perhaps she will confirm that or not.

The circuit as originally drawn is wrong and will give problems.
 
Yes I am using an op-amp, the LM2904. Im not using the IC.
I put a resistor(Ry) at the output of the op-amp and the circuit is functional now.
 
jenifer85 said:
Yes I am using an op-amp, the LM2904. Im not using the IC.
I put a resistor(Ry) at the output of the op-amp and the circuit is functional now.
Click to expand...

hi Jenifer,
Pleased to hear it works, I would be interested to see the final circuit.?
Is it giving the required 4 to 20mA for the input voltage.?
 
Is your power supply isolated as I asked in my previous post? If not, that would explain the heating.
 
crutschow said:
Is your power supply isolated as I asked in my previous post? If not, that would explain the heating.
Click to expand...


What do you mean by isolated power supply?
I send you my full schematic.

V+ of op-amp is 24V and V- of op-amp is GNDV( virtual ground)
 

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Isolated means the minus side of the power supply is not connected to ground. Your "full" schematic does not show where the minus side of the power supply is connected. It should be connected to the output, as I previously stated, and as the Maxim article shows.

It would be a courtesy to read my previous posts before replying.
 
Ok im sorry I have read your posts. But in my schematics I can't connect something on the op amp V+ and V- but I said that V+ was at 24V and V- at GNDV.

But Ill try to connect Vminus to the output instead.

Thanks
 
I did switch the Vminus connected to the output. And it's the same result transistor is heatins :S

Any other ideas about the problem?
 
I simulated you circuit (see below), and it worked if I changed the value of R11 to 1k ohm. Since this is a 20mA loop, it's obvious that R11 can't be greater than that or you can't get 20mA from a 24V supply.

I also put a 1k ohm pot between 5V and ground with the wiper connected to R7 to vary the input. The output current goes from about 4mA to 20mA as is desired when the input goes from 0 to 5V.

Edit: So if your circuit is not working (after changing R11) I suspect you have a misswire somewhere.
 
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Hi,

thanks for showing me that circuit!

I tried it and it's true, it is working I get 4mA to 20mA.

The transistor and the op-amp are not heating! But it's R11 that is heating now. I used a 1/4W.

Correct me if Im wrong but. In R11 there's 24V with a maximum of 20mA. So 20mA*24V = 0,48mW
If I take a resistor of 1W I should be ok?
 
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