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brumjam  
#1 Posted : Saturday, June 6, 2015 10:10:03 PM(UTC)
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last night a built a LCDPS and connected the 6v outputs from my transformer to supply 1 & 2. When I measure the output with my multimeter I can only get a voltage range (adjusting the pot) of between 6.1v and 7.5v. I was expecting to get down to 5v to power my Beaglebone Black.

Does this mean I've made an error when assembling the board? I've looked at photos of the assembled board online and mine looks the same in terms of components and parts orientation on the board.

Photo of board can be found here:

https://flic.kr/p/tXBdcN


Any ideas?

Thanks,

James

Edited by user Saturday, June 6, 2015 10:17:07 PM(UTC)  | Reason: Not specified

avr300  
#2 Posted : Sunday, June 7, 2015 8:03:09 AM(UTC)
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Should be no problem getting 5v out.

Check orientation of diodes d1-d4. From your picture its hard to tell.
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brumjam on 6/7/2015(UTC)
brumjam  
#3 Posted : Sunday, June 7, 2015 3:54:56 PM(UTC)
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Originally Posted by: avr300 Go to Quoted Post
Should be no problem getting 5v out.

Check orientation of diodes d1-d4. From your picture its hard to tell.


Here is a pic of D1. D3 has same orientation, with the light ring on the left-hand side nearer to where 'D3' is printed.

https://flic.kr/p/tkxxJ3
avr300  
#4 Posted : Sunday, June 7, 2015 4:58:52 PM(UTC)
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Well for sure its possible. Here you have it, with 8v AC input. Easy going down to 5v DC.

(never mind the crappy DMM)
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avr300  
#5 Posted : Sunday, June 7, 2015 5:08:49 PM(UTC)
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The circuit is so simple. If both diodes are in the correct orientation them.... hmm.

You might have a crappy pot.

When the pot goes towards 0 ohm, the output goes towards 1.25 volt.

You could place a jumper across the pot (see attachment), just to see if you can get 1.25v out. Just solder a piece of wire across on the back of the PCB.
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avr300  
#6 Posted : Sunday, June 7, 2015 5:11:25 PM(UTC)
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Yup
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avr300  
#7 Posted : Sunday, June 7, 2015 5:19:35 PM(UTC)
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If so, either replace the bad pot, or replace the bad pot with a resistor, ~720 ohm. This value gives you 5v out.
brumjam  
#8 Posted : Sunday, June 7, 2015 6:07:38 PM(UTC)
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Originally Posted by: avr300 Go to Quoted Post
If so, either replace the bad pot, or replace the bad pot with a resistor, ~720 ohm. This value gives you 5v out.


Thanks. So do I have the diodes correctly orientated? If yes then I have another pot here that I can try.
avr300  
#9 Posted : Sunday, June 7, 2015 6:35:47 PM(UTC)
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D3 silver ring pointing towards ac input side.
.

Edited by user Sunday, June 7, 2015 6:36:30 PM(UTC)  | Reason: Not specified

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brumjam on 6/7/2015(UTC)
brumjam  
#10 Posted : Sunday, June 7, 2015 8:38:05 PM(UTC)
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Originally Posted by: avr300 Go to Quoted Post
D3 silver ring pointing towards ac input side.
.


With a piece of wire across the pot pins as shown in your attachment I have 6.1v Brick wall As you say, this circuit is so simple yet I don't understand why it's not working.
avr300  
#11 Posted : Monday, June 8, 2015 4:03:20 PM(UTC)
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Since it's a LCDPS (low current DUAL power supply) it has a second side.

What about the other side. Is the problem also present there ?

.

Edited by user Monday, June 8, 2015 4:04:21 PM(UTC)  | Reason: Not specified

Brian Donegan  
#12 Posted : Wednesday, June 10, 2015 6:24:43 PM(UTC)
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Can you confirm that the two voltage regulators are both LM317, not LM337?
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