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Delay off time for a 12V relay

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I have a water pump controller that turns on/off a 120VAC water pump. The original controller has a relay output, but I did not like the 120VAC being connected directly to the board so I added an extrernal SSR. The relay is driven by the original circuit and the SSR is driven by the same power source (12VDC). I'd like to delay the off time of the SSR (relay goes off now and SSR goes off x seconds later). My original though was a large cap between the relay and the SSR, but the number of caps required has made that option unfeasible. I would like to the circuit to be 12V. What do you guys have for options?
 

colin55

Well-Known Member
RELAY OFF DELAY
This circuit turns ON a relay when the input is above 2v and the relay turns OFF after 2 seconds when the signal is removed. The OFF delay can be increased or decreased.

 
:rolleyes: Duh on me. That would be purpose #1 of transistors.

Two questions.

1) Assuming the relay in your pic represents my SSR, the input to the 1N4004 is the original relay ouput. Do I need the diode? What purpose is it serving?

2) When the input relay closes, the cap in the pic is discharged. What effect will that have on the power supply voltage? Seems like that's going to be a very sudden very large load. I'm thinking there needs to be a resistor between the diode in the pic and the cap/1k junction, or in place of the diode. The on time will be large (tens of seconds), so there would be plenty of time to charge the cap. Any on-time slew (time delay) between input relay on and SSR on, if only a few seconds will be acceptable.
 
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ljcox

Well-Known Member
RELAY OFF DELAY
This circuit turns ON a relay when the input is above 2v and the relay turns OFF after 2 seconds when the signal is removed. The OFF delay can be increased or decreased.
This circuit needs a diode across the relay coil to protect the transistor from the back EMF.

A diode will reduce the back EMF to about 0.7 Volt.
 
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ljcox

Well-Known Member
In the case of a well designed circuit where the transistor is turned on & off quickly - Yes.

In the case where the transistor is turned on & off slowly - Probably not.
 
I've gotten to the point to implement this circuit and found the current required to drive the transistor (2n2222a) to be atleast 0.5mA, which is now pushing the size of the capacitor to large values (>4700uF) for the off time delay required. I was thinking a darlington setup may help this situation. I can withstand the increase in Vbe. Based on prelimiary testing it is reducing Ib to less than 0.1mA. Are there any other trade-offs that I'm missing?
 

alec_t

Well-Known Member
Most Helpful Member
Are there any other trade-offs that I'm missing?
If you replaced the BC547 with a suitable NMOSFET your cap could have a very low value.
 

ccurtis

Well-Known Member
I would expect you to have much more improvement than an Ib reduction to 0.1mA from 0.5mA using a Darlington pair. Be that as it may, in addition to the trade-off of increased Vbe, there is also an increased collector-emitter saturation voltage of approximately 0.7V over a single transistor.
 
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That's what I meant. I meant Vce not Vbe (although I know that increases as well, it not as much of a factor).

I haven't tried to lower IB any further. I was starting to thinking about cap leakage losses, along the lines the reason why very high R and C values don't work well on the 555 chip.
 
I think point got missed. A stright NPN is drawing too much current throught the base. I have to have a minimum of 0.5mA which limits Rb. With Rb limited, I can only play with C, but C was getting to large. I have some Vce I can give up, so I was thinking of a darlington configuration. I should be able to reduce Ib way way down. But I'm wondering what else I'm giving up or I have to watch out for.
 
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