Power puzzle: 5V drops to 4.5V

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Power puzzle: 5V drops to 4.5V

Postby pistrix » Oct 26th, '17, 11:53

I have two Yocto's, and am having a problem with the 5V line on one of my IO boards that I just can't figure out.

My Main Board #1 and Main Board #2 are both functioning 100% when used with my IO Board #2.

But my IO Board #1 causes the sequencers to malfunction and frequently reset itself or lockup. It had worked fine for awhile, but I may have damaged something at some point while working on mods or fixing wiring etc.

Measuring the voltage of IO board #1, all measures correctly when disconnected from Main Board: 5V, 15V, -15V... And the 5V line still measures 5V when the power cable is connected to the Main Board. But it drops to 4.5V when the sequencer cable is connected to the main board. (Specifically, it drops to 4.5V when ground is connected to the sequencer circuit.)

This only happens with one of my IO boards. IO Board #2 holds steady at 5V and powers each Main Board with no sequencer problems..so logic dictates the problem is on IO Board #1, but I can't figure out what is causing it. I replaced the 2sa733's for Trig2 & Trig3, double-checked the resistors, redid the cabling, and even pulled the 6n138 out completely for now but the 5v still drops to 4.5v...The capacitors look fine..Could a diode be failing? What am I missing?

It seems that I have enough observations to be guided by logic to where the problem is, especially since the circuit on the IO Board is so simple, but I'm really stumped...Perhaps it is the 6n138, and that's why removing it didn't change the symptoms, and I need to replace it...I only have 6n139's on hand though..

Any help at all hugely appreciated!
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Re: Power puzzle: 5V drops to 4.5V

Postby pistrix » Oct 26th, '17, 17:17

I tried swapping a 6n139 for the 6n138, but my 5V still drops to 4.5V

This really seems like a current/power draw issue -- could the LM2596 be failing and no longer supplying 3A, causing the voltage drop when the sequencer increases the current being pulled? Seems like a long shot but I am desperate for a solution...
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Re: Power puzzle: 5V drops to 4.5V

Postby e-licktronic » Oct 27th, '17, 01:38

pistrix wrote:I tried swapping a 6n139 for the 6n138, but my 5V still drops to 4.5V

This really seems like a current/power draw issue -- could the LM2596 be failing and no longer supplying 3A, causing the voltage drop when the sequencer increases the current being pulled? Seems like a long shot but I am desperate for a solution...



Have a look on 1N4004s and capacitors way on the IO board. Then it could be the LM2596.
Best regards,
e-licktronic
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Re: Power puzzle: 5V drops to 4.5V

Postby pistrix » Oct 27th, '17, 15:54

Thanks for the reply !

The diodes and caps are all in correctly..I'm putting an order in for a replacement LM2596T today, hope it does the trick
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Re: Power puzzle: 5V drops to 4.5V

Postby pistrix » Nov 6th, '17, 00:36

well the new LM2596 didn't change anything

but i figured it out! after working backwards thru the 5V line on both of my IO boards, i found that the 680uf capacitor on the problem IO board was knocking the input voltage going to the LM2596 down to 8.3V...and it should be about 22V..i replaced it and finally i am back to steady 5V and correct working sequencer again...hurray!
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