Problem with the ad8420 configured as a transmitter to get 4-20mA at its output

ever.q21@gmail.com on Jan 4, 2020

Hi. I have a problem with a design that I made with the AD8420. I proceed to place the schematic diagram to explain more clearly my problem.

Design a transmitter that receives a voltage of 0 to 10V at its input. The output must obtain a current in the range 4mA to 20mA. For this I have used the AD8420 and I have been guided by the application note CN0314, I have followed its indications and the design I have made is the schematic diagram of the image that I placed previously.

The problem I have is that transistor Q5 (ZXT13N50DE6TA) overheats just as soon as I power the circuit with 24V and it happens that the power supply that I use to supply the 24V is placed in a short circuit and does not deliver the 24V. Upon receiving the temperature of each component I found that Q5 is the only one that overheats, so I proceeded to extract this component and verify the operation of the circuit. By feeding the circuit without Q5 present, the power supply is no longer short and delivers the 24V correctly.
I would like to receive your help in this unusual failure and listen to any suggestions to solve the problem.

0 Hooman

on Jan 7, 2020 1:58 AM
Hi ever.q21_4000_gmail.com,

Is your 24V supply isolated from your board ground? It should be or otherwise you have a short across your load resistor (R44, 50ohm). Here is what CN0314.pdf states:

"It is important for proper loop operation that the circuit board ground and the loop ground are not connected except for the R9 (R44 in your case), 50 Ω sense resistor."

You could use an isolated bench supply for the 24V source as an experiment and use the 250ohm RLOOP resistor shown CN0314.pdf.

Please let me know.

Regards,

Hooman
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0 ever.q21@gmail.com on Jan 8, 2020 10:49 PM in reply to Hooman

Dear @hooman thank you very much for your reply.

The 24v that I am using is not in isolation, now I understand part of the problem. I have made the indication he gave me and I have not had good results, I placed a 240ohm Rloop because I do not have a 250ohm on hand and the 24v are not present in the circuit. The resistance is very hot and the 24V input only receives 1.7V.
I have looked at the reference board CN0314 and I have noticed that the connection scheme is a little different from that of the application note. The difference I could see is that the FB PIN is not connected to GND and is directly connected to the emitter transistor, in the case of applying these changes could the problem be solved?

Should I still use an isolation in the 24V source?
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0 Hooman

on Jan 9, 2020 1:02 AM in reply to ever.q21@gmail.com
Hi ever.q21_4000_gmail.com,

Sorry, I don't exactly follow what steps you've tried and what results you are getting.

Can you tell me how your 24V source is produced? This source has to be floating with respect to your board ground. If you have access to a DC lab power supply then that would be a good source to use because its black / return terminal will be floating with respect to your board ground.

When you say there is a difference in the FB pin (your schematic pin 7 which is also tied to Q5 emitter and board ground) connection between CN0314 and the application note, what application note are you referring to?

Regards,

Hooman
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