measuring DC current

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Proberly a silly question, but is there a cheap and easy way of measuring current on 12 volt circuits, between about 4 and 15 amps, dont have to be accurate within 1/2 amp will do.
I used to have a multimeter, that measured ma and had a copper shunt that allowed higher current but lost it.
My first thoughts was a car ammeter, but not sure how precise they are.
Any better ideas, something digital and cheap would be nice.
I want to fit croc clips, and just temporary connect it in series, to check the current draw on LED strips, as Im getting a lot of driver failures and not sure whether the drivers, were powerfull enough when fitted.
Its mainly custom made hard to access led signs so not a simple task to see the leds,
 
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Proberly a silly question, but is there a cheap and easy way of measuring current on 12 volt circuits, between about 4 and 15 amps, dont have to be accurate within 1/2 amp will do. ... My first thoughts was a car ammeter, but not sure how precise they are.
Probably OK to within a 1/2 amp or so.
Any better ideas, something digital and cheap would be nice.
Many of the very cheapo digital multimeters have a 10A DC range, but that may not be high enough for you. ... but be careful, since they are often not fused!
I want to fit croc clips, and just temporary connect it in series, to check the current draw on LED strips, as Im getting a lot of driver failures and not sure whether the drivers, were powerfull enough when fitted.
Its mainly custom made hard to access led signs so not a simple task to see the leds,
What sort of operating voltage are we talking about? If you have a DMM which can measure low voltages reasonably accurately, you could put a very small resistor in series with the load and measure the voltage across it - but the voltage across the resistor would obviously have to be low in comparison with the supply voltage, so as not to affect the circuit much ... for example, a 0.1Ω resistor would give you 1V at 10A, or a 0.05Ω resistor (maybe two 0.1Ω's in parallel) would give you 0.5V at 10A.

Kind Regards, John
 
Its 12 volt from a Led driver on a 230 volt supply, I asume there all electronic inside, would that have any effect on what I need to measure the current.
Is there like a clampmeter on the market, I would have thought a Dc one would be simpler than making an Ac one, or am i wrong.
Think i will check ebay and see what scale is on a car one, I have an old one out of a battery charger but the scales go 5, 10 15 up to 30, would that work, and be quite accurate
 
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How urgent? I had a similar challenge for my boat, with deep cycle and winch/motor batteries, so put cheap displays inline on switches, its very easy to see draw, they were 3 quid each, but took a few weeks to arrive from china.
http://www.ebay.co.uk/itm/Dual-Disp...hash=item51b990d6ee:m:mAZ0LYzojz34Qy444DQmPSQ
No rush, I will try one of those, for that price
thanks

would that work ok, with the messed up output from a driver, do you think.

Looking at the drawing it looks like you put a sensing coil round your cable , does that come with it
 
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Its 12 volt from a Led driver on a 230 volt supply, I asume there all electronic inside, would that have any effect on what I need to measure the current.
Not really. I assume you're sure that it is (at least 'sort-of'!)) DC output?
Is there like a clampmeter on the market, I would have thought a Dc one would be simpler than making an Ac one, or am i wrong.
You're sort-of wrong. The inductive coupling used for AC clamp meters won't work with DC. There are DC clamp meters, but they have to use something like Hall Effect devices.
Think i will check ebay and see what scale is on a car one, I have an old one out of a battery charger but the scales go 5, 10 15 up to 30, would that work, and be quite accurate
One which goes up to 30A would not give you much resolution - probably no better than about ±1A at best.

Do you not like my idea of measuring the voltage across a very low value resistor?

Kind Regards, John
 
Do you not like my idea of measuring the voltage across a very low value resistor?

Kind Regards, John
bit tired at the mo, will have to digest that later :)

Had a look at these, they seem to do higher current, not that i need it,was thinking get the 50amp, though possibly they are all the same anyway, reading the gumf it sounds like they have a built in shunt, so maybe quicker to connect. only about pound dearer than ebay, but ive used the company before and been ok.
See what you think please.
We use good quality drivers, better than some of the cheapo lighting ones around, more like the units that run your computer,
they have 12vDC on them and usually jump 20 30 50 60 80watt etc, so Accuracy can be give or take a little, we just want a rough idea, rather than just asuming the failed one was initially man enough for the job.
At the moment ive been replacing failed 2 year old 60watt units with 80 watt units as we have stock of them, if anything that should be an improvement, if excessive, shouldnt it.
as they are made up signs we have no details for the Led load, only the fact it needs 12volt dc

http://www.banggood.com/0_28-Inch-D...er-Ammeter-1-100A-p-1093413.html?rmmds=search
 
Had a look at these, they seem to do higher current, not that i need it,was thinking get the 50amp, though possibly they are all the same anyway, reading the gumf it sounds like they have a built in shunt, so maybe quicker to connect. only about pound dearer than ebay, but ive used the company before and been ok. See what you think please.
It certainly looks as if they are worth a try. I'm a bit confused by the wiring diagram which appears to show an external shunt (do they provide that?) but it will probably becom clearer once you've got one (and the instructions) in your hand.

My main reservation is that they would appear to be ridiculously cheap, particularly given that the price appears to include shipping half way around the world!

Kind Regards, John
 
Those meters certainly look sexy, I will probably get some myself and make my wife jealous, can't go wrong for that price.

But for now If you do not have an ammeter, John's idea of using a shunt resistor of a known value like 0.1 ohms or even lower value for higher currents to 0.05ohms, so for example if you had 5 amps flowing through a 0.1ohm resistor (wire wound 5watt resistor) it would drop 0.5volts, so you could then measure volt drop across this resistor to work out current flowing through it. So for instant if you measured 300mV across a 0.1 ohm resistor, you take volts and divide it by your resistor (0.1) you get amps, so 300mV = 0.3/0.1 = 3amps.

Trouble is you don't know if the LED lights are blowing out the power supplies,, there could be many other reasons not necessarily driver rating, temperature increase can cause premature failure, as well you do not know if the 12v output is a smooth DC or pulsed average DC, where the voltage may be much higher than 12v for a brief period, so it may be drawing peak currents and getting destructed.
 
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Seems like you are into LED signs, using LED strips or even individual LEDs, years ago I worked for a company which was basically a Fire Alarm Manufacturing company, my Boss was a genius and he always ventured into other things, wanted to get rich quickly, but sadly all his ideas failed to take off because he did not have access to cheap large scale manufacturing facilities in UK, he went out to China and Hong kong but they would only be interested if he paid for tooling charges and placed orders container full of gadgets, for which he needed loads of wonga! He tried to get a few investors interested but no one backed him or his ideas.

As far as I know, and I am now talking of mid 80s to early 90s, (between 1985-1992) he was the first one to ever create a Fiber Optic Christmas tree, using strands of fibre optic loom and a colour wheel, and a halogen light source, these trees were produced by him, his brainwave, naturally I worked for him so got involved with all his ideas, wiring 250 strands of fiber optic loom and tieing each strand to a plastic 6' tree using black cable ties, it was absolutely horrendously boring job, we detested it so much, took around 4 hours to complete a tree, and then he had got a company to fabricate a base made of steel and powder coated, which housed a fiber optic light and wheel source and and it ran off a torriodal 12v 5 amp power unit that i helped making, the fiber optic and loom cost us £250 alone! and the bare plastic tree from B&Q cost around £60 then, and by the time his concept was finished it cost £400.00 and it looked so magical, it was first ever, no one had seen anything like It, I still have mine I bought of him, but as I build it myself, I only had to pay for materials, he could not sell them to anyone other than a handful of rich households and a few large hotels, but now if you look around you could buy these trees with fiber optics for under £50 complete!

He was also the first one to use LEDs into signs, he came up with an OPEN sign well before anyone even dreamed of it, again we used to make OPEN sign for about £200.00 then, but now you can buy these damn things for under £20 from China! and you see loads of them everywhere, poor man retired now and i still think of him highly inventive person, but luck and finance was not on his side.


So I can see why switched mode power supplies you are using is giving you a lot of grief, SMP's are at best rubbish, so why not spend a little more and get a robust toroidal transformer based power supply. If you need any help in custom made power supplies let me know.
 
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Buy a 0.1 ohm resistor (readily available cheapish on ebay in a wired ceramic package) and measure the voltage across it with a multimeter.
 
SMP's are at best rubbish,

Low cost expendable Switch Mode Power Supplies as supplied with LED strings and similar are very poor in both design and build quality. That said some credit has to be given for the igenuity of getting it so cheap while appearing to function as a SMPS.

There are many high quality SMPS that provide smoothed DC current at precise voltages for the equipment they are supplying. Almost all computers rely on multi output SMPSs for their power supplies.

Buy a 0.1 ohm resistor (readily available cheapish on ebay in a wired ceramic package) and measure the voltage across it with a multimeter.

That will not provide an accurate reading of the current. In almost all low cost LED drivers ( and some of the more expensive ones ) the output is far from steady smoothed DC. A driver with an output labelled 12 Vdc (eff) is effectively 12 volts but in reality could be as bad as 36 volt pulses with gaps between such that the average is 12 volts. This has the advantage that the LED elements appear to be three times as bright as they would be on a steady smoothed DC supply.

To get a true indication of the pulsing current and it's waveform ( size, shape and timing of the pulses ) would require an oscilloscope in differential mode ( input 1 subtracted from input 2 ) and a very low value non-inductive resistor with a high impedance scope probe at each end.
 
In almost all low cost LED drivers ( and some of the more expensive ones ) the output is far from steady smoothed DC. A driver with an output labelled 12 Vdc (eff) is effectively 12 volts but in reality could be as bad as 36 volt pulses with gaps between such that the average is 12 volts. This has the advantage that the LED elements appear to be three times as bright as they would be on a steady smoothed DC supply.
That presumably depends upon what current-controlling circuitry exists within the LED lamps? What you say would be true if current 'control' within the lamp was simply a resistor, but the closer the control circuitry came to being 'constant current', the more would it eliminate the effects of such high voltage pulses (if present).
To get a true indication of the pulsing current and it's waveform ( size, shape and timing of the pulses ) would require an oscilloscope in differential mode ( input 1 subtracted from input 2 ) and a very low value non-inductive resistor with a high impedance scope probe at each end.
That's theoretically true, and would be appropriate in a laboratory - but, I suspect, totally impractical for Rocky's 'in the field' application.

Kind Regards, John
 
LED lamp is LED element + LED driver. LED element is just the light emitting diode
We know that, but we are talking about ELV LED lamps, which makes the full picture more complicated. In other words, in addition to the current control ('driver') within the 'LED lamp', there is also an external 'LED driver', providing an 'effective 12V' supply to the LED lamp (LED element + 'internal' driver). It is presumably that 'external driver' that we have been talking about here.

Kind Regards, John'
 

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