Relay switching current rating question - Page 1
Forum section: electronics / projects

author: Red Squirrel
author_rank: Super Contributor
timestamp: March 12, 2016, 12:51:38 am
content: When buying relays for various purposes, there is a switching current rating, however it never specifies if it's AC or DC, what is the rule of thumb here, is it assumed that it's AC, and you need to say, double the rating for DC? From my understanding you need heavier switching contacts for DC as it has a higher tendency of arcing.Like say I want to switch 10 amps at 12VDC I can't imagine being able to use a little clicker relay that's rated at 10 amps, vs a bigger heavier duty automotive relay. Or can I? Are they designed in such a way that the arcing is mostly mitigated?

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 01:34:03 am
content: Quote from: Red Squirrel on March 12, 2016, 12:51:38 amHiAt least on the relays I buy, it always specifies if it's AC or DC and the voltage it applies at. If you have one that is not showing all the ratings, then it's likely from a vendor who does not quite know what they are selling.Best guess is that the rating is a low voltage DC number if nothing else is mentioned. AC ratings will be higher than DC ratings, so this is a bit of a risky assumption.Bob

author: zlymex
author_rank: Frequent Contributor
timestamp: March 12, 2016, 12:49:07 pm
content: See attached photo.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 12, 2016, 03:53:13 pm
content: There will be a few specs for a relay that matter.See this table for example.It's not only the current that matters.

author: nctnico
author_rank: Super Contributor
timestamp: March 12, 2016, 06:19:13 pm
content: The current specified is always the best case maximum for AC and a resistive load.For DC there should be a DC derating graph. Generally speaking the maximum DC current derates quickly over 24V due to arcing.

author: SeanB
author_rank: Super Contributor
timestamp: March 12, 2016, 07:45:12 pm
content: Generally the current ratings are given ( for relays that are not marked DC) as an AC resistive load, like a heater which draws a current that is not higher at turn on like a lamp which heats up and draws a lower current. It is only going to have a limited life at that maximum loading, unless noted it can be as little as 100 cycles at full loading, though often it is a thousand or so cycles to 100k cycles for good quality relays.Inductive loads require derating , often your 10A AC 250VAC relay ( with 100k cycles at this load, as it is a quality branded part and not a knock off with 1000 cycle life) might only be rated 4A with an inductive or tungsten lamp load, because of the inrush current every switching cycle. DC loading often is very much lower, or not given at all, indicating the contacts move apart too slowly to reliably break a DC arc. Voltage is very derated, so you see a relay with a 250VAC rating only having a 30VDC rating because of the arc created over around 50VDC being capable of keeping the relay conducting current from both NC and NO contacts simultaneously to common for either long enough to melt the relay or to severely reduce operational life. You do get high voltage DC relays, but the construction has a fast moving contact with a very large separation, along with arc quenching that breaks up the arc on opening very quickly so it does not burn the relay.

author: Red Squirrel
author_rank: Super Contributor
timestamp: March 13, 2016, 11:25:41 pm
content: Actually now that you mention it there is typically a voltage rating in DC and AC, so if there is just a general amp rating but two separate voltage ratings then the amp rating is for both? Ex: 250vac and 25vdc, with 5 amps for either voltage.

author: uncle_bob
author_rank: Unknown
timestamp: March 14, 2016, 12:39:46 am
content: Quote from: Red Squirrel on March 13, 2016, 11:25:41 pmHiYes, that is the rating on the contacts. 5A at low voltage DC and 5A at high voltage AC.Bob

author: calexanian
author_rank: Super Contributor
timestamp: March 14, 2016, 12:43:32 am
content: Any relay worth its salt will have all this clearly spelled out in its datasheet. Remember it is always harder to break DC than AC. Some relay designs are just better than others so a bigger one may not be better. Contact plating makes a huge difference in making and breaking current. Also be careful cause come companies list a much higher "Carrying current" that really does not mean any more than the terminals and conductors will melt at this current but don't expect it to survive making and breaking there. That rating only meant anything for motors in a locked rotor situation and such where a breaker was going to open and handle the fault. That is a topic for another post though.

author: Red Squirrel
author_rank: Super Contributor
timestamp: April 01, 2016, 07:32:15 am
content: Actually for increasing life of relays is it common to have multiple in parallel, ex: have a "master" one that is on, but if you want to turn it off you switch the other ones on, then switch them all off at once. Or is that a waste of time because it's likely they won't all turn off that the exact instant?I think I'm going to just stick to the automotive relay I originally picked though. I just felt the coil current was kinda high (like 100ish mA) but it's for a small solar setup with 2 7.2ah batteries and 2 30w panels so in the grand scheme of things it's not THAT bad. The system would typically be used over a camping session and at 14.4 amp hours and the fact that a night is less than 14 hours in summer I could be drawing an amp all night and solar panel would pick back up before the battery is dead. Of course the whole point of the relay is that it will kill power when voltage gets too low. I think what would be ideal for this setup is a latching relay though, but maybe next time. I am not happy with the size I made this system so I might build another later and sell this one. It's kinda a fun little setup to have.

author: uncle_bob
author_rank: Unknown
timestamp: April 02, 2016, 05:15:11 pm
content: Quote from: Red Squirrel on April 01, 2016, 07:32:15 amHiTrying to get relays to all switch at the same time .... not going to happen.Relays are dirt cheap and available all over the place. It's very easy to find the right part for whatever job you need to do. Rather than force a design to use the wrong part.... just get the right one.Bob