Dual Voltage power supply ??

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bobrown14

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Joined 2007
I've got an issue with a dual voltage power supply. I have a full wave bridge with 4 diodes fed by 2 12vac secondaries with a center tap. at each output from the bridge I have (or should have) negative 12v and positive 12v referenced to the center tap ground. I have 2 capacitors on each leg coming off the bridge and instead of getting 12v outputs they are out of balance with say +13.5vdc and -9.65vdc. Where should I be looking to get the balanced voltage out that the circuit requires. The circuit works sort of .... its a led light bank on a citation16/19. Could this cause of the imbalance be due to a possible cold solder joint to ground somewhere in the circuit??

I've been working on trying to get this circuit to work properly for a few days and I'm missing something ...

Voltage from the secondaries is the same measured at the input to the 12v circuit, tested diodes all measure the same, tested capacitors both the same uF (new replaced), with no load the measurements are different and with load the measurements are still different (same as without load).

Any help would be appreciated.

Cheers,
Bob
 
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bobrown14

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Joined 2007
I got it - pulled a leg on each diode to double check and after re-soldering them to the board I've got balanced voltage!! Light board still not working properly tho.... more cold solder joints probable......I couldn't tell even under VERY close look I couldn't see the bad solder joint, I was looking I swear!

Cheers,
Bob
 
rayma

rayma

Moderator
Joined 2011
bobrown14 said:
I got it - pulled a leg on each diode to double check and after re-soldering them to the board I've got balanced voltage!! Light board still not working properly tho.... more cold solder joints probable......I couldn't tell even under VERY close look I couldn't see the bad solder joint, I was looking I swear!

Cheers,
Bob
Click to expand...

Try getting some eutectic solder, 63/37 tin/lead. It's hard to go wrong with this type.
 
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bobrown14

Member
Joined 2007
rayma said:
Try getting some eutectic solder, 63/37 tin/lead. It's hard to go wrong with this type.
Click to expand...

I use Cardas Quad Silver Eutectic Solder - get it cheap of epay, I love it.

Been touching up all the solder joints on the LED board. Still have an issue. One of my ICs on the problem side is putting out negative voltage at the output when I want to see positive voltage - so I've got some more trouble learning to do with the LM741 ic. I've most likely got another cold solder joint I've yet to find. I did find a bad 1N914 diode thats on the output side of this chip... I'm going to have to figure out how to trouble shoot/test the ic. Lots of parts to test ....

Cheers,
Bob
 
rayma

rayma

Moderator
Joined 2011
bobrown14 said:
I use Cardas Quad Silver Eutectic Solder - get it cheap of epay, I love it.

Been touching up all the solder joints on the LED board. Still have an issue. One of my ICs on the problem side is putting out negative voltage at the output when I want to see positive voltage - so I've got some more trouble learning to do with the LM741 ic. I've most likely got another cold solder joint I've yet to find.
Click to expand...

Them you must have a soldering tip that's not hot enough, or corroded, or dirty. Before each connection, wipe the tip on a wet sponge,
and it should look shiny and clean. Hold the tip on the junction of the lead and pcb, and flow solder into the joint without touching the solder
directly to the tip. The metal should be hot enough to melt the solder by itself.
 
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bobrown14

Member
Joined 2007
AndrewT said:
$11.75 is not cheap.
That's $235 for a 500gm reel.

And then he adds on $10 for postage !
Click to expand...

Wow.. seriously? I bought 4 sets or 100g and it was free shipping there.
Good quality silver solder is getting more and more $$. I used to use the WBT German stuff that is like 2x the money or more @ $165/250gm roll, so I'm actually slumming it with this stuff. Thought I'd give it a try before buying a bunch. I like the WBT probably best but this is not too bad pretty close actually so for the price hard to beat unless I get a roll.

Cheers,
Bob
 
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bobrown14

Member
Joined 2007
DF96 said:
Why use silver solder? As you have discovered, it is harder to make a good joint. In electronic soldering the quality of the joint is far far more important than which alloy is used.
Click to expand...

The silver content 2-4% doesn't make the joint harder to make. Eutectic silver solder melts like butter and gets a nice silver finish to it so you know your joint is perfect. I do not and have never had any issues with silver solder and prefer it over tin/lead or lead free, not even close comparison IMHO. I do a fair amount of soldering.. probably every day or almost every day for several years.

Cheers,
Bob
 
D

DF96

R.I.P.
Joined 2007
myweb.tiscali.co.uk/g8hqp/
bobrown14 said:
I got it - pulled a leg on each diode to double check and after re-soldering them to the board I've got balanced voltage!! Light board still not working properly tho.... more cold solder joints probable......I couldn't tell even under VERY close look I couldn't see the bad solder joint, I was looking I swear!
Click to expand...
bobrown14 said:
The silver content 2-4% doesn't make the joint harder to make. Eutectic silver solder melts like butter and gets a nice silver finish to it so you know your joint is perfect. I do not and have never had any issues with silver solder and prefer it over tin/lead or lead free, not even close comparison IMHO. I do a fair amount of soldering.. probably every day or almost every day for several years.
Click to expand...
So using silver solder has the advantage that an experienced solderer using the right equipment can produce bad joints which look exactly like perfect joints?
 
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