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low resistance between power and ground on digital circuit normal?

oysteroid

Feb 16, 2016
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Feb 16, 2016
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Is it unusual to see a low resistance like 6 ohms between power and ground on a complex digital PCB, such as a mainboard for a TV? I measure 5.4 ohms between 1.3VB and ground and 17.8 ohms between 3.3VB and ground on a mainboard for a Samsung TV that I am troubleshooting and I don't know enough to know if this is possibly a problem. These power lines feed part of the big processor on the board.
 

AnalogKid

Jun 10, 2015
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Jun 10, 2015
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Measuring the resistance of solid state components rarely gives an accurate indication of the current drawn when powered up. But, both of those "resistances) lead to currents at or below 1/4 A, certainly a reasonable number for low-power electronics, if not maybe too low. Do the supply voltage sag under load? If so, then they might be overloaded. If not, can't begin to say without a schematic, and even then it probably has some custom parts for which there is no data.

ak
 

oysteroid

Feb 16, 2016
4
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Feb 16, 2016
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Thanks for the reply, AnalogKid!

Right now, I have a bunch of caps off the board as I've been testing them, and I had to order a new soldering iron (a Hakko FX-888D to replace my $20 Radio Shack piece of junk!), so I can't test anything at the moment, but I'll check when I get them back on. I am pretty new to this stuff!

The 1.3VB and 3.3VB circuits on the mainboard are converted on the mainboard using MP1583DN chips from 12V coming from the power supply. That 12V is measured as specified. I never thought to measure the 1.3V and 3.3V while running. I only measured the power coming from the power supply. Once I get it together, I'll check those.

Looking at the schematic, on the 1.3VB line, there are a handful of electrolytic caps in parallel and a bunch of little SMD ceramics. That 1.3VB supplies part of the main chip on the board. It also goes to part of the tuner riser. I am not sure where it goes or what it does inside there.

Here are all the places on the schematic that I can see where this 1.3VB shows up:
MPS buck converter:
1_3VB conversion.jpg

Here is the datasheet to that converter:
http://www.mouser.com/ds/2/277/MP1583_r3.1-371539.pdf


Feed to big heat-sinked processor:
processor feed.jpg


Feed to RF tuner:
tuner.jpg
 
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