Selection of a high current toggle switch - Page 1
Forum section: electronics / beginners

author: rfengg
author_rank: Regular Contributor
timestamp: August 20, 2019, 12:08:42 am
content: In my project I need to switch 20A d.c. (+12V) through a mechanical toggle/rocker switch.There is no dearth of 20V switches but what worries me is the contact resistance of the commonly available switches.Most switches i have looked at on Mouser have a maximum (initial) contact resistance of 50 milliohms......passing 20A thru these would result in a dissipation of 20W!!!Isn't there something amiss in my understanding......20W would surely damage the switch in no time?How can these switches be rated for 20A when the contact resistance is specified at 50 milliohms?

author: Jwillis
author_rank: Super Contributor
timestamp: August 20, 2019, 01:07:54 am
content: Passing that many amps directly though a toggle is not the best practice. Normally when switching that many amps you usually use the toggle to trip a solenoid.At least that's the way I would do it.

author: rfengg
author_rank: Regular Contributor
timestamp: August 20, 2019, 02:35:26 am
content: Quote from: Jwillis on August 20, 2019, 01:07:54 amFair enuf Sir.....so do you imply that 20A toggle switches are not manufactured the way they are rated for?If I am buying a 20A rated switch, why should I not be able to pass 20A thru it?

author: Circlotron
author_rank: Super Contributor
timestamp: August 20, 2019, 02:49:12 am
content: My guess is the *initial* contact resistance is before it has switched rated load a number of times. During several full load make and break cycles the contacts may alternately microweld and tear apart and this will cause the domed contact faces to bed in and give a greater contact area and much lower resistance. That's my guess anyway.

author: Psi
author_rank: Super Contributor
timestamp: August 20, 2019, 03:13:44 am
content: Yes, specs like that will trash most switches very fastYou basically have the worst case possible.-Low voltage-High amps-DCSwitches do exist but they are expensive and so almost everyone opts to use a chunky relay or solenoid instead.Automotive relays are rated for those sort of requirements.Then you can use pretty much any switch you want to pull in the relay as it only has to handle mA's.

author: Jwillis
author_rank: Super Contributor
timestamp: August 20, 2019, 04:25:39 am
content: Quote from: rfengg on August 20, 2019, 02:35:26 am20amp rated switches are rated to pass up to 20amps is right, but as Circlotron mentions it's the initial contact that does the damage by arcing. It quickly deteriorates the contacts and the switch will fail prematurely. DC is hard on switches so usually if you opt to use a switch you probably want to double the rated amperage rating. 12volts is nothing compared to 24V.I've seen 3/4 inch wrenches burned in half from people attempting to bypass ignition solenoids. But that of course is an extreme case because there was around 2000 amps at the batteries.I laughed my arse off.Another bad idea is to use AC switches for DC. Slide Contact is better than Point Contact for DC.AC switches tend to be point contact.You don't really need to use clunky mechanical solenoids because there are solid state relays that are rated for that current or more.And are quite compact.

author: floobydust
author_rank: Super Contributor
timestamp: August 20, 2019, 05:09:52 am
content: 50 milliohm is the contact maximum resistance. A new switch/relay contact resistance is always less, and with age it increases. They have a 6,000 cycle life.https://www.e-switch.com/product-catalog/toggle/product-lines/st1-series-high-amp-toggle-switchesIt's easier to use a high current relays - lower cost than a big switch and you can concentrate the high current wiring away from the front panel, a shorter wire run.