# Dual Power supply switch with 5-10 sec delay

#### CircutScoper

Mar 29, 2022
300
Yes, this and a couple other electronic forums are a good place to learn the basics along with some tricks.

For example, when I first started here I knew little about the 555, but there were many requests for timing circuits where the 555 was a good choice, so I learned more about it.
Once you understand that it is just a FF latch with level-sensitive set (TRIG) and reset (THRS) inputs, along with a pin (DIS) to discharge the timing capacitor when the reset input is triggered, it's then fairly easy to understand and design various types of timing circuits, such as the delay-on / delay-off circuit I used here.

Plus, the 555, unlike most FFs, has an output that can handle bi-directional load currents up to 200mA while including inductive kick-back protection. I'm not sure that last feature is mentioned in any of the datasheets, either.

#### CircutScoper

Mar 29, 2022
300
...
If not, I'll get to work and see if I can make what you laid out. Thanks again gentlemen and I think it's a perfectly decent way to spend a Saturday.

If you do, and just in case it wasn't obvious, you should go with crutschow's circuit. It's far superior to my pathetic attempt.

#### CircutScoper

Mar 29, 2022
300
...

If not, I'll get to work and see if I can make what you laid out. Thanks again gentlemen and I think it's a perfectly decent way to spend a Saturday.

Here's another idea. If constructing an electronic version of the circuit is too much of a pain, this stone-axe version might (emphasis on MIGHT) work adequately. The -ND numbers are DigiKey stock numbers. Total cost (sans shipping) is about $12. The (stupid-simple) operating principle is that coupling through the (ginormous) capacitor actuates RLY2, thus disconnecting power from the load, for 5 to 10 sec when B comes on and again when it turns off. Capacitor 495-76905-ND 22,000uF @16V$2.89
Relay PB1690-ND12V 360ohm 12A \$4.26

https://www.digikey.com/

Relay characteristics,,,
Must Operate Voltage 8.4 VDC
Must Release Voltage 1.2 VDC

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#### crutschow

May 7, 2021
517
The (stupid-simple) operating principle is that coupling through the (ginormous) capacitor actuates RLY2, thus disconnecting power from the load, for 5 to 10 sec when B comes on and again when it turns off.
It's brute force crude, but I like it.
The one caveat is that you have to make sure there is a path for the current when the B supply goes off that will turn on RLY2, for example if the B supply were simply just disconnected.

To avoid that possible problem, below is the simulation of a one transistor, one diode circuit, that rapidly pulls the voltage to near ground, even if the B supply is abruptly disconnected (here by switch S1) so that RLY2 will get the proper pulse from the capacitor.
(Since my relay models have a polarized coil and do not respond to a negative voltages, I had to use two relays in series to simulate a non-polarized coil that will respond to both the positive and negative pulse from the capacitor.)

#### CircutScoper

Mar 29, 2022
300
It's brute force crude, but I like it.
The one caveat is that you have to make sure there is a path for the current when the B supply goes off that will turn on RLY2, for example if the B supply were simply just disconnected.
View attachment 55077

Point taken. But until RLY1 drops out, the cap' is still connected to the load, which should (?) provide enough current to pull RLY2 in. Then the cap' is at least connected to ground through RLY1's coil. Given that must-release is only 1.2V, it's reasonable (?) to expect that the typical drop-out V is well below 6V, and we only need a fraction of 0.022F x 2 x 360 = 16sec to provide more than 5 sec of delay.

Hands waving rapidly.

#### crutschow

May 7, 2021
517
All sounds a little dicey by depending upon the load and where the relays actually drop out.

Note that 1.2V is the minimum, not the maximum voltage (which is not given) for the relay to open.
I would expect significant variation in that voltage, depending upon mechanical and spring tension tolerances.

#### CircutScoper

Mar 29, 2022
300
All sounds a little dicey by depending upon the load and where the relays actually drop out.

Note that 1.2V is the minimum, not the maximum voltage (which is not given) for the relay to open.
I would expect significant variation in that voltage, depending upon mechanical and spring tension tolerances.

All true. But don't forget that the OP's spec' wasn't exactly for timekeeping accuracy. It was ~+/-50%

A good rule to remembed (and live by): Better is the enemy of good enough.

#### crutschow

May 7, 2021
517
Certainly the TS could experiment with the simple capacitor circuit and see if it meets his needs.
If not then perhaps add the diode and transistor.

#### CircutScoper

Mar 29, 2022
300
Certainly the TS could experiment with the simple capacitor circuit and see if it meets his needs.
If not then perhaps add the diode and transistor.
Good enough!

#### NuclearOnion

Apr 28, 2022
5
If you do, and just in case it wasn't obvious, you should go with crutschow's circuit. It's far superior to my pathetic attempt.

10-4. All attempts here are superior to mine. Thanks guys. When I get it put together, I'll post back. Thanks again.

Mar 29, 2022
300

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