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Old 3rd February 2012, 06:21 PM   #1
mjock3 is offline mjock3  United States
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Default Power Supply problem

I have built 2- 5 volt DC power supplies. One seems just fine the other is running hot and noisy. I check the voltage coming out of both transformers(11.7 & 11.9) and they are within .2 v ac. After rectification 1 is 9.4 v dc, and the other is 13.85. What can be causing that? I am thinking that is why one of the regulators is running hot.

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Old 3rd February 2012, 06:30 PM   #2
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Regulator from China?
Replace and it works?
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Old 3rd February 2012, 06:50 PM   #3
mjock3 is offline mjock3  United States
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I am not following. After regulator voltage is fine. It is between rectification and regulation that is showing problems.?

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Old 3rd February 2012, 06:59 PM   #4
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Quote:
Originally Posted by mjock3 View Post
I have built 2- 5 volt DC power supplies.
Schematic? What type of PSU? A schematic or enough detail to know exactly what it is, is very useful.

Quote:
One seems just fine the other is running hot and noisy.
What does noisy mean exactly?

Quote:
I check the voltage coming out of both transformers(11.7 & 11.9) and they are within .2 v ac. After rectification 1 is 9.4 v dc, and the other is 13.85. What can be causing that? I am thinking that is why one of the regulators is running hot.

Thanks!
Are these PSU connected to a load? If they are switching PSU, they may need to be connected to a load to regulate properly, or at least have a ballast resistor between output and ground to cause a few dozens of mA current flow.

If they are not switching PSU, your rectified DC is too low for both of them, "probably" but without a schematic we can only assume. The second PSU measuring 13.85VDC, is not too far off an expected value if it is powering a load. The first measuring 9.4V seems to have a problem, perhaps a wiring or soldering fault, shorted capacitor or regulator.

Measure the voltage before and after the regulator. Is it the correct/expected value?
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Old 3rd February 2012, 07:19 PM   #5
mjock3 is offline mjock3  United States
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Click the image to open in full size.Not sure if the post is right as I have not done this before. But this is the first Linear power supply. The second is a variation of the first, in that it is done without the coil.
They both have a load on them at time of checking. Regulators are LT1084's.







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Old 3rd February 2012, 07:37 PM   #6
agdr is offline agdr  United States
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Measure the DC voltage at the output of the regulator chip on the "bad" supply. Remove the load and then measure it again, along with the voltage going into the regulator. What do you measure?
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Old 3rd February 2012, 07:39 PM   #7
DF96 is offline DF96  England
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You haven't told us which one works and which one fails. The one with the coil will behave a bit like a choke input supply, although modified by the small input cap. The one without the coil is a cap input supply so it will have much larger but shorter charging pulses. This could make the transformer heat up, also the regulator as it will have a higher input voltage.
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Old 3rd February 2012, 07:40 PM   #8
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Quote:
Originally Posted by mjock3 View Post
Not sure if the post is right as I have not done this before. But this is the first Linear power supply. The second is a variation of the first, in that it is done without the coil.
They both have a load on them at time of checking. Regulators are LT1084's
I usually run some low value (0.1uF - 1.0 uF) caps right next to linear regulators. They have a different time constant than the larger value caps and thus help suppress possible oscillations. I'd also try running the pair unloaded for a while to eliminate a possible loading problem.

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Old 3rd February 2012, 07:48 PM   #9
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mjock--

You may have the wiring of the transformer secondaries wrong. Check the phasing.
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Old 3rd February 2012, 08:07 PM   #10
mjock3 is offline mjock3  United States
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ChicagoJTW I looked at both trans and the appear to be wired the same. I am not quite sure what phase means though.

agdr: the voltages are as follows with load 13.78 in 5.64 out; no load 5.64 out 15.48 in. Let me explain the out. I am trying to use a music rail at the end of supply and it uses .5 volts.

DF96: the one that is acting up is the one above, I am just guessing it has to do with extra voltage.

thaumaturge: you know I was wondering if that could play a part, but I just didn't know.

Thank you all! Please let me know if anything I have said above gives cause for comment.

Mark
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