Flashing output to drive a relay, help please!

Thread Starter

jay4708

Joined Jul 28, 2014
12
Hi

My electronics are a little rusty, I need a bit of help with the following if you don't mind, thanks all.

I have an output that once triggered pulses low with respect to +24vdc as shown below:

(The lamp is just a load for this example drawing)


What I need is for that flashing output to drive a relay for as long as the output is active, I don't want the relay to turn on and off with the output I need the relay to be on whilst the output continues to flash.

Your input is much appreciated thanks.

John
 

inwo

Joined Nov 7, 2013
2,419
If source is stiff enough you just need a 1000uf or so across a small relay coil to keep it on for a 1/2sec.

If not, and the high charging current could damage the source, you will need a transistor or ic driver and access to the +&-24
vdc.
 

Thread Starter

jay4708

Joined Jul 28, 2014
12
Do you mean like this:



I would be interested in seeing a circuit of how I could get round this with a transistor just in case the output is not up to driving the relay and yes I do have access to the +24vdc and ov, there is no -v.

John
 

AnalogKid

Joined Aug 1, 2013
9,494
When converting a pulse train to a steady state signal, a common issue is turn-off delay. Whatever circuit you end up with, it is functioning as a missing-pulse detector - the relay turns off when the pulses go missing. The problem is how long to wait to make sure the pulses really have stopped. With a high speec digital counting circuit you still have to wait longer than 1/2 cycle to be sure. Analog timer circuits like putting a cap across the coil work very well, but the cap probably has a 20& tolerance so you have to oversize it to make sure it never is too small. So, what kind of turn-off accuracy do you need?

ak
 

Thread Starter

jay4708

Joined Jul 28, 2014
12
The pulsed 0v is an output from a nurse call system control board, when an emergency call is activated the output is pulsed low to 0v and lamps are energised with respect to +24vdc.

Thanks for any help.

John
 

inwo

Joined Nov 7, 2013
2,419
If pulse drives lamps directly, then it should supply enough average current to hold in a small relay.

If a second or so extra hold time isn't issue, then try the cap.
 
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