Flyback diode with mechanical switch
Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=143720
Printed Date: May 02, 2024 at 9:16 AM
Topic: Flyback diode with mechanical switch
Posted By: thenzero
Subject: Flyback diode with mechanical switch
Date Posted: November 04, 2017 at 1:34 PM
I've seen people (read: Howie) on here say pretty forcefully that you need to put a flyback diode across relay coils to avoid destroying transistors etc. My question is do you also need the diode with a mechanical switch such as this one. I opened this switch up and it's basically just some pieces of copper that squish together when you click the switch. Apart from avoiding a nasty shock if you touch it, is there any need for the diode?
Replies:
Posted By: thenzero
Date Posted: November 04, 2017 at 1:38 PM
Looks like my link keeps getting hosed when I post so here it is as text:
https://www.ebay.com/itm/Universal-Car-Power-Door-Lock-Unlock-Switch-Spring-Back-Return-Hoist-5-Pin-DPDT/182626154472
Posted By: i am an idiot
Date Posted: November 04, 2017 at 8:10 PM
You do not need a diode if you are using that switch.
Posted By: thenzero
Date Posted: November 04, 2017 at 8:23 PM
Cool, thanks.
Posted By: howie ll
Date Posted: November 05, 2017 at 4:19 AM
The fly back or quenching diode is to block a "back EMF" or spike of about 200 volts going back down the trigger wire when the relay opens, where it will fry any electronics.
Since a switch is mechanical and open circuit at rest, as Mr. I says you don't need a diode.
The switch in the link looks like a SPAL and that's a good price!
If you'll be using relays after the switch, source to the centre terminals, outputs from the outer terminals, you'll only need 3 of 5 terminals.
-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
Posted By: howie ll
Date Posted: November 05, 2017 at 4:29 AM
Can I ask:-
1) Is this simply for power locks?
2) Is their a supplied specialist lock relay with aux inputs, e.g. from an alarm/keyless entry?
3) Are you using an alarm/keyless entry and if so does it have 5 or 6 wires for the locks or just 2?
Tell me which and I'll draw you a circuit.
-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
Posted By: thenzero
Date Posted: November 05, 2017 at 6:48 AM
Thanks I really appreciate that. This is something I already installed, just making sure I did it right. Yeah it was just for the power locks. The install my truck came with had keyless but no lock button inside the cab. I wired it exactly how you described with three lines used and two unused. I used regular relays whose output is in line with the NC (87a) line on the 2101 we were talking about it another thread.
|