Relay contact ratings.

"John Fields" .

** Problem being, there is simply not enough gap available to break the arc that forms if the DC ratings are exceeded.

For the type of relay in question, a permanent arc will form with DC if the current flow is say 10 amps and the voltage across the contacts is over 40 volts. This means there is over 400 watts of heat, mostly being dissipated into the contacts and destroying them in a few seconds.

In the world of high powered audio amplifiers it is still common to see such relays used to protect speakers from DC high fault currents and turn on/off transients. In the case of former, the relay is not capable of doing the job.

... Phil

**Unlikely. In fact, IME, TRIACs (appropriately rated ones) are vastly more reliable than relays when driving highly inductive loads. In fact, I've been using several for around 30 years, without issue. That is not to say that TRIACs cannot fail. They can and do and usually shorted. Which, in Sylvia's case, may prove inconvenient.

and since fridges are turned on for

**IME, the vast majority of lightning problems occur via TV antennas. Power line issues are massively over-stated. Again, the only time I can pin point a power line "surge" as the direct cause of a problem was a very long time ago, when a 5kV bearer fell across the 240VAC main overhead lines. The damage was considerable and affected several blocks. **I agree. However, in terms of longevity, TRIACs win hands down.

-- Trevor Wilson

**Tapco CP500 anyone? Crap design, with useless relay protection.

-- Trevor Wilson

"Trevor Wilson"

** Plus the telephone line - modems and TAMs drop like flies when there is a thunderstorm. ** It's an issue in many rural areas - the solution to which is fitting varistors in the power box. ** Funny how microwave ovens all seem to have relays turning on the big tranny.

... Phil

I would not use a 12V automotive relay to switch mains voltage, particularly inductive loads.. coil-to-contact dielectric strength of those POS relays is

Seen that happen. Contacts weld, amp and speakers go boom.

nd

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Is the house (hosting the fridge) required to be fire retarded?

"swanny" "Phil Allison"

** I have made a bit of a study of this in an attempt to find a solution.

Conclusions are:

1. There is no readily available relay capable of breaking the voltages and currents involved if a 1000 wpc amp goes DC.

1. If you wire a changeover relay so it shorts the speaker and disconnects the amp at the same time - chances are good the speakers will be OK, but the relay contacts will be burnt by the resulting arc to ground. The amp must have DC rail fuses if this is to work.

2. A large ( ie octal base ) relay with 4mm clearance contacts PLUS a strong magnet next to the contacts CAN work with amps up to about 400wpc. The magnet pulls at the arc and helps break it.

1. A triac "crowbar" will also work but gives no on/off transient suppression. The amp must be able to take a dead short without internal failure and not have response down to subsonics or DC.

... Phil

**Quite true.

**I'm a city boy. I don't see many country products.

**Might be something to do with the different failure modes of the two devices.

-- Trevor Wilson

**Mains shut down works fine in most cases, provided there is not a very large energy storage employed. A few hundred joules should be do-able by a serious bass driver which is normally connected to such a large amp.

-- Trevor Wilson

"Trevor Wilson"

** Thinking about it - it is more to do with failure mode in the oven itself.

If a diode or high voltage cap goes west, or there is an internal short the HRC fuse in the AC will blow.

The current surge during such events would destroy any low cost triac - but not a relay.

Also, a relay provides high isolation from the control PCB to the AC supply.

... Phil

**Good points.

**All academic now anyway. I've noticed that MOs are starting to employ SMPS nowadays. I scored a broken, high end model from a neighbour. Lots of IGBTs, but no relays to be seen. Naturally, the fault lies with the IGBTs. :-(

-- Trevor Wilson

"Trevor Wilson"

** So the AC goes off when single polarity voltage on the output passes a threshold in voltage and time - then what?

There has to be a warning LED and an auxiliary power supply to do the AC relay control. Ideally, it should not be possible to switch the amp back on after DC is detected.

And there still has to be relays for de thumping.

BTW

I was after a solution that could be affordably retro-fitted to any amp.

The only time a customer is willing to pay for speaker protection to be fitted is when they see the bill for the repair of a couple of JBL 2226s etc.

... Phil

**Irrelvant. Whilst I don't know the specific legislation pertaining to relays, here in Australia, ALL mains connected components (from transformers to power leads) MUST have the requisite approval numbers and, usually, fire retardant protection, or specific materials used in their construction. Other jurisdictions may not have such legislation, for whatever reason. Sylvia resides in Australia. As such, connecting non-approved components may carry an insurance liability risk.

-- Trevor Wilson

Come to think of it, do you really need two contacts? If the heater is just a few watts maybe it could be connected in series with the compressor and shorted out to turn the compressor on.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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**Correct. I'll email you a schematic Peter Stein used 30 years ago. Simple, cheap, reliable and highly effective. BTW: Mains switching is accomplished by a TRIAC. **Ah, I forgot about crappy designed amps. Perhaps another relay and appropriate sensing will be required for that purpose. In fact, perhaps an existing design could be utilised, using a small auxiliary power supply and an extra relay. I guess complexity and cost needs to be weighed up against burnt speakers. **Depends on what you call "affordable". **Funny about that.

-- Trevor Wilson

**Emails to you are bouncing. Is there a trick to your email addy?

-- Trevor Wilson

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You are missing the point. Once they (insurance co) determined that the fire is caused by mis-speced component, they will deny the whole claim. They will tell you to sue the manufacturer, or whoever made the modification. It is utterly irrespondable to use auto components for house main.

**No, I am not, but continue....

Once they (insurance co) determined that

**I believe that is pretty much what I said.

-- Trevor Wilson