latching relays and logic

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salomonander

Well-known member
Joined
Aug 28, 2011
Messages
913
hey

i have a question regarding bipolar/latching relays. im aware that one normally sends a pulse to either coil to active/deactivate it.
but how would i trigger one when having steady 5v logic outputs instead (say from a flip flop)?

on a regular realy id simply use a transistor to drive the coil. but since latching relays work with pulses im confused. could i run a steady voltage to either coil and switch by alternating between the two logic outputs? or would i need to convert my continous outputs back to a pulse somehow. if yes, how would i do that?
 
depending on the model, once you trip a latching relay, it does not matter if the trip coil remains energized or not,

as long as you do not pull in the un latch coil, the relay will stay latched via a mechanical lever inside.

once the lever is in place, you need to energize the un-latch coil to reset the device.

if the trip coil remains energized due to a non corrected fault,  then the un-latch portion of the relay will not work. the trip coil will pull in the relay again.

with a regular relay, once you energize the fault coil, the output section usually kills the fault, which means that the relay resets, and the whole cycle starts all over again,

you can hear it as mechanical chatter

the latching relay is designed to break this feedback cycle

you need to switch some open collector transistors with the 5 volts,

find Ib sat and wire it for that,

most relays are 24 volts, then 12, then 48, then 5, so it will be hard to find a 5 volt latching that will switch much of a load,

don't use fets, rely coils can spike into the gate,



 
If you have steady drive and constant power, use a NON-latching relay. They are more robust, and generally cheaper and more selection. (This is changing.)

If you must use a latcher, you can drive it steady IF it is rated for the continuous power. Read the sheet.

For pulse operation, drive one-shot monostable from your logic levels. While there are some very fine canned one-shots, it may be enough to use a cap to feed a transistor Base (and a resistor to bleed the cap-charge and transistor leakage).
 
you can get a Failsafe setup that will revert to the tripped state when power is removed,

you are worried about restoring power and not knowing if the relay is tripped, correct?

 
CJ said:
you can get a Failsafe setup that will revert to the tripped state when power is removed,

you are worried about restoring power and not knowing if the relay is tripped, correct?


thanks guys

no, actually that would be great - if the relays remain in the position they were in when the power was shut down (which should be the case right?).

one more thing though. if i want to trigger a single coil latching relay from one logic output i do need an h-bridge right?
i only have +5V outputs and if i understand correctly the single coil latching relays need a reversed impulse to switch. please let me know if this is wrong.

thanks. you guys are great!
 
Jacob,

With a single supply you will require H bridge.  However, with a micro you can steer it without the H bridge.

If you are thinking it for your console, then to be perfectly honest you are unnecessarily making things difficult for yourself. Just spend the extra money and effort on beefing up your power supply if power is the problem.

 
sahib said:
Jacob,

With a single supply you will require H bridge.  However, with a micro you can steer it without the H bridge.

If you are thinking it for your console, then to be perfectly honest you are unnecessarily making things difficult for yourself. Just spend the extra money and effort on beefing up your power supply if power is the problem.

Hey Sahib

thanks for the feedback. i guess you are right. my power supply isn't the problem. it has plenty of juice. i was thinking about keeping the power supply bills low (ill have  32 relays enaged at all time). plus i need to trigger both channels of each relay independantly. i did not find any non-latching relay that has two independant coils.
 
> i did not find any non-latching relay that has two independant coils.

Two independent coils (unless used to simplify latching) means two relays.

> keeping the power supply bills low (ill have  32 relays

So 64 relays?

IF they are on ALL the time, why have them? If on "most" of the time, wire them so no-power is the normal connection, power is the exception.

You can get a good small-signal relay with 50mW power consumption. 64*0.050W= 3.2 Watts. Figuring power supply efficiency as 50%, 6.4 Watts on the electric bill.

I pay $0.16 per KiloWatt-Hour. $0.000,16 per Watt-Hour. 6.4 Watts at that rate is $0.001 per hour, $0.73 per month or $9 per year if you *never* turn the thing off. If you run it 8 hours a day 5 days a week it's $0.43 a year.

Doesn't put much dent in the beer-budget. Does hasten global warming, but much-much less than the lights in the studio.
 
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