volt free input protection - Page 1
Forum section: electronics / projects

author: veryevil
author_rank: Unknown
timestamp: October 21, 2014, 09:40:58 am
content: Hey, I'm looking to create a volt free input on something I'm working on but I'm not sure how best to design it and protect it.I'm looking to have a signal terminal and a GND terminal and have it externally connected to a relay as the volt free contactThe issue is I want to protect the terminals to stop idiots who wire 24VDC or even 240VAC into it.Any suggestions on this?Thanks

author: Jeroen3
author_rank: Super Contributor
timestamp: October 21, 2014, 10:09:06 am
content: Quote from: veryevil on October 21, 2014, 09:40:58 amYou cannot prevent damage from those idiots. But you can start with mov's and ultra rapid fuses.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2014, 02:17:33 pm
content: What is "volt free"?

author: veryevil
author_rank: Unknown
timestamp: October 21, 2014, 02:36:25 pm
content: It means that its connected to a switch or a relay

author: Codemonkey
author_rank: Regular Contributor
timestamp: October 21, 2014, 03:03:26 pm
content: "Volt free contacts" generally refers to outputs provided on equipment like fire alarm panels etc. For example, there may be a fault output relay that provides "volt free contacts", i.e. just clean contacts not connected to anything else, so you can use them to connect to other equipment.As suggested above, just stick a fuse inline with the common contact. You can't idiot proof everything!

author: rexxar
author_rank: Frequent Contributor
timestamp: October 21, 2014, 03:09:51 pm
content: Quote from: veryevil on October 21, 2014, 09:40:58 amHonestly, if someone puts 240VAC up an input, they fully deserve to have it blow up in their face.Is this going to be in an application where the user is likely to have wires lying about with 240V? Otherwise I probably wouldn't bother. Like Jeroen3 said, you can't protect against stupidity, they'll break it one way or another.

author: Rufus
author_rank: Super Contributor
timestamp: October 21, 2014, 03:22:25 pm
content: Quote from: veryevil on October 21, 2014, 09:40:58 amYou can get low current 250vac rated PTC fuses (even a few with 600vac). Such a fuse followed by a zener or TVS clamp will keep voltages applied to the inputs at manageable levels. Consider fusing the GND side of the connection(s) as well.

author: ConKbot
author_rank: Super Contributor
timestamp: October 21, 2014, 04:44:50 pm
content: A series diode, 350V+ jfet/depletion mode mosfet + gate-source resistor setup as a current source, and a zener can make a voltage clamp. Ive made one, (minus the zener, it was driving a LED in an optoisolator so the current source was desirable) for a DAQ system trigger that had to be limited. It would reliably trigger from 2.2v up to over 200V. (stopped testing it there... the clearance of everything inside from line to neutral wasnt the best, it had to fit in a connector backshell) I even plugged it into 120V mains and everything worked fine. It probably couldnt handle transients that it would need to in a cat II situation like that, but it could handle at least a short duration connection to mains (in case it somehow ended up on the BNC connector in the input...)This would probably be overkill though as youre looking for 'not damaged' rather than 'operating normally'Mind your dissipation in the FET though, dropping 200V @ 5mA is a decent amount of heat :p

author: SL4P
author_rank: Super Contributor
timestamp: October 21, 2014, 09:20:45 pm
content: VOLT-FREE inputsare typically an optocoupler with the LED anode tied high with a resistor pullup.When you apply 0V to the cathode of the LED through some external contacts or driver - that causes the opto's output transistor to 'turn on'. What you do after that is your business.Simple isolated contact-closure input -no external voltagerequired.A reverse biased diode, caps and/or zener / MOV will help, and it won't stop you putting mains AC in, but will probably protect (most of) the controlled equipment.There are further strategies to over-voltage protect the input, but how far do you need to go?

author: Jeroen3
author_rank: Super Contributor
timestamp: October 22, 2014, 08:26:26 am
content: I've seen products with an input using an optocoupler suitable for anything between 10 and 250 VDC/AC as a HIGH level.

author: SL4P
author_rank: Super Contributor
timestamp: October 22, 2014, 08:44:59 am
content: Quote from: Jeroen3 on October 22, 2014, 08:26:26 amThen it'snotaNO VOLTinput...

author: Jeroen3
author_rank: Super Contributor
timestamp: October 22, 2014, 09:49:45 am
content: He's asking for a input isn't hè? Or an output?Ah I see, I wan't informed about the true meaning of a no volt input....It's just a switch input.

author: SL4P
author_rank: Super Contributor
timestamp: October 22, 2014, 10:01:58 am
content: Correct - a simple clean contact closure (often to 0V - provided as the second input pin).Analternativeto a NO VOLT input is adriveninput, where you provide say 12V across 2 pins as the trigger signal... or in the case mentioned above 240V!)