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Old 27th November 2006, 08:30 PM   #1
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Default A simple speaker protection circuit

Hi,

I have seen a few speaker protection circuits, but they all seem to be designed for a particular power amp, or just don't seem very flexible.

I have been searching the past week or so and found several designs which look workable, and some that kinda scare me.

Anyway, based an what I have learned I have come up with this (this is just the basic schematic, there will be slightly more to it).


Does this look sane?

In particular I am wondering about the zener diodes and the Schottky's I have to prevent the zeners from shorting each other out. I am not sure if what I am doing there is necessary. The idea was to protect T1 and T2 from full power amp rail voltages at the input.

Comments welcome.

Cheers!
Russ
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Old 27th November 2006, 10:33 PM   #2
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Ok so I like this a bit better, the speaker is now at GND when the relay is off.
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Old 28th November 2006, 03:15 AM   #3
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Nice and simple! Have you tested it? The last one I did I connected it to my amp and put a signal gen through it at full taps and tested it down to around 10Hz, where it would trigger.

Yours is a bit different to what I have used in the past, but I can't see why it wont work.
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Old 1st December 2006, 08:37 AM   #4
d3imlay is offline d3imlay  United States
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I looked at the relay you're using (Future Elec's site) and noted that it's contacts are only rated for 24 VDC. The JQX115's they list are DPDT and rated for 125 VDC. The ability to break more than 24 VDC is very important. I'm working on a similar design and will give the JQX115 serious consideration.
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Old 1st December 2006, 11:17 AM   #5
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I thnk the Futurlec specs are a bit unclear the way they summarized them. Here are the specs from the Omron G5LB dtasheet (same form factor relay):

Rated load: 10 A at 120 VAC; 8 A at 30 VDC & 10A at 250 VDC
Rated carry current: 10 A
Max. switching voltage: 250 VAC, 125 VDC (30 VDC when UL/CSA standard is applied)
Max. switching current: AC: 10A; DC: 8A
Max. switching capacity: 1,200 VA, 240 W & 2,500 VA
Min. permissible load: 100 mA at 5 VDC

So, you can switch more than 25/30VDC, just with less than 10A current. Although, the first line does strike me as strange: 8A@30VDC and 10A@250VDC?

Anyway, this is the relay we use on the RevC Monoblocks for speaker protection (Siemens version).
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Old 2nd December 2006, 12:19 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
I think the 250Vdc is a misprint.
The spec reads correctly if it meant 250Vac
check down to the 1200VA, 240W, 2500VA line.
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Old 2nd December 2006, 12:27 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
do you realise you have 4V across the BE junction of t3?

The grounding of the speaker output is far better. It will help blow the amplifier fuse if the arc across the contacts refuses to extinguish. You need rail fuses to take advantage of this.

How can you latch the protection ON after activation? You don't want it chattering ON OFF ON with one supply rail sending offset to the speaker.

I'm not sure the Zeners are quite right.

I think C2 will delay ON till it's too late (or over current T1 and/or T2).

D1 slows down the release of the relay.
You can speed up the release by running it at lower voltage with a dropper resistor and then add a cap from top of relay coil to T3 emitter. The 24V on the extra cap prior to T3 closing fires the relay and then voltage falls to some lower value (above guaranteed hold in V).
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Old 2nd December 2006, 02:13 PM   #8
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Quote:
Originally posted by AndrewT
Hi,
1) do you realise you have 4V across the BE junction of t3?

2) The grounding of the speaker output is far better. It will help blow the amplifier fuse if the arc across the contacts refuses to extinguish. You need rail fuses to take advantage of this.

3)How can you latch the protection ON after activation? You don't want it chattering ON OFF ON with one supply rail sending offset to the speaker.

4) I'm not sure the Zeners are quite right.

5) I think C2 will delay ON till it's too late (or over current T1 and/or T2).

6) D1 slows down the release of the relay.
You can speed up the release by running it at lower voltage with a dropper resistor and then add a cap from top of relay coil to T3 emitter. The 24V on the extra cap prior to T3 closing fires the relay and then voltage falls to some lower value (above guaranteed hold in V).

1) I really hadn't finalized the resistor values yet, thanks for spotting that that was pretty high. I have changed it.

2) So at least I got that part right?

3) I am not sure how do accomplish this without defeating one of my design goals which is to make the circuit self resetting. I am open to suggestions.

4) Me either, the way I have them they should protect T1 and T2 from VBE > 6V. That was the idea.

5) It could be I will have to test. I have changed the cap and resistor values.

6) I believe I have understood you here, and it makes a lot of sense. Thanks for the excellent suggestion. I hope I have implemented it correctly. I will have to calculate R4 for the correct voltage drop.

Thanks very much for all the input so far!
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Old 2nd December 2006, 02:49 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
try breadboarding one that works.
When you understand how it detects and hold, and closes/opens the relay then think about redesigning/improving the circuit.

The r2/r3 divider will either prevent t3 switching on or risk killing it if the supply voltage rises a little.

You have implemented C3 correctly. You can now run at more than 24Vdc and adjust R4 to hold in under all normal operating conditions.
That 24V supply can come from ONE diode on the main PSU rectifier bridge with just a tiny bit of smoothing to give near instant drop out on loss of mains voltage.

But, I fear the circuit, as is, is struggling to work.
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Old 5th December 2006, 09:45 AM   #10
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Quote:
Originally posted by AndrewT
Hi,
do you realise you have 4V across the BE junction of t3?
No, he hasn't.

There's 0.6V, limited by the VBE. Under these conditions, about 0.1mA is available to turn on the transistor. Assuming a (generous) worst-case HFE of 50, that only results in 5mA to drive the relay coil.

I haven't looked up the relay, but I would determine the coil current, then double it. Divide that by the worst-case HFE to find the base current, then double that to be sure. You're trying to saturate the transistor! I would suggest that a value around 22K would be a good starting point - 470K will not work! Remember - you're *not* trying to make a potential divider here, you are providing T3 with base current...

R3 doesn't play that much of a role in this calculation, as it only steals 12uA from the contribution of R2.

C2 provides a power-on delay, but in the event of a DC fault, one of the two input transistors will have to discharge it. Maybe fine for T1, but in the case of T2 doing this, the only means it has to sink current is via the input 100K (R1). I would suggest that this is a problem... For that reason, move the on-delay elsewhere - perhaps add another transistor after T3. This also makes it easier to implement the latch:

Another transistor is needed, forming a simple bistable latch with T3. Some sort of power-up reset is needed, probably a series RC combination.

BUT, do you need the latch? If this circuit triggers, the fault condition will almost certainly persist once the speaker has been disconnected. In fact, the DC fault voltage will probably rise once the OPS has been relieved of supplying many amps to the poor voice coil. You are correctly sampling before the relay contacts

For what it's worth, I implemented this DC-detect circuit in my quad LM4780 amp, only I interfaced it to a PIC (overkill, but the PIC was there doing other things). During testing, I found it works well, but you need to avoid trying to pull too much current from the collectors of the transistors if you want the detection thresholds to remain symmetrical (although C1 helps, of course). Which is what could happen with this simple circuit if you stick with 3 transistors (so perhaps a Darlington would be better for T3?)

Mark
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