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Bentham 605 Current stabilised Power Supply

dperry81

Jan 31, 2023
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Hi all,

I am trying to repair this Bentham power supply.
The FETs keep on blowing up. I have replaced them and same problem - I've measured everything else and it seems to be fine when compared against the datasheets. I have also checked for shorts and cannot find anything.
I am connecting a lamp between "red" and "black" on the right side of the schematic and it's supposed to give 12V when connected (we have another unit that is working).
When the FETs were replaced, they started at 12V, but then slowly ramped up to 38V and blew the lamp.

Any suggestions to why this may be happening?605 Circuit diagram[6006].jpg
 

kellys_eye

Jun 25, 2010
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The board schematic indicates the off-board devices are transistors (i.e. connections to 'e' and 'b'), not FETs. Do you have the part number for the original devices used?
 

dperry81

Jan 31, 2023
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The board schematic indicates the off-board devices are transistors (i.e. connections to 'e' and 'b'), not FETs. Do you have the part number for the original devices used?
I'm very confused by your question - I can see the "E" connection in the top left of the schematic, but not the "B"

The E in the top left refers to earth doesn't it?
 

kellys_eye

Jun 25, 2010
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Where you have drawn the FET symbols, their connections (i.e. the gate) are connected to the 'board' at points marked 'b' and the sources at points marked 'e'.

'b' and 'e' usually refer to transistor connections - base and emitter, not FET connections.
 

bertus

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Nov 8, 2019
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Hello,

As @kellys_eye said, there must be power transistors used and not fets.
The schematic clearly shows the Base and Emittor connections:
Bentham output driver.png

Bertus
 

dperry81

Jan 31, 2023
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Apologies - my mistake.
No part of the schematic was drawn or edited by me, It was provided by the manufacturer.

TR1 and TR2 are w45nm50 (as shown) and not power transistors. The only other "transistor" type thing in the circuit is IC1 voltage reg.
 

kellys_eye

Jun 25, 2010
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No part of the schematic was drawn or edited by me, It was provided by the manufacturer.
The fact that the schematic has been 'modified' by hand-drawing the FETs in makes me suspicious..... it may relate to a later model and/or upgraded device rather than the original unit.

I'd (temporarily) wire in some transistors to prove correct operation first....
 

dperry81

Jan 31, 2023
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Thanks for your help.

I've checked the schematic against the unit and it matches up.
Which kind of transistors would you use? Will any generic NPN do?
 
Last edited:

kellys_eye

Jun 25, 2010
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'Any' reasonably large NPN device will work (TIP3055 for example) but all you're going to do at this stage is fit a pair to prove that the system works - if you can vary the voltage and limit the current with 'any' transistor (bearing in mind the current/voltage restrictions based on the transistors you use i.e. say 10A and 50V for the TIP3055) then you can find/fit whatever devices are readily available and/or fit the power range you use.
 
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