Driver for 2 coils latching relay

[iw1au]

Hi to all!
I found a nice relay for RF, with great insolation crosstalk, that I wish to use for my 1.2 GHz band. This relay is double coil 12 V latching, one to be dedicated to TX and the other for RX. I have fortunately the 2 signals TX and RX that remains stable until change of status (if RX is ON, the signal is 12V, while TX is at 0V and viceversa).
The relay absorbes 250 mA that it's not the case to keep supplied, after commutation!

So I need a very simple circuit (no CMOS or IC, possibly) based on RC networks, transistors and ausilliary relays, if needed, in order to generate a positive pulse of 50/100 ms for both coils, syncronized to a simple positive (0 -> 12V) rise front step, keeping in mind that these signals are able to drive the necessary current for the time required directly to the coils.

I hope to have been sufficient clear, being not my mother language!
Thanks for attention.
Gian M., IW1AU

 

[wp100]

Hi,

You could probably do all that with just two cheapo chips.

A 556 dual timer where each timer, connected up in Monostable mode, is triggered by the rising edge of the +12v control signal and generates the 100ms pulse.

This pulse is passed to the high current relay driver chip, say a unl2002/4 or 2802/4 which has all the diodes built in so no other parts needed.

 

[iw1au]

Hi,

You could probably do all that with just two cheapo chips.

A 556 dual timer where each timer, connected up in Monostable mode, is triggered by the rising edge of the +12v control signal and generates the 100ms pulse.

This pulse is passed to the high current relay driver chip, say a unl2002/4 or 2802/4 which has all the diodes built in so no other parts needed.

Sorry for late answer (business trip!).
It seems a really good start! :idea:
I did some fast checks and I noticed that capability of 555 is up to 200 mA (not so far from my need, taking into account 50 ms or less only!) and the 555, in Monostable mode, when powered on, do a single cycle! So the "power line", when activated, gives via 555 the requested pulse... only to check now if 50 ms are sufficient to burn 555!!!
By the way a simple buffer transistor could solve the issue!
Other ideas are always welcome!
Gian M.

 

[wp100]

Hi,

Well if your relay is rated at 250ma , it will probably draw at lot more than that at switch on, so the 555 will surely burn out.
I always rate the current of a part at twice what its expected to pass, so for your 250ma the relay drive chip can handle 500ma per leg

Yes you can use a transistor but you will need a resistor from the 555 to the transistors base and then a diode across the relays coil, so that an extra 6 parts, if you use the relay driver chip it just the one part - so much simpler.

 

[iw1au]

Hi,

Well if your relay is rated at 250ma , it will probably draw at lot more than that at switch on, so the 555 will surely burn out.
I always rate the current of a part at twice what its expected to pass, so for your 250ma the relay drive chip can handle 500ma per leg.

To be honest I am a little bit surprise; why a "discharged" inductor should ask more current at the beginning? This is for capacitance, as far as I know...

 

[jiripolivka]

Hello there: For a similar task I used a SPDT toggle switch which discharged a 1000 uF capacitor through one of the coils. The capacitor was trickle-charged from 12 V power supply through a 10 kOhm resistor. You can use a 1 kOhm resistor for a faster switching.
Also I used IRF520 "logic" MOSFET instead of the toggle switch: it opens by TTL input and presents ~10 milliOhm resistance. In this case, coils are connected from +12 VDC through the MOSFET to GND. You can generate the alternating TTL controls from a suitable 74XX chip.

 

[frankliner]

Something like the attached driven by your tx and rx 12V signals might work. You may have to buffer the signals driving the caps.

 

[iw1au]

Something like the attached driven by your tx and rx 12V signals might work. You may have to buffer the signals driving the caps.

Sorry for late reply, but you gave me an idea, using a simple capacitor... I have to find an half of hour to test with real relay and, if it works, I will post the results...