Electrical Circuit Testing

Submitted: Saturday, Sep 03, 2016 at 18:56
ThreadID: 133370 Views:3802 Replies:6 FollowUps:7
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OK all you electrical dudes out there, please explain the method of testing say an LED light Bar for Amps output. I do understand the theory of Ohlms Law and that you test the item 'in series' but I'm talking about the physical application of using the multi meter to read the actual amps being used.

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Reply By: Zippo - Saturday, Sep 03, 2016 at 19:11

Saturday, Sep 03, 2016 at 19:11
Well to start with, a LED lamp will have light/heat OUTPUT vs electrical INPUT.

Put the unspecified multimeter on a suitable current scale and the leads into the corresponding meter sockets. You can have an estimate (WAG) at the required meter range capability by dividing the claimed wattage by 12.

Insert the meter into the circuit in SERIES with EITHER lead. Convention suggests the positive lead (but either will do IF the lamp elements/driver are not earthed to the lamp body metalwork).

Read the current on the meter when the lamp is energised. Be surprised by the current draw, more so if the lamp was purchased on ebay from an overseas source - they habitually exaggerate the power output, and the power input follows that.

Be also aware that *most* LED lamps - automotive or not - use switchmode drivers for the actual LEDs. These consume less current as the voltage is increased (and vice versa). The benefit is that they tend to be less affected by - almost immune to - voltage drop in their supply circuitry, but the presence of your meter when testing will skew the accuracy of your reading slightly.
AnswerID: 604075

Reply By: Notso - Saturday, Sep 03, 2016 at 20:28

Saturday, Sep 03, 2016 at 20:28
Or Get yourself a DC Clamp meter and put it over the active lead only and read the amps.
AnswerID: 604076

Follow Up By: The Bantam - Sunday, Sep 04, 2016 at 11:49

Sunday, Sep 04, 2016 at 11:49
Um ... I have one ( a DC clamp meter) .... but most people who don't have a daily use will find them way too expensive.

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Follow Up By: Zippo - Sunday, Sep 04, 2016 at 13:08

Sunday, Sep 04, 2016 at 13:08
Ditto to Bantam's comment.
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Follow Up By: Notso - Sunday, Sep 04, 2016 at 18:44

Sunday, Sep 04, 2016 at 18:44
Ah, I dunno, I have one $99.00 at Jaycar a few years back. Makes all this on the road testing a breeze.
FollowupID: 873796

Follow Up By: The Bantam - Monday, Sep 05, 2016 at 00:47

Monday, Sep 05, 2016 at 00:47
When you consider most ordinary people will not be paying $50 for their multimeter and they can buy a digital that works with a 10 amp current range for as little as $10
$100 for something they will rarely use is expensive.

Yes Jaycar Do have a AC/DC clamp meter for about $129 .... but it only has a 40 amp and a 400 amp range with only a 2.5% accuracy ... on the 40amp scale that is a 1 amp full scale error on the 400 amp scale its a 10 amp full scale error

When respectable digital meters will manage better than 0.5% on their DC voltage scales and better than 1% on their DC current scales.

Lack of sensitivity and lack of accuracy is a issue with clamp meters

you can buy a digital with a 10 amp scale error of 1% for $20

Most of what we want to measure around cars will either be under 10 amps or way above

If you want to measure more than 10 amps with your $20 Multi-meter $13 will buy you a 50 amp shunt ... that reads 50mV for 50 amps

I have a high sensitivty high accuracy DC clamp meter that has a 4 amp range .... but it is a dedicated current instrument and does nothing else ... it also cost me about $300 ...... not for the average traveler.

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Follow Up By: Notso - Monday, Sep 05, 2016 at 18:57

Monday, Sep 05, 2016 at 18:57
Hmmm, I suppose those concerned with exactitude might worry about .1 of an amp at 4 amps, but simple folk like me are quite happy with that level of accuracy.
FollowupID: 873835

Follow Up By: The Bantam - Wednesday, Sep 07, 2016 at 10:05

Wednesday, Sep 07, 2016 at 10:05
But most of the DC clamps peter you will buy it is 1 amp in 40 ...... that is working at full scale.

The specifications will not show this, but, the high current clamp meters suffer badly in the low end of the low current range.
Unless you are using a high sensitivty, high resolution clamp meter ...... it may be quite inaccurate under 10 amps and even worse under 5 amps.

FollowupID: 873894

Follow Up By: Notso - Thursday, Sep 08, 2016 at 19:03

Thursday, Sep 08, 2016 at 19:03
Yeah, I know the error ranges but really, are we talking about something significant in the context of this discussion, let's just imagine the error level was 10%, now at 4 amps which is the range we are talking about that would amount to only .4 of an amp. Now my device claims an error level of 2% so really is that an issue? As I said above if you are a professional searching for exact figures that may affect you, but for us mortals well I'm happy to use the convenience of the clamp meter.
FollowupID: 873958

Reply By: Genny - Saturday, Sep 03, 2016 at 22:14

Saturday, Sep 03, 2016 at 22:14
Measuring amps howto. Be aware of the limitations of your multimeter!
AnswerID: 604082

Reply By: Member - Bigfish - Sunday, Sep 04, 2016 at 07:27

Sunday, Sep 04, 2016 at 07:27
Go to you-tube. There are several very good clips on how to measure the output using multimeters, clamp meters or a watt meter. A cheap wattmeter is a handy tool. Naturally every car owner should have a multimeter...
AnswerID: 604088

Reply By: The Bantam - Sunday, Sep 04, 2016 at 12:19

Sunday, Sep 04, 2016 at 12:19
Lets start at the very beginning, because it is a very good place to start.

Current is always measured in series.

Ohms law is very simple ...... don't make it harder than it is. .... there are two sets of equasions ..... don't let this complicate and confuse things.

First set
Ohms law, voltage, current and resistance ( or pressure, flow and resistance)
It comes in 3 forms ( and their inverses) depending on what you have and what you want to find
V ( voltage) = I ( current) x R ( resistance)
I ( current) = V ( voltage) / R ( resistance)
R (resistance) = V ( voltage) / I (current)

There is a second set of equasions for power the one you want to know about is

P (power) = I( current ) x V (voltage)

If you have power and voltage ..... Divide power with voltage to get current.
So 100Watts, divided by 12 Volts equals 8.33 Amps

Many small cheap multi-meters will not measure very much in the way of current. under an Amp.

Most low to midrange digital multimeters will measure 10 amps maximum ..... and have no fuse in the high current circuit ....... so be aware of approximately how much you are measuring.

Note that most real loads current draw varies with voltage .. so you need to be aware that a few volts difference on a 12 volt circuit and the current can change quite a bit. .... The 12 volt system in your car will be around 12 volts at rest and as high as 15 volts with the engine running ..... that is a 25% variance, it can change the current drawn quite dramaticaly.

If you have a muiti-meter ..... there are on line articles and utube vids ..... OR there are relativly cheap "how to use your multimeter" books at places like Jaycar and Altronics

AnswerID: 604095

Reply By: Member - Trouper (NSW) - Sunday, Sep 04, 2016 at 18:34

Sunday, Sep 04, 2016 at 18:34
Thankyou, much appreciated, the utube thing was helpful
AnswerID: 604110

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