Suitable relay for delay cicuit

[Nuuk]

Wow, an intersting read even if I didn't understand all of it!

So,if I understand correctly, the supply needs to be 1.7 volts more than the relay requires, ie 13.7 volts for a 12 volt relay.

[tool49]

Yes and no, you can use a higher supply to compensate for the losses, but usually, your relay will clamp at a lower voltage than the rated one.

If you look at your relay specsheet, the coil operating range is 70%-150% or for the 12V version 8.4V to 18V. Any voltage in between those two will suffice to make the relay clamp.

I never really used the NE555 to drive the relays directly, I always buffer them with a transistor or a mosfet. This allows the coil to see practically the full swing of the power supply, especially with the mosfet solution. This way I can use a 12V supply and the relay will have it's rated 12V.

Hope this helps!
Sébastien

EDIT: More info here:
http://home.cogeco.ca/~rpaisley4/LM555.html

[Nuuk]

I have now built and installed the circuit shown at the start of this thread and it works! No noise at all on power up (or power down).

I get a delay of about 3 seconds using a 47 F cap. Adjusting the trimmer doesn't seem to make any difference to the delay so I guess it is to do with the voltage that the relay is triggered at. Perhaps Sébastien could confirm that if he is still around.

The circuit is powered from the buffers supply (15 volts) and I have used a 12 volt relay.

In my buffers, I have 15 volt rails, R3 is 120R and the output series resistor is 75 ohms. With BC547's there is no problem. I did wonder if higher currents were involved whether it would be a good idea to use an additional resistor between the output and ground.

Anyway, at last I am pleased to be able to turn on the power amps and buffers at the same time (with the same switch!) and not get any nasty noises.

[Nuuk]

I have just noticed that I forgot the protection diode on my delay circuit. But which way does it go?

[Nuuk]

Is the above correct please?

[Russ White]

Nuuk,

I cannot answer your question ,but I can't wait to try the circuit.

[Nuuk]

I cannot answer your question ,but I can't wait to try the circuit.

Hi Russ, as I said, the circuit works fine without the diodes but it's better to have them to protect the NE555.

If you want to order the parts, then go ahead and don't forget the IN4148 and IN4002 diodes too!

When you build the circuit leave room for the diodes. The IN4148 could be soldered underneath the PCB across the legs of the relay.

[Yoghourt]

Originally posted by Nuuk Is the above correct please?

No:
1) no current may pass trough the relay due to your 1n4001
2) 1n4148 diode cannot withstand relay back-emf. If it does not burn at 1st use, it will for sur burn one day or another. Since diodes tend to burn as short-circuit, relay will not be operated, but ne555 will burn. It may burn as a short-circuit vs. supply, which won't like it at all, etc. My guess is that you don't really want that experience

Cure:
1) remove or reverse your 1n4001
2) use a 4001 instead of 4148.

[peranders]

A 1N4148 is quite enough for a freewheeling diode. The coil current is less then the diode current rating.

Parameters to have in mind is that the diode must be able to take used voltage, like > 15 volts, times 2 or 3 at least and in case it's 75 volts.

The current for the diode must be at least 0.25 times the relay current and in the case a 10-25 mA would be enough but good engineering pratice would be to choose a diode with at least the same current rating as the coil and that wouild be in this case 30-50 mA. A 1N4148 has 100 mA as limit, note continuous current limit

[Nuuk]

Thanks for your help chaps!

I found a circuit using the IN4148 and that's what I copied. As I have IN4002's in my parts box that's what I will use. And as it's easier to order one type rather than two, I'll suggest IN4001's on Decibel Dungeon.