another protection circuit

[sakis]

a friend of mine designed a turn on delay +protection circuit based on 555 any body has an idea if any drawbacks ????

opinions please

thank you

[Jan Dupont]

Hi sakis

That should be ok. Maybe something is missing to discharge the C1 ?
I like the DC protection

[sakis]

yes ... i will look in to it thanks Jan

[Jan Dupont]

I think it might be needed if you quickly turn the amp off and on again......
A simple test can clear that question

[AndrewT]

very similar to what I have used for soft start and for DC detection/protection.

I have not checked the circuit.

[sakis]

thanks again i will make this tests update the schematic with any changes ...

hello Andrew !!! happy new year .... nice to see you I hope you are doing fine

Regards sakis

[indianajo]

Thanks Sakis for bringing protection circuits up. What is the 220k resistor on the trigger of the 555 with a slash through it and the T beside it? It's not a thermistor attached to the heat sink, is it? They have invented some stuff since I got out of school, and wikipedia didn't show this symbol under thermistor. I was looking at the Newark.com catalog Sunday and found a honeywell 140-103LAC-RB1 thermistor with 25 C resistance of 10Kohm and a beta of 3974. I think this means it goes 1070 ohm at 60 deg C, is that about right? I've been disassembling motor drives trying to find the temperature sensor, but I think they are integrated into the thyristor block where I can't play with them, and honeywell doesn't have and ap note online, so I am trying to get a free education. Thanks in advance.

[Greg Erskine]

Originally Posted by indianajo

Thanks Sakis for bringing protection circuits up. What is the 220k resistor on the trigger of the 555 with a slash through it and the T beside it? It's not a thermistor attached to the heat sink, is it? They have invented some stuff since I got out of school, and wikipedia didn't show this symbol under thermistor. I was looking at the Newark.com catalog Sunday and found a honeywell 140-103LAC-RB1 thermistor with 25 C resistance of 10Kohm and a beta of 3974. I think this means it goes 1070 ohm at 60 deg C, is that about right? I've been disassembling motor drives trying to find the temperature sensor, but I think they are integrated into the thyristor block where I can't play with them, and honeywell doesn't have and ap note online, so I am trying to get a free education. Thanks in advance.

It's a pot.

If a PCB is generated from this schematic have a close look around R5.

[stratus46]

Originally Posted by Greg Erskine

It's a pot.

If a PCB is generated from this schematic have a close look around R5.

Agreed. By the time a positive offset triggers the protection, it will likely have damaged the speakers or simply blown some fuses. I'm not sure you can get a negative offset to even trigger it. I would think a window comparator would do a better job of it but in all my years of working with audio gear, I personally have never run into a amp that put out significant DC without taking out its own fuses and I've never seen speakers damaged by continuous DC.

G²

[Jan Dupont]

I see no problem around R5 except that value of R4 and R5 maybe has to be lowered...

I'm not sure you can get a negative offset to even trigger it.

As T2 is used in it's Grounded base configuration it have a very high Current gain. It should work fine