Testing Open circuits

Discussion in 'General Electronics Chat' started by relicmarks, May 22, 2008.

1. relicmarks Thread Starter Active Member

Oct 13, 2006
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How do you troubleshoot,check or test open circuits?

I know you use continiuity to test,check or troubleshoot shorts to ground or VCC to ground but what about testing for open circuits how would you guys test this?

OPen Circuit voltage check? Whats this?

2. mik3 Senior Member

Feb 4, 2008
4,846
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There is not a standard method to find open circuit faults. it depends on your circuit and what components you use. If you know how a component works and you think it is faulty then you make some measurements and test to see if it is good.

3. thingmaker3 Retired Moderator

May 16, 2005
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An "open circuit voltage check" is simply a measure the source EMF with load removed. This check can often show whether a problem is in the power supply or in the load circuit.

4. Caveman Active Member

Apr 15, 2008
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One interesting method that I learned from an old engineer years ago works well. Let's say I think a node is possibly open, so I put a voltmeter across this pin to ground. It reads 0V. The problem now is that I don't know if the actual voltage is 0V or it is open and my multimeter's 10Meg resistance pulled it down to ground.

The trick is to take the negative of my multimeter off of ground and put it at the 5V (or other power) node. If I read 0V again, then the multimeter has pulled it up to 5V, and it is open. If I read -5V, it at least has enough drive to hold itself to ground so it is not technically open.

Remember, you have to think about what you are doing here. I had one guy come to me after I showed him this method. He had done it right, but when he moved the negative lead to a power, he had moved it to 12V. He was reading about -7V and was confused. Looking at the circuit, it was going into a part with a 5V supply and the protection diodes were holding the line to about 5.3V, so his voltmeter was from 5.3 to 12 = -6.7V. The node was otherwise open of course.

5. relicmarks Thread Starter Active Member

Oct 13, 2006
355
0
Can i use a external 5 volt battery or 9volt battery thats not part of the circuit and put my negative probe on it and my RED probe on the "Open node" to see it if reads 0 zero volts?

Why would i have to put my negative probe on a +5volts and not +12 volts supply?

I don't get this pulling up to +5 volts to = zero volts how so and why pulling it up?

6. Caveman Active Member

Apr 15, 2008
471
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A multimeter usually has about a 10Meg resistance between the positive and negative leads. If a circuit node is open, the multimeter will actually be able to affect the node. Once the multimeter pulls the two leads together, it will read zero volts.

You cannot just use a random battery floating in space. It must be somehow related to the circuit ground. If you connect a 9V battery ground to the circuit ground, then you can use the ground and the 9V positive nodes as your references.

7. relicmarks Thread Starter Active Member

Oct 13, 2006
355
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If you connect a 9V battery ground to the circuit ground, then you can use the ground and the 9V positive nodes as your references.

Yes this is what i was thinking

So if i have a 9 volt battery connected to the circuit ground, i put my Black probe on the + 9 volts and the REd on the Open node, what should i read please? if its a true open circuit/network?

If the supply voltage is +12 the open node will read?
if the supply voltage is +18 the open node will read?
If the supply voltage is +9 the open node will read?

8. Caveman Active Member

Apr 15, 2008
471
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0V in all cases. This is assuming that the node is truly open.

May 16, 2005
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10. relicmarks Thread Starter Active Member

Oct 13, 2006
355
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So if i have a 9 volt battery connected to the circuit ground, i put my Black probe on the + 9 volts and the REd on the Open node, what should i read please? if its a true open circuit/network?

1.) If the supply voltage is +12 the open node will read? ZERO VOLTS
2.) if the supply voltage is +18 the open node will read? ZERO vOLTS
3.) If the supply voltage is +9 the open node will read? ZERO VOLTS

Why is all 1,2,3 questions zero volts? i don't get it

If the Meter has a 10 meg ohm resistance wouldn't there be a "voltage drop" between the floating battery terminal and open node?

Or you only get a "voltage drop" if the node is normal or not open

11. relicmarks Thread Starter Active Member

Oct 13, 2006
355
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Can't i just measure from the + power supply rail RED probe to the Open node? is it measures zero volts its open if there is a voltage drop then its good and not open right?

What confuses me is that a shorted node will read zero volts and a open circuit node will read zero volts , how can you tell the difference or if its a short or open because they are both zero volts

12. Caveman Active Member

Apr 15, 2008
471
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Man, I wish you would use freakin' quote boxes! I don't get it.

Anyways, let's look at 1. The supply voltage is +12V, the open node has a voltage with reference to the +12V (because everything has a voltage!). Now when I go to measure it, I am putting a 10Meg resistor between the open node (at whatever it's voltage is) and the +12V. Since the node is open, there is no current flowing into or out of it. Therefore by ohm's law, the voltage across the 10Meg resistor must be 0V, so the negative and positive nodes of the multimeter must be the same, and the voltage reading is 0V. Because the node is open, the multimeter is actually driving it to the same voltage as the negative lead, which in this case is +12V.

13. thingmaker3 Retired Moderator

May 16, 2005
5,072
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Get yourself a battery, a breadboard, a voltmeter, and some resistors.

Hook up a circuit with some shorted nodes and some open nodes.

Measure them.

Books and theory will only get you so far. You have to get your hands into the thing or you'll never know what's going on.

14. relicmarks Thread Starter Active Member

Oct 13, 2006
355
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Hook up a circuit with some shorted nodes and some open nodes.

1.) Can you please give me examples of OPEN nodes ? from your experience what have you guys seen that are open nodes examples?

I am putting a 10Meg resistor between the open node (at whatever it's voltage is) and the +12V. Since the node is open, there is no current flowing into or out of it. Therefore by ohm's law, the voltage across the 10Meg resistor must be 0V,

2.) How do you know if its a short or open? they both would read ZERO volts so how would i know if its a short or open node? thats the trick question

15. Caveman Active Member

Apr 15, 2008
471
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In this situation I know it is bloody open because you said it was open! Read the post again. We were talking about an open node. This is how I know.

If I read with red on a node and black on ground and I get 0V, there are two possibilities: It is actually being driven to 0V, or it is being driven so weakly that that meter pulled it to 0V.

If I move the black to +12V and I get -12V, then the node stayed at 0V, and is not floating. However, if I get a reading of 0V, then the mode is pulled to 12V (with reference to the original ground.) and the voltage across the meter is 0V (just like it said). If a node can be pulled around by a 10Meg resistor, it is generally considered open.

16. relicmarks Thread Starter Active Member

Oct 13, 2006
355
0
If I read with red on a node and black on ground and I get 0V, there are two possibilities: It is actually being driven to 0V, or it is being driven so weakly that that meter pulled it to 0V.

1.) What do u mean by how the meter PULLED it to 0v i don't understand about this meter pulling effect

If I move the black to +12V and I get -12V, then the node stayed at 0V, and is not floating. However, if I get a reading of 0V, then the mode is pulled to 12V (with reference to the original ground.) and the voltage across the meter is 0V (just like it said). If a node can be pulled around by a 10Meg resistor, it is generally considered open.

2.) Pulled around by a 10meg resistor i don't get it

3.) So a SHORT is zero volts from Node to ground
4.) A Open is zero volts from +VCC to Node