resistor on fire

[jmasciswannabe]

I have run into a problem with my deluxe electric mistress build. The 27ohm resistor is frying directly after the voltage regulator 7815. I am using an NTE968 15v positive voltage regulator. I have gone through three resistors and the meter is showing around 14v all the way up to it....any ideas on what could cause the resistor to burn up on the other side?

http://www.generalguitargadgets.com/diagrams/dmistlo.gif

There would be a lot less diy effects in my setup without this forum...thanks everyone!

Ian

[j0shua]

i think is to heat for 27 Ohm resistor , it's 18V power transformer but how many Ampere is ?

try to put 1k resistor or 47 Ohm

[pjwhite]

Something in your circuit is drawing too much current.  If your 27 ohm resistor is rated at 1/4 watt, you should have no more than 100mA going through it.  More than that and it will generate enough heat to burn up.  Put a voltmeter across the two resistor leads and measure the DC voltage.  It should be no more than 2.5 volts.
I'd guess that something is shorted, or wired incorrectly.

[jmasciswannabe]

the transformer I am using is this:

http://www.mouser.com/catalog/specsheets/114135.pdf

it was suggested by ggg....

I changed the resistor to a 1k and no more burning up....yay! I have found that the other side of the resistor is not showing any voltage. I am assuming this is because it is being grounded out somewhere along the way. Using the beep funciton on the multimeter, I am getting beep when I touch one lead after the resistor and the other to where the transformer is grounded. Unfortunatly looking at the pcb I see no obvious stray solder traces, but the lines get pretty close....looks like I will be getting out the magnifying glass. Besides the grounding issue, what else could cause the voltage to be 0 on the other side of the resistor?

Thanks pj and joshua!!

[Paul Perry (Frostwave)]

. Besides the grounding issue, what else could cause the voltage to be 0 on the other side of the resistor?

It's a short to ground.
Probably via solder (it can happen that it is VERY hard to see where this happens. Sometimes you have to cut through various power rail or ground tracks to isolate the point).
Less likely, is short through a faulty component, eg a chip in backwards. If that has happened,, the chip is no use now! (if it is shorting thru a chip, the chip will be hot enough to burn your finger).

I've been there many, many, MANY times :x