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Author Topic: NPN or PNP?  (Read 279 times)

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Online Bealman

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NPN or PNP?
« on: May 25, 2020, 06:52:07 AM »
As documented on my layout thread, I discovered an old CDU which was given to me a long time ago. I have cleaned it up and put screw terminals on it ready for installation.

It is a standard design, using two capacitors, two resistors, three diodes and a power transistor.

I haven't tested it yet, but have a couple of curiosity type questions. I traced the PCB out this afternoon:



I'm thinking I may have the base, collector and emitter labelling incorrect. I cannot see what the transistor is, but it is in a TO126 package, making it either a BD681(NPN) or a BD682 (PNP) power transistor, I think. Both are rated at 40W 100V 4A.

If the unit works when I test it, I guess it doesn't really matter, but I'm curious as to which transistor it is, if anyone has any ideas.

Other question - the larger resistor is 390 ohms (I'm guessing half a watt, as it's physically larger than the 2.2 kilohm one).

Most designs of this nature I look at seem to use two 1k resistors. Is there any reason this circuit has 'strange' resistor values? Is it anything to do with the transistor?  :thumbsup:

Oh, and as I was editing some typos, I realised that I have placed the actual PCB upside down in relation to the diagram. Sorry!  :-[

Actually, have I? The diagram is from the copper track side. It's like right and left on anatomical diagrams. I always get mixed up!  :uneasy:
« Last Edit: May 25, 2020, 07:58:36 AM by Bealman »
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Offline Dorsetmike

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Re: NPN or PNP?
« Reply #1 on: May 25, 2020, 10:54:04 AM »
My guess would be NPN  connected as an emitter follower, R2 is either 390 or 39 ohms (orange/3, white/9 and either brown/ 1 zero or black/no zero) difficult to determine between brown and black from the image, again only a guess probably 39 ohms
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Offline ronp849

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Re: NPN or PNP?
« Reply #2 on: May 25, 2020, 12:29:28 PM »
I agree with NPN but I think the Base and Emitter labels are swapped. As it is, the main capacitors have no way of being rapidly charged. (Other than through the base  :goggleeyes:).

Ron
 

Offline Platy767

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Re: NPN or PNP?
« Reply #3 on: May 29, 2020, 04:55:03 AM »
NPN for sure and if it is a BD681 your pinout is reversed. From left to right, looking at the front of the device, the leads are ECB. The resistor on the LED is just to limit the current through the LED to a safe value. A 2k2 value will give about 5mA. I had a google and your circuit seems similar to this one that I found on this site:
http://www.eastbank.org.uk/circuits.htm



Mark

Online Bealman

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Re: NPN or PNP?
« Reply #4 on: May 29, 2020, 05:41:43 AM »
Thanks, Mark!

You've basically answered my question - when I traced it out, I had a feeling I didn't have the transistor leads labelled correctly.

As I say, this technicality basically doesn't matter if it works, though I do like things documented, in case things go wrong later. Ok, I could replace it with a whole new unit - it's not exactly gonna break the bank - but I just like fixing things!  :thumbsup:
« Last Edit: May 29, 2020, 05:43:40 AM by Bealman »
Vision over visibility. Bono, U2.

 

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