# Polarity change for linear actuator logic question

#### riccardo

Nov 21, 2015
2
I want to make a 5v~ touch plate trigger for a linear actuator that uses a relay to switch 12v for the actuator.

(Crude?) Circuit I drew...

How I want it to work...

Touch metal object that is connected to the base of a npn transistor which goes through a 'security stopper switch' (s1), then that activates the thyristor that latches, so now we have continuous current flow to a relay which is powering a actuator that will extend. It will extend and hit the "presumed limit switch built inside of it" which opens the thyristor due the interrupted current flow and stops current flow completely Everything works good!

Now I want the next time I touch the metal object for the linear actuator to go the opposite way, this is my problem. How can I get the polarity to be inverted without using a DPDT switch? Then when I touch it again for it to go back the other way, etc.

Please can someone explain to me how I can do this?

Thanks very much

- Ric

Last edited:

#### Alec_t

Jul 7, 2015
3,647
How can I get the polarity to be inverted without using a spdt switch?
You can use either a DPDT relay, an H-bridge, or dual power supplies.

#### riccardo

Nov 21, 2015
2
You can use either a DPDT relay, an H-bridge, or dual power supplies.

#### Colin Mitchell

Aug 31, 2014
1,416
Do you want to use two touch plates?

#### Minder

Apr 24, 2015
3,479
Just opening the gate on DC the SCR will stay latched?
Or is that 5vac?
In any case Q1 is redundant.
M.

Last edited:

#### GPG

Sep 18, 2015
452
Just opening the gate on DC the SCR will stay latched?
SCRs are available that will trigger on a few μA and will hold on a few mA
EG: http://www.digikey.com/catalog/en/partgroup/sensitive-gate-scr/16257
To reverse the motor will need 2 spdt relays.( or a bridge driver) The voltage drop across the SCR ca be as high as 2V so relays that have a reliable pull in at 10V (12V supply) are needed. How much motor current? relay current? Do you have limit switches in both directions? If there is a power failure in between limits do you want an override switch, or detect and drive to limit in one direction?

#### AnalogKid

Jun 10, 2015
2,922
With all unipolar power supplies and relay control, there are two or three basic ways to do this and all of them take at least two relays. Performing two different tasks (power control and direction control) takes two switches of some kind.

Separate from that - if S1 is built into the actuator and opens when it reaches the end of its travel, how can power be reapplied, let alone with a different polarity?

ak

#### Minder

Apr 24, 2015
3,479
SCRs are available that will trigger on a few μA and will hold on a few mA
According to the spec sheet they will still stay latched on DC unless it transitions to a very low level. Once triggered.
M.

#### GPG

Sep 18, 2015
452
Yes, his limit switch is in the wrong place.

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