Relay voltage max - Page 1
Forum section: electronics / beginners

author: electronics man
author_rank: Frequent Contributor
timestamp: February 05, 2014, 06:23:29 pm
content: Why do relays have a lower maximum voltage for ac than for dc and what what can I do to up the maximum voltage.

author: G7PSK
author_rank: Super Contributor
timestamp: February 05, 2014, 06:37:02 pm
content: Most relays have a higher AC voltage than DC, that's down to the ability to draw an arc is greater with DC than AC as every time AC crosses zero the arc is extinguished. You can put a capacitor ac cross the relay contacts which will help with contracting the arc, otherwise air blast or oil immersion.

author: Simon
author_rank: Unknown
timestamp: February 05, 2014, 06:37:33 pm
content: actually it will be the other way around, DC makes much bigger sparks so you will easily find that the DV voltage is 1/10 the AC one for the same current.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 05, 2014, 07:01:21 pm
content: Even at 30v dc

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 05, 2014, 07:15:19 pm
content: The requirements for switching high DC currents with inductive loads are VERY severe. I have worked on large industrial DC motor drives, where the output may be 0-500V and hundreds of amperes, into a motor winding. In the event of an emergency stop or other fault condition, where the output contactor gets dropped out under load, special construction is needed to dissipate the resulting arc energy. Large DC contactors often incorporate magnets near the separating contacts to help "stretch" and break up the arc before it vaporizes too much of the contact surfaces. Some systems use contacts sealed inside vacuum chambers, to prevent the formation of an arc through ionized air.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 05, 2014, 07:58:21 pm
content: Quote from: N2IXK on February 05, 2014, 07:15:19 pmBut that's not what I'm doing whith this relay,all I am doing is switching a 1.2 to 37v at 0 to 3A on and off it will not be switching inductive loads like your talking about I might drive a 6v hobby motor or a pc fan and I will have a diode across the output.

author: SeanB
author_rank: Super Contributor
timestamp: February 05, 2014, 08:14:34 pm
content: 6V motor or fan then use a 30V relay rated at 6A or so. Will do most likely the electrical life then, probably around 10 000 cycles.I have been changing some industrial ones switching AC that did around 3 million cycles, which was quite poor as the old one that was there did around 50 million cycles before the frame wore out. The contacts were still usable. The new one had contact failure even with RC snubbers, but was at least cheaper and more importantly obtainable off the shelf. Plug in makes changing easy though.

author: Len
author_rank: Frequent Contributor
timestamp: February 05, 2014, 08:16:07 pm
content: Quote from: electronics man on February 05, 2014, 06:23:29 pmHere's the answer:https://www.eevblog.com/forum/beginners/relay-contact-ratings/QuoteUse a different relay, I guess. The maximum voltage is based on the mechanical design of the relay.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 05, 2014, 08:24:49 pm
content: For arcing to happen there needs to be an electric field of about 3MV/m so d = v/e this showers that at 37v this would mean a distance of 0.0123mm would form an arc I find it hard to believe the contacts are this close together when open

author: SeanB
author_rank: Super Contributor
timestamp: February 05, 2014, 08:36:09 pm
content: With a standard AC welder you can draw an arc of a half meter or more on a windless day. With DC you can do more as the arc will tend to not stop ionising the air. Welder open circuit voltage is 50VAC.

author: Kjelt
author_rank: Super Contributor
timestamp: February 05, 2014, 08:42:40 pm
content: For DC loads you can also go for a SSR, a bit more expensive though.

author: Len
author_rank: Frequent Contributor
timestamp: February 05, 2014, 09:06:47 pm
content: Quote from: electronics man on February 05, 2014, 08:24:49 pmThat's tostartan arc, isn't it? Once an arc starts it can be maintained at a much lower voltage. When the relay contacts begin to open, the distance between them is infinitesimal so an arc starts, then it persists until the contacts are too far apart. Even when the rated voltage is not exceeded, a relay's contacts can eventually get welded together. That's why they have a rated lifetime (number of cycles).

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 05, 2014, 09:08:06 pm
content: If you use an SSR, you need to use one specifically made for DC. Standard SSRs are intended for AC switching, with a triac or 2 inverse parallel SCRs for an output stage. DC current will cause these to latch on once activated, until the current is removed.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 05, 2014, 09:13:09 pm
content: i think i will just buy a new relay

author: Kjelt
author_rank: Super Contributor
timestamp: February 08, 2014, 12:57:02 am
content: Quote from: N2IXK on February 05, 2014, 09:08:06 pmIndeed, the DC ones use a FET, they are not that expensive anymore.