Hi guys,
I recently did my first zner modon a 7*7135 driver, and I am using it with a xhp70. I tested the driver today with a new heatsik for the xhp70, but the 7135 chips got really hot after a few seconds. Is that normal or did I do something wrong? I use a 2s 3p battery pack for the driver
Yes that is normal, but not optimal. When used with 2s emitter like XHP70 there is more voltage to drop so there is more power dissipated in the 7135 chip. I believe these 7135 chips are rated to 7V output so they are in spec for most 2s emitters, but unless they are heat sinked well they will get very hot. I think some 7135 chips have a thermal protection which lowers the current when they get too hot.
Do you have a suggestion, how I could heatsink the chips?
Hello Devryd,
The ACM7135s are only rated for 6V input maximum (7V is the absolute maximum rating, you should not be running the device at 7Vin). I'm assuming you're using a 6V configuration XHP70, which has a forward voltage of 6.0V at 3000mA (from datasheet). With a 2S lithium pack, input voltage is up to 8.4V - this exceeds the ratings of the AMC7135 and you're at a risk of damaging the driver.
http://illuminationsupply.com/AMC7135.pdf
I'd recommend using a suitable constant current buck driver if your XHP70 is in 6V configuration. Alternatively, a boost driver will work too if your LED is in 12V configuration.
Good luck!
There really isn’t a easy way to heatsink them except to install the driver in a flashlight. The flashlight pill or driver shelf takes heat from the driver and dissipates it into the flashlight body. If you are running the diver in open air, that’s really hard on those overdriven 7135’s and they can either fail or throttle. As was said, you really don’t want to run those over 6V for long.
Ideally, you’re better off with a buck driver. You’d be limited to around 5A output unless you went with Lexel’s 8A driver. I run a buck driver off a 3S2P in my LED converted spotlight and it’s _plenty _bright. It also stays bright for as long as the batteries can supply 5.7 to 5.9 volts! They’re cheap too, but you still need to cool it somehow.
If you’re really ambitious, get a 12V MCPCB, reflow the emitter on it, and run it on 4S direct drive with a FET driver.
I just had the Idea to use a DC-DC stepdown converter in between the battery pack and the dirver.
Would that be useable?
When I use a lower mode, the 7135s stay cool to touch. Would it be a solution, to pack a lot of them on the driver and only run them at 50% each?
I would have to reflash the driver for that, I know, but would that solve my problem?
Total power dissipation is still the same, so without also increasing cooling you’ll have just as much heat.
But the heat would be dissipated (not sure if spelled correctly) over a larger area
I mean yeah, if you give the driver more surface area then you’re increasing the cooling. My point is; n 7135’s at 100% make the same amount of heat as 2n 7135’s at 50%. Packing twice as many into the same physical environment won’t change a lot. Probably you’ll mitigate short-term overheating of single 7135’s (due to relatively high thermal resistance from 7135 to PCB), but the entire driver will eventually heat up to high temperature. It’s not really a good solution. The right solution is a buck/boost rather than a dissipative driver at these power levels.
As loneoceans says, not only are you exceeding the supply voltage for 7135’s, you’re also asking them to drop quite a large voltage (up to 2.5V). With a full 2S battery and running 7x7135 (or 14 at 50%) you’re going to be dissipating 6W in the driver.
I had to remind myself more about how these chips work. The Zener mod lowers the supply voltage to the Vdd pin (and to the MCU) within spec. The 7135 chip then never sees the full 8.4V of the battery; it only cares about the voltage it has to drop. So it’s just an issue of whether the chip can dissipate the heat well enough.
Using 7135 chips with 2s LED and battery is a somewhat common mod, even sold by mtnelectronics:
http://www.mtnelectronics.com/index.php?route=product/product&product_id=53
Devryd, how is your driver mounted to the flashlight? It needs to be well mounted so it can transport the heat to the flashlight. To help with heat dissipation you can look into silicone heat spreading cubes and silicone thermal potting.
This should help, but another option, as others have suggested, is to just use a suitable buck or boost driver.
I have a 7135+FET driver in my Convoy L6. It was from MTN but they don’t seem to list the exact “30mm FET+7135 Driver - 2S Momentary - Convoy L6 FET7135-30-L6” driver any longer.
There’s some vaguely strange behavior involved, but the 7135s are holding up alright. The strange behavior I’ve noticed is that moonlight mode (7135-only) is brighter when the cell voltage is lower. At full charge (8.4V) I can stare directly at the moonlit LED, but below 7.8-ish it’s ever so slightly too bright for that.
They don’t have that one anymore I don’t think. I have the 26mm version for 4S clicky and it doesn’t even have a 7135 since it’s low modes are controlled by firmware. I guess you got one depending on what FW you wanted. Either way, very high quality drivers.