Hi All,
What should the correct tail amp measurement be for a Convoy C8 with 8x7135? I'm asking because the readings that I'm getting don't make sense to me. A 8x7135 should be pulling in the neighborhood of 2.8A. I'm getting almost half that at the tail, more like what I would expect from 4x7135.
I'm estimating an output of around 1,000 lumens when calibrating against other sources and if it was a 4x7135 I'd be expecting around 500 lumens.
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How are you measuring the current? Standard long DMM leads will eat up a lot, & give a lower reading.
Check out this vid by Djozz to see the differing methods/readings.
Yeah, I’m using the standard leads right now so that might be the problem. I’ll have to look into getting heavier gauge wires to see if that addresses the issue.
Yep, the long leads have a lot of resistance and will vastly reduce the amp readings. This is why the output still looks right, with the tailcap in place you are getting the correct 2.8A and the full output.
Yeah, just grab some banana plugs (“solderless” screw okay, solder better), and no more than about 1’ each of 16ga wire. (Red and black if you want to be fancy, else who cares if the beastie reads 2.796A or –2.796A, eh?, so the same color is fine.)
That can handle up to a few amps or so w/o too much degradation. If you have a DD quad that should be pulling close to 10A, forget it. It might handle the current fine, but also it’d be reading lower going through the meter than if directly shorted. Ie, the more current pulled, the less accurate the meter would be.
Thanks for all the feedback and also thanks for the tip on using banana plugs. It just so happens that I have extra banana plugs and I have plenty of wire. I think I might even have 10 or 12 gauge that I used to use on my truck.
I’m getting 2.85 amps from my clear XP-L Hi version.
Update - I found some old speaker wire that was 14 gauge and conveniently already had banana plugs attached. I cut them down to less than 8 inches. I’m now getting 2.84A from my Convoy C8.
There ya go…
Banana plugs and some speaker wire! Never thought of that. I think I will try that out myself. Much cheaper than buying a clamp meter.
Clamp meter can measure much higher current(mine can measure upto 400A DC/ac)
The drawback is it not as accuracy as tradition one when measure low current
Okay, here is my final solution. 8 inches of 16 gauge speaker wire. Color-coded banana plugs on the DMM end. On the probe side, I’m using an O-connector for positive and a flat connector for negative.
I found that using these connectors as “probes” got consistently better results than using either bare wire or banana plugs as probes. I bent the O-connector slightly so that I could apply even pressure against the negative end of a battery for amp testing. I’m no longer having to push something pointy into the back of the battery and the contact area using these probes is perfect too.
I’m now getting exactly 2.85A on the Convoy C8 every time and without the fluctuations I had before.
Looking good. :+1: … Working good too it sounds like. 
Yeah, that setup will work fine for regulated setups. For direct drive turbo setups the readings will be a bit lower then they actually are but still acceptable.
Why will they be lower in DD Turbo T_A??
Just curious…. 
Those wires while reasonable thick and short are still going to have more resistance then a spring bypassed tailcap. Any extra resistance will reduce the current. So the readings will be lower accordingly.
Although with 16awg it should not be a big enough difference to really matter on a single LED light. Just don’t take the readings as Gospel.
I've compared DD/FET setups with my 14 AWG lead meter and clamp meter, and I think I saw difference of like 1A - 6A on heavy leads, 7A on the clamp meter, in that ballpark. The low Vf LEDs like XPL2's, 219C's can get up there. Of course when measuring a multi LED DD/FET setup (triples, quads, etc.), a clamp meter is the only way to go. Differences at 8A and above get really significant.
A guy a while back went into thorough detail on the resistance inside the meters themselves, even the Flukes. Dunno where that thread/post is, but he could basically calculate the amp drop we see in high amp measurements with heavy leads.
My budget tail cap amp draw station,,, Free giveaway HF meter, 11" #12 wire, some solder/flux, = $.35... lol
#12 wire, ends tinned to fit tightly in HF meter, seems to work well and within .02 of an amp compared to my clamp meter on any draw below 10amps.
Keep in mind that 16 gauge wire has 2.5 times the resistance as 12 gauge. Although, like I said, for a single LED (aka, less then ~10A or so) the readings should be fine.
Agree 100% Tex…
iirr you are the one that explained this to me when I first started on this ‘enlightened’ journey, and helped me to figure things out with this low voltage dc stuff… thanks.