LM 317 help

[lazer]

I'm trying to power a 1.5 volt R/C glow plug off a 7.4 lipo battery, and was told I could use a LM 317 to do it. I took the positive off the Battery and soldered it to the In terminal and hooked the the glow plug driver to the out terminal and soldered the negative to the adjustable leed and then also soldered a ground to the adjustable leed that goes to the R/C glow engine. so I hooked it up and it worked for abour 6 to 7 seconds and then over heated the 317 and fried it. can one of electrical genius's help me out . thanks

[jeffs555]

When building a linear regulator you have to be very aware of power dissipation(ie heat). To calculate power dissipation in the regulator, you take the input voltage, then subtract the output voltage, and then multiply by the current. With the adjust lead connected to ground, the LM317 will output 1.25 volts. Starting with 8.4 volts for a fully charged two cell lipo, then subtracting the 1.25 volts output voltage, you are left with 7.15 volts. Not sure how much current a glow plug takes, but even if it is only 1 amp, when you multiply 7.15 volts by 1 amp you get 7.15 watts. This is way too much for a to220 package without a good heat sink. Without a heat sink, the to220 package on the LM317 will only dissipate about 2 watts before overheating.

[lazer]

so you think all I'm missing is a heat sink to make it work properly. thanks

[jeffs555]

Probably, but without knowing the current draw of the glow plug, can't say for sure. It for sure won't work without a good heat sink.

PS You should also be aware that only about 1/8 of the battery power is going to the plug and 7/8 is being wasted as heat. You would be better off with a nicad or else using a switching regulator with the lipo.

[tomskk]

Just for information, at 1.25V the current draw will be arround 4 Amps.

[jeffs555]

If that is the case, he would need an LM138 in a TO-3 package and a large heat sink. Don't know if there is a sink efficient enough to dissipate 25 watts in a TO-220 package.

[tomskk]

Better to use a NE555 circuit and a Mosfet.

[jeffs555]

Agree 100%

[lazer]

thanks for your input guys,

[lazer]

Need help again, I bought a switching voltage regulator from dimensional engineering and hooked it up properly I believe, then set the output voltage to 1.5 volts and I get no glow to the plug but still read 1.5 volts. can someone help please

[rmteo]

Which one did you get?

In any event, you would need 3-4A to light a glow plug. The ones from DE seem to be limited to 0.5-1.0A.

[lazer]

It's under there 3 and 5 volt regulators listed as there adjustable switching regulator. If what you said is true is there anyway to up the amps? thanks

[rmteo]

I don't think that there is any way to increase it's output current.

[Wayne V]

Why don't you just use a single cell NIMH with a large capacity, it will probably be the same weight as a 2S LIPO.

[lazer]

I already have the lipo and am powering a ducted fan unit and servos off of the lipo and want to power the glow plug off of the same power source.

[jeffs555]

How about these?
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXRYK2&P=0
This one doesn't say what voltage it works on, so might not handle lipo's.
http://electrodynam.com/rc/EDR-103/index.shtml

However, are you really sure you want to power the glow plug and the servos off the same battery? It is one thing to lose an engine, but if you lose all control trying to keep the engine running that would be disastrous.