the MRH Forum

Resistors for lights MRH product support
Reply
New member 6 posts ollietom1944 New member 6 posts
ollietom1944

Hi folks,

Love this site to bits, and all the information provided. Thanks. Now I have a question I hope you can help answer. I am not in anyway electrically competent, so please spell out the answer for me thanks. I bought some 12V               (I thought) mini globes off the net, and they turned out to be extremely bright, and hot. Contacted the seller, in Hong Kong) and he advised I used resistors on them. So back to the net, and bought a pack of 560 Ohm 1/4 W 0.25 W resistors which now make the light barely visible! Can someone help me what size resistors I need, Please! Thanks in advance folks. And in case I miss the answer, can you answer me via my email address:

ollietom1944@yahoo.com

Thanks folks.
Reply 0
Prof_Klyzlr Moderator 7,956 posts
Prof_Klyzlr

Dear ??? (Ollie), The

Dear ??? (Ollie), The underlying maths for this have been covered here on-fora previously. May I suggest using the MRH Search function at top right of the MRH page, with the search terms "LED resistor". (I know you aren't working with LEDs, but the same maths applies). In your particular instance, firstly we need some info on the actual specs of the bulbs. Specifically, we need: - voltage of the bulb - current (Amps or milliAmps) or wattage (Watts) of the bulbs Once we have these values, we can plug them into some simple formulas, and get some useful, meaningful answers, to "go-forward" with. If you're a "ignore the maths, just lemme have a go" kind of modeller, you could start (in relative safety) by taking a 2nd 560Ohm resistor, and connecting it in parallel with the first one. The result will act as a 280 Ohm 1/2 Watt resistor. (Simple resistor maths: 2x resistors of same value in parallel = half the resistance, with twice the wattage capability). Hope this helps, Happy Modelling, Aim to Improve, Prof Klyzlr
Reply 0
lexon Prolific poster 1,211 posts
lexon

Lights

Buy a 1k potentiometer and a digital multi meter. Do some measurements.

I have a 2.5 k pot out of the junk box that I have used a lot for light bulb and LED lighting.

I keep an assortment of resistors on hand.

I have used the below meters for some years. Started with a VOM in 1953 and have used all kinds of VOM's, VTVM's and expensive meters over the years and this style meter works very good for most home use. I keep one in the car. I have four of these meters.

I have one expensive meter for measuring AC current but i rarely ever have to do that.

http://www.trainelectronics.com/Meter_Workshop/index.htm

http://www.trainelectronics.com/Meter_HF/index.htm

Very seldom have bothered with the math, though I do know how to.

Ricgh
Reply 0
ollietom1944 New member 6 posts
ollietom1944

Hi Prof. Thanks so much for

Hi Prof.

Thanks so much for your help. 2 Resistors in parallel worked! Brighter light, no where near the heat. I'm very illiterate regarding this side of electronics, so am so glad you were able to help. I didn't know what voltage the globes were to start with, so that was a problem in itself! Thanks again,

Regards, Tom (ollietom)
Reply 0
ollietom1944 New member 6 posts
ollietom1944

Hi Ricgh, Thanks so much

Hi Ricgh,

Thanks so much for your help and your further info on meters. Prof was able to alleviate my problem with his answer, as I am pretty illiterate regarding this side of electronics! Thanks again,

Regards, Tom (ollie)
Reply 0
duckdogger Super User 808 posts
duckdogger

One more time

Board style Soundtraxx mobile decoder, 1.5 volt bulbs and 3 volt at the common. What resistor? Please.
Reply 0
kjd Frequent poster 575 posts
kjd

Mr Dogger

You need to know the current drawn by the bulb.  You are dropping the difference between the supply and what is used by the bulb with the resistor.  Divide that difference by the current and you get resistance.  For example if it is a 20ma bulb, divide 1.5 (3-1.5=1.5) by .02 and you get 75 ohm

Paul
Reply 0
Graeme Nitz OKGraeme Prolific poster 1,695 posts
Graeme Nitz OKGraeme

Or put 2 in series

If using 1.5v bulbs in a 3v circuit put 2 in series. Voila! No resistor needed

Graeme Nitz

An Aussie living in Owasso OK

K NO W Trains

K NO W Fun

 

There are 10 types of people in this world,

Those that understand Binary and those that Don't!
Reply 0
duckdogger Super User 808 posts
duckdogger

I like it.

Two in series it is as I have no 75 ohm resistors. Thank you.
Reply 0
Reply