Discharging capacitors, PSU-testing with dummy loads

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Pacman
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Discharging capacitors, PSU-testing with dummy loads

Post by Pacman » Tue Aug 29, 2017 12:05 pm

I have two questions concerning before and after recapping an Atari PSU:

1) what is the safe and proper way to discharge a (high voltage) capacitor in the PSU?
I prefer not being electrocuted while attempting to desolder it ;)

2) Regarding the use of PSU dummy load resistors (so I can test the PSU alone without any risk to the computer) you explain to connect the 10 Ohm resistor between blue and black (if the colours are standardized between the various Atari PSUs this should mean to put it between the -12V and GND outputs, right?).
Likewise, the 2.2 Ohm resistor between red and black (+5V and GND?).

Does this only allow for testing the +5V and -12V outputs?
For testing the +12V and -5V outputs, do I need to move the resistors to those pins and GND, or will the above allow the entire PSU to be tested?
I suppose 30 seconds should be enough to test if the voltages are right. Is there a 10% tolerance?
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by exxos » Tue Aug 29, 2017 2:57 pm

Pacman wrote:
Tue Aug 29, 2017 12:05 pm
1) what is the safe and proper way to discharge a (high voltage) capacitor in the PSU?
I prefer not being electrocuted while attempting to desolder it ;)
The PSU should self discharge in under 10mins. So always check the voltage on capacitors before desoldering.

Most PSU's have a discharge resistor, normally 10K - 100K across the capacitor to discharge it, but I do not see it on the schematic. But it may discharge anyway as the switchmode will still operate when disconnected from the mains, as the voltage will still be 400V. The capacitor will discharge as the switchmode "runs out of juice". So its time at 400V after mains disconnection should be low. But I have not tested how long this takes on that PSU.

Should the voltage be higher than 50volts after 10mins (I would expect unlikely), the proper way is with a "discharge wand". This is just some isolation probes (like found on multimeters) with a 10K 5watt resistor. You hold on the capacitor terminals until the voltage drops to near zero. This may take 30 seconds or more depending on charge left in the capacitor.
Pacman wrote:
Tue Aug 29, 2017 12:05 pm
2) Regarding the use of PSU dummy load resistors (so I can test the PSU alone without any risk to the computer) you explain to connect the 10 Ohm resistor between blue and black (if the colours are standardized between the various Atari PSUs this should mean to put it between the -12V and GND outputs, right?).
Likewise, the 2.2 Ohm resistor between red and black (+5V and GND?).

Does this only allow for testing the +5V and -12V outputs?
For testing the +12V and -5V outputs, do I need to move the resistors to those pins and GND, or will the above allow the entire PSU to be tested?
I suppose 30 seconds should be enough to test if the voltages are right. Is there a 10% tolerance?
Those resistors were to test the STF/E PSU so other voltages wouldn't be tested. You can still use them to test the 5V and 12V rails. The -12V needs something like a 330R resistor to try.

I am not sure what he colour codes are for the MSTE PSU, I only had the PSU on loan for a short time and I never thought to document it.
Though I would assume red is 5V and yellow is +12V.

Loading those with resistors should be enough to start the PSU (you will need to tie PG to 0v to turn on the PSU also).

I would suggest you use a meter on resistance check (low ohms range) and trace back the colours on the connector to what they are connected to on the PCB (powered off of course).

Then you can trace which colour goes where on the schematic. Then you will find PG and -12 & -5V.

From what I can see on my image on my site, orange wire looks near R32, which is the PG circuit. So assume that is correct. I also make assumption white is -5 and blue is -12.

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Re: Discharging capacitors, PSU-testing with dummy loads

Post by troed » Wed Aug 30, 2017 6:28 pm

Also, the PSU might not start up as it should if you only connect the 12V test resistor. You need to load the 5V rail as well with the other one.

(I don't recall which PSU I did when I realized this, either Falcon or MSTE I think)

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Re: Discharging capacitors, PSU-testing with dummy loads

Post by exxos » Wed Aug 30, 2017 10:00 pm

Loading the 5V alone should be enough to start the PSU as the feedback opto is coupled to the 5V rail. The 12V rail is likely based on turns ratio on the core (which is also why the 12V is mostly "screwy" sometimes with output voltage). But one should really load the 5V and 12V for initial testing. Once thats verified, load the other voltages (once the wire colour codes have been identified).
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by Pacman » Thu Aug 31, 2017 12:27 am

Thanks guys.

So you're saying the PSU needs to have a load on the +5V line (a 2.2 Ohm high-wattage resistor between +5V and GND)
and...
a wire connected between the "PG" (Power Good) line
or else the PSU won't turn on at all?
And with that I can also check the +5V line (DC voltage check between +5V and GND with a digital multimeter)?

And to continue checking the rest of the PSU (the -5V, +12V and -12V lines) I have to (while still keeping the above two connections) put a resistor across each line, one by one (or at once if I have several suitable resistors) to GND, which will allow me to check their output voltages?
Atari_PSU_testing_setup.png
Atari_PSU_testing_setup.png (48.54 KiB) Viewed 174 times
Exxos, you suggested 330 Ohms for checking the -12V line and not 10 Ohms as in the dummy resistor kit you sell. Is this because the MSTE has a more powerful PSU, hence I need to put more load on the voltage lines?
How about the resistor values for +12V, -5V and +5V? Do you sell dummy resistor kits for this as well? I'll check my spares box, but I don't think I have much of those high wattage resistors.
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by Pacman » Thu Aug 31, 2017 12:42 am

Oh, I almost forgot...

the MSTE PSU (Phihong PSM-5341) service manual/schematic lists the following wire colours:

+12V (yellow)
-12V (blue)
+5V (red)
-5V (white)
GND (black)
PG (orange)

But I never trust this sort of information 100% on its own, so I'm going to check the voltage outputs with a multimeter before recapping.

Furthermore, the manual lists up following which might be useful in determining the correct dummy load resistor values:

+12V (0.2A typ/2A max load current)
-12V (0A typ/0.3A max load current)
+5V (2A typ/7A max load current)
-5V (0A typ/0.4A max load current)
64.6W (total output power)


And I'll be sure to check the capacitor voltages when it's time to recap. I've read horror stories about people getting electrocuted days after having stuff disconnected from the mains, but perhaps switching PSUs are better at discharging. It's probably smart to check anyway.
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by exxos » Thu Aug 31, 2017 2:23 pm

Pacman wrote:
Thu Aug 31, 2017 12:42 am
And I'll be sure to check the capacitor voltages when it's time to recap. I've read horror stories about people getting electrocuted days after having stuff disconnected from the mains, but perhaps switching PSUs are better at discharging. It's probably smart to check anyway.

Looks correct wire colours, but better to check real voltages.

I think you may be confusing -12 with +12V. The resistors kit I sell is for +5 and +12V, for the STF PSU's etc.

+12V 10R = 1.2amps. ~15watts.
+5V 2.2R = 2.27amps. ~12 watts
-12 330R = 36mA. ~ 0.5 watts.
-5 100R = 50mA. ~ 0.25watts.
Total load ~28watts.

You could load the 5v line more, but I don't think there is any real need.
+5V 1.0R = 5amps. ~25watts.

Pacman wrote:
Thu Aug 31, 2017 12:27 am
So you're saying the PSU needs to have a load on the +5V line (a 2.2 Ohm high-wattage resistor between +5V and GND)
and...
a wire connected between the "PG" (Power Good) line
or else the PSU won't turn on at all?
And with that I can also check the +5V line (DC voltage check between +5V and GND with a digital multimeter)?
PG is remote turn on for the PSU. For example, if the PSU is not connected to anything, it does not power up, switchmodes do not like being driven without a load, so its safety feature. Though also motherboards use the line to turn on/off the PSU as well.

I will never forget I purchased 2 very large switchmode power supplies years ago, They came with diagnostic information, lots of sheets passing all tests and safety regulations. I connected one to the mains and BANG. The thing exploded. Of course I learned a valuable lesson that day ;)

So you have to load the +5V rail for the PSU to regulate the voltage output, mostly the PSU's protection will kick in otherwise and turn the PSU off. The PSU will probably make a ticking sounds and the fan will "judder". Some people think the PSU has died when it does that, but actually its normal under fault conditions like no load on the 5V rail.

As a side note, this is why the STF/E PSU's mostly have a high(ish) wattage resistor on the output, its to always have a load on the PSU to at least make some sensible regulation on the output. Otherwise, you can have a spike of 100V or more on the 5V rail for example, and that is why PSU's explode. Some PSU's have good protection against that, some do not.

Its also why its important to have good capacitors on the outputs as it helps regulate the voltage and eliminates the voltage spikes. Again this is what I was showing before on the Falcon PSU. If the PSU wasn't serviced, the spikes would grow and continue getting higher in voltage until it kills something.

In case of Falcon owners, they gamble on the PSU, rather than buy a £15 kit and service the PSU, they roll the dice on a now £1,000+ priced machine.
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by Pacman » Fri Sep 15, 2017 1:13 am

Thanks Exxos.
I just checked my Mega STe PSU and the colours are correct according to previous discussions. The output seems to be within spec as well, which is good, although I will recap. Here are my measurements (before recapping) which I got from the PSU power connector, attached to the computer:

+12V (yellow): 12.040V
-12V (blue): -12.619V
+5V (red): 5.040V
-5V (white): -5.040V
GND (black)
PG (orange): 4.904V


Actually, I think I might just order that recapping kit of yours for the STe as well (SR98). I'll eventually replace it with something better when I recase the computer, but it's probably a good idea to feed it with "clean" power while I'm working on it, so as not risk damage the computer.

Exploding PSUs! I don't want to risk that, so let me summarize to make absolutely sure I've understood what you're saying:
In order to test the MSTE PSU (while disconnected from the MSTE) I need to:

a) connect a resistor between PG and GND.
and ....
b) connect a resistor between +5V and GND.
This will allow me to power up the PSU safely and let me check the PG and +5V output voltages. Its absolutely essential to have these two resistors in place when powering up the PSU (or it'll explode/get damaged?).

in addition to the above:
c) for checking the +12V line I need to (in addition to steps A and B) put a resistor between +12V and GND
d) for checking the -5V line I need to (in addition to steps A and B) put a resistor between -5V and GND
I only need to put a resistor across one of the lines at a time (the line I'm measuring), so after checking one line and powering off I can move the resistor over to the other line, power on again and check that line.
Did I get it all right?
I think you may be confusing -12 with +12V. The resistors kit I sell is for +5 and +12V, for the STF PSU's etc.

+12V 10R = 1.2amps. ~15watts.
+5V 2.2R = 2.27amps. ~12 watts
-12 330R = 36mA. ~ 0.5 watts.
-5 100R = 50mA. ~ 0.25watts.
Total load ~28watts.
The first two (10 Ohms and 2.2 Ohms) are the same as in your SR98 dummy load kit -is the Watt rating suitable for using them with the MSTE PSU as well (for testing +12V and +5V)?

Yes, I'm a little confused about + and -12V, and I see why:
Looking at the SR98 (for my STe) schematic I see that its PSU only outputs +5V (red) and -12V (blue).
But the MSTE PSU outputs the following in addition to those: -5V (white), +5V (orange -for "Power good") and +12V (yellow).
I assumed the dummy load kit was for any Atari PSU, so when I read:
5V uses 2.2R (loads 5V rail with 2.2amps) and 12V uses 10R (loads 12V rail with 1.2amps)
.....I assumed you were talking about +12V since there was no mention of a negative voltage. But that's cleared up now, so if I've understood it right I just need to buy the following resistors in addition to your ST PSU dummy load kit so I can test both PSUs?:

330 Ohms/0.5W or more (for testing -12V from the MSTE PSU)
100 Ohms/0.25W or more (for testing -5V from the MSTE PSU)

How about the PG (+5V) line? Do I need another 100 Ohm/0.25W resistor there, or another 2.2 Ohm/12W resistor?
Judging by the choice of the MSTE -12V dummy load resistor above (330 Ohms/0.5W) being different from the -12V dummy load resistor for use the with ST PSU (10 Ohms/?? W) I suppose you need different resistors for each switched mode PSU you're testing?

Hopefully all my questions will help others with the details as well so they don't need to ask as much ;)
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by exxos » Fri Sep 15, 2017 10:44 am

According to the schematic, the minimum load on the 5 V is 2 A and 0.2 A on the 12 V. The dummy load resistors I sell for the ST power supplies will meet that requirement so there's no reason not to use them initially anyway. you could use other values but I don't think there is really any need. this is all you need to power up a power supply.

The negative voltages, I would not worry about these for now anyway. The power good line is actually an output so you do not connect it to anything, I miss read what you wrote originally and thought you wrote power on signal which is used on ATX power supplies, but is not actually used on this power supply anyway.

Just try to load resistors on the power supply as it is now make sure it powers up check the voltages then you can re-cap. Your biggest responsibility is mainly making sure you put the capacitors in the right way around, and you sold them properly, otherwise inserting electrolytics in backwards will cause them to explode.
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Re: Discharging capacitors, PSU-testing with dummy loads

Post by Pacman » Fri Sep 15, 2017 11:09 am

Thanks for explaining, Exxos.
Good idea to check the PSU with the dummy load resistors before recapping as well as afterwards.

I've just completed an order now for the (SR98) dummy load resistor kit and other goodies.
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