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Back emf protection on a single coil latching relay

post #1 of 6
Thread Starter 
As the title says, how would you provide back emf protection on a relay if it's a single coil latching, so that the coil polarity is reversed to change states ?

I'm really confusing myself thinking about it, as what I keep thinking up would pretty much put a short across the coil.
post #2 of 6
I think what you'd do is to drive each end of the coil with a 'half circuit' and those halves would have the diodes in them for 'their' direction.

each buffer would drive 0 or 'high' volts and if one is 0 and the other is high (5v or etc) then you latch one way. swap them and it latches the other way.

2 buffers for each relay. plus you put both buffers at the same level after you latch (so the effective is 0 across them).
post #3 of 6
Thread Starter 
I sort of get what you mean... however what exactly do you mean by buffer ?

Would you by able to show me a schematic ?
post #4 of 6
post #5 of 6
Quote:
Originally Posted by splaz View Post
As the title says, how would you provide back emf protection on a relay if it's a single coil latching, so that the coil polarity is reversed to change states ?

I'm really confusing myself thinking about it, as what I keep thinking up would pretty much put a short across the coil.
This is quite simple but first I want to know which relay type you are going to use and your control signal.
post #6 of 6
Thread Starter 
Thanks for the help guys, I found to do it with a darlington array is slightly different to that schematic shown, have both sides of the coil at +5v in my case, then pull down whichever side you want to latch it to, emf protection is in the ic itself.

However when looking I've found something... more expensive, however better imo. MAX4820/4821, with serial/parallel interface. I've got quite a few relays and a microcontroller overseeing things, so that ic cuts down a lot on the amount of pins needed plus has the same advantages of a darlington driver in terms of more current to drive the relays, internal emf protection and isolating the relays from the microcontroller.
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