Voice Coil Protection for Speakers

[EWorkshop1708]

I'm designing a circuit to shut off the amplifier if the Voice Coil becomes overheated, that way if I get too heavy on the volume control, my speaker is saved.

Some speakers have vents in them, so I plan to put a 10K thermistor inside one vent, and put it as close to the VC as possible without touching it. Then I'd glue the wires from the outside of the speaker to hold it in place.

I already vaporized a VC on a 350W/1200W subwoofer, and I don't want this to happen again!

I have a +350W RMS amplifier and it already uses LM339 Quad-Comparator to shut the AMP off when overheated, and I have 3 unused comparators left - for future uses. I want to use one of these to detect the Voice Coil temperature.

I'd use a voltage divider w/POT from regulated +5V as a voltage reference, and a resistor in series with the Thermistor, then feed that to one of the LM339 inputs.

Questions I have:
What is a safe temperature for Voice Coils? I want the temp to cut off below the glue-melting point - long before the copper melts.

Also, what resistance would an avg 10K thermistor read under the highest allowed VC temperature?

Thanks for your responses.

[just a guy]

Questions I have:
What is a safe temperature for Voice Coils? I want the temp to cut off below the glue-melting point - long before the copper melts.

Also, what resistance would an avg 10K thermistor read under the highest allowed VC temperature?

Can't help with that stuff, sorry, but maybe I might be of some service by asking a different question. Do you know why the last one burned?

For example, if you have a ported box and you blast it with 1200 watts at the port tuning frequency you can fry it very quickly due to the lack of cone motion. You can burn it with much less than the driver's rated power handling.

Maybe you know all this already, but it might be worth investigating the cause of the problem instead of adding a kill switch. Maybe you already have.

[AndrewT]

VC insulation becomes a problem long before copper melts.

But, the VC expands when heated. How far can it expand before it starts to rub on the pole?

[EWorkshop1708]

It is in a 1.25cu ft sealed box, exactly what the speaker calls for in the Sony papers that come with the woofer.

The speaker had lots of excursion. I was clipping the amp a bit playing "war pigs" from Black Sabbath. I love playing hard rock with heavy drum and guitar notes.

Also, I need to measure the output of the amp, because this is the 2nd 350W RMS speaker (2 different types) that I've destroyed with this amp. I think I underestimated the real RMS power. It uses 5 pairs of MJL4281/4302 outputs with no SOA protection, and is fan cooled.

Transformer is a 9lb 55-0-55V 350VA toroid transformer (weighs as much as an Avel 500VA) equipment transformer, I used extra voltage preparing for sagging rails, but after building the amp, the rails didn't sag as much as I thought they would when driving 4 ohms. The transformer is never even warm under any load. I have a feeling the VA is not accurate on the toroid due to the weight. Unloaded, my rails are usually 75-0-75 or sometimes 77V on a high mains outlet.

The speaker didn't die right away, but over time from me cooking the coil with loud music. The speaker has vents, so I want to insert a thermistor on the new speaker when I get it, to keep track of the temp.

[EWorkshop1708]

Originally posted by AndrewT
VC insulation becomes a problem long before copper melts.

But, the VC expands when heated. How far can it expand before it starts to rub on the pole?

Never had that rubbing problem with this speaker. The generic speaker before it developed that problem, which is why it got replaced. However, the Sony played problem free with no warning up to the point the coil blew.

I agree with you on the VC insulation, that's why I want the temp protect to cut off the amp if it gets close to that temp, that way the coil doesn't die over time.

[janneman]

Are you sure this is a 350W RMS speaker? Does it actually say RMS on the spec sheet? It may be 350W music power or something similar, with lower continuous power (that's how it is usually spec'd).

Jan Didden

[EWorkshop1708]

350W RMS 1200W Max

Even says 1200W on the cone. However, I know better, and went by the RMS rating.



There is a higher priced version that's 380W RMS 1300W MAX, I may also consider, the speakers are nearly identical, except the 1300W one is black, and goes down to 18hz.

[EWorkshop1708]

Regardless of the speaker used, I want to implement voice coil protection, because all voice coils get hot, and deteriorate over time from playing too loud.

[AndrewT]

and the magnets deteriorate as well. Alnico is much more resistant than ferrite.

[EWorkshop1708]

Interesting, didn't know that.

Sounds like another good reason to be able to monitor the temp of the motor assy.