Problem with DC Protect

[opor]

I have build this as per ESP project 33, DC protect circuit. Then I tested the circuit by connect 12VDC supply to AMP terminal .It turn off the relay then I disconnect 12VDC it never turn the relay on.So I power off and leave it untill supply down to 0volt and power it back .It turn the relay no again. Do I have made something wrong?

[tomchr]

In my opinion, that's an overly complicated circuit to perform a relatively simple task. But that's beside the point, I guess...

As far as I can tell, you have two time constants in the system, RN1*CN1 and RN7*CN2. Those time constants are 4.7 seconds each. I would expect this circuit to take several 10's of seconds to reset. In order for this to happen, you need to turn the input voltage to zero (don't just turn it off). If it doesn't reset within, say, 30~60 seconds I'd say something's wrong. It doesn't look like a latching circuit, so it should reset eventually if the input voltage is turned to zero.

The reason for the delay you're seeing if you just disconnect the trigger voltage (marked AMP in the schematic) is that CN1 is then only discharged by the base currents of QN1 and QN2. That's a tiny current, hence CN1 will stay charged for quite a while. By grounding the AMP connection (turning the input voltage to zero) allows CN1 to discharge via RN1.

Even if the circuit requires you to power cycle the amp to reset, that might actually be a feature rather than a bug. You want the amp to turn off and stay off in the event of DC on the output.

~Tom

[nigelwright7557]

I hate those protection circuits with loads of transistors in.
I prefer a PIC based circuit which at least will have quite a bit of inteligence.
I built one with 4 resistors to detect positive and negative transitions.
The PIC also had a 4 second power up delay.

[sregor]

Minor point - if you have it hooked up to an amplifier and their is no DC in the output, the amplifier itself will discharge the cap and allow the relay to come back on more quickly. (for your test set up - grounding the AMP pin would simulate a normal functioning amplifier with no DC on the output - just as applying 12 volts was used to simulate a malfunctioning amplifier YMMV.

[smartx21]

Are you using 6V relays?

[smartx21]

Originally Posted by nigelwright7557

I hate those protection circuits with loads of transistors in.
I prefer a PIC based circuit which at least will have quite a bit of inteligence.
I built one with 4 resistors to detect positive and negative transitions.
The PIC also had a 4 second power up delay.

BTW, Nigel, are you used to share your designs?

[opor]

Hi Sregor,
Even I grounding the AMP pin to simulate 0volt DC (power supply is still running),it never turn relay on.