Loss of voltage when switch is activated - Page 1
Forum section: electronics / beginners

author: dennisyuki
author_rank: Contributor
timestamp: January 12, 2016, 04:01:18 am
content: Heres a problem to solve.I installed in my car a switch, powered from accessory power, that controls a 12v power outlet. It has been working for 1 year but last week the power died and I can power anything. The switch has a led and it powers on when the switch is activated, but power outlet does not work.So this evening I opened up the wiring. When the switch is off I get 10.47 volts between the ground and accesory power line. and .5 v from ground to the load line (to the power outlet)When I switch it on. The voltage from the ground to the accesory line is 1.87 volts. and the voltage from ground to the load line is 1.8v.With the switch on, I touched the ground line on the switch and the positive line in the power outlet I get the same 1.8v. I checked the continuity of the the ground and power line and it was complete.Could the switch be bad. and that is why the voltage drops when turned on. Or could the power outlet be bad. I opened the power outlet and everything looked fine, no burnt components.Any ideas would be appreciated.Thanks.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 12, 2016, 04:23:45 am
content: A "good" continuity test with a multimeter doesn't mean the switch is good.You can have "continuity" with tens of Ohms - not good enough for a switch though.

author: Brumby
author_rank: Unknown
timestamp: January 12, 2016, 04:41:55 am
content: What sort of load was connected to the accessory socket?Was thereanyload connected?

author: kolbep
author_rank: Frequent Contributor
timestamp: January 12, 2016, 04:51:43 am
content: There is a high resistance somewhere between your source of supply, and the switch.When you try to draw power (ie when the switch is on), the resistance causes the voltage go drop.You need to trace back from your switch (while you have the load on), and see at which point the voltage dissapears.To do this, just hook your tester to the ground line, and then probe the other wire whereever you can. When you see the voltage increase suddenly, you have found your culprit...Could be a blown car fuse, a broken wire, or a loose connection somewhere...

author: Brumby
author_rank: Unknown
timestamp: January 12, 2016, 04:56:56 am
content: Check and make sure your accessory 'ground' is, in fact, properly connected to the vehicle ground.A few years ago my son installed a stereo in his car and wondered why it wasn't working too well. Bad ground connection - which measured 7v with respect to the chassis. Re-terminating the massive earth cable fixed everything.

author: dennisyuki
author_rank: Contributor
timestamp: January 12, 2016, 11:36:41 am
content: Quote from: Brumby on January 12, 2016, 04:41:55 amNo load. I was just probing the socket.Sent from my SAMSUNG-SM-G920A using Tapatalk

author: Audioguru
author_rank: Super Contributor
timestamp: January 12, 2016, 06:59:11 pm
content: I think the switch is cheap and its contacts have corroded. Replace it with one designed for a car's humid environment.

author: electr_peter
author_rank: Unknown
timestamp: January 12, 2016, 08:50:37 pm
content: Switch intended for mains switching or high current DC switching can have resistance of 1-5 Ohms or more (when contacts are compromised). Replace switch or clean contacts.

author: Audioguru
author_rank: Super Contributor
timestamp: January 13, 2016, 12:18:24 am
content: For switching high impedance audio or DC voltages I always used switches with low power gold plated contacts that never corrode like the high current switches with silver contacts that turn black and need a spark to clean away the corrosion. The prices are the same.

author: dennisyuki
author_rank: Contributor
timestamp: January 13, 2016, 01:03:28 am
content: Quote from: Audioguru on January 12, 2016, 06:59:11 pmThat is what I will try. This was a switch for cars but it was cheap. Thanks.Sent from my SAMSUNG-SM-G920A using Tapatalk