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aparatusonitus 5th September 2012 10:36 AM

Speaker power protector/limiter by David Tilbrook
2 Attachment(s)

Can anyone confirm that this is an accurate scheme of David Tilbrook speaker overload protection circuit published in ETI Oct '82 magazine? If not, what needs to be corrected in order to become useful for limiting the power delivered by the amplifier in the range from 5W to 15W/8R?

Ian Finch 5th September 2012 02:25 PM

This is an Australian edition ETI design for self-powered DC protection. At least, that's all it did in my old speakers. To open the relay when power (voltage) exceeded 5-15W/8R would require more accurate RMS conversion, an adjustable reference and a comparator to drive the relay. That's a redesign of the lower half of the schematic and probably quite a challenge. A Polyswitch would be a simpler solution, and not just become a source of on-off relay noises, I think.

aparatusonitus 5th September 2012 03:34 PM

Thanks for the reply Ian,

May I assume from your response that the schematic, as it is, is ok for a power greater than 15W/8R?
Also, is it possible to connect another HEF4050 (after the 1/2 HEF4050 ralay driver) also wired as double triple buffer in parallel, so I can run two output relays instead of one as it is now?

Ian Finch 5th September 2012 08:45 PM

Yes, it was developed originally for a DIY 12" 4-Way Philips loudspeaker rated at 100W nominal. I suggest that was a maximum rating as the relay was a common single pole 10A DC type with a 12V 200R coil. There would be plenty of current available to drive another 3 buffers of a second 4050 but the current to power a second, similar relay may not be adequate. At the least, R1 rating and C1 capacitance probably need to be doubled.

However, you may not need to bother, as at the lower power, this could all be done with just a change to a similar frame relay but with double pole (DPDT) contacts.

aparatusonitus 5th September 2012 10:19 PM

Thanks Ian for the extra info...I thought that this limiter was exclusively made ​​for Ozzy's gear.:)
In any case, I intend to use DPDT relays, as you suggested, but the problem is I have to switch four speakers simultaneously, not only two. In addition, I'd like to put the relay contacts in parallel if possible, that's why I asked for a second HEF4050. Do you think it would be better to use a relay with four-way contacts and be done? What relay specifications should be in this case?

aparatusonitus 5th September 2012 10:32 PM

BTW, I found one patent (US6201680) with the same task (and something extra, also self-powered), but the lower limit is set to 25W/8R. Fortunately, all of the components values are given, so I was wondering what should be changed to extend the lower limit below 25W/8R if possible?

Ian Finch 6th September 2012 12:33 AM

Hmm... 1A contacts would be OK for 5W/8R but 15W/8R might be too much for reliable use on a 2A rated relay. 3-5A rating would be sensible.

I don't follow that you have 4 speakers to switch so you want four sets of contacts but you also want to put them in parallel?? That seems to defeat its purpose. Remember, this device is signal powered but senses DC faults on the output. Ideally you need to sense each channel separately and sum the inputs at the 4050 to do that with 1 relay or better, have individual relay circuits on each channel output (if you do actually have separate channels here)

I had forgotten to say that 5W/8R power would not generate enough voltage after the rectifier, to drive the circuit. I suspect that a nominal 15W amplifier wouldn't either. Schottky diodes would help, but how much will depend on relatively high signal levels.

I doubt now that this is going to work properly, whatever changes are made to the circuit. It's possible a 6V relay may work with a 15W amplifier if the available current can be increased but realistically, a conventionally powered circuit would be simpler and safer with low power audio. :o

tauro0221 6th September 2012 02:08 AM

1 Attachment(s)
I attached it is a modified sketch of your drawing that allow you to enable few relays with no problem. The only it is that you need a supply voltage for the relays. The TIP31 can drive a max voltage of 60 volts.

andrewlebon 6th September 2012 04:24 AM

Hi Tauro0221
greetings can you share any simple low impedance detector schematic and
short circuit protect if amp impedance is below 2 ohms and if speaker wires touch each other by mistake
cant find any schematic on the net but proffessional amps have it
warm regards
andrew lebon

tauro0221 6th September 2012 05:00 AM

I have been working in a prototype that started reading the speaker voltage and if it is out of limits the micro will disable the output relay. right now the final one it is reading only the current. If the current it is out of the limits the micro will disable the output relay and shutdown the amplifier to protect it from further damage. Up to now nobody as show any interest. I already done some test by increasing the current to the limit and does what it is suppose to do.

You can check/read it at this thread.

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