denis5556
Dec 31, 2013, 01:09 AM
I am installing a "Hot Water Bypass" system in my motor-home to conserve water.Instead of running cold water from the hot tap down the drain I have installed a solenoid to return it to the water-tank for a set time using a 555 timer circuit.
On its own the 555 timer (off a 12 v DC supply) drives a little relay and works fine with a small test buzzer across the relay contacts, BUT connect 24vdc to the relay contacts to drive the solenoid and the timer gets false triggering to keep it on for a random number of cycles. The relay coil and contacts are electrically isolated.
I guess there is induction happening causing the circuit to re-operate.
I connected a test light across the solenoid coil and observed that the timer ceases but re-triggers before the solenoid releases effectively keeping it operated. But with the testlight connected I noticed a decrease in the number of "re-triggers".
In an effort to desensitize the trigger circuit I doubled the resister value to 20Kohms between +12dc and pin 2, the trigger pin.
With the 20Kohms and a 24v 3 watt bulb across the relay contacts in parallel with the solenoid coil the system has operated 100 plus times without a re-trigger.
I think the lightbulb is absorbing the back emf spike at the opening of the relay contacts effectively
1 letting the solenoid release, and
2 preventing "induction" re-triggering the 555 timer
I am happy now it works as I wanted it to, but would a capacitor do the same job as the bulb? If so what type of capacitor and size?
Alternatively is there another method?
Many Thanks
Denis Hibbs
Napier NZ
On its own the 555 timer (off a 12 v DC supply) drives a little relay and works fine with a small test buzzer across the relay contacts, BUT connect 24vdc to the relay contacts to drive the solenoid and the timer gets false triggering to keep it on for a random number of cycles. The relay coil and contacts are electrically isolated.
I guess there is induction happening causing the circuit to re-operate.
I connected a test light across the solenoid coil and observed that the timer ceases but re-triggers before the solenoid releases effectively keeping it operated. But with the testlight connected I noticed a decrease in the number of "re-triggers".
In an effort to desensitize the trigger circuit I doubled the resister value to 20Kohms between +12dc and pin 2, the trigger pin.
With the 20Kohms and a 24v 3 watt bulb across the relay contacts in parallel with the solenoid coil the system has operated 100 plus times without a re-trigger.
I think the lightbulb is absorbing the back emf spike at the opening of the relay contacts effectively
1 letting the solenoid release, and
2 preventing "induction" re-triggering the 555 timer
I am happy now it works as I wanted it to, but would a capacitor do the same job as the bulb? If so what type of capacitor and size?
Alternatively is there another method?
Many Thanks
Denis Hibbs
Napier NZ