AMC7135 Issue

[Zeebit]

I got an AMC7135 based driver from Fasttech that is supposed to be set to 700mA. I hooked up a 3W white LED to the driver powered by a single Li-ion cell but its only putting out 225mA. I got the reading by hooking my multimeter in series with the LED.

Any ideas why the driver isn't putting out 700mA?

This is the driver: https://www.fasttech.com/products/0/10001685/1122403-2-amc7135-led-flashlight-driver-circuit

 

[djQUAN]

here are a few things that came to my mind:

1. battery voltage drops when loaded not giving enough headroom for the AMC dropout voltage.
2. your multimeter shunt resistance might be too high causing a similar effect as 1.
3. Your multimeter leads are long and thin, also same as 1.
4. Your multimeter leads are long causing inductance and the AMC oscillates. You'll need an oscilloscope to check that.

Can you post a pic of your test setup?

 

[Zeebit]

Even when I don't hook up the multimeter, the LED isn't any brighter. The cell is also fully charged.

I'll try to post pics but my camera is not here right now.

 

[djQUAN]

what cell are you using? it may be that the cell has high internal resistance, reading fully charged unloaded and drops when connected to a load.

What's the voltage across the battery and LED when the circuit is wired and operating?

 

[LarryDFW]

Zeebit;

Two things could be responsible for the low current:

1. LED requires higher voltage than normal.

2. Battery voltage is sagging under load (more likely).

Check the voltage on LED when lit.

LarryDFW

 

[Zeebit]

I'll update later. I'm kinda busy right now but thanks for the input guys.

Update:

I've tried other batteries and LEDs but I get the same result.

Some details:
Cell resting voltage: 3.95V
Loaded: 3.92V
LED Vf: 3.28V
LED current draw: 231mA
Driver current draw: 233mA

I still don't know why I'm not getting 700mA.

 

[Zeebit]

Bump.

I checked the driver board and everything is ok except it is still not giving me 700mA. Anyone knows why? Maybe I'll just have to buy a different one.

 

[Wolfman29]

One possible problem is that you're using alligator clips. Those things have notoriously high resistance.

 

[Zeebit]

I'll solder it in a moment. I've tried to measure the resistances and I get 1 milliohm at most.

 

[djQUAN]

I think those wires are too long causing inductance and the AMC chips are oscillating.

try adding a 1uF or larger capacitor across the battery+ and - right at the driver board.

 

[Zeebit]

When I added a 1uF cap on the input something had shorted out so I removed it but the cap was fine. After that, the driver is now giving out 460mA. :thinking:

 

[djQUAN]

was that with the cap removed?

I still recommend adding a cap as it is suggested in the datasheet. I also had problems with the AMC chips with long wires (more than a couple inches). Cheap flashlights get away without one since the battery wires are very short.

 

[Zeebit]

Yep, 460mA is w/o the cap. With the cap soldered on the input side, something shorts out and I dont know what it is or why.

 

[djQUAN]

how were you able to tell something shorts out? does it get hot? smokes? or simply doesn't have output?

Edit: I have a few boards and AMC chips here. I'll go and try to replicate your problems.

 

[Zeebit]

The led does not light and I burned the tip of my finger from the spark when I connected the wires to the battery.

If I can't get this right then I'll just buy a few more. No need to sweat about this.

 

[djQUAN]

That's weird. I tried making a driver with 2 chips. 4 inch wires from the battery to driver and another 4 inches from driver to LED. ran at 720mA.

Tried adding an inductor in series with the LED or battery without any decoupling capacitor to simulate long wires but still a solid 718mA. (battery though is a protected panasonic 3400)

There is one difference though. My PCB doesn't have the tiny reverse polarity diode (little brown glass bead thing in your pic in the driver PCB) if you bypass (short) it, see if it works any better, careful with reverse polarity though. If it still doesn't work, then call it quits and the AMC chips are probably damaged in some way.