Measuring 425mA on a KD 2280mA driver?
I'm wiring up an XML LED in a test configuration with KD 2280mA driver and I wanted to get an amperage reading before permanently wiring it together. I put one of my meters in-line with the positive lead to the LED and measured about 3.5ma on low (what I expected). But, when I switched to medium (supposed to be 35% of the max) I only read about 115mA. On high I read 425mA.
I thought maybe I had somehow knocked out part of this driver so I wired up a spare KD 3040mA driver. On high I read about 490mA in-line with the LED.
Still uncertain of the readings, I pulled out my spare meter and found almost exactly the same measurements.
I'm very confused and hesitant to finish up the wiring until I better understand what I'm seeing. Can anyone shed some light on this for me? I'm probably making some dumb mistake or observation. Any help would be appreciated.
Last edited by marpilli; 11-20-2012 at 08:50 PM.
how did you solder/short the connections to adjust what mode you wanted?
EDIT: I just now went out and gathered this data.
Tenma 72-2050 (w/fresh 9v installed)
Beckman Industrial HD100
Single cell voltage reading prior to start:
2280 Driver on High - Meter in-line with +LED lead
2280 Driver on High - Meter in-line with +BAT lead
3040 Driver on High - Meter in-line with +LED lead
3040 Driver on High - Meter in-line with +BAT lead
Single cell voltage reading after finished:
Last edited by marpilli; 11-20-2012 at 09:18 PM.
There may be too much voltage drop in the meter you're using to measure the current. Does the LED get significantly brighter with all the meters out of the circuit?
You can also use another meter to measure the voltage before and after the current meter to see if that's the problem. Just make all the voltage measurements relative to the same ground point. I would use the negative terminal of the battery.
If that doesn't fix the problem, then you probably have a voltage drop somewhere else in the circuit. Measure voltages at multiple points in the circuit to see if you have drop at an unexpected place. You should have a drop across the regulator and across the LED equal to the expected Vf. You shouldn't have drops anywhere else.
I edited a previous post in the thread to try and get some realistic data. I hate not attacking something in a logical manner.
EDIT: I didn't notice the LED being significantly brighter with the meters out of the circuit.
As a matter of fact, I was quite happy with that blazing LED until I checked the amperage. Now all I've done is cast doubt on my creation.
To look at the LED (on high) with the naked eye it's quite blinding...
Using the Tenma meter in-line with the +LED lead.
Originally Posted by MtbMacgyver
Using the Beckman to check the voltage before and after the Tenma meter (both measurements had ground at the negative terminal of the battery as recommended).
Tenma measuring: .84A
Beckman measuring before Tenma: 3.98v
Beckman measuring after Tenma: 2.98v
That's the problem, you can't afford to drop a volt in the meter in a circuit powered by a 1s battery. You need more than 3v going to the LED to get higher currents. It's likely working correctly without the meter in the circuit but you can't tell the increase in brightness by looking directly at the LED.
Originally Posted by marpilli
MtbMacgyver, thank you very much for your responses. That makes a lot of sense and I'm less concerned about it now. Even if there wasn't a reasonable explanation, the driver is going to be mounted in a project box near the battery pack (no room in the head assembly of the light). So, I'll be able to replace/troubleshoot it very easily if it's ever necessary.
I thought about it some more after my last post and I can't recall using either of my meters to measure < 48v DC amperage before. For all I know both of these meters are whacked. It was then I decided the reading didn't really matter and I wasn't going to worry about it. I was happy with how it was coming along before and nothing changed (except more (and possibly erroneous) information).
I have another (semi-related) question.
Can anyone recommend a good meter (make & model, please) that would allow me to get accurate amperage readings at the 3~4v level?
I need to add one to my holiday wish list.
Originally Posted by marpilli
Fluke DMMs have really high reputation.
But for a start I would try with some decent test leads. Yours are more than obvious really bad.
Toaster79, roger wilco... Thanks!
Get a clamp ammeter. They do not introduce resistance in the circuit.
Clamp meter might be clumsy in most situations in our business. I own a cheap Nimex multimeter with good test leads and always get accurate readings comparing to datasheets although I use pointy probes. Voltages are within mV and so is current. Oh yes, and leads are about 70cm long.
If you have a known accurate voltmeter, just measure the voltage across the XML. Then go to the Cree chart that shows current vs voltage and you will get a reasonably good idea what the current is.
When I use mine, I solder in line a 10 turn coil of 16AWG wire for the meter to clamp on. Then divide all readings by 10 for actual current.
Originally Posted by Toaster79
Last edited by Vancbiker; 11-21-2012 at 01:05 PM.
Reason: spelling correction
This is a meter designed for RC usage, but it works well for bike light stuff. You have to add your own connectors, but once you do, it's really easy to just insert it between the battery and light to get measurements. It has an extremely low voltage drop for measuring current and can measure up to 100a. It also shows voltage, watts, current, amp-hours, and watt-hours. So if you leave it connected while doing a test run of a light, it'll give you total amp-hours and watt-hours of the test run at the end. I also put a female connectors on both ends of the meter and then make a short male to male adapter cable. That way I can use the meter both for measuring charge and discharge. I just move the adapter cable from the source side of the meter to the load side of the meter depending on whether the source / load is battery / light or charger / battery.
TowerHobbies.com | RC Electronics Watt's Up Watt Meter Blue
Don't forget one very important feature when measuring voltage or current. We have been assuming in our discussions here that this is "pure" DC. If the current or voltage is modulated in any way, then you must use a "true" rms reading meter to get accurate results. If your particular driver adjusts the current by pwm or other waveform changing, then you can no longer use the "DC" meter function.
Another trick is to order some precision resistors in values such as 0.1, 0.2, 0.5, 1.0 ohms and the appropriate wattage based on how much current you want to measure. Then you just solder them to connectors, or just temporarily solder them into the circuit. Then you just measure the voltage drop across the resistor and use ohms law to calculate current. That's exactly what a meter does anyway, which is why they introduce a voltage drop.
There is always a tradeoff between accuracy and voltage drop, so having multiple resistance values on hand is a good idea. If you try to use a 0.1 ohm resistor to measure a half amp, you'll be measuring a drop of 0.05 volts which is below the accuracy of most meters. With multiple resistors, you can pick a good compromise between accuracy and the voltage drop you can tolerate.
If you want to get really fancy, you can build a little test rig with connectors that match your light system and connectors that plug into the voltage inputs on a meter.