# Automatic electrical switch to between two paths

## Main Question or Discussion Point

I need a switch that will alternate the flow of electrical current between two diverging wires at a decided rate. Are there switches that function this way? What are they called?

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davenn
Gold Member
2019 Award
you could use a relay connected to a square wave oscillator would probably be the easiest
could be done with transistor switching as well using the same square wave oscillator

meBigGuy
Gold Member
The function you need is a single pole double throw switch. As dave said, that could be a relay, or it could be two MOSFET or transistor switches.

What sorts of loads are you switching (voltage and current).
What sort of control circuit are you using to produce the "decided rate" .
What is the frequency range and duty cycle of the decided rate?
How accurate does the duty cycle need to be?
Does there also need to be an OFF position?
Does there need to be a guaranteed off time when switching (time between loads being powered)?
Can there be overlap when switching load? Or is guaranteed non overlap required?
Does the circuit really need to operate upside down :)

We will probably think of a few more things that need to be decided as we continue.

sophiecentaur
Gold Member
Yes. A bit of (quantitative) context would help. In engineering, it's the numbers that count,

anorlunda
Mentor
The word demultiplexer also comes to mind.

sophiecentaur
Gold Member
The word demultiplexer also comes to mind.
Perhaps . . . . from kW to mW.

The function you need is a single pole double throw switch. As dave said, that could be a relay, or it could be two MOSFET or transistor switches.

What sorts of loads are you switching (voltage and current).
What sort of control circuit are you using to produce the "decided rate" .
What is the frequency range and duty cycle of the decided rate?
How accurate does the duty cycle need to be?
Does there also need to be an OFF position?
Does there need to be a guaranteed off time when switching (time between loads being powered)?
Can there be overlap when switching load? Or is guaranteed non overlap required?
Does the circuit really need to operate upside down :)

We will probably think of a few more things that need to be decided as we continue.
What I know:

The power source is a 12 v car battery, so the amplitude is unknown until I can test it , it needs to oscillate every .006 seconds (167 hz)so the switch needs accuracy to the thousandth of a second , duty cycle .5, duty cycle Should be as accurate as possible, there should be no time between the current changing directions, guaranteed non overlap is required, thanks for the help

berkeman
Mentor
oscillate every .006 seconds
That's too fast for a mechanical relay, so you need to use pass transistors. Can you say more about the application? What is the load current? Are the two load currents equal? What is the nature of the load -- is it inductive?

davenn
That's too fast for a mechanical relay, so you need to use pass transistors. Can you say more about the application? What is the load current? Are the two load currents equal? What is the nature of the load -- is it inductive?
The load current is 1.25 amps on both sides, the load is purely resistive

berkeman
Mentor
So you need to use 2 power transistors (probably power Darlingtons based on the current), with the base drive circuits based off of a square wave oscillator at your frequency. You will want to tune the drive circuits to make the current switchover smooth, with little glitching of the total current out of the battery. Are you comfortable taking it from here?

meBigGuy
Gold Member
I'd recommend PMOS power mosfets. Lower voltage drop, but driving power MOSFET gate capacitance can be tricky. The non overlap part can be tricky in either case (BJT or PMOSFET)

Look at circuits on google (2 phase clock generator) to create non-overlapping waveforms.

On the other hand, It would be a lot easier to use low side switching. then you can interface logic directly to NMOS or NPN transistors (Connect both loads to +12 and ground one or the other to turn it on).

http://reibot.org/2011/09/06/a-beginners-guide-to-the-mosfet/ show PMOS and NMOS switches.

berkeman
So you need to use 2 power transistors (probably power Darlingtons based on the current), with the base drive circuits based off of a square wave oscillator at your frequency. You will want to tune the drive circuits to make the current switchover smooth, with little glitching of the total current out of the battery. Are you comfortable taking it from here?
yes thank you