My LED and resistor went up in FLAMES

Brentsp · Post by **Brentsp** » Fri Dec 09, 2005 12:12 am

Actually it just started smoking a bit. I'm using the LED in place of the indicator light. I was told I needed 500 ohm or less resistor in series with the LED. I used a 470 ohm. Turned it on and the LED lights up but then the resistor starts smoking. I thought maybe it needs more like 200 ohms. I then used two 470 ohm resistors piggy backed on top of each other for a total of 235 ohms. Turned it on and the LED flashes and burns out......its dead. I then wire up my Fender style jewel light. Turn it on and the bulb blows. Then I went back to the normal Marshall indicator light and it worked fine. What am I doing wrong? I oringinally want the LED but figured the Jewel light would have worked fine.

Brentsp · Post by **Brentsp** » Fri Dec 09, 2005 12:23 am

You know what? After looking at my indicator and how its wired up I'm thinking its a 120v light. The red wire is going to the power switch and the black is going to the end of the Mains fuse.

Edit: I just turned it on and measured. It is 120 volt. No wonder it blew. How big of a resistor would I need now?

Post by **VelvetGeorge** » Fri Dec 09, 2005 1:16 am

Brentsp wrote:
Edit: I just turned it on and measured. It is 120 volt. No wonder it blew. How big of a resistor would I need now?

About 14".

Just kidding. You'll be better off running new wires to your heater voltage.

George

novosibir · Post by **novosibir** » Fri Dec 09, 2005 6:55 am

I can't imagine, what you did

My calculator says, that if you'd use a (red) LED in series with a 235 ohms resistor parallel to the other tube filaments, then the resistor wouldt dissipate 0.1 watt - so a 0.25 watt resistor wouldt be big enoug.

Is it possible, that your 470 ohms resistors in real have been 47 ohms? Did you read or measure them?

Larry G

Brentsp · Post by **Brentsp** » Fri Dec 09, 2005 7:20 am

novosibir wrote:I can't imagine, what you did

My calculator says, that if you'd use a (red) LED in series with a 235 ohms resistor parallel to the other tube filaments, then the resistor wouldt dissipate 0.1 watt - so a 0.25 watt resistor wouldt be big enoug.

Is it possible, that your 470 ohms resistors in real have been 47 ohms? Did you read or measure them?

Larry G

Hey Larry,

I was running 120v instead of 6.3v thats why she blew. I'd have to have a 4k or 40k resistor I'd assume.

Brentsp · Post by **Brentsp** » Fri Dec 09, 2005 7:22 am

VelvetGeorge wrote:
Brentsp wrote:
Edit: I just turned it on and measured. It is 120 volt. No wonder it blew. How big of a resistor would I need now?
About 14".

Just kidding. You'll be better off running new wires to your heater voltage.

George

So leave the black wire connected to the end of the mains and run a red wire from one of the 12ax7's pin 4?

novosibir · Post by **novosibir** » Fri Dec 09, 2005 7:26 am

Brentsp wrote:I was running 120v instead of 6.3v thats why she blew. I'd have to have a 4k or 40k resistor I'd assume.

6.8K - 5.0 watts (!!!)

Larry G

Brentsp · Post by **Brentsp** » Fri Dec 09, 2005 8:44 am

novosibir wrote:
Brentsp wrote:I was running 120v instead of 6.3v thats why she blew. I'd have to have a 4k or 40k resistor I'd assume.
6.8K - 5.0 watts (!!!)

Larry G

So I'd need a 5 watt 6.8k resistor? LOL

Dude · Post by **Dude** » Thu Dec 22, 2005 9:36 am

Brent, the way I normaly go about this is use a rectifier diode before the resistor, this will drop the voltage by half (seeing you are using AC) and reduce the size of the resistor to use.

The formula for LED resistor selection is:

R = (VS - VLED) / ILED

VS = Voltage Source
VLED = Voltage drop of LED
ILED = Current draw of LED

As George says you are far better off (and safer) to source your LED current from the heater line as you have that already available. Use a rectifier diode here also as LEDs are not designed to rectify AC.

Brentsp · Post by **Brentsp** » Wed Dec 28, 2005 1:29 pm

Dude wrote:Brent, the way I normaly go about this is..........

Thanks. I ended up using a 200 ohm resistor from the heaters. It seems to be working fine. http://img.photobucket.com/albums/v160/ ... wlight.jpg
http://img.photobucket.com/albums/v160/ ... wlight.jpg

sub · Post by **sub** » Wed Dec 28, 2005 2:11 pm

Ahh..... very nice!