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June 17, 2019, 08:09:53 pm

Author Topic: How long is too long for resistors to bleed off the voltage?  (Read 166 times)

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Offline PS2500

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When I run the preliminary voltage tests on the transformer and power supply stage of the Eros it takes far longer for the capacitor to lose its charge after the 375V measurement. According to the manual "The filter capacitors store charge and the 270K bleeder resistors take several seconds to 'bleed off' that voltage".

In my case it takes a few minutes. For example, having just tested a third time, with the power on for around 30 seconds, it took 35 seconds after power off to drop to 200V, and then around 1'30" to get to 99V. And still reading 28V after 3 minutes.

Something wrong there. What do I need to check?

Offline Paul Birkeland

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #1 on: May 30, 2019, 07:46:22 pm »
I would be checking the solder joints on the 270K resistors.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline PS2500

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #2 on: May 30, 2019, 09:17:30 pm »
I couldn't see any problem with them (checked before posting the question; however I'm no expert at visually judging a good joint). Perhaps I just need to unsolder the wires from the power supply so that I can remove it, and then redo those two resistors.

Offline Adrian

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #3 on: May 30, 2019, 11:09:46 pm »
Perhaps just "re-flowing" the solder joints will help instead of completely removing them.
Reheat them and then add just a tiny bit more solder (or just reheating the existing solder only).
Adrian C.

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Offline PS2500

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #4 on: May 31, 2019, 03:24:26 am »
I did think about reflowing, but access is awkward and it's pretty tight in there to get both ends of the resistors - I want to avoid nicking other wires (insulation) with the soldering iron. I think it's actually easier in the long run, albeit slower, to just desolder them with the power supply removed and reinstall them. There's only a couple of wires that have to be disconnected to get the power supply off.

Offline Paul Birkeland

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #5 on: May 31, 2019, 03:53:06 am »
You can bend the diodes down a little bit so they are out of the way and just flow a little solder onto each end of those resistors from the top side of the board.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline PS2500

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #6 on: May 31, 2019, 04:48:11 am »
Thanks. I'll try that first then.

Offline PS2500

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #7 on: June 01, 2019, 05:11:17 pm »
No luck.

I have unsoldered the two 270 kOhm resistors, rechecked their resistance values with the meter, and resoldered them. I also added some solder from the top of the board to both leads of the 270 Ohm resistor (which is measured at the right lead) to make sure that connection's good.

 The voltage is still taking more than a couple of minutes to bleed off - still at around 140V after a full minute.

What would I need to look at next?

Offline Paul Birkeland

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #8 on: June 01, 2019, 05:21:10 pm »
Measure the voltages across each 270K resistor as the supply bleeds off.

The supply will bleed down incredibly quickly once you build the high voltage regulator and get it running, so provided you see the same voltage across each 270K resistor, I wouldn't get too hung up on it. 
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline PS2500

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #9 on: June 01, 2019, 05:49:52 pm »
Okay, thanks.

Offline PS2500

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Re: How long is too long for resistors to bleed off the voltage?
« Reply #10 on: June 01, 2019, 06:34:59 pm »
Yep, that checks out, so I'll go on to building the shunt reglator.

Thanks!