I stripped a nanjg 105c of the 7135 chips and replaced it with a fet. I tried to buy the fets that get recommended on this forum, but couldn’t locate any at a reasonable price in Australia. I got an irlr3717, which seems to have good specs as far as I could tell.
I wired direct drive a inr 20c at about 4.0v to an XML t6. Got 6 amps to the led. Then I wired an xpg2 direct drive and got 5 amps. Once I wired in the driver(thick wire, positive direct to led and board, negative to fet pin) I got 4amps to the xpg2. So I seem to be losing about 1 amp through the use of the fet driver. Is this normal? Is it caused by a poor spec fet? (I have stock firmware on the driver, in case that makes any difference).
The driver is still outside the flashlight so there is no switch resistance? Being direct drive the battery is going to go down like a rock, so it was recharged to 4.0V for second test?
Looking at an irlr3717 data sheet that I have, there might be a problem with the gate drive voltage. As a quick check, you could bypass the polarity protection diode on the 105C with a small piece of wire. The idea here is between the battery voltage sag, and the 0,6V drop across the diode, makes for less gate drive voltage and thus less drain source amps?
Thanks for having a look at the specs of the fet. So maybe I didn’t quite get the right type. I’ll look into the detail you mention.
I didn’t write it, but I changed a few times between direct drive and using the driver. It was clear that direct drive was each time giving me about 1amp more.
If you used the capacitor location as shown in my nanjg-092 thread, it's already taken care of, bypassing the diode will just raise the MCU input & output voltage (to the FET's gate) slightly.
You can also try just jumping B+ straight to the FET gate, it'll do the same thing.
If you’re using a stock Nanjg with the cap in its normal location (C1) there is an issue that causes the FET gate to see >6v spikes, this can cause very strange / poor behavior, especially with an unknown FET. (If you moved the CAP from its pad’s to between BATT+ and GND then the issue goes away).
So if you have C1 in its stock nanjg 105c location yes you should try jumpering the polarity diode as it will temporarily “fix” the >6v on the gate issue (which causes lots of different issues).
So we know exactly what you’ve done you should post a macro pic of your driver if you can.
OK thanks. No I didn’t move the capacitor. I thought this was just so you could have the spring side clear of components. But I clearly didn’t look close enough. Thanks for the suggestion. I’ll report back on the result once i get a chance to make the change. Thanks.
Thanks. I will move the capacitor. My mistake not realizing with was required.
Yes I will have to look into somewhere I could place photos, as I tried previously on this forum but gather you need to upload them to another location first.
FYI photobucket and imgur are both extremely easy to use. If you have a pic email it to me (email address in my signature), I’ll host it and post it here for you.
OK so I finally got around to moving that capacitor this morning. It managed 4.3a with both a xpg2 and an XML, which is still low. Funny thing is this same driver (well not exactly the same driver, but same setup)in an srk is drawing over 10 amps. I suppose due to the 4 batteries vs 1 in this application. As I need it to run from 1 battery only I guess I need to chase up a different fet. Thanks all for your help.