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Old 8th October 2011, 02:45 PM   #1
spot is offline spot  Australia
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Default Voice Coil Protection - thermal time constant

I would like to protect a sub from overheating by monitoring the power going into it.
The amplifier used on the sub would be capable of substantially larger power than the subs continous rating to allow the peaks. This circuit would need to limit the average power to a lower level than the peak power.
I am thinking of putting the measured power through a filter, say a single time constant, then a comparator to decide if its too much.
The question is what to make the time constant?
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Old 8th October 2011, 03:35 PM   #2
Ron E is offline Ron E  United States
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It could be much easier to measure coil temp directly, or perhaps you could infer by comparing Voltage and current?
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Old 8th October 2011, 09:28 PM   #3
spot is offline spot  Australia
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Originally Posted by Ron E View Post
It could be much easier to measure coil temp directly, or perhaps you could infer by comparing Voltage and current?
I'm trying to avoid cutting into the subwoofer itself. What I'm thinking of is essentially taking the voltage, squaring it (to get VxV= VxIxR) and then working on the worstcase assumption that R=8Ohms always I have an approximation of the power in. Putting this through a low pass filter before deciding if its too much would then allow peaks through but hopefully prevent it from overheating (as the voice coil has thermal inertia). I just dont have a feel for even an approximate idea of the thermal inertia.
I can use approximations as I'm not trying to squeeze every ounce of power out of the subwoofer, just don't want to fry it or buy a $$$ super power capable model.
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Old 8th October 2011, 10:22 PM   #4
gootee is offline gootee  United States
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It takes about a minute or so for the over-temp-warning lights to come on when I way-overdrive my Vandersteen 2Ce speakers. Then they don't go all the way off unless I turn off the input completely, or turn it WAY down, immediately. (Just turning it down to the max level that normally would never make the lights come on makes them stay on and I haven't wanted to leave them like that to see how long it would take for them to go off.) I don't know if that will help but it's the only data point I have.

Wouldn't a worst-case assumption be LOWER than 8 Ohms?
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Old 8th October 2011, 10:46 PM   #5
forr is offline forr  France
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I have some papers by Penkov dealing with such circuits.
They are in french but the figures may suffice for your application.
They look more or less like the attached picture and are available from Selectronic in France :
Kit de protection pour haut parleur pas cher | Acheter Kit de protection pour haut parleur discount
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File Type: gif SpkrProtectAutonom.gif (7.4 KB, 134 views)
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Old 8th October 2011, 11:40 PM   #6
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You could just get a compressor/limiter with variable attack and release.
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Old 9th October 2011, 10:37 PM   #7
NEO Dan is offline NEO Dan  United States
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Has anyone had a look at the protection gizmo that eminence has come out with?
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Old 10th October 2011, 04:32 PM   #8
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There are various protective gizmos sold by kit houses.

Please correct me, but doesn't Class D kind of amps handle big peaks but poop out when longer notes are played?

I believe there is also a spec about headroom in amps which is usually consulted in order to buy an amp which has muscular power rails so that it doesn't droop on long notes. The OP is worried about those long notes being too strong.

Without endorsing Class D (which, as it happens, power all 6 of my amps), the logic of buying an amp which can play loud brief notes but which DOES droop on longer notes, makes sense for speaker protection and sensible enough given the millisecond peaks in music waveforms (such as soprano choristers).

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Last edited by bentoronto; 10th October 2011 at 04:35 PM.
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Old 10th October 2011, 06:09 PM   #9
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Originally Posted by NEO Dan View Post
Has anyone had a look at the protection gizmo that eminence has come out with?
I looked at one of the videos, looks like it works well at protecting speakers, but would require proper programming which requires destructive testing if you want to get the most out of a speaker.

Spot's original question:
"The question is what to make the time constant? "
still needs to be answered, the time constant is dependent on voice coil heat dissipation capability, mass, motor mass, and frequency (for starters).

Some drivers are far better at getting rid of heat than others.

An RMS time constant of a second or two may be appropriate for a smallish woofer, but would be too long for a light tweeter.
Also, the question of what is "substantially larger power" than a driver can take on peaks comes into play, what works at 3 dB above the RMS level won't work for 6 dB.

Eminence designers have probably figured out the protection parameters for their drivers, using other drivers similar to theirs could use similar parameters with their device.
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Old 10th October 2011, 06:33 PM   #10
sreten is offline sreten  United Kingdom
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Ball park numbers for a 2" voicecoil driver are a time constant of ~ 20 seconds,
and thermal resistivity of 3 degrees C per watt, with something like 300 degrees
C peak allowed and something like 250 degrees C continuous allowed.

These numbers of course will vary between drivers. However it all depends on
how high in frequency the subwoofer is used. Excursion not thermal issues are
the deciding factor in the low bass. Typical sub amps will take the driver to
its excursion limits and there is no point in having any extra peak power.

Might be for the bass unit in a 3-way, as that enters the thermal region.
But IMHO for a sub you should forget about thermal issues for excursion.

A sub drivers continuous thermal rating practically will never be approached.
The more "hifi" the bass, with higher peaks, the lower the average power.
The low bass peaks should not exceed the excursion capabilities of the
driver, this can have very little to do with the drivers thermal rating.

rgds, sreten.

For AV some subs predict driver excursion and apply progressive compression
to prevent overexcursion, not really hifi, but the sub goes a lot "louder" FWIW.

Last edited by sreten; 10th October 2011 at 06:49 PM.
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