Pass Transistor? - Page 1
Forum section: electronics / projects

author: mbear2k
author_rank: Regular Contributor
timestamp: January 08, 2014, 08:56:14 pm
content: Hello - I am working on a simple circuit to be used for capacitor leakage testing. My current working voltage is 0-76vdc and I am using a voltage doubler with a small 38v transformer. I borrowed the circuit below using what I think is functionally a Pass transistor. The transistor I'm using is on old silicon 2N1536.Two questions:(1) would someone be willing to describe how this transistor is being used in this circuit?(2) I'd like to swap transformers and get to around 300vdc so I can test the higher voltage caps. Can I safely do this with this transistor? And is the 2k, 5 watt pot (R1) sufficient? Current draw is minimal hence the usage of the voltage doubler.

author: mbear2k
author_rank: Regular Contributor
timestamp: January 10, 2014, 03:03:36 pm
content: Any thoughts - or is my primary question to vague?

author: don.r
author_rank: Frequent Contributor
timestamp: January 10, 2014, 03:24:37 pm
content: R1 acts as a variable voltage divider setting the base voltage of the pass transistor. This controls the current flow through ground to the DUT.You would need to add a resistor above R1 to ensure that the voltage to the base never exceeds the transistors specs if R1 is wound fully up.

author: SeanB
author_rank: Super Contributor
timestamp: January 10, 2014, 03:59:59 pm
content: Basically the transistor is being used as a voltage regulator, so that the output is regulated roughly to whatever is on the setting of the variable pot. I actually have some of those transistors around still, NOS, doing nothing.You will need a few changes to get to 300VDC rating. First would be a small isolating transformer, or easier is to get another 38V secondary transformer of around 10VA rating and use it in reverse to make an isolated 115VAC supply at low current using the existing transformer as a 38VAC source. The diode bridge would have to be a 1000V 1A unit, and the capacitors would be around 47uF 400V each. The pot would have to be a 20k to 100k 5-10W wirewound or carbon film pot, just to get the voltage withstand ability. Digikey CT2160-ND is a likely candidate. Caution it will run warm. Knob and such is your choice.Bear in mind at this voltage contact is very likely to be lethal, so be careful. No second warnings, you should not be doing this if you are not at least somewhat experienced. Plenty enough to make you very dead at this voltage, at best incredibly painful.You would basically exchange the meter and all the associated resistors and shunt switches to the negative rail, as you will have to use a high voltage high power transistor to provide the power supply. High voltage PNP transistors are both very rare and very expensive.You probably will be using a NPN transistor instead, likely a BUX85GOS-ND ( digikey part no) in a TO220 package, isolated from a small heat sink, as the pass device. Then it will be upgraded to a 300V unit. Power rating will be limited, and you would be best to place a 4k7 0.25W resistor in the base lead to provide some current limit ability to the supply. In any case it will not be a precision supply, but will be reasonably stable. It will not have a very high current capacity, as there only needs to be a limited current ( mostly from the transformers impedance) to initially charge the capacitor under test.

author: qno
author_rank: Frequent Contributor
timestamp: January 10, 2014, 04:47:03 pm
content: The transistor is used here as a current controller.With the potmeter you set the input current.You need to put in a base resistor o you will blow up the transistor.This means the capacitor is charged by current giving a linear voltage across the capacitor under test.this is a faster method than charging it only with a resistor.A good capacitor has leakage in the uA range. so your current shunts look a bit to low in ohms.

author: edavid
author_rank: Super Contributor
timestamp: January 10, 2014, 04:57:13 pm
content: Quote from: qno on January 10, 2014, 04:47:03 pmThis is wrong, the transistor is an emitter follower. A base resistor is not needed since the meter circuit resistance limits the current.BTW the 2N1536 has a VCEOspec of 60V, so it's not a good choice even for the 76V version.

author: mbear2k
author_rank: Regular Contributor
timestamp: January 10, 2014, 05:28:43 pm
content: Thanks guys - this is big help.I was aware of the current VCEO limitation and have only been testing with ~50v.The way this currently operates, the pot adjusts the voltage. I'm open to design ideas though - especially around current limitation.I'm very aware and respectful of these voltages - and while lacking a bit of theoretical knowledge, have been tinkering a long time. I'll admit I'm a parts swapper and pretty good at 'engineering' the physical builds, but want to learn more!Ideally, I'd like to build this as simply as possible with some safeguards in place for both equipment and the user. The purpose is primarily to test old caps as I remove and replace from vintage radios, audio and TV's - but also to help identify/troubleshoot where a found leaky cap is affecting a circuit. Much more trouble than it's worth probably, but something I'd like to do.

author: dannyf
author_rank: Super Contributor
timestamp: January 10, 2014, 05:56:10 pm
content: Actually all the posters above are correct: the wiper and the upper terminal of the pot determines the voltage applied to the dut. All the transistor does is to "isolate" the load current / charge-up current from the pot. Sliding the wiper up reduces the voltage applied on the dut.I would have built it with a npn, and I would have a resistor on the collector to soften the charge-up current.

author: mbear2k
author_rank: Regular Contributor
timestamp: January 10, 2014, 06:26:16 pm
content: Quote from: dannyf on January 10, 2014, 05:56:10 pmSo does the transistor still have a VCEO requirement equivalent to the working voltage?

author: dannyf
author_rank: Super Contributor
timestamp: January 10, 2014, 06:37:26 pm
content: Yes. Typically it is 2x of the highest rail. I would use a 13001 (they are easily found in a used CFL lightbulb).