Fatman Technical Resistance.

I was wondering if anyone new what resistance to dial in on the fatman for pure 1 amp power to the LED's? Using 8AA batts to power the beast. In the technical it says 13.50K but according to candlepower forums various views some seem to think higher. Please help with this conudrum of conudrums!!!

If you're not using an external pot then 13.5K for the onboard pot will give you 1000mA.
If you're using a 50K external pot (like some of us) for dimming then set the onboard to 18.5K Ohms.

The way you calculate the onboard pot is as follows.

1/Ronboard = 1/Rdesired - 1/Rexternal

Ronbard - the resistance across the small onboard pot
Rdesired - derived from current desired on table (eg 1000mA = 13.5K)
Rexternal - the resistance of the external pot

I hope this helps

Well I will be darn, I learn something every day. I am using the external 50K pot and set the resistance at about 13.5 or a little less. I will reset the internal trim pot this weekend and see what it does. Thanks modamag, this is not covered in the technical data on the Fatman on the Taskled website.

Would you be able to acheive the same results by connecting the external pot first turning it to its maximum and then setting the resistance onboard to 13.5K?

Thanks,
Chris
.308

Mine seems to be behaving oddly. It's running 3 series-connected Luxeon IIIs @ 1000mA (I checked and re-checked the resistance before epoxying it in place). The Vf for those Luxeons totals 10.5V @ 1000mA. The voltage on my 6-cell battery pack measures ~8V fully-charged... but when I measure current, I get ~2000mA. Unless the voltage is sagging under load, that's ~16W going into a 10.5W load - and I thought the fatman was a bit more efficient.

.308 said:

Would you be able to acheive the same results by connecting the external pot first turning it to its maximum and then setting the resistance onboard to 13.5K?

Thanks,
Chris
.308

Click to expand...

Yes - that is the CORRECT way to do it. Since it takes into account the typical +/-20% tolerance of Pots...

And YES it IS on the taskled website... www.taskled.com/techfatman.html

To quote:
----
Using the Fatman driver with an external Potentiometer to provide dimming control.

The following picture shows the hookup to an external Pot. One wire connects to the Wiper contact of the Pot, the other wire connects to one end of the Pot. The Pot should have a Logarithmic Taper (similar to Audio pots), this will allow the intensity to appear to change linearly to the human eye. The onboard trimpot is adjusted as per the table and equation above. Prior to adjusting the trimpot ensure the external Pot is adjusted to its maximum resistance. Then the procedure is the same as above, measure the resistance across the VIN- and POT and adjust the trimpot as above.
---

george.

There is definitely a voltage sag under 2A load. Depending on which cell brand but it's like 1.1-1.2V under such load. You need to measure the Vin on the fatman to be sure of the Vin. Don't rely on the unloaded voltage.

This means that you'll get 6.6-7.2V. So 2A @ 7V is ~14W.
Most of my 3x3W (TW0J) are like 12.5V-Vf @ 1000mA. (I think you're using T@@J with the spec of 3.5V @ 700mA).

So my guess is that your setup will experience ~89%. Which is inline with the fatman driver specs.

Efficiency = Pout / Pin
Pout = 12.5W (12.5V * 1A)
Pin = 14W (7V * 2A)

I measured voltage drop across each LED when the setup was running and got 3.4V, 3.5V, & 3.6V : 10.5V

Assuming 1.2V loaded, that's 14.4W : 73% efficient. 1.1V, 13.2W : 79%.

Perhaps I should discount my cheap DMM's readings - I had to short the 10A fuse slot since the fuse was blown when I got it and it reads between 200 & 600 mOhms resistance when the probes are shorted.

Thanks everyone for the input it seems easy enough now just turn the pot to max resist and then dial the onboard pot to 13.5k. Now that I see how easy it is I'm worried about the stars I chose, there J Binned 3 watters and I have 3 with 8 Nimh batts powering it. Do you think they will have a high enough Vf for the batteries I chose running them at 1 amp.? I asume they'll run at 3.51 Vf at 1 amp total 10.53and the batts should be total of 10.4. So hopefully this is true and I get a near perfect conversion of power.

8NiMh perfect soultion for 3x lux3..

8NimH is nominal 9.6.. but you want lower Vbat than Vf.. else you can not dim them. I have that problem with 3xLiON running 4x Lux3.. the 'low beam' won't work properly 'til the batteries are worn in a bit.

Don't expect more than about 92% efficiency you can work backwards to get runtime estimates.. and if you use the dimmer.. it can exponentially increase your runtime as you dim the light to just what you need.

-awr

elfen1: One dummy AA should fix you right up for low power mode.

that is true.. i never think of using dummy cells.. no can do in most of my hosts.. i use LiON.. i'd need to have 1/3 of a dummy cell, those are hard to come by

-awr