Headlight Upgrade Relay P/N?

[RRoller123]

Anyone have a Radio Shack, etc. part number for the relays that are/can be used for the headlight wiring upgrade? Obviously 12VDC, SPST I think, but a part number would be helpful. Our local Pep Boys around here had nada. Are you guys using quenching diodes? What is typical Relay resistance for this application? Any guidance appreciated. Thanks. Pete.

[4uall]

http://www.youtube.com/watch?v=JkG22KgciwY

Radio Shack micro relay part number 275-241

[124JOE]

thanks that vid was helpfull

[RRoller123]

Yes that was a good video! There is quite a bit of stuff on YouTube. Here is another web resource, with good explanatory text regarding automotive applications, and an assortment of relay diagrams for lighting and other various auto applications:

http://www.parts-express.com/resources/ ... relays.cfm

I went over to Radio Shack to look for the 275-241 relays, but while there I think I found a better relay for the lighting application. The Radio Shack 275-226 has an advantage over the 275-241 by having a mounting tab, being specifically designed for automotive lighting applications. (The 275-241 is intended for PCB plated thru-hole solder application.) The 275-226 also has a capacity of 30A switched at 12VDC. This is more than enough for the (2) high beams. The (2) 50W high beams will draw ~8.3A total (50W @12VDC = ~4.17A each), while the (2) low beams draw ~5.8A total (35W@12VDC=~2.92A each) so there is plenty of cushion there. The relay coil R is 66 ohms, so it will only draw around 180mA. Since it is SPST, the switched output can be ganged to feed both lights from the single relay with plenty of amperage cushion. (1 relay for highbeams, 1 relay for lowbeams). I looked at sockets for these relays, but they all seem to have lighter AWG wiring than I wish to use, (16 and 18 typically) so I intend to wire directly to the relay tabs with Faston type connectors. Fastons with 12AWG wire can typically carry about 20A continuous, so this also seems to have enough cushion.

I intend to try this out tomorrow unless the cold here is unbearable (supposed to be in the 20's tomorrow), and I'll report back. Pete.

[boogiedude]

you can get relays that are suitible for horns. or lights or whatever else you're installing at any checkers/napa/autozone. Probably at about half the price of radioshack.

[4uall]

@RRoller123

Thanks for the clarification. I am looking at putting in some relays over the winter so I will follow your info. How much for your relays? I am always amazed at some of the price differences between Canada and US etc. Where are you located again?

[RRoller123]

Hi 4uall: Glad to hear from you! I am in Foxboro, Massachusetts. My local Automart, etc. didn't have the relays needed, nor did Pep Boys, so I went to Radio Shack. They were there on the rack at $6.98 (probably twice what you could get them for elsewhere, but who cares? It is such a small purchase. I spent more than that in gas wasting my time driving around looking for them.) I also bought a good pair of RATCHETING crimping pliers to make sure the crimps are good. I may solder the leads, but I must admit (I know, this is HERESY!) that I am not fully convinced of the need. Automotive crimps have worked for many decades, and the spec sheet data from AMP, Amphenol, etc (Faston type connctors) clearly indicate enough carrying capacity for this (and most) application. The Brown wire fix is a whole 'nother beast. That probably should be soldered.

I still don't have an answer regarding the need for quenching diodes, but after giving it more thought, I expect that they are not at all necessary if you use the original lighting switch for control. The diodes stop the back surge of current when the relay coil is de-energized, (i.e. you turn off the lights) which in a microelectronics PCB application could harm upstream components, but I can't for the life of me see how it would hurt an automotive lighting switch. I did see in the literature that many relay sockets have the protective circuitry in them, but the socket wire gauges are generally too light, so I am not using them.

It is so darned cold here that I didn't get to this over the weekend, so I can't yet report upon the outcome, but I will. I am still trying to sort out my odd ignition switch wiring, as per another post.

Here's a great resource for AWG data and info regarding current capacity:

http://en.wikipedia.org/wiki/American_wire_gauge

Here's a great calculator for DC current/volt/amp calculations as well:

http://www.sengpielaudio.com/calculator-ohmslaw.htm

Here is the best Relay Primer that I could find, where I first heard of quenching diodes:

http://www.bcae1.com/relays.htm

Hope this all helps our collective condition! Pete.

[4uall]

Awesome thanks.
fyi
http://www.thesource.ca/estore/product. ... ct=2750226