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

Eva · 21st October 2006, 07:23 AM

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.

Danko · 21st October 2006, 09:15 AM

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.

Nordic · 21st October 2006, 01:02 PM

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.

sawreyrw · 21st October 2006, 02:36 PM

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

N-Channel · 21st October 2006, 04:09 PM

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

astouffer · 21st October 2006, 04:17 PM

Looks like you need to use one of these

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

Eva · 21st October 2006, 05:01 PM

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.

TOINO · 21st October 2006, 07:01 PM

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…

davidallancole · 21st October 2006, 07:22 PM

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.

Irakli · 22nd October 2006, 12:24 AM

Hello

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

Thanks for starting a post on very important subject.

21st October 2006, 09:15 AM	#2
Danko diyAudio Member Join Date: Apr 2004 Location: Hungary	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. __________________ Best regards, Danko

21st October 2006, 05:01 PM	#7
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	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. __________________ I use to feel like the small child in The Emperor's New Clothes tale

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21st October 2006, 01:02 PM	#3
Nordic diyAudio Member Join Date: Sep 2005	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.

21st October 2006, 02:36 PM	#4
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	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

21st October 2006, 04:09 PM	#5
N-Channel diyAudio Member Join Date: Mar 2003 Location: Sol System	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

21st October 2006, 04:17 PM	#6
astouffer diyAudio Member Join Date: Jun 2004 Location: Pittsburgh, crumbling wasteland	Looks like you need to use one of these http://www.powerlabs.org/images/scr.jpg

21st October 2006, 07:01 PM	#8
TOINO diyAudio Member Join Date: Oct 2004 Location: Coimbra	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…

21st October 2006, 07:22 PM	#9
davidallancole diyAudio Member Join Date: Nov 2005	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.

22nd October 2006, 12:24 AM	#10
Irakli diyAudio Member Join Date: Aug 2004 Location: washington, DC Blog Entries: 1	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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