I have a two filament turn signal/parking lamp on my car (3157A bulb). When the parking lights are on, the turn signal is not very noticeable, so I am trying to get the parking filament to turn off when turn signal is on. I understand I can achieve this by using a normally closed 12V relay. The 3157 bulb draws about 0.6amps for the parking lamp filament. I have two questions: 1) Since this is a very low current circuit would it be ok to use a generic automotive relay ie: bosch 30 or 40amp? Or should I use a smaller rated like 10amp or 5amp PC relay like this one https://www.e-sonic.com/ds/505330.pdf?p=45319483 2)Do I need to install a flyback diode in parallel with the coil to prevent damage, if so what size does it need to be?

Thanks in advance.

  • \$\begingroup\$ If you draw out how you intend to wire the relay it would make it easier to confirm component suitability. \$\endgroup\$ – AngeloQ Feb 8 '17 at 23:12
  • \$\begingroup\$ If you're controlling the relay with solid state components you need a flyback diode. Other specs to consider: coil voltage and current required to actuate the relay. \$\endgroup\$ – Ryan Griggs Feb 9 '17 at 3:34
  • \$\begingroup\$ Ok so I have added links to 2 images, one showing how I plan to use the relay connections and the other showing the factory wiring of the turn signal circuit which will be used to trigger the relay. hightail.com/download/dDZITkFnMm10TW52bk1UQw hightail.com/download/dDZITkFnMm1Fc0x2WnNUQw \$\endgroup\$ – Gordyg Feb 9 '17 at 5:11

It's okay if you use a car relay as long as it handles more current than your load, you'll have the advantage of having terminals to easily connect all the wiring but it will cost you some more current when the relay is on, car relays can draw up to 2amps from your battery, so that is something you should keep in mind considering your load only draws 600mA.

Using pcb relays will work as well and draw much less current, but they're hard to connect if they're not soldered on a PCB

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  • \$\begingroup\$ Oh I see, I didn't think about the extra current draw of the coil. I think I will first see if I can enclose the pcb relay into a small circuit box. thanks for your input. much appreciated. \$\endgroup\$ – Gordyg Feb 9 '17 at 4:43

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