DC protection relay flashes

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I've made a DC protection board using a microcontroller. When I detect DC or too low frequency I open the output relay. I have tested it a about full power (nearly 600/700W) and when the relay open it creates an arc. The relay is a Finder one (a big type). I open both contact that breaks the electric connection to of the speakers (which is for my test a resistive load in the water).
I supose my relay will destroy quickly. Is there special relay which are made for this purpose ?

Any comments would be appreciated.
First of all if you break any load at rather high current you will get an arc. That's normal. But did your protection trigger only at normal AC voltage at 700 W?

Note that DC doesn't exist. It's AC with low frequency.

First you have a LP-filter Must be chosen with taste

Then you fullwave rectify it

Then you feed a comparator with or without some delay.

Can you show us how you have done?

My DC protection constists of 4th order Thebusjev at 7 Hz, 1.5 V trigger voltage, no delay. The filter is aimed for 300 W at >20 Hz at 4 ohms.
Peranders=>I've made some kind of digital LPF. Then if the output of this filter goes to high it means I have DC or low frequency in the input and so I open the relay. It started to trig at about 20Hz with significant power.

Why did you use 4th order ? I suppose it is to have a better selection of the frequency, but doesn't it rises the delay to trig ?
Minimum frequency at max out power? 20 Hz at 300 W, 4 ohms, i.e

Max speed and min trigger voltage? In my case 7 Hz, 1.5 volt peak

When you know this you just have to look at some filter charts in order to determine how sharp filter you will need.

49 V peak, at 20 Hz, should not be more than 1.5 V peak after the filter. 20/7 = 2.9 => 0.35 from the corner of the filter it should attentuate 30 dB. => 4th order Tchebushev

I designed it in this way because I wanted a fast DC protection.

Have you checked this?


I forgot to say that you must see to it that your filter can take full AC voltage at any frequency.
Arcing cannot be avoided (unless you use a capacitor across the relay). However, some relays are designed to withstand it. They use materials that can stand the high temperatures involved.

The important thing is to read the ratings of the relay correctly.
Example: A relay is rated 250V ac, 5 (1) A
This rating means that is tested with a certain number of cycles at 250V, 5 A purely resistive load. (This is not at all the case for a loudspeaker).
The "(1)" in the rating means that it is tested to 1 A partly inductive load. Such a derating of relays is very common, as few relays can make/break full load (5A in this example) if it's not purely resistive.
Some years ago I worked at a testing company, and have seen nasty results of switches/relays that did not pass these tests (smoke, fire).

Then again, you don't plan to operate your relay frequently at full load, do you? :rolleyes:

Joined 2004
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Hi computeruser,
Try to mute the input first. 1mS should do it. That way you only arc on an output stage failure which is unavoidable. Relays are cheaper than speakers ... right? A cap across the contacts (or RC) will help quench the arc on opening.

Reminds of the city music shop a few decades ago that was building up big PA's (4ohm everything) mostly for bands, using the pretty little Marantz 140 (I think?) Hi Fi power amplifiers - the ones with the big blue power meters on the front panel. Looked impressive - until the whole bank came back with melted relay contacts. Light duty design, heavy duty application.
Joined 2004
Paid Member
140's, 240's. The 500 (total animal). They were pretty, and designed for the home market.

Lots of Marantz units were used to beat up speakers and the darn things survived a long time. The relays didn't if the amps were routinely overdriven - as you saw.

The answer is to mute the input before breaking the output connection.

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