Simple Diode Reality Check

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Apr 2002
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Just a bit northeast of nowhere
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I really hate smoking components, so I thought I'd get a few opinions on this circuit I've drawn.

Can you confirm if it acts as I describe?

circuit2.JPG



With the switch closed :

1. If A is positive and B is negative, relay 1 is off, relay 2 is on

2. If A is negative and B is positive, relay 1 is on, relay 2 is off

If the switch opens power is applied in either case, the flyback diode of the active circuit will short out the reverse emf and spare the switch.

Yes? No? Maybe? It depends? (Hi Terry)


TM
 
Last edited:
Leadfoot said:
What exactly are you trying to accomplish with this?

I've seen this technique used for simple remote control of two functions using only two wires. Usually low voltage (24V) over small gauge wire. e.g. one pair of telecom multicore. The relays need to have sensitive low power coils if any significant distance is involved.
 
Nothing so elaborate I'm afraid :)

I have a 24 VDC motor controller that can reverse the motor on an input signal. However, the controller does not have enough output power to run the motor at full speed, although my 24 vdc supply can provide it.

So, I'm using the reversable output of the dc drive to switch a pair of relays in the manner shown. The relays will connect the motor leads directly to the power supply, by way of suitable breakers, for full speed performance without the current limits of the weaker drive.

My concern in this instance was protecting the low-amperage output from the original DC drive from the flyback of the coils, while also ensuring that only one coil turned on, depending on the polarity of the output.

Thanks guys! I feel better now.

TM
 
You don't need to worry about supressing back EMF. A motor has more than a relay. Protection is built in.
To simplify your circuit (2 diodes instead of 4), connect the 2 relay coils in series with the flyback diodes back-to-back (or front-to-front). One diode will always be forward biased, bypassing the coil. I belive your motor controller will be happier with this circuit as well.
 

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