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Old 28th December 2011, 02:40 AM   #17751
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Hi guys,

I built a PSU for my Fleas yesterday. I'm quite proud of it except for the fact that the 7818s don't seem to be behaving properly and I can't figure out why. According to my meter, they're both putting out 26.2 volts. I played with it all day today to no avail; tried inserting resistors to drop the input voltage and the output just scales accordingly, subtracting the voltage drop of the reg but not regulating it to 18v. I'm running out of ideas. Anyway, it looks quite nice :-)

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Old 28th December 2011, 06:17 AM   #17752
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Hi Ben,

Try a small capacitor at the output, something like 10uF. If that doesn't help, check if the GND pin isn't floating.

Ray
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Old 28th December 2011, 06:31 AM   #17753
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Hi Ray, thanks. I tried 10uF and 47uF caps but they made no difference. Probably a stupid question, but what should I ground to? I have no mains earth.
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Old 28th December 2011, 07:32 AM   #17754
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Look here. The center pin of a 78xx is known as GND. It should be tied to the - of the buffer cap and the - of the output. If it's left floating, the regulator can't regulate (?) . It will just output the voltage that's present at the input, minus a small drop. Looks like that's your problem here.

You need an output cap for stability as well.

Ray
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Old 28th December 2011, 09:24 AM   #17755
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Hey Ray, that's exactly how I have it set up. I thought you meant it needed some other ground connection, e.g. to the mains or chassis. The differences to your schematic are 2200uF input cap versus 1000uF (just what I had handy), no output cap (I did try 10uF and 47uF with no effect), and a 1k resistor and blue LED connected in series between OUT and GND.
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Old 28th December 2011, 09:30 AM   #17756
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Meter says input to reg is 37v, which is over max. Not sure why, as secondaries are 15v. That's slightly over the 35v limit. I wonder if I fried them...? Seems unlikely. I will add output caps again, perhaps an input resistor to bring to more like 25v and maybe take a trip to buy new regs.
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Old 28th December 2011, 10:05 AM   #17757
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Ben,

Look at the schematic and double check that you have wired the bridge with the diodes the correct way around.

If the transformer has 15V secondary windings have you mistakenly connected the wires from two different secondary windings into the same bridge (2x15xsqrt(2) = 42v) and thus have a floating ground ?

Jon
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Old 28th December 2011, 10:54 AM   #17758
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37V is a bit high for a 15V winding indeed, even with no load. Maybe you accidentally built a voltage-doubler
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Old 28th December 2011, 01:22 PM   #17759
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Thanks for the ideas everyone. I woke up at 5am thinking about this thing, so now I'm up and multimetering.

DC measures 32v at the board input, but 37v across the cap terminals and reg input pins. I've checked the diode directions, and double checked that the ground pins are connected. I've confirmed that there's no connectivity between the two circuits and that the secondaries are wired correctly. I can run either or both circuits with no effect. I've tried different output caps in the range 10uF to 100uF, also with no effect. I've tried powering from both 110v and 220v inputs with the same results: a steady 26.2v output on both sides as measured by my meter across the output terminals.

I'm conscious of the fact that I'm taking the thread a bit off topic and that I should probably move this somewhere else until I'm at installation stage.
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Last edited by Ben Hanke; 28th December 2011 at 01:26 PM. Reason: Typo (blaming cracked laptop screen)
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Old 28th December 2011, 01:32 PM   #17760
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Conclusion: your meter is defective. That's what you get when you buy a muktimeter...

You say 'DC measures 32v at the board input' but I assume that must be AC? If so, this is still way too high for a single 15V winding. You're using 120V input at Blue and Grey? There should be about 19V AC present between Red/Black and Orange/Yellow. Try measuring without the board connected.

Ray
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