Question about resistors and electronics.

[Dox_21]

Didn't know where else to post this, so here it is.

I have a raptor heli, and I'm rigging up an on board glow charger. I have a switch with an 'on' led, which requires 12 volts. This is obviously too much voltage for the glow plug (the battery from my glow charger is 1.2volt), so I need a resistor between the switch and plug.

Also I have found that the plug glows well with 2.25 amps through it.

Now, I know pretty much nothing about all of this, but I found a formula which supposedly tells you the required resistance.

Volts (supply) minus Volts required devided by amps required.

This leaves me with a figure of 4.8 ohms of resistance. Is this correct? Where in the uk can I get the required resistor.

(BTW, I am aware that I could change the resistor in the switch to a weaker one, so that the led could run off 1.2 volts, but this way seems easier. Plus I'll get more charge time out of a 12v battery)

[HeliDan]

I am assuming your voltage and current numbers are correct as I don't know them my self.

That being said, that means that with a 1.2V battery with a current of 2.25A, your plug resistance is 0.533-Ohms

So, with a 12VDC source, you would need a 4.8-Ohm resistor in series to limit with current to 2.25A. Since the power dissipation will be almost 11watts, I would use a 15W or 20W resistor.

Being in the US, I cant help you with where to buy the resistor. Beware, it will get very hot very fast!!

Good luck

[Dox_21]

Ok, thanks.

So I was correct in thinking that I need a 4.8 ohm resistor.

So what I need is a resistor with 4.8 ohms resistance, that can handle 15 to 20 watts?

[HeliDan]

Yes sir. That is correct.

How ever you decide to mount it, make sure that there is plenty of space around the resistor as if you if it runs more than a few seconds, it will get very hot and you don't want to burn or melt anything.

One other thing, make sure that you don't have the resistor in an area that keeps it from getting plenty of air to allow convection cooling. This should be enough, but if you find it getting too hot it will shorten its life. You may consider relocation or air holes above and below the resistor.

You can also use many lower power resistors in a series parallel configuration if you must.

Good Luck! :D

[Dox_21]

Thanks for the help Dan.

I'll do plenty of testing off the heli to make sure It all works.
I've already had it running in 2 configurations.

1, With the 1.2v batt out of the glow charger. The plug worked fine, but the led on the switch wouldn't.

2, A small 11.1V lipo, the switch led worked, but I burned the glow plug out.

I figure a small heatsink made for a PC RAM module might do the trick for cooling.

Thanks again

[MarkD]

R=V/I

R = (12-1.2)/2,25
= 4.8 Ohms

Which you got correct :smokin:

The Wattage rating of this resistor is as follows

P=VI
= 10.8 x 2.25

= 24.3W

So you'll need a 4.8 ohm rated at 25Watts min

www.maplin.co.uk seem do only do max 10W

RS Components do High Power ones

http://rswww.com/cgi-bin/bv/rswww/searchBrowseAction.do?Nr=avl%3auk&N=4294739326%204294624939%204294681482&name=SiteStandard&forwardingPage=line&R=2252950&callingPage=/jsp/search/search.jsp&BV_SessionID=@@@@0213457036.1165600465@@@@&BV_EngineID=cccgaddjhhdmmjfcefeceeldgkidhgh.0&cacheID=uknetscape

[HeliDan]

Holy Crap! Your right, don't know what I did wrong, I just recalculated it and it is ~25W.

this means you need a 30W to 35W resistor. Lot of power dude!

Sorry for the error, I don't know what happened! I must have dumb thumbed my calculator! :arggg:

Thanx for catching my error!! :Chair

[Pinecone]

LOTS of wasted power.

Better to use a proper voltage regulator. Or find another switch or LED.

[Dox_21]

I think you guys may be right, it's a bit of a rough workaround. I'd probably be better off useing the 1.2v batt and changing the resistor in the switch. That way I should be able to get away with one of those small resistors that cost about 5 pence. So now I need to know how much current the led needs in order to run off the 1.2v batt.

Plus, I've looked on RS, and they want 5 quid for the 25 watt resistor, which isn't too bad, but then they want 10 pounds just for postage!

Also, it's not going to be a great idea to have something that hot connected to my plastic frames.

[MarkD]

The standard 5mm/3mm LED's take around 10 - 20mA

Or just get one of these

http://www.justengines.unseen.org/onbglow.htm

[HeliDan]

To run one of the newer style LEDs, you need about 12-15mA.

With a 12VDC battery, at 12mA you would need a 1K-ohm resistor and a 1/4W resistor should be fine.

If the LED needs more current (You need to find the spec on the LED) then use the formulas:

R=E/I (R=resistance, E=voltage and I=current)

For the power dissipation
P=E*I (or I use this: I2 (means current value squared) * R

If you need more like 30mA (older LEDs) then you need a 1/2W resistor.

ps I double checked my numbers this time. :mrgreen:

[Dox_21]

Once again guys, much thanks for the informative replys.

I've taken the switch apart, and I can't even find the resistor. I think it is buried inside the plastic stalk of the toggle, along with the led.

I've seen the shop bought on board glow chargers, but I only started this to give myself a little 'project' to do. Kind of like buying a 'rtf' as opposed to a kit.

I think I am just going to install the switch as is, with the 1.2v batt. Everything works fine apart from the led, so I just need to make sure I remember to turn it off!

[spork]

Dox,

If the plug works well on 2 amps with a 12V battery, it will work well at 2 amps with anything. It's possible that a 1.2V battery won't be able to produce the 2 amps (but it probably will).

Incidentally, wasting 25 watts as heat is an unforgivable sin. :mrgreen:

A regular (linear) voltage regulator will waste exactly the same amount of energy as a resistor. If you want to drop the voltage without losing the excess as heat, you need a switching type regulator