# Convert 12 v timer circuit to 24 v

#### DonMuncy

Joined Dec 18, 2011
20
Apologies in advance. I erroneously posted this in the digital circuit design section. Sorry.
I have a circuit that when powered, activates for about 10 minutes and shuts off. It is powered with 12v, which charges a 470uf capacitor. The capacitor discharges through a 47.5k resistor through a Darlington pair of transistors. I need to adapt it to operate on 24 volts and do not have the knowledge to determine what components and values would be needed.
Any help would be appreciated.

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#### ScottWang

Joined Aug 23, 2012
6,881

#### WBahn

Joined Mar 31, 2012
25,544
Assuming you are now using a relay and lamp that are suitable for 24 V operation and your cap is also suitably rated, you may find that it is still close enough to ten minutes for your purposes. Try it and let us know how long the light stays on.

#### Dodgydave

Joined Jun 22, 2012
9,064
The time constant will be be the same, just change the relay to 24v, and the bulb also, make sure C1 is 35V or 50V rated.

#### MisterBill2

Joined Jan 23, 2018
5,733
If the circuit is actually like the drawing then it does not work at all. It would work if both of the lines shown connected to the battery were on the positive side, and the battery negative was connected to the ground symbol point. Then, there are other considerations. First, the relay will need to either be changed to a 24 volt relay or else have a series resistor with a value equal to the relay coil resistance put in series, so as to keep the relay current at the proper value. Next, the capacitor does need to have a working voltage above 30 volts so that it will not be subject to overvoltage breakdown failure. Probably the 100 ohm resistor that is used to limit the charging inrush current should be increased, I suggest 220 ohms because that is a standard value. The last consideration is the 4.7K ohm resistor that sets the delay time. Since the capacitor will have close to twice the voltage while the voltage point where the relay switches off will not change very much, the time will be quite a bit longer with the same capacitor. So if you need to change the capacitor anyway, substitute a 220MFD one.
An alternative, if it is convenient, which I doubt, would be to put a 12 volt zener diode in series with the whole circuit. But probably you will need two of them, one for the relay and another for the trigger input.

#### MisterBill2

Joined Jan 23, 2018
5,733
And now a suggestion for the moderators: How about adding a date of the original post so that all can see if it is an old post as soon as we look at it? Just a suggestion as I seem to be finding a lot of old posts today.