Voltage measurement

William_H

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A post by Jordanbasset on water cooled fridges mentioned that he measured voltage at the compressor as 12v however the terminal would not carry the current for the compressor.
It struck me that a reminder about digital voltmeters (DVM) might be in order.
Almost everyone has a DVM and uses them to check for 12v supply.

The DVM very accurately indicates the voltage that is present. However this does not always indicate a good 12v supply. The DVM draws a little current to measure. However this is incredibly tiny current like about 1/10 of a microamp. This kind of current can easily creep through a mass of corrosion or dirty contact.
However as soon as you seek to use the 12v supply so measured at some amps you get nothing through. You have been misled by the DVM. If however you measure the voltage while the device be it fridge or lamp is connected (and drawing current) you will get a more accurate picture of the voltage through the switch etc. ie none if it is faulty.

One way to overcome this problem is to connect a lamp of a few watts in parallel with the DVM. The current drawn by the lamp will be commensurate with the current the circuit is expected to carry so will give a more realistic assessment of the supply capability.

Or just be aware that voltage measured does not always mean a good supply. olewill
 

VicS

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You highlight the very reasons I often suggest a using a test lamp on a couple of leads rather than a digital voltmeter.

I made a fool of myself a few years ago through using a digital meter.

A friend's son had launched is fathers boat ( craned in) but then could not start the engine despite having fitted two new batteries. Dead as a dodo.

I'd gone along armed with my meter in case anything went wrong.
Power supplies all checked out OK of course. I started delving into the control panel. We missed the tide so the boat ended up alongside at the yard for two weeks

The trouble was caused by bad connections to both batteries. Almost beyond belief that both batteries should have bad connections but they did. If I'd used a test lamp instead of the meter I'd have probably found the cause straight away and got the boat way 2 weeks earlier.
 

CelebrityScandel

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Just extending from William and VicS's comments to measuring voltages between supposedly isolated metals with DVM's. Due to their needing only a very small current to make a measurement a digital voltmeter may still measure 12v between these even if they are insulated from each other but over which a very high resistance path exists, say from slight salty dampness.

I often see people getting into all sorts of difficulties and panics measuring these apparant voltages and then incorrectly assuming they are the cause of some corrosion problem they have.
 

charles_reed

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You highlight the very reasons I often suggest a using a test lamp on a couple of leads rather than a digital voltmeter.

I made a fool of myself a few years ago through using a digital meter.

A friend's son had launched is fathers boat ( craned in) but then could not start the engine despite having fitted two new batteries. Dead as a dodo.

I'd gone along armed with my meter in case anything went wrong.
Power supplies all checked out OK of course. I started delving into the control panel. We missed the tide so the boat ended up alongside at the yard for two weeks

The trouble was caused by bad connections to both batteries. Almost beyond belief that both batteries should have bad connections but they did. If I'd used a test lamp instead of the meter I'd have probably found the cause straight away and got the boat way 2 weeks earlier.
I doubt you would have unless it was a 60 watt bulb
 

starfire

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A very useful thing to have, and cheap, is an old car headlamp bulb with leads & croc clips soldered on.


Useful for doing a known discharge on suspect batteries as well as providing a load when tracing suspect wiring or connections.


I use this as well as a more general test lamp consisting of 10w sidelight bulb.


Anyone prodding around low voltage systems should have one !
 

sarabande

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I always get the theoretical flow mixed up with the physical one ! + to - , that is. Or is it - to + ?


Stupid early morning question (because I have been lambing most of the night): Why connect the lamp in parallel with one of the leads, rather than series, please ?
 

prv

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The multimeter in my tool locker is an analogue moving-needle one. Obviously this isn't going to draw as much current as a 60w headlight bulb, but is it less susceptible to this phantom-voltage issue than a digital one?

Pete
 

jordanbasset

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Yes, I was thinking of something along the same lines after the issue I mentioned which prompted this post. It could have been a an expensive purchase for a new compressor, so a few minutes spent checking the power under load would be my next action if the voltmeter shows power coming through. Just grateful for Terry on Sea Dragon by sheer chance being next to me when the problem occurred.
Have now checked all my connections in and behind the switch panel.
 

john_morris_uk

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I always get the theoretical flow mixed up with the physical one ! + to - , that is. Or is it - to + ?


Stupid early morning question (because I have been lambing most of the night): Why connect the lamp in parallel with one of the leads, rather than series, please ?
Theoretical flow was always considered positive to negative. In reality, the electrons (negative charge) are attracted to the positive charge and flow in that direction.

The reason the lamp is suggested as being in parallel is to make sure that a real current is being drawn by the lamp and making sure that the high impedance of the DVM isn't being fooled by bad connections into telling you that there is 12 volts there when there isn't.

Do you want me to send my daughter round to help with the lambing. She's very experienced.
 

VicS

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I always get the theoretical flow mixed up with the physical one ! + to - , that is. Or is it - to + ?


Stupid early morning question (because I have been lambing most of the night): Why connect the lamp in parallel with one of the leads, rather than series, please ?

Flow usually considered from + to - but electron flow really is - to +

William is suggesting a bulb in parallel with a digital voltmeter meter so that it is drawing some current. Overcomes the snag with using a digital voltmeter namely that is does not show up bad connections because it requires next to f.a. current to give a reading.
 

starfire

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The multimeter in my tool locker is an analogue moving-needle one. Obviously this isn't going to draw as much current as a 60w headlight bulb, but is it less susceptible to this phantom-voltage issue than a digital one?

Pete


Less susceptible, but still prone to the same problems.


Putting it in perspective, an analogue meter on low voltage ranges would draw say
0.0005 A while measuring something, if you are dealing with a circuit that supplies, in the op's case, a fridge compressor, you are talking several amps.

Any bad connections, (corroded crimp, corroded wire, bad contact between a fuse & it's holder etc) would show full supply volts on just a meter, but with the test lamp imposing a load would show a voltage drop.

I would start at the load end, with the meter & lamp connected, if there was a large voltage drop, I would move up the wiring towards the panel until I found almost full supply volts, The problem then lies somewhere between where you got full voltage & the load connections.


Had just this sort of problem on a mates passerelle, where it would not operate correctly, voltage at the pump was down to 10v or so, moving up the wiring, checking voltage at the breaker, that was low, other end of the cable to the breaker voltage was good, eventually found a bad crimp in that cable.
 

charles_reed

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A post by Jordanbasset on water cooled fridges mentioned that he measured voltage at the compressor as 12v however the terminal would not carry the current for the compressor.
It struck me that a reminder about digital voltmeters (DVM) might be in order.
Almost everyone has a DVM and uses them to check for 12v supply.

The DVM very accurately indicates the voltage that is present. However this does not always indicate a good 12v supply. The DVM draws a little current to measure. However this is incredibly tiny current like about 1/10 of a microamp. This kind of current can easily creep through a mass of corrosion or dirty contact.
However as soon as you seek to use the 12v supply so measured at some amps you get nothing through. You have been misled by the DVM. If however you measure the voltage while the device be it fridge or lamp is connected (and drawing current) you will get a more accurate picture of the voltage through the switch etc. ie none if it is faulty.

One way to overcome this problem is to connect a lamp of a few watts in parallel with the DVM. The current drawn by the lamp will be commensurate with the current the circuit is expected to carry so will give a more realistic assessment of the supply capability.

Or just be aware that voltage measured does not always mean a good supply. olewill
A sorry tale, which illustrates the fickle nature of things electrical and would not have been discovered by any of the precautions outlined in the posts.

My pressure water pump stopped working - in the nature of these things it did it after I'd thoroughly soaped myself but before rinsing. This had occasionally happened before and a sharp tap on the pump usually got it working again, on this occasion, nix.

After I'd sponged myself off using water out of one of the 25 litre cans, I started to trouble-shoot.
12.4 v on the digital meter - using an 18w bulb across the +ve and -ve feeds produced a light. Attaching the pump direct to a battery produced an healthy chuckle and the pump worked perfectly when disconnected from the pipework and attached to its wires.

On re-assembly in the system the pump worked perfectly, until I'd run the water in the accumulator down when it refused to restart.

Testing the pressure switch found that to be perfectly in order. Dis-assembly of the pump and testing the components showed everything totally in order.

I did solve the problem in the end - after I'd ordered a pump on special delivery from the UK to Greece - and had the old pump working again 2 days before the replacement arrived in Kalamata.

Perhaps the interested would be prepared to hazard a guess as to the eventual problem.
 

LittleSister

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A sorry tale, which illustrates the fickle nature of things electrical and would not have been discovered by any of the precautions outlined in the posts.

My pressure water pump stopped working - in the nature of these things it did it after I'd thoroughly soaped myself but before rinsing. This had occasionally happened before and a sharp tap on the pump usually got it working again, on this occasion, nix.

After I'd sponged myself off using water out of one of the 25 litre cans, I started to trouble-shoot.
12.4 v on the digital meter - using an 18w bulb across the +ve and -ve feeds produced a light. Attaching the pump direct to a battery produced an healthy chuckle and the pump worked perfectly when disconnected from the pipework and attached to its wires.

On re-assembly in the system the pump worked perfectly, until I'd run the water in the accumulator down when it refused to restart.

Testing the pressure switch found that to be perfectly in order. Dis-assembly of the pump and testing the components showed everything totally in order.

I did solve the problem in the end - after I'd ordered a pump on special delivery from the UK to Greece - and had the old pump working again 2 days before the replacement arrived in Kalamata.

Perhaps the interested would be prepared to hazard a guess as to the eventual problem.

Sounds like the start of a new series - 'What now, Bosun?'
 

VicMallows

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The multimeter in my tool locker is an analogue moving-needle one. Obviously this isn't going to draw as much current as a 60w headlight bulb, but is it less susceptible to this phantom-voltage issue than a digital one?

Pete

I think you will find that I have often mentioned 'reaching for the old AVO 8' when analysing apparently strange voltage readings. However testing under load is equally important. Vic.
 
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I think you will find that I have often mentioned 'reaching for the old AVO 8' when analysing apparently strange voltage readings. However testing under load is equally important. Vic.

Would some kindly and tolerant 'lecky initiate' care to spell out ( lickle wurds ) in detail an Idiots' Guide to such 'testing' under load AND using an AVO, so this 'bear of little brain' can find what's what and what isn't inside an absent mate's 'snakes nest'....?

As in A, B, C, p'rhaps?

Puleez? :eek:
 

William_H

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Voltmeter

An AVO model 8 was the standard multimeter of the 60s and 70s. It is a moving coil meter movement. The current drawn to measure would be typically about 1/10 of a milliamp at full scale deflection. Compared to perhaps .001 ma for digital multimeter. The current drawn by an AVO would still possibly indicate a good supply when the supply is really bad but obviously would in some cases show a bad supply compared to DVM.
In electronic circuits of very high resistance the DVM more accurately indicates what is there. So low current hence low interference with the circuit is desirable. But of course boat 12v supplies are a lot different.
In a boat far better is to use a lamp or similar load on the meter or in lieu of the meter.
I think the AVO model 8 should reside in a museum. The DVM will be as accurate or better, certainly lighter and with clearer readout. The AVO had a lovely mirror behind the needle so you could carefully discern exactly what the reading was.
Just be aware the DVM reads out exactly what is there. But does not reflect actual operating conditions. olewill
 
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