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Henry H

Sat Feb 27 2010, 03:57AM

Registered Member #2298 Joined: Sat Aug 15 2009, 08:16PM
Location: ex UK, now Santa Cruz, CA
Posts: 35

Have a comparator to detect when your 18V starts to droop. Once it drops below say 17V, use a logic AND gate to change the gate drive input to low, so that the gate itself gets clamped to zero quickly before the supply drops any further.

Nik

Sat Feb 27 2010, 03:32PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

Some times when a DC contractor isn't available you can get away with looping one wire through a 3 phase AC contractor twice. So both + and - go through 1 contact each and then one of them gets looped back to go through the last contact reducing the likelihood of an arc being drawn out. He have done this at work when waiting on parts but the machine has to run. We never left anything like that longer then a month but that was under constant switching so you could probably get away with it a lot longer.

MinorityCarrier

Sat Feb 27 2010, 05:03PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Here's a nice contactor model that I have a few of. This one has a 24VDC coil, others I have use 120VAC coils. Contact rating conditions are conveniently printed on the contactor body.

IamSmooth

Sat Feb 27 2010, 07:13PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Nik wrote ...

go through 1 contact each and then one of them gets looped back to go through the last contact reducing the likelihood of an arc being drawn out.

Sounds intesting. Can this be done with a single phase relay? Any way you can make s simple sketch of what you are talking about?

I have reduced the timing delay on my shunt controller to increase the chance that I have enough voltage on the gate before turn-off. I also put a diode on positive output lead in case there is a ghost voltage coming in. I think the burn-out is happening when the inverter is still drawing power.

Renesis

Sat Feb 27 2010, 08:41PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Motorcontactors are sturdy little bastards, and like Nik says they have three individual power contacts, which could be cleverly exploited. A large capacitor in paralell with the contacts can also help reduce sparking as the contacts opens. Your current circuit is using semiconductors, so it should be a simple task to make your MOSFET/IGBT drive the contactors coil, and let it take the punishment instead.

Renesis

Sat Feb 27 2010, 09:35PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Terribly sorry for doubleposting, i cant edit my post with a picture without using imageshack.

I drew up a simple circuit here, which only need a 5V control signal. I used a similar circuit when i made a silicone-free power controller for my desk, using the 5V supply in my laptops USB port as the control signal. The drive voltage for the contactor would in most cases be mains, but you can find 24V and probably 12V versions too.

radiotech

Sat Feb 27 2010, 10:59PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

One nifty way to get a relay to handle inductive contact loads is to stick a magnet close to the contact to push the arc away. This was used in a pole changer on a bank of DC operated 30 watt germicidal
T8 lamps each having an inductive balast in series with the resistor.
The AC rated relay had an AC coil run by a timer.

Like this one. The magnet was a factory mod.

,16354,16453&LG=1&PG=1&IDS=113861,113871,113970,11 3975,113990,113983,110555,115350,115351,115352,115 353,115358,115359,374659,115479,115480,115481,1154 87,115488,115489&N=2#features

IamSmooth

Sun Feb 28 2010, 03:28AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

The load is non-inductive. I don't think the capacitor would help much.

Renesis

Sun Feb 28 2010, 11:39AM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

The capacitor will reduse the voltage across the contacts in the critical moment when the contacts opens. The induction from your wires, or even the switching voltage alone may be enough to create a tiny little spark as the contacts opens, and this will eventually ruin the contact surfaces. If you had an inductive DC load then you would have to use freewheeling diodes as well.

Steve Conner

Sun Feb 28 2010, 12:03PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If the load isn't inductive, then DC probably isn't too much of an issue. I've destroyed switches with 600V DC and an inductive load, though. The contacts just drew an arc that continued until the switch was a flaming puddle of molten plastic. Sometimes with an inductive load, you can figure out a place to add a flywheel diode, to clamp the voltage that would otherwise drive an arc. I've done this in tube amplifier and OLTC power supplies.

A generator is an inductive source, because of the inductance of the windings. If you suddenly interrupt the current, the voltage will shoot up.

Using several sets of contacts in series in a relay, switch or contactor works reasonably well. The DC voltage rating adds, so if one set of contacts is rated at 32V DC, then three sets in series can do 96V.

Triac solid-state relays won't turn off DC, they'll just stick on indefinitely. You need a MOSFET or IGBT SSR for DC, and buying semiconductors to protect semiconductors seems a little stupid. (maybe you should just have bought a MOSFET SSR in the first place.)

Semiconductors are quite capable of blowing up before any relay or breaker can open to save them. The old joke about the transistor blowing to save the fuse applies here, too.

MinorityCarrier: Is that a mercury contactor? It certainly looks like it.

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