rheostat question

Thread Starter

mavrick

Joined Dec 10, 2006
16
i have been trying and trying and trying to build me a 20 volt rechargable light for hunting. i have the batteries and charger and every thing else i need. but i can not wire a brightness control switch. can anyone help me please. ive been told i need a rheostat switch. but i dont know what size or where to get one. or how its wired up. to buy a light like this in the stores is about 300+ dollars. i hate to pay that much for a light. thats just rediculous.
could someone tell me how to do it please.
 

Thread Starter

mavrick

Joined Dec 10, 2006
16
i tried a 10k potentiometer switch on the light. it would dim and brighten the bulb but the potentiometer started smoking. it had to much voltage to it or i didnt wire it right.
 

EEMajor

Joined Aug 9, 2006
67
Well, if 10K is giving you the brightness/dimming you desire (As in it dimmed and brightened enough for what you want) then thats fine, stick with that resistance. However, you are going to need a fairly heavy duty one to handle the current from that load. If the bulb is really 12 volts, then it will not last very long on a 20 volt power supply cranked up at full brightness.

Also, if it was doing the dimming, then you wired it correctly. The problem was too much current going through it.

I will assume the 30 means 30 watts. In that case, with 20 volts, your current is: CURRENT = WATTS/VOLTAGE
so 30 W / 20 V = 1.5 Amps. So for safety sake I would suggest rating your equipment for at least 2 amps, if not 3.
 

EEMajor

Joined Aug 9, 2006
67
You need to find one rated for lower voltage. Sounds like the one you are using is rated for household power. I'm not sure how to downrate it to 20volts however. Either way, we know you need one with a higher rating than that, since you let the smoke out of that one :)

I guess technically its a rheostat, here is a link to three of them that would handle your load, but they are about $50.00 each! Guess thats part of why they charge so much for the lights, eh!

http://www.mouser.com/search/Refine.aspx?Ne=1447464+254016+688632+688665+254359&N=1323038+4294958529+4294955051+254359+0&GetRecs=1&Msb=0&Mkw=potentiometer&Ns=P_SField&Ntx=mode matchall&Ntk=Mouser_Wildcards&Ntt=*potentiometer*
 

wireaddict

Joined Nov 1, 2006
133
The 3A, 125V & 1A,250V ratings are for the power [off/on] switch switch contacts & have nothing to do with the pot.

If the bulb is rated at 12V why are you using 20V? Is that the charger output voltage? If possible, I'd lower the operating voltage to 12-14V to protect the bulb.

If the bulb is rated at 30W then you need a rheostat rated at 30W or the next size larger to minimize smoke, if that's a problem. The resistance doesn't need to be anywhere near 10K; if the bulb draws 2.5A then a 4 ohm rheostat in series with it will drop 10V across it, leaving about 2V for the bulb [with a 12V battery]. So, knowing the bulb wattage now [which I didn't previously when I suggested 100 ohm, 10W, as I recall] I'd go with a 5-10 ohm, 30+ W rheostat.

Connect everytning in series: the batteries, switch, rheostat & the light & you should be in good shape. Hope you can get it built before hunting season ends!
 

Sirropo

Joined Nov 24, 2006
3
May well be easier to get a power transister with a small pot to control it.

May be a few more parts but likely cheaper and less bulky than a large 30+ watt pot ?
 
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