digital circuit with a short stuck at +5 volts

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dannydavis

Joined: 01 Nov 2008
Posts: 14

04 Nov 2008 5:46 digital circuit with a short stuck at +5 volts

When troubleshooting digital circuits, most shorts are either stuck at VCC +5 volts or shorted to ground

example: of a digital circuit with a short stuck at +5 volts

Its hard to know if the short is on the input stages or output stages because all the stages of each IC chip inputs and outputs have +5 volts on the inputs and outputs of each stage

so how would i isolate the area of the +5 volt short?

So i was thinking can't i just "isolate each section one by one" into areas by grounding the inputs and outputs of each stage? or would this just short out the inputs and outputs and cause damage? but it would help me "isolate" the area/section from the other sections before and after it

Taking a jumper from the input of section#1 and grounding it, and than taking another jumper from output of section#1 and grounding it
or would this damage the IC chips and board?

Than using a volt meter or lamp and measure the input and outputs of that section#1 to see if i read 5Volts or if the Lamp lights up? if the Lamp bulb does light/glow than its NOT that section/stage so its either before or after. But if the Lamp bulb lights up than its that section/stage thats causing the direct short to +5 volts

How would u use jumpers to create a OPEN or BREAK in a circuit without cutting traces?

how do you guys "Isolate areas" into section without cutting traces?

example: of a digital circuit with a short at ground, that Lamp is not lighting up at the main output

1.) How would u guys "isolate" the section or area where the short is to ground from input to output of multiple sections and stages?

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Colbhaidh

Joined: 10 Aug 2004
Posts: 232
Helped: 35
Location: Scotland

04 Nov 2008 21:51 Re: digital circuit with a short stuck at +5 volts

With the chip powered, I would ramp each input pin from 0 to 1.0V and 0 to -1.0V and watch if there is a diode short clamping the voltage at around 0.6V. This would indicate the input gate is blown shorting one of the pmos or nmos transistors of that input to Supply or ground. For CMOS inputs, they should pass this test.(assuming none of the circuitry connected to the input would mimic such a fail).
Do the same on each output pin but this time look for a resistive short through zero i.e. there is none of the expected diode behavious. The outputs should look like diodes and not like resistors again depending on what external circuitry is connected to that outpin pin.