Re: single coil latching relay


Bo wrote:
"Didi" <dp@xxxxxxxxxxx> wrote in message
news:1140202979.813965.95880@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Bo wrote:
<cbarn24050@xxxxxxx> wrote in message
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With both drivers on there is 5 volts to each side of the coil.
Isn't
there a diode from the coil to the driver?

It would not be a problem - you do need the diodes to 5V and to
GND at both sides of the coil, but they will only conduct when
"discharging" the relay coil.
However, using open drain outputs and resistors is an archaic
approach. You do get HCT chips capable of driving both sides
with enough current for small relays for decades now, and you
do get sot-23 individual buffers which are small enough
to hide beneath any relay I have seen (Teledyne included) for
about a decade.

Dimiter

Yes I had it drawn wrong, and yes it does need some extra diodes. A
driver chip is the best solution but the op wanted something cheap and
easy.


I'm confused.

Where are extra diodes needed? and why?

Paul

If your relay coil is between two buffers, you need two diodes
at each side of the coil. One of the diodes has its cathode connected
to the coil and its anode to GND, the other has its anode connected
to the coil and its cathode to +5V (this circuit at both ends of the
coil).
A pair of these diodes will conduct only while discharging the
coil - which pair depends on the direction you have been
driving the coil before switching off.
If you do not understand why the diodes are necessary I
strongly suggest you just put them there, this will save you
burned buffers etc. Perhaps looking up something like "flywheel diode"
could help (I have not tried Google on it, though).

Dimiter

Didi,

Thanks for the extra details. I understand the need for the diodes that have
their anodes tied to the coil. I do not understand the diodes between coil
and ground (with anode=gnd, and cathode tied to coil). In what circumstances
are these diodes forward biased? The word 'flyback' catches my attention. Is
this the 'std' diode that goes across a coil to snub inductance spikes from
the coil?

Paul

Paul,
when the diode with its anode to the coil is forward biased (the
one you understand), the diode at the other side of the coil
with its anode to GND (cathode to coil) is also forward biased,
thus closing the current path.

Dimiter

------------------------------------------------------
Dimiter Popoff Transgalactic Instruments

http://www.tgi-sci.com
------------------------------------------------------

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