3 ms·

One of the big issues with 40-60V systems, which have been right around the corner for 20 years now, maybe more, has been that typical relay contacts arc over a

One of the big issues with 40-60V systems, which have been right around the corner for 20 years now, maybe more, has been that typical relay contacts arc over at about 28V DC. Yes, there are ways to adress this. No, none of them are as practical as relays yet.

idiotsecant 5y ago
This isn't really a fundamental problem. It just means using relays with contacts rated for higher voltages. It's probably the easiest part of the bom on a typical vehicle to move to a higher voltage range. I use 125VDC contact/coil rated relays all the time.
- formerly_proven 5y ago
  It is a fundamental problem (it's physics). The current rating for relays drops off sharply as the DC voltage increases; you might see 125 V DC rating, but the current will be a fraction of the nominal current at the rated AC voltage. If you're using an relay rated for up to 250 VAC/DC, then the breaking capacity with DC might be as low as 1 % compared to AC. Bad relay datasheets don't mention this at all, or maybe only for 30 V. Good datasheets have plots of capacity vs. voltage and current vs. life (rated capacity of relays is generally for around 100k cycles).
  - idiotsecant 5y ago
    I am an engineer in a field that has been using >100VDC relays for a hundred years. There is a sizable supply chain out there for these things. You can buy them in whatever amperage you need, they just use different construction from typical low frequency AC relays. Wipers are often high quality metallurgy to extend life and they make use of simple passive magnetic snubbers to 'blow out' the arc that is self-extinguishing in AC circuits. They aren't substantially more expensive than regular high quality AC relays and use standard form factors.
  - Gracana 5y ago
    Speaking of good relay datasheets, some will also show contact resistance vs current. I recall one which noted that for small signals the resistance was very low, but if higher current was applied, the contact coating would burn off and the resistance rises permanently.
    - formerly_proven 5y ago
      Yep, those are gold-over-silver(-over-nickel-on-contact-alloy) plated contacts. Good for both signal and power switching, as long as you use the same relay only for one of those things.

idiotsecant 5y ago

This isn't really a fundamental problem. It just means using relays with contacts rated for higher voltages. It's probably the easiest part of the bom on a typical vehicle to move to a higher voltage range. I use 125VDC contact/coil rated relays all the time.

formerly_proven 5y ago
It is a fundamental problem (it's physics). The current rating for relays drops off sharply as the DC voltage increases; you might see 125 V DC rating, but the current will be a fraction of the nominal current at the rated AC voltage. If you're using an relay rated for up to 250 VAC/DC, then the breaking capacity with DC might be as low as 1 % compared to AC. Bad relay datasheets don't mention this at all, or maybe only for 30 V. Good datasheets have plots of capacity vs. voltage and current vs. life (rated capacity of relays is generally for around 100k cycles).
- idiotsecant 5y ago
  I am an engineer in a field that has been using >100VDC relays for a hundred years. There is a sizable supply chain out there for these things. You can buy them in whatever amperage you need, they just use different construction from typical low frequency AC relays. Wipers are often high quality metallurgy to extend life and they make use of simple passive magnetic snubbers to 'blow out' the arc that is self-extinguishing in AC circuits. They aren't substantially more expensive than regular high quality AC relays and use standard form factors.
- Gracana 5y ago
  Speaking of good relay datasheets, some will also show contact resistance vs current. I recall one which noted that for small signals the resistance was very low, but if higher current was applied, the contact coating would burn off and the resistance rises permanently.
  - formerly_proven 5y ago
    Yep, those are gold-over-silver(-over-nickel-on-contact-alloy) plated contacts. Good for both signal and power switching, as long as you use the same relay only for one of those things.