I need some help selecting a driver for a custom USB powered light. I’ve tried an AMC7135 board with 2 chips which works well (shown in picture below), but the board overheats very quickly due to the excess voltage it needs to burn off. There are a few other boards that can handle the input voltage, such as this 18V driver from DX, but I don’t want to drive this >250mA due heat issues and I don't need a blinding light.
Desired driver specs:
3-6V input
1-3 modes
250mA max
Any suggestions would be helpful.
Thanks!
Hill
The light engine with USB connector attached
Here is the light running off a 4x18650 USB power supply.
This Buck/Boost driver reviewed by HKJ would be very efficient at 5V input. The current sense resistor could be increased slightly to get a lower medium mode. Has mode memory, SOS mode could be a deal breaker.
I saw that driver and yes, it would do the job. Problem is I'm trying to keep expenses down by using what I have on hand.
I figure there must be some resistor mod to lower the voltage a bit, but I'm not an electronics expert. Could you give me some help in figuring out what type of diode to get?
I could just buy one of those, but what's the fun in that?
I had a similar problem with running a red XP-E from a 18650 cell. I ended up using 2 diodes in series to drop ~.7V to the LED, so the 7135 chips had less voltage to burn off. Can’t for the life of me remember what thread that was or what their name is, but I can dig ’em out when I get home. If you want a couple I can just pop ’em in the post, I have plenty of them (and stamps :laughing:.
Here is how the final USB light will look when fully assembled. I will probably fill in the area around the USB plug to make it look cleaner but you get the idea.
no worries, I bought 10 for a dollar or so, so they’re not especially expensive
Yes, each diode drops between 0.35 and 0.4V, although the light they’re in oscillates between 0.35 and 1.4A (flashing rear bike light), so that’s just a rough guess. Total voltage dropped by the 7135 chips should be around 0.8V and they’ve been totally fine with that for the last 6mths or so.
Given that you’ll have some sag in your power supply (I think?) you’ll probably only need 3. Vf of 3V, plus 0.8V or so in the 7135 chips, plus 1.2V in the diodes will get you to ~5V easy. The diodes are large enough that you wouldn’t want to have to stuff too many in your pill. PM me your address and I’ll dig them out tonight.
Housing looks awesome too! Why fill in those cooling fins with o-rings though? Or does it cool well enough that they’re not needed?
Thanks for the compliment. I have plenty of space inside the housing if needed. Power supply will be either from a 4x18650 USB power supply (see pic in post #1) or PC USB port. Either way, I wouldn't expect to much voltage sag. I will play around with the number of diodes to get it just right.
The light does not run overly hot since the XML is driven by only 2 x 7135 chips and it's mounted to a copper sinkpad on top of a rather sizable aluminum heatsink (rather overkill but I often over engineer my lights ). I thought the O-rings were kinda cool and added some grip to the light.
that’s what the ones I used were supposed to drop, but measured with my fluke only dropped 0.35-0.4V (the light oscillates between 0.35A and 1.4A drive current, hence the variation), which is why I used two. I’ll post the part no. when I dig them out for Hill.
I didn’t realise that it was a Schottky diode, I have a vague memory that these have a lower voltage drop than the regular ones?
Oh, and as an aside, I’ve been running a modified desklamp with a XR-E (now Nichia 119), 2 AMC7135 chips and a wallwart phone charger (5V 0.8A) for several months without problems. It’s been on for several hours at a time (it’s a garage/ workshop work light) with no issues. Although adding a diode or two will be good insurance, you’d probably be fine as is if my experience is anything to go by. I think AMC7135 chips shut off at 150C (give or take 15C or so), so they’re pretty robust temperature wise.
