Speaker relay protection

[Gareth Magennis]

Have a LEM powered sub for repair.

Speaker toast.
Amp blown/DC. (2 amps bridged for sub, one probably OK)
Relay shorted from amp to speaker.


Not sure of the chronological order of events here, but for sure the relay
did not do its job.

More of this arcing under fault conditions shenanigans I presume.




Gareth.

 

[Phil Allison]

Gareth Magennis wrote:

Have a LEM powered sub for repair.

Speaker toast.
Amp blown/DC. (2 amps bridged for sub, one probably OK)
Relay shorted from amp to speaker.

Not sure of the chronological order of events here, but for sure the relay
did not do its job.

More of this arcing under fault conditions shenanigans I presume.

** My colleague Rod Elliot of ESP

http://sound.westhost.com/

is currently preparing an article on relays with particular reference to audio applications - one of which is dealing with large DC voltages and currents.

The site has a number of articles and construction projects written by yours truly - like this one.

http://sound.westhost.com/articles/speaker-failure.html

Lots of other tech stuff there is from me too, much of it with no formal acknowledgement.

I'll post a link to the new article when it is up.


..... Phil

 

"Speaker toast.
Amp blown/DC. (2 amps bridged for sub, one probably OK)
Relay shorted from amp to speaker. "

Ho boy. I remember the thread about the magnets now.

Incidentally I suggested a cure for that welded relay problem. Just throw a good crossover cap across the contacts. So what if the amp still produces sound when the protection kicks in, you are blocking the DC which is the object.

I think like a 22 uF/200 volt cap designed for speaker crossovers would do it. Two hundred volts is ALOT of power into a speaker so it should be fairly immune to shorting out, which would defeat the prupose.

If you got more than 400 volts P-P gont to a dynamic speaker, just figure out an SCR crowbar circuit for it.

 

[Phil Allison]

"Speaker toast.

Amp blown/DC. (2 amps bridged for sub, one probably OK)
Relay shorted from amp to speaker. "


Incidentally I suggested a cure for that welded relay problem. Just throw a good crossover cap across the contacts. So what if the amp still produces sound when the protection kicks in, you are blocking the DC which is the object.

I think like a 22 uF/200 volt cap designed for speaker crossovers would do it. Two hundred volts is ALOT of power into a speaker so it should be fairly immune to shorting out, which would defeat the prupose.

** I have just been experimenting with a PCB mount, 240VAC/12amp relay made by "Schrack" and a variable PSU - consisting of a 300VA tranny, bridge rectifier and 10,000uf cap. The tranny was fed from a Variac.

The relay, when energised, connected a 4ohm high power load to the PSU and tried to disconnect it when de-energised. With no cap across the contacts, serious flash arcing occurred at 30VDC. With 50VDC, you can normally expect the arc to become continuous, first shot. The maker's rating for DC switching is 24V at 10 amps max.

However, with a 20uF film cap across the contacts, all signs of arcing at switch off disappeared. Amazingly, this was still the case when tried with 6, 3 & 1uF instead. When I tried 0.22uF, flash arcs appeared on about 1 out of 3 tries.

To simulate a *bad* inductive speaker load, I added a 5mH air core choke in series with the 4 ohm load and saw slight flash arcing with a 1uF cap but none with 3uF.

With a 8 ohm load and 100VDC, 6uF was enough to reduce arcing to minor flashes.

So, a film cap across the relay contacts made a huge difference when breaking DC current at voltages well above the relay's ratings.

FYI: the film cap delays the voltage rise across the relay's contacts for the first 10 to 50 *microseconds* after opening - which is when the arc forms. Delay that rise enough and there is no arc.



..... Phil