Resistor sizes

[chartrock]

I need help from an electronic whizz and I am sure we must have some out there in Expanda Land. :help:

I bought a couple of LED goose neck lamps from IKEA. They plug into 240V with a piggy back transformer setup, where the cable just has 2 a 2 pin plug to go into the back of the transformer. The rating on the side of the T/F says it is 4V output at .75A and 3W which to me adds up correctly.

I figured I could put a resistor in series to drop the voltage from 13 to 4 and I could then use it hard wired in the van. To my mind a 12 ohm resistor pulling .75A would use up 9V leaving 4 for the lamp. I found an 18 ohm, rated at 230W according to its wording, and wired it up for testing. The lamp worked fine but in about 15 seconds the resistor was smoking. I tried a larger resistor but it also overheated. :smokin:

I certainly don't want anything starting fires so, where am I going wrong?

Paul

 

[boots33]

Paul try checking the light with a meter and test its actual current draw. 750ma is quite a lot for a small led lamp and i don't think they would draw the maximum that the powerpac can supply. I am not at home now but from memory to work out the resistor value it's

(supply voltage minus led voltage) divided by current

will check that when i get home

Edit
formula below (as stated on Jaycar data sheet)

R = (VS-Vf)/If

where

R is the resistor value in ohms
VS is the source voltage
Vf is the forward voltage drop of the led
If is the current draw of the led light (in amps)

Which is basically what you have used.
Standard resistor wattages range between .25w and 10w while resistors can be obtained outside these ratings they are not so common. 4v and .75 amp is indeed 3w so you would need at least a 5w resistor and you may still find that will get warm, if you have room i would use 10w instead. But I would still check the draw first as it will make a big difference to the resistor required. 5w resistors are in the Jaycar catalogue here http://www.jaycar.com.au/productRes...m=ADVANCED&pageNumber=0&priceMin=0&priceMax=0
You should also use 14v as your supply voltage reference just to be sure you don't put undue strain on you new leds.
Jaycar now have a store at Labrador

hope this is of some help
shane

 

[grinner914]

Rather than a resistor, why not try a voltage regulator

http://www.jaycar.com.au/productView.asp?ID=ZV1505&keywords=voltage+regulator&form=KEYWORD

Basically it is a small 3 leg chip, looks similar to a transistor, leg 1 is voltage in, leg 2 is the ground (or negative) and leg 3 is the voltage out, in the above link, 5 volt.

The problem with a resistor is that as the input voltage changes, the output voltage changes in proportion. With a voltage regualtor, the output voltage remains constant, provided the input voltage is at least 2 volts above the output.

Jason

 

[bigman0510]

remember you must get rid of the extra energy some how... heat:flame:

 

[boots33]

Rather than a resistor, why not try a voltage regulator

Jason

Yep that would work also, though maybe an lm317t would be more suitable as it will get you 4v. any of those regulator ic's would need good heat sinking to handle 750ma.

I agree with you bigman the ratings on these devices tells you how much power they can dissipate and the tradeoff for that is heat

 

[chartrock]

Thanks for all that guys.

Shane, I did measure the current at 44mA after which I visited Jaycar at Labrador, just after you posted and spoke to one of the people, explaining my problem. He told me LED's are current dependant, sold me 2 lots of resistors 20 ohm and 30 ohm at .5W.

I set the 30 ohm up in series with the lamp and it worked but the resistor got quite hot. I then set the second one up and the resistor smoked and before I could switch it off the LED also went up in a puff of smoke. :flame: It is now dead but the other one is still working. I have it connected through a terminal block which is housed in a plastic box and has been working for a couple of hours at night. We are now in Cobar (NSW) so if there are any problems with this one then no reading in bed.

 

[boots33]

Thanks for all that guys.

Shane, I did measure the current at 44mA after which I visited Jaycar at Labrador, just after you posted and spoke to one of the people, explaining my problem. He told me LED's are current dependant, sold me 2 lots of resistors 20 ohm and 30 ohm at .5W.

I set the 30 ohm up in series with the lamp and it worked but the resistor got quite hot. I then set the second one up and the resistor smoked and before I could switch it off the LED also went up in a puff of smoke. :flame: It is now dead but the other one is still working. I have it connected through a terminal block which is housed in a plastic box and has been working for a couple of hours at night. We are now in Cobar (NSW) so if there are any problems with this one then no reading in bed.

Glad to hear you are under way hope all is going ok, we are off this weekend to try out the new van by my reckoning at 44ma you should be using around a 220 ohm resistor. the 20 and 30 ohm will allow way too much current through and cause rapid failure. they have worked out the value on 440ma which would require a 22 ohm resistor.
Don't forget to post a few pics when you can.

 

[chartrock]

Thanks Shane. The light is working OK at the moment and has been used every night (4 nights so far) but will purchase new resistors when in a larger town.

As for pictures, by the time I have read all the new posts when I get to a reception area, it is time to get my head down. As yet not too many interesting subjects. I'll post a few to keep you happy and prove I am not justed holed up somewhere. :wave: We are now at Port Germein, about 25Km north of Pt Pirie.

The largest turntable in the country, could hold a Garret engine


All roads into Peterborough have a model train. It's the home of Steamtown.

Mannahill Station. No longer used though on the India Pacific Line.

A view of the India Pacific Line track at Olary SA

 

[boots33]

Thanks for the pics Paul. you will have to start up a blog so we can keep track