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Old 14th January 2007, 10:04 PM   #1
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Default Problem building a power supply, no output after smoothing caps?

Hi all,

I am building a preamp and am having a bit of trouble with the power supply. I have a little 30VA toroid, dual secondary 15VAC, single primary.

I have measured (with my multimeter) the secondaries connected to absolutely nothing (not even rectifier), and they measure about right. Then I hook it up to a bridge rectifier (both one I built myself with 1N4007's and a 'store bought' 35A metal square thing), I see voltage on the multimeter (~22V DC).

Now, here's the problem. When I connect the output of the bridge rectifier to my PCB of filter caps.... I get nothing!

*cue bad ascii attempt at filter cap PCB*

PHP Code:
br + ||--------|------------|----------|----------|--------|| out +
           [
2.2 k]     [680uF]     [680uF]     [680uF]
br - ||--------|------------|----------|----------|--------|| out 


I am wondering, might this just be because of the 2.2k resistor? I am a bit of a newbie, and copied this from part of the supply I used for my power amp. That had 2x2200uF though...

The fuse isn't blowing (1A quick), the capacitors aren't exploding, nothing seems to be getting hot, there doesn't appear to be any shorts on the filter cap PCB (checked with multimeter)... I'm lost, quite frankly!
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Old 14th January 2007, 10:53 PM   #2
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I don't want to sound silly, but is it plugged in?

Try measuring the transformer output voltage when it is connected to the bridge and PCB. If the transformer voltage is OK and you're getting no output, your rectifier is wired wrong or you've got open diodes.

Are you sure the bridge is wired correctly and that you are connecting the transformer to the right places on the bridge(s)?

2.2k is a little low for a bleeder resistor, but it isn't the cause of your problem here. Wait, are you sure it is 2.2K and not 2.2 Ohms?

I_F
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Old 14th January 2007, 11:10 PM   #3
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Quote:
Originally posted by I_Forgot
I don't want to sound silly, but is it plugged in?
Heh, yep

Quote:
Try measuring the transformer output voltage when it is connected to the bridge and PCB. If the transformer voltage is OK and you're getting no output, your rectifier is wired wrong or you've got open diodes.
Well, that's the thing. It's doing the same thing with my DIY bridge rectifier *and* an actual proper bridge rectifier. But it only seems to do it when I connect the filter cap PCB. Measuring the transformer secondaries while connected... I also get nothing.

Quote:
Are you sure the bridge is wired correctly and that you are connecting the transformer to the right places on the bridge(s)?
Well, I have triple checked. I can't see what's wrong!

Quote:
2.2k is a little low for a bleeder resistor, but it isn't the cause of your problem here. Wait, are you sure it is 2.2K and not 2.2 Ohms?
Good question. Multimeter between -ve and +ve on the filter board read 2.2k when I checked... and resistors from the same pack also measure 2.2k, so ... yep!
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Old 15th January 2007, 12:05 AM   #4
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You have dual secondaries on the transformer. Are you sure you're connecting the right wires to the bridge? A volt meter will read strange things when it is the only load on the circuit.

Try putting a load resistor across the transformer winding and measure the AC voltage.

I_F
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Old 15th January 2007, 06:00 AM   #5
Nordic is offline Nordic  South Africa
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I have some transformers with weird output windings... with wire 1 and 3 forming a secondary and 2 and 4.... normaly one expect the pairs to be 1 and 2, and 3 and 4.....
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Old 15th January 2007, 07:05 AM   #6
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Quote:
Originally posted by I_Forgot
You have dual secondaries on the transformer. Are you sure you're connecting the right wires to the bridge? A volt meter will read strange things when it is the only load on the circuit.

Try putting a load resistor across the transformer winding and measure the AC voltage.

I_F
I really do hate to question the experts, but... what would a resistor across the secondary acheive? Should I do this with the bridge / filter cap PCB connected?

To clarify, all seems to measure OK (unless I am missing something) until I connect the filter PCB

Quote:
Originally posted by Nordic
I have some transformers with weird output windings... with wire 1 and 3 forming a secondary and 2 and 4.... normaly one expect the pairs to be 1 and 2, and 3 and 4.....
Well, I am reading right off the label. It says Red = 15v, Black = 0v, Yellow = 15v, Orange = 0v. Fairly sure I haven't got it wrong there


EDIT: Got the 'scope out. Straight from the secondary windings, I see a solid constant (rather dirty looking) sine wave. When I connect the bridge (either mine or the commercial standard 35A metal type), I see what looks like a half-wave (ie flat for a period, small bump, flat, small bump etc) which very slowly drops in amplitude until it might aswell be a straight line.

Any ideas?
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Old 15th January 2007, 08:19 AM   #7
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Quote:
Originally posted by markiemrboo
EDIT: When I connect the bridge (either mine or the commercial standard 35A metal type), I see what looks like a half-wave (ie flat for a period, small bump, flat, small bump etc) which very slowly drops in amplitude until it might aswell be a straight line.
Scratch that. I think my probe leads are just a bit finicky or something. I see a half wave, but it is a steady constant amplitude. It doesn't decrease over time. I'm not sure it's supposed to be a half wave mind...?

EDIT: figured out what it is now. It's steady when set to DC, but falls over time when set to AC [ on the scope ].
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Old 15th January 2007, 01:24 PM   #8
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Ah ha. Turns out to be loose connections, would you believe it! I am using so-called 'fast-on' connectors, and it was badly crimped it seems!

Probably better to use terminal blocks, or even solder I guess. I think I am gonna solder
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Old 15th January 2007, 01:44 PM   #9
radtech is offline radtech  United States
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The resistor is to put a load on the secondary, say for example there was a fault that caused a high resistance in the transformer, when there is no or little load on it you may read the correct AC voltage, but when loaded down it won't be able to supply the current and you'll read low or zero AC voltage.

You should do this with the bridge & caps disconnected, what you're trying to do here is determine if the problem is in the transformer.
You can try it with 2.2K, as that's the load you're bleeder is putting on it, to give it a bit more loading try something like 470ohms, at least 2 watts.

If this causes the secondary voltage to drop significantly, check the primary side (use appropriate caution) right at the transformer leads, there could be a poor connection in your wiring or fuse.

If the transformer checks ok, the next culprit would be the caps, make sure they're polarity is correct, and try connecting them one at a time.


Quote:
Originally posted by markiemrboo

I really do hate to question the experts, but... what would a resistor across the secondary acheive? Should I do this with the bridge / filter cap PCB connected?
EDIT: crud I have to learn to type faster.... well, I'll leave this as it may help someone else. Glad you figured it out.
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Old 15th January 2007, 02:35 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
it's safer practice to use a light bulb in series with the mains live lead to supply all new and maintenance projects at first start up.

Build up a plug top to light bulb holder to socket outlet.
The earth wire and neutral wire go direct from plug top to socket outlet.
The live lead from plug top goes to bulbholder terminal 1 and from bulb holder terminal 2 to socket outlet live connection.

The bulb glows bright or full on if there is a wiring fault.
It glows dim or off if there is almost no current flowing.
If it flashes briefly and then goes off it indicates a high initial current that subsides as transformer charges up or develops correct magnetic flux.

This saves major blow ups and severe overheating.
Keep it handy and use it regularly.
__________________
regards Andrew T.
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