Built in plug to measure LED current?

[sygyzy]

Hi,

I am building a P7 mag with a Der Wichtel driver. I understand the only way to measure current across the LED is to put the DMM in series with the LED. In order to do this, I need to break the LED lead. Since I have to adjust the board for various battery setups, I need to be able to measure current multiple times.

Has anyone thought about soldering some sort of plug to one of the LED leads that can be separated when you want to measure current?

 

[LukeA]

Sure you could do this. You'd want to cut the wire going to the LED and connect each half of the cut wire to one leg of a small connector, say, a Deans Micro. Then assemble a mating connector (could be as simple as clipping the DMM lead clips to the connector legs). To run the LED without the DMM, you'd want another connector with the legs connected to each other.

 

[DonShock]

I installed a current measurement point in several of the early mods I did. Since I had some computer junk laying around, that's what I used. I used a piece of header with two 90 degree pins. I put it between the driver and the LED on the positive lead. I epoxied it to the heatsink between a couple of the LEDs. I just used a regular computer pin jumper to make the connection during normal use. To take a current reading, I just had to pull the jumper and connect the multimeter. I'll see if I can find an old picture that included that setup and post it later if I do.

 

[sygyzy]

Please do, I would love to see some pictures or a diagram.

 

[DonShock]

No luck finding a picture of the prior builds with the current measurement point installed. But I did a quick mockup that shows the parts and function and snapped a pic of that. The piece off to the side is the piece of computer pin header I use. I just cut off one segment to use for the jumper pins.

 

[georges80]

IF you have a DC capable clamp meter with a 40A range (usually means you have 10mA resolution) that is a very good way to measure the LED current without introducing voltage drops due to the multimeter. Of course you need a wired spot that you can put the clamp meter over, but it will give you a reasonably non-invasive current measurement.

You could leave a couple of inch wire section in your light/housing etc that could be pulled out for clamp meter access and then pushed back into the housing when done.

This becomes more important with a direct drive design since the multimeter can easily give you a false sense of security that your current isn't too high - due to the very steep VI curve at the high current end.

cheers,
george.

 

[sygyzy]

George - How does your technique work? You mean a meter that can measure current (amps) without actually needing to physically touch or be in series with the circuit? Where can I get such a device. Narnia?

 

[DonShock]

Clamp on current sensors rely on measuring the magnetic field around the wire that is caused by the currrent flowing through it. The higher the current, the stronger the magnetic field.

 

[georges80]

George - How does your technique work? You mean a meter that can measure current (amps) without actually needing to physically touch or be in series with the circuit? Where can I get such a device. Narnia?

http://www.extech.com/instruments/categories.asp?catid=26 shows some various styles. Make SURE you get one that can measure DC current (not just AC). There's also various sensitivities available. For <$100 you can find a decent one that can resolve to 1mA, works very well for most measurements we need to do with LEDs.

cheers,
george.

 

[TigerhawkT3]

I have a Craftsman clamp meter. The clamp's current measurements don't seem more precise/reliable than to the nearest 100mA or so, but I mostly use it when measuring currents higher than what my Extech 411 can handle. The Craftsman's voltmeter is autoranging, too, which is pretty nice.

 

[sygyzy]

George - Would you mind recommending a specific model? I am overwhelmed with choices and don't want to make a mistake on something so expensive. What's a reasonable amount? Say $100?

 

[TigerhawkT3]

Well, the MA220 has a resolution of 10mA, max 40/400A, and is about $100, while the 380950 has a resolution of 1mA, max 30A, and is $130 at sears.com. I checked out the other models in georges80's link, and let's just say that they were quite a bit over $100.

The only way you can make a mistake buying an Extech (or Fluke, or other good DMM - even Craftsman, if the one I have is any indication) is if you buy one that doesn't match your intended usage. Pick a model with the features you want at a price point that sounds reasonable, and you'll have a reliable instrument for years.

Just... don't let your friends drop it into a bucket of water. And if you do, or they do it while your back is turned, open it up, check the fuses and let it dry out for a few days.

 

[Benson]

Just... don't let your friends drop it into a bucket of water. And if you do, or they do it while your back is turned, open it up, check the fuses and let it dry out for a few days.

Sounds like the voice of experience? Ouch! :sick2:

 

[TigerhawkT3]

Sounds like the voice of experience? Ouch! :sick2:

Yep, but it wasn't too bad, I guess. It was an Extech 411, BTW, and it's still working fine.

 

[Packhorse]

I use this one. does not seem to be 100% accurate ( or my inline meter isnt) but for the price its a good value meter. ( I also have 2 Flukes one at $7k NZD )

 

[sygyzy]

Thanks for your tips! I am going to buy the 380950!