# making a delay off timer

#### sdowney717

Joined Jul 18, 2012
711
Running into wall on these things but found one willing to make.
http://www.bowdenshobbycircuits.info/page2.htm

The description about the TC is a little chalenging to me

What I want to do is take the circuit on the right and make the resistor variable. What value ohms variability to get the delay off from 1 to 20 seconds?

I also have some NTE123 trans and lots of 330uf caps.

Can I use the NTE123 in place of theses suggested 2N2222? Circuit modify to use them?
Is the diode across the relay coil needed?
I plan to put 3 of the caps in parallel to make 1000uf.

his description below which having some trouble with

"The time delay for the common emitter will be approximately 3 time constants or 3*R*C.
The capacitor value for a 15 second delay will be 15/3R = 1327 uF."

Can someone explain this to me, how he comes up with the values?

Power-Off Time Delay Relay

The two circuits below illustrate opening a relay contact a short time after the ignition or ligh switch is turned off. The capacitor is charged and the relay is closed when the voltage at the diode anode rises to +12 volts. The circuit on the left is a common collector or emitter follower and has the advantage of one less part since a resistor is not needed in series with the transistor base. However the voltage across the relay coil will be two diode drops less than the supply voltage, or about 11 volts for a 12.5 volt input. The common emitter configuration on the right offers the advantage of the full supply voltage across the load for most of the delay time, which makes the relay pull-in and drop-out voltages less of a concern but requires an extra resistor in series with transistor base. The common emitter (circuit on the right) is the better circuit since the series base resistor can be selected to obtain the desired delay time whereas the capacitor must be selected for the common collector (or an additional resistor used in parallel with the capacitor). The time delay for the common emitter will be approximately 3 time constants or 3*R*C. The capacitor/resistor values can be worked out from the relay coil current and transistor gain. For example a 120 ohm relay coil will draw 100 mA at 12 volts and assumming a transistor gain of 30, the base current will be 100/30 = 3 mA. The voltage across the resistor will be the supply voltage minus two diode drops or 12-1.4 = 10.6. The resistor value will be the voltage/current = 10.6/0.003 = 3533 or about 3.6K. The capacitor value for a 15 second delay will be 15/3R = 1327 uF. We can use a standard 1000 uF capacitor and increase the resistor proportionally to get 15 seconds.

#### sdowney717

Joined Jul 18, 2012
711
I put this together and it works!
Powered with an old 12v car battery charger

I Used these parts

1 - 1N4007 single diode
1 - 2.2K resistor
3 - 330uf cap = 990uf
1 - relay with 156 ohm 12vdc coil

1 - NTE123

with 2.2k resistor power off delay is ~10 seconds
with 1.0k resistor power off delay is ~6 seconds

#### sdowney717

Joined Jul 18, 2012
711
further testing
only 2 caps is 4 seconds
only 1 cap is 2 seconds.

This will be perfect for my fuel pump issue on the boat.

I am thinking different relay coil resistances will affect the power off delay time?

#### BSomer

Joined Dec 28, 2011
434
Good to see that you got it worked out. You should use a diode across the relay though. This will help in protecting the circuit from spikes when the relay shuts off. When the magnetic field around the coil of the relay collapses there could be some large voltage spikes that can go back into the circuit and ruin the transistor or other components.

#### sdowney717

Joined Jul 18, 2012
711
does adding the diode across the relay affect the time delay?

#### Dodgydave

Joined Jun 22, 2012
11,318
does adding the diode across the relay affect the time delay?
No, it prevents back emf from the relay damaging the transistor.

#### ErnieM

Joined Apr 24, 2011
8,378
No, it prevents back emf from the relay damaging the transistor.
Which means STOP using this circuit until you add that diode as you are killing the transistor.

Don't use that transistor in your boat: get a fresh one that never saw the relay open without that diode.

Aside: I happen to have designed the delay timer inside some brands of MIL-PRF-83726-28 thru -31 relays used on commercial and military aircraft, so I know all about driving relays.

#### sdowney717

Joined Jul 18, 2012
711
I took a poor picture of my little project board but it gives a general idea.
black wire is ground
purple is the trigger
red is the relay and 12v positive
blue is the other side of relay.

I added the diode across the relay on the board. And used a new transistor.

I am using a standard automotive 40 amp relay which has an 83 ohm coil. So this will wire into the fuel pump relay control circuit already on the boat.
The case is from a broken Omron timer relay. For the little board I used the Omron board and removed all components and copper tracing. There are 2 little slots on this board which I passed the wires through which serve well as a strain relief.

The device is in the wheel house well protected from the weather.

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#### sdowney717

Joined Jul 18, 2012
711
I made the timer for the electric head flush pump. With the var resistor mini pot I can vary from 8 to 12 seconds which should be enough flush time. Raritan Lectrasan has a flush timer of 1 to 20 seconds with it factory set at 10 seconds. Minipot taken off old ATX PS, not enough resistance range to increase the delay time further. Would be nice to have one with range from 1 to 10,000 ohms. Would old VCR have those minipots?
Fixed resistor is 1K. Cap is 1000uf. All these parts salvaged from various old useless pieces of electric stuff.

It is important to have a resistor in series with minipot as if you turn minipot resistance to low, the NTE123 will be destroyed too much current flowing from base to emitter.

showing the cutup salvaged part of a board from an old broken PC power supply. Easy to drill new holes where needed.
I removed some copper trace by grinding, others by peeling off the board. Cut the piece off board using a grit cut off wheel.

fits well inside an old pill bottle. Will be mounted inside a dry cabinet in the head.
Will use a 40 amp auto relay.
Red wire to 12vdc pos and one side of relay coil
Black to ground negative
Orange to other side of relay coil
Green to trigger pump switch 12vdc pos.

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#### The Question

Joined Oct 16, 2009
1
I have a question. what is the purpose of the relay coil? is it just there to draw amperage?

#### KMoffett

Joined Dec 19, 2007
2,918
I have a question. what is the purpose of the relay coil? is it just there to draw amperage?
The relay coil is there to produce the magnetic field, when a current is passed through it, that pulls the relay's contacts together.

Ken

#### cajunsniper

Joined Jul 29, 2013
1
how can i make it delay 5 minutes
and how can i make it delay 45 minues

de 5 minutes is for a bathroom fan
and the 45 minutes its for the AC of my room
thanks

#### merzatt

Joined Jan 15, 2009
43
I hope someone will tell me what I am doing wrong.

I built the circuit on the right hand side using 1 kOhm resistor, 1000uf capacitor, NTE101 transistor and 12V tyco relay.

Connected 12V constant to the pin 86 bridged to the pin 30 of the relay. As soon as the power is on, relay turns on. When I connect the 12v signal to the base side of the transistor, nothing changes. What am I doing wrong?

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#### dannyf

Joined Sep 13, 2015
2,197
it is meant to be driven by a high output impedance source.

#### merzatt

Joined Jan 15, 2009
43
Sorry but I don't know what you mean. Could you explain what I need to do to make it work?

#### eetech00

Joined Jun 8, 2013
4,006
I hope someone will tell me what I am doing wrong.

I built the circuit on the right hand side using 1 kOhm resistor, 1000uf capacitor, NTE101 transistor and 12V tyco relay.

Connected 12V constant to the pin 86 bridged to the pin 30 of the relay. As soon as the power is on, relay turns on. When I connect the 12v signal to the base side of the transistor, nothing changes. What am I doing wrong?
How do you expect it to work?

#### merzatt

Joined Jan 15, 2009
43
I want to delay turning off the power for 2-6 seconds after the ignition switch is turned off.

I don't know how to connect the 12v and ground to the relay and circuit.

#### AnalogKid

Joined Aug 1, 2013
11,147
This thread is over 4 years old. You should start a new thread, state your question clearly, and describe what you have tried so far.

ak