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SHORTS, voltage across resistor converts to current

danny davis

May 9, 2012
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That's Why I asked this to make it simpler for me to help me out

wouldn't a SERIES LAMP help me find the SHORT?

Because the short is some component on the PCB or power supply

What kind of lamp should i get?
 

danny davis

May 9, 2012
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Do diodes have a forward biased they have to overcome before they pass any negligible current? What happens bellow that threshold?

yes it's .7 volts to turn on the diode

Below that threshold the diode it turned off
 

CocaCola

Apr 7, 2012
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what kind of LAMP can i use to measure the current of the power supply?

What kind of resolution are your eyes going to be able to resolve from the lamps brightness, and what kind of mystery math are you going to to use the resolve your eyes perceived brightness back to a real world measured current?

Why are you on this lamp tangent? Did you fry your multimeter?
 

Rleo6965

Jan 22, 2012
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How about desolder components one by one and test resistance if shorted then solder it back again on pcb. I'm sure within a day you will surely find that shorted component.
 

CocaCola

Apr 7, 2012
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yes it's .7 volts to turn on the diode

Below that threshold the diode it turned off

So, since you have suggested you use a Fluke 87 series meter and the Fluke 87 series owners manual states that in the To 4.0MΩ measuring range it supplies <450 mV DC as the test voltage... FYI: Most digital multimeters supply this same half volt or less voltage on the Ohm setting so they don't damages stuff...

With that said, revisit the circuit I posted, what happens to the diodes when you are testing the resistance on each side with your multimeter and <450 mV DC? How does this effect you ability to test for the supply side 'short' when you are testing on the 'stage one' side of the circuit?

Have you been listing to us when we keep telling you that you NEED TO KNOW THE CIRCUIT you are testing? Have you been listening to us when we tell you there is no single black and white answer to how to test this stuff, the circuit will introduce variables and other issues that have to be taken into consideration and will vary test results...
 

danny davis

May 9, 2012
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How about desolder components one by one and test resistance if shorted then solder it back again on pcb. I'm sure within a day you will surely find that shorted component

Yes thats what i have been doing, but my manager says it takes way to long

Why are you on this lamp tangent?

Because the LAMP will tell me if i have a SHORT
Plus when I remember a transistor or FET in a stage it will tell me if its good or bad , because if the LAMP is still LITE and BRIGHT that means there is a SHORT still on the PCB

Or I can lift up transistor and FET pins to disconnect each stage one by one until the LAMP it back to normal light.
 

CocaCola

Apr 7, 2012
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Because the LAMP will tell me if i have a SHORT
Plus when I remember a transistor or FET in a stage it will tell me if its good or bad , because if the LAMP is still LITE and BRIGHT that means there is a SHORT still on the PCB

You are adamant that using an ammeter in series with the device caused it to fail (remember that?) now you want to add a bulb in series, to what end? Spontaneous combustion? :p
 

danny davis

May 9, 2012
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what happens to the diodes when you are testing the resistance on each side with your multimeter and <450 mV DC?

It turns on the Diodes, which suck some of the current , changing the resistance measured

How does this effect you ability to test for the supply side 'short' when testing on the stage one side of the circuit?

It won't read Zero ohms but the 450mv has turned on the diode
 

danny davis

May 9, 2012
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You are adamant that using an ammeter in series with the device caused it to fail (remember that?) now you want to add a bulb in series, to what end?

Block Diagram::

Battery 4.5 volts -----> LAMP in series ---> power supply on PCB
 

danny davis

May 9, 2012
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well what does the 450mV in the ohm range on the fluke meter do than to the circuit or diodes?

You're saying that on the OHMs setting to measure resistance , the meter is outputing a 450mV voltage?
 

CocaCola

Apr 7, 2012
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You're saying that on the OHMs setting to measure resistance , the meter is outputing a 450mV voltage?

Yeah according to the owners manual...

well what does the 450mV in the ohm range on the fluke meter do than to the circuit or diodes?

See attached...
 

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Rleo6965

Jan 22, 2012
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The 450mv of the fluke meter probe will be applied on the resistor or diode under test. A certain amount of current flow on resistor / diode and this will be computed / converted in resistance value on LCD display of Fluke 87..
 

danny davis

May 9, 2012
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Today at work i found some shorts

On the PCB boards there is 2 regulators in parallel
one outputs 5 volts
the 2nd outputs 3 volts

I was getting 5 volts everywhere , where i was suppost to get it

I was getting 5 volts on stages that were suppost to have 3 volts for +Vcc

How would u start to troubleshooting and what would u do please?

How would u narrow down to which component was causing a short from 5 volts on the 3 volts rail?

1.) I started on a Vcc rail that was suppost to be 3 volts ( even tho it was shorted to 5 volts) , I just used that as a reference point
2.) I used the other probe of my DVM meter to measure the ohms of a component going to +Vcc 5 volts.
3.) the problem was that every component that was tied to the 5 volts +Vcc measure zero ohms and the continuity check was beeping
4.) So the only thing I could do was disconnect each component that was tied to 5 volts +Vcc one by one until I found the short

Was this the best way and how would u have done this please?
 

CocaCola

Apr 7, 2012
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How would u narrow down to which component was causing a short from 5 volts on the 3 volts rail?

I would look at the schematic and the board for points of concern, but wait you can't share, oh well...
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Was this the best way and how would u have done this please?

Haven't we been here before?

Get a multimeter that can measure small fractions of an ohm (or make yourself a low-ohms adapter).

watch this:
 

danny davis

May 9, 2012
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whats an low ohms adapter?

If all components tied up to either the +Vcc 5 volts and +Vcc 3 volts was testing SHORTED

Because The +5 volts rail was shorted to the +3 volt rail

but Only one component was doing that

How would u guys find that component please?
 
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