Electrical Circuit Testing

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.

Or Get yourself a DC Clamp meter and put it over the active lead only and read the amps.

Measuring amps howto. Be aware of the limitations of your multimeter!

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...

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

cheers

Thankyou, much appreciated, the utube thing was helpful