Never try to switch 450V DC at 3A with a conventional mains switch...

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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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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
 
Eva,

You've let the magic smoke out! :rolleyes: 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! :hot:

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
 
Hey Steve, that's really funny :D:D

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 :D:D:D

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

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

BTW: Check my next post about paralleling IGBT and MOSFET for improved switching.
 
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.
 
The switching situation isn't so bad for AC, as any arc created gets interrupted each 1/2 cycle and thus has a chance (a chance, mind you) to extinguish. If the arc is severe enough (especially inside a closed space) so that the flame/plasma cloud has no chance to dissipate, even an AC arc can continue until all the nearby consumables are, so to say, consumed...

Back in school, the labs were supplied with a 200VDC bus to run things like electromagnets. I remember being able to draw a beautiful green arc with an electromagnet and its associated knife switch. I didn't let the arc run too long, though, as I didn't want to chew up the switch too badly. That was back in 1974 - a safety inspector these days would have a heart attack looking at all the lab setups back then, especially since most of the students playing with them didn't know which end of a soldering iron to grab, and were mostly unsupervised....
 
exploding fets and current transfomer

eva,
what was the result of the switch transistors with the removal of the load?

mine explode..... i am trying to find out whether there is some transient turning on opposite pairs (full bridge) or some type of spike exceeding the max voltage level of the fets.

i dont have my current transformer set yet and i was thinking about a mov across the 400v bus.

speaking of current transformers where do you think is the best place for it to live? in the bus to the brige or in series with the bridge transformer.

jimbo:hot:
 
Now I am scared, lol.
I am making a power conditioner with 1.5KVA transformer. I was wondering do I need something special for a master power switch (which I assume will be upstream the transformer's primary). Or just robust, high quality, properly rated DPDT switch will do?

I saw people using NTC resistors for soft start, but my heart is not in it. I do not think they are reliable enough.

Thanks for your comments.
 
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