# Preventing a possible 12v short circuit

#### smilegr8

Dec 30, 2021
9
Good evening everyone !

My very first post here
I was wondering if somebody can spare few min to have a look at the diagram / sketch I have attached and advise how to solve the issue I have .
Basically I need to power a 12v liniar actuator via 2 different timer switches , one switch has the outputs reversed so the actuator can retract

1 timer switch will be on for 60sec at 14:30 and the other timer switch will be on for 60 sec at 21:30
A short circuit is certain if somehow both timer switches will be on at the same time ( let say one switch fails )

Happy to pay for a coffee if somebody can solve the mystery for me in a simple way as i'm a total beginner

Happy 2022

#### Attachments

• switch-problem.jpg
124 KB · Views: 15

#### Minder

Apr 24, 2015
3,318
Two suitable interlock relays?

#### smilegr8

Dec 30, 2021
9
Two suitable interlock relays?

Kind regards
Iurie

#### Bluejets

Oct 5, 2014
5,985
The way how I see I think you have the diodes will not work.
Cannot see actual diode polarity in your so-called diagram.
Most here don't work for beans....

Dec 30, 2021
9
thanks

#### AnalogKid

Jun 10, 2015
2,670
That timer has + and - inputs and + and - outputs. When the timer is *off*, is there any continuity between the input and output terminals? For example, are the two - terminals connected all the time, and the timer breaks only the connection between the + terminals?

ak

#### smilegr8

Dec 30, 2021
9
That timer has + and - inputs and + and - outputs. When the timer is *off*, is there any continuity between the input and output terminals? For example, are the two - terminals connected all the time, and the timer breaks only the connection between the + terminals?

ak

Good evening- yes I think so at least that's what i can understand from the diagram below .
Thank you Iurie

#### Attachments

• Screenshot_20211230-223528_Chrome.jpg
226.2 KB · Views: 12

#### crutschow

May 7, 2021
511
If both timers are on at the same time, there won't be a short if they are applying power from the same 12v supply.

#### Bluejets

Oct 5, 2014
5,985
If both timers are on at the same time, there won't be a short if they are applying power from the same 12v supply.
There will be if the output is wired as in #1.

#### smilegr8

Dec 30, 2021
9
If both timers are on at the same time, there won't be a short if they are applying power from the same 12v supply.
Yes both timers will have the same source however for the actuator ( dc motor ) to travel in both directions second timer has the output polarity the other round

#### smilegr8

Dec 30, 2021
9
That timer has + and - inputs and + and - outputs. When the timer is *off*, is there any continuity between the input and output terminals? For example, are the two - terminals connected all the time, and the timer breaks only the connection between the + terminals?

ak

Thanks for asking about continuity, considering that two terminals are connected all the time - my already poor designed diagram is a total failure
I will probably go for a manual DPDT momentary switch instead

Regards Iurie

Last edited:

#### crutschow

May 7, 2021
511
I will probably go for a manual DPDT momentary switch instead
Sorry I didn't properly understand your requirements, thus my incorrect previous response.

You can use two 12Vdc SPDT relays in a bridge configuration, activated by the timers to control the actuator as shown below:
It will not cause a short even if both relays are on at the same time (it would just apply +12V to both actuator connections which means it's OFF).

Last edited:

#### smilegr8

Dec 30, 2021
9
Sorry I'd didn't properly understand your requirements, thus my incorrect previous response.

You can use two 12Vdc SPDT relays in a bridge configuration, activated by the timers to control the actuator as shown below:
It will not cause a short even if both relays are on at the same time (it would just apply +12V to both actuator connections which means it's OFF).
View attachment 53737

Thanks Crutschow

I ll print it out and ask my neighbour ( electrician) to read it for me before i mess it up : )

Happy 2022 !

#### Tha fios agaibh

Aug 11, 2014
2,241
I always pick relays with a normally closed contacts so they can be interlocked like this.

Dec 30, 2021
9

#### crutschow

May 7, 2021
511
I always pick relays with a normally closed contacts so they can be interlocked like this.View attachment 53738
I think that's essentially that same circuit as my post #12.

The diodes in my circuit are to prevent contact arcing and erosion from the high reverse transient voltages otherwise generated by the relay coil and actuator inductances.

Last edited:

#### crutschow

May 7, 2021
511
I ll print it out and ask my neighbour ( electrician) to read it for me before i mess it up : )
If you aren't aware, the lines in the schematic represent wire connections, and a dot on the wire indicates the two wires are connected together. The two wires can be connected together at any convenient point in the physical circuit (such as a relay terminal).

#### smilegr8

Dec 30, 2021
9
If you aren't aware, the lines in the schematic represent wire connections, and a dot on the wire indicates the two wires are connected together. The two wires can be connected together at any convenient point in the physical circuit (such as a relay terminal).
Thank you !

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