Never try to switch 450V DC at 3A with a conventional mains switch... - diyAudio
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Old 21st October 2006, 06:23 AM   #1
Eva is offline Eva  Spain
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Talking Never try to switch 450V DC at 3A with a conventional mains switch...

It happened while I was testing my new 3KW PFC frontend with a 1200W electric heater as a load. The two 230V 600W heating elements were connected in series to match the 450V output of the PFC. I wanted to investigate the transient response of the converter, so I decided to turn off the electric heater through its original switch without much thinking...

After five seconds of arcing I got a melted mains switch and a funny smell... Breaking DC currents is not easy at all.
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Old 21st October 2006, 08:15 AM   #2
Danko is offline Danko  Hungary
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The opposite happened to me a few months ago. I turned on with a mains powered SMPS with a switch. I couldn't switch off any more :-) The switch's contacts welded together.

A few houndred uF was on the primary side of the SMPS.
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Old 21st October 2006, 12:02 PM   #3
Nordic is offline Nordic  South Africa
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You can use DC and salsa to eat through prison window bars, they showed on mythbusters.. Most switches and relays have a very limited DC capacity.
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Old 21st October 2006, 01:36 PM   #4
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Eva,

I'm somewhat surprised you would tell us about your mistake. I assume you have done so to caution others against doing the same thing. Good for you.

Interrupting large DC currents with either a mechanical switch or a semiconductor device is a real problem. Every circuit has some inductance and stored energy so you must somehow clamp the voltage and provide a place for the energy to go. Some combination of a snubber and MOVs or transzorbs will take care of the problem; the details depend on the specific situation.

Rick
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Old 21st October 2006, 03:09 PM   #5
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Eva,

You've let the magic smoke out! As a member of the SAE 42V Committee, I have seen, firsthand, the effects of trying to switch large (over 1/2A) DC Currents above 35V. Relay and soleniod contacts trying to switch a few amperes at 42-48VDC usually wear out after about 5-10 cycles (ouch!), and switches have melted in the same amount of time. It was generally agreed that, for anything over 35VDC, conventional switching means would not do.

Two possible solutions were to try and enclose the contact area in some kind of inert gas, like Nitrogen or Argon, to minimize the arcing. The other was to use exotic metals that designed withstand repeated sustained arcing, which has been demonstrated almost 500mS!

Even a new type automotive fuse had to be developed because of the arcing issue. At least you didn't let the magic smoke out of you.

Thank You for sharing this with us. Like Rick, I appreciate the caution, too.

Regards,

Steve
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Old 21st October 2006, 03:17 PM   #6
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Looks like you need to use one of these

http://www.powerlabs.org/images/scr.jpg

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Old 21st October 2006, 04:01 PM   #7
Eva is offline Eva  Spain
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Hey Steve, that's really funny

I have yet to tell some car-electrician friend of mine that he will no longer be able to use his conventional relays and switches in those new 42V cars

After that, he is going to hate latest generation cars with microcontrollers, can-bus, solid state switching and PWM-driven light bulbs even more

So I'll have to build some solid state load gating system for testing the transient response of the PFC

BTW: Check my next post about paralleling IGBT and MOSFET for improved switching.
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Old 21st October 2006, 06:01 PM   #8
TOINO is offline TOINO  Portugal
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I have see permanent magnets glued near to the contacts of relays...
Apparently the magnetic field forces the ionised air molecules to travel a longer distance, helping to extinguish the electrical arc
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Old 21st October 2006, 06:22 PM   #9
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You should see the breaker used in the exciter at our power plant. The voltage going to the rotor from the exciter is ~200 V and the current is ~2000 A so the breaker has to be able to trip this off without burning out every time. The arc shuts are about a foot tall and the contacts are 1-2 inches thick and 5-6 inches wide. There is a hole in it to put a solid inch round stock in it 4 ft long to manually operate the breaker. It takes a lot of effort to move it.
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Old 21st October 2006, 11:24 PM   #10
Irakli is offline Irakli  United States
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Hello

Anybody can comment how severe is the problem with AC currents?

Thanks for starting a post on very important subject.

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