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So I got a multimeter and am ok with voltage measuring but just been playing around measuring current in a low powered LED torch.
When I measure on A scale I get about .61A but if I measure on mA I get 350mA. I checked withy dad's meter and his is about the same, he doesn't know why either as he just uses his for voltage and continuity.
Why do you get different readings on different scales, and is it best to check current first on the A scale then if below the max mA rating on the meter use that and take the reading on theat scale? Are the differences due to different resistance paths on the A and Ma settings?
I have no intention of measuring anything with large currents btw just curious as I've been playing about with it today. Only really want it for voltage, reactance and continuity
Surely it should be the same reading just 1000 times greater or lesser, have you got the cables in the correct ports.
Well that's what I thought. Cables definitely in correct ports, there's an A one and mA one which light up when you select the range so idiots like me get the right ones!
a low powered LED torch
Well, LED resistance changes with temperature, so when you turn them on they get hotter and current draw changes. LED drivers are current management circuits. That's why we don't talk about watts any more with bike lights, just lumens. This may have an effect on the current you are measuring - perhaps it cools down as you've disconnected it to change the leads around on the multimeter or something.
Just a guess but error in the devices?
I'd have thought you should use the scale most appropriate to the current you are measuring & that will probably give you the most accurate value.
So measuring the current when boiling a kettle, you probably aren't bothered whether it's 12.6A or 12.9A so some error in the larger range scale is acceptable.
When measuring the current through an LED, the larger scale will tell you that there's current there, but not very accurately. The mA scale will tell you what's there with higher accuracy.
Similar to checking tyre pressure on a track pump gauge that goes from 0-150psi compared to one that only reads 0-30psi.
It will be caused by pulse width modulation and the sampling rate of the meter at different scales. An LED doesn't draw a continuous current, instead it is controlled by a series of pulses measured in Hz and the current each if those pulses draws. Think of it as switching a light on and off incredibly quickly while also having a current (not voltage) dimmer in there too. The meter cannot sample the current quickly enough to see each pulse so gets an average, the different ranges if the meter will measure it over a different timeframe so get slightly different results. It's why an LED light moving across your field of vision quickly seems to pulse while a traditional DC-powered bulb does not in torches and car lights. Much easier to see and understand what's going on with an oscilloscope.
How are you measuring it, where are you sticking the probes?
Current must be measured in circuit. voltage accross a circuit. I.e. when you measure current, the meter completes the circuit. Are you using it like that?
... which is why I was asking. 😁
Reluctant jumper has the answer, just tried it on a non LED device. 0.039A or 36.8 on the mA scale, accuracy on A is 1.2% and +/- 3 counts so that would mean the A reading matches once the error is considered.
Thanks!
Some multimeters have a separate input for measuring currents over 10A.
@Bruce, that's to put a larger value shunt resistance in the measurement circuit ( needed because of the larger current)
Thanks Stretch, I used to be an electronics design engineer, but a usefull point. 🙂
It’s why an LED light moving across your field of vision quickly seems to pulse
Useful stuff Mr Jumper - I'd noticed this effect in Top Gear videos of cars with LED lights.
Occasionally they run a sequence in slow-motion and it becomes even more obvious :o)
An LED doesn’t draw a continuous current, instead it is controlled by a series of pulses measured in Hz and the current each if those pulses draws
That's only the case if they're using PWM to modulate brightness. An LED will happily draw a continuous current if given one.
It’s why an LED light moving across your field of vision quickly seems to pulse
It's also why LED torches are bloody dangerous if you're looking at moving machinery and don't have your wits about you.
The meter cannot sample the current quickly enough
My Fluke oscilloscope would say otherwise!