Author Topic: FMMT597 transistor not working  (Read 1500 times)

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Offline Chris HaslerTopic starter

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FMMT597 transistor not working
« on: July 12, 2018, 02:01:04 pm »
I have a transistor circuit which is not working properly. There are six of them all failing the same way.

There is a PNP transistor (FMMT597, datasheet https://www.diodes.com/assets/Datasheets/FMMT597.pdf) switching a 24V line to a LED and resistor.

This is switched on/off from a 24V line via a potential divider.
24V - 1k5 - Base - 12k - NPN transistor - 0V

The NPN transistor is triggering correctly, but the PNP transistor is continually passing 15V.

Does anyone have any idea what I have managed to screw up on this?


edit: I have swapped out the PNP transistor for a MPSA92G and achieved the same result, but with 15.6V passing the current.
« Last Edit: July 12, 2018, 02:20:47 pm by Chris Hasler »
 

Offline Zero999

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Re: FMMT597 transistor not working
« Reply #1 on: July 12, 2018, 04:13:53 pm »
Please post a schematic. It's impossible to know what the problem is, without one.
 
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Offline Chris HaslerTopic starter

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Re: FMMT597 transistor not working
« Reply #2 on: July 13, 2018, 11:36:38 am »
Please see attached image with the schematic and measurement points highlighted. The rest of the circuit is not attached to the connector at the moment.

 

Offline Zero999

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Re: FMMT597 transistor not working
« Reply #3 on: July 13, 2018, 12:36:45 pm »
The PNP transistors are connected backwards, so are behaving like 7V zener diodes. If the collector emitter connections are corrected, it should work, but it's also possible the transistors have been damaged, due to the reverse voltage. I recommend replacing the PNP transistors with new ones, connected correctly.

The schematic is also drawn in a confusing manner, which makes mistakes more likely.
 
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Offline CJay

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Re: FMMT597 transistor not working
« Reply #4 on: July 13, 2018, 12:47:34 pm »
Sure that schematic is correct only it looks wrong to me, emitters of the transistors should be connected to +V and collectors to 0V surely?

*edit* damnit, Hero999 posted while I was editing :)
 

Offline Zero999

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Re: FMMT597 transistor not working
« Reply #5 on: July 13, 2018, 12:59:03 pm »
Dough, I wanted him to figure out the correct places for the collector and emitter connections.
 

Offline CJay

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Re: FMMT597 transistor not working
« Reply #6 on: July 13, 2018, 01:08:34 pm »
Ooops....  |O

'Left as an exercise for the reader'
 

Offline Chris HaslerTopic starter

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Re: FMMT597 transistor not working
« Reply #7 on: July 13, 2018, 01:15:30 pm »
Didn't realise that the schematic would be wrong as its a rework of a through hole board to smd.

So the next question is how did it work on the last 2 board revisions?

I will have to work out if the boards are salvageable.
 

Offline Zero999

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Re: FMMT597 transistor not working
« Reply #8 on: July 13, 2018, 01:50:40 pm »
One possibility is the footprints for the transistors, on the previous board revisions, had their collector and emitter connections reversed, so it worked.
 

Offline CJay

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Re: FMMT597 transistor not working
« Reply #9 on: July 13, 2018, 02:22:53 pm »
One possibility is the footprints for the transistors, on the previous board revisions, had their collector and emitter connections reversed, so it worked.

Or if the last boards were through hole the transistors might just have been fitted bass ackwards and the schematic has never been correct...
 


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