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Electronics => Projects, Designs, and Technical Stuff => Topic started by: Bratster on October 03, 2017, 01:29:31 am

Title: Would like some help understanding power supply circuit
Post by: Bratster on October 03, 2017, 01:29:31 am

Hello,
I would like some help understanding what this circuit is doing.

This is from a fluke BC 190 power supply / battery charger for the 19x scopemeter series.

It seems to be mostly unregulated so I'm not sure why they put so much circuitry in this.

Two out of the three Active Components I'm not sure what they are. I just used a transistor schematic symbol, but I don't actually know what they are.

Thanks

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Title: Re: Would like some help understanding power supply circuit
Post by: rmacintosh on October 03, 2017, 01:45:19 am

The 431 makes it a 4.3v zener
The other marking DGs makes it a generic PNP transistor

Looking up small SMD markings parts is really easy with this cross reference
http://www.dl7avf.info/charts/smdcode/cc.html#TOC (http://www.dl7avf.info/charts/smdcode/cc.html#TOC)

Title: Re: Would like some help understanding power supply circuit
Post by: amspire on October 03, 2017, 01:55:46 am

The 431 is a TL431 regulator IC.

http://www.ti.com/lit/ds/symlink/tl431.pdf (http://www.ti.com/lit/ds/symlink/tl431.pdf)

It regulates so that the voltage between the REF to Anode is 2.5V.

Title: Re: Would like some help understanding power supply circuit
Post by: expinkolator on October 03, 2017, 03:47:39 am

The circuit probably looks like the attached. The value you have for R1 looks wrong.

Q3 emitter is held at about half supply which means about 2mA flows through R4. Some of that flows though Q3 and R5 generating a +ve voltage on Q1 gate to turn it on.  Q1 turning on pulls OUT- low until there is enough voltage between OUT+ and OUT- for the TL431 to start drawing current from R4 and tend to turn Q1 off. There should be 2.5v across R2 and 17.8 - 2.5 across R1, R1 and R2 values should have that ratio.

Your voltages indicate there is no regulation, Q1 has probably failed short circuit.

Title: Re: Would like some help understanding power supply circuit
Post by: Bratster on October 05, 2017, 05:49:15 am

Thanks everyone for the input!

I should be able to take another look at this this weekend.

I will look and see if that mosfet is shorted as well as check the other two of these that I have and see if they're output voltages line up with this one or not.


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