Power supply regulator tabs and panel continuity

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waldenpotato
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Power supply regulator tabs and panel continuity

Post by waldenpotato » Mon May 31, 2021 11:39 am

Just checking if this is normal. For the PSU2 and MU Dizzy boards, when testing continuity between the panel and the metal tabs I was_not testing positive for a connection. After connecting the grounding post to the panel and wiring everything to the power supply I now get a positive continuity test for two of the three total regulator tabs. Is this normal? The builder guides do say to test continuity before wiring everything but I haven’t read anything about afterwards. Are the tabs of the regulators connected to their ground pins? Why might only 2/3 of the tabs test positive for panel continuity but not the third?

Thanks.

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Katherine Alicia
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Re: Power supply regulator tabs and panel continuity

Post by Katherine Alicia » Mon May 31, 2021 11:58 am

be careful, some of the T0-220 cases have the metal tab connected internally and will need to be insulated from direct contact with the heatsink, this especially important if you`re mixing 78xx and 79xx regs on the same heatsink ;)
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waldenpotato
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Re: Power supply regulator tabs and panel continuity

Post by waldenpotato » Mon May 31, 2021 12:04 pm

Katherine Alicia wrote:
Mon May 31, 2021 11:58 am
be careful, some of the T0-220 cases have the metal tab connected internally and will need to be insulated from direct contact with the heatsink, this especially important if you`re mixing 78xx and 79xx regs on the same heatsink ;)
Just to note, I am using T0-220 insulating pads and bushings.

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Synthbuilder
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Re: Power supply regulator tabs and panel continuity

Post by Synthbuilder » Mon May 31, 2021 3:44 pm

The metal mounting tab is connected to the middle pin of the regulator IC. Ideally the middle pin and the tab should not be electrically connected to the metal panel the IC is mounted on. Now it so happens that the middle pin is connected to 0V on the 7805 used on the MU Dizzy. So if you have already connected the metal work to 0V you should find that there appears to be a connection between the middle pin of the 7805 and the panel. That's because both are connected to 0V via the wiring.

Now the LM317's pin 2 (middle pin) is connected to the +15V output, and the LM337's pin 2 (middle pin) is connected to the -15V's input supply. Neither are connected to 0V directly. But the LM337's input is connected to 0V via a very big capacitance (C7 and C8). This may make a continuity meter beep (or read a low resistance) while the capacitor charges up slowly. If you leave the meter connected long enough, the beeping should stop and the resistance goes up to near infinite.

waldenpotato
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Re: Power supply regulator tabs and panel continuity

Post by waldenpotato » Mon May 31, 2021 5:40 pm

Synthbuilder wrote:
Mon May 31, 2021 3:44 pm
The metal mounting tab is connected to the middle pin of the regulator IC. Ideally the middle pin and the tab should not be electrically connected to the metal panel the IC is mounted on. Now it so happens that the middle pin is connected to 0V on the 7805 used on the MU Dizzy. So if you have already connected the metal work to 0V you should find that there appears to be a connection between the middle pin of the 7805 and the panel. That's because both are connected to 0V via the wiring.

Now the LM317's pin 2 (middle pin) is connected to the +15V output, and the LM337's pin 2 (middle pin) is connected to the -15V's input supply. Neither are connected to 0V directly. But the LM337's input is connected to 0V via a very big capacitance (C7 and C8). This may make a continuity meter beep (or read a low resistance) while the capacitor charges up slowly. If you leave the meter connected long enough, the beeping should stop and the resistance goes up to near infinite.
Okay, this makes sense. So this is normal and okay? I'm letting myself get a little confused because we went to the effort of insulating the regulators.

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Synthbuilder
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Re: Power supply regulator tabs and panel continuity

Post by Synthbuilder » Tue Jun 01, 2021 11:17 am

waldenpotato wrote:
Mon May 31, 2021 5:40 pm
So this is normal and okay?
Only the 78M05 should have continuity between its tab and the metal panel and this should only happen once the wiring is complete.

The other two power ICs should not have continuity between the tab and panel. If the meter beeps or reads low, it should only do so for a short time. If it is still beeping after ten seconds you probably have a short
I'm letting myself get a little confused because we went to the effort of insulating the regulators.
It's about getting the current to go where we want it to. The plan is to make any connected module's return currents travel through the wiring only, and not the panel.

In practice it probably won't be that bad if the 78M05 on the MU Dizzy is bolted directly to the panel. However, if you bolted the LM317 or LM337 directly to the panel the fuses would blow.

waldenpotato
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Re: Power supply regulator tabs and panel continuity

Post by waldenpotato » Tue Jun 01, 2021 12:09 pm

Synthbuilder wrote:
Tue Jun 01, 2021 11:17 am
Only the 78M05 should have continuity between its tab and the metal panel and this should only happen once the wiring is complete.

The other two power ICs should not have continuity between the tab and panel. If the meter beeps or reads low, it should only do so for a short time. If it is still beeping after ten seconds you probably have a short
I retested the continuity yesterday and it did appear that the regulators on the PSU2 board stopped beeping after a second and then didn't beep at all after that. I'll double-re-check that and report back when I get the chance.

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