astable

[jcajca]

Hi

I wanted a circuit with a timer controlling an output preamp relay. A LED should blinking meanwhile and stay ON when the relay
get closed.
I designed this with RL3 reseting U4. Q41 also starts RL2, the output relay.
It works but after a while, D7 restart blinking. Maybe there is not enough current for 2 relays.
How can I fix this ?
Mainly, is there a smarter way to achieve the goal ?

 

[Sadlercomfort]

It might be best to have two transistors for each relay, the base of each transistors connected together and each relay with its own diode across the coil. The relays will still switch at the same time.

 

[Miguel Lopez]

I don't think that the 1,7Mohm resistor in the base of Q41 would be enough to saturate it, in order for the relay to work. I think that transistor is working on its limit of saturation.

You're using an NE555. Couldn't you use another 555 timer in monostable configuration to provide such delay?

 

[Sadlercomfort]

The HFE gain for the MPSA13 transistor is very high (5000 min)

As an example:
A 12V relay with a coil 960Ω need around 2.5μA going through the base this transistor at 5000 HFE. Therefore R61 would need to be around 1.6MΩ for a single relay of this type.

So this seems okay.. but not for two relays. What relays are you using?

 

[Miguel Lopez]

need around 2.5μA going through the base this transistor at 5000 HFE.

In my modest opinion, 2,5μA could be induced by noise y the transistor could get cut, if the noise induced in the oposite direction, so the relay could get "crazy".
My two cents.

 

[Miguel Lopez]

need around 2.5μA going through the base this transistor at 5000 HFE.

In my modest opinion, 2,5μA could be induced by noise y the transistor could get cut, if the noise induced in the oposite direction, so the relay could get "crazy".
My two cents.

 

[jcajca]

OK sadlercomfort.
Redo the math for I use 2 x 12V-90Ω relays.
I already thought about one NPN to command one relay. By the way it's not "two transistors for each relay"
but one.
I just hoped that there is a more clever way to ground the "reset pin" a the end of countdown.