Driving relays from a CMOS IC and vice-versa

[usbcd36]

Ok, here goes.

I need to drive some relays from the output of a CMOS SRAM IC. The relays are some little 6v armature things (Tyco TSC-106D3H), and the SRAM part number is IDT71016.

I know I need transistors to drive the relays (the IC can source 50 mA total), but how does one choose a transistor for a particular application? Also, I know I need diodes backward across the coils, but what kind?

For the reverse, switching the inputs via relays, I am planning on using a simple RC switch debouncing circuit with Schmitt trigger ICs (2 ICs of 8 Schmitt triggers each). The question is, what are typical values for the resistor and capacitor?

Finally, bypass caps: I have read that the typical value is 0.1 µF, but beyond that, what type of capacitor?

Oh, and obviously through-hole parts are preferable. This is going on protoboard.

 

[mvs sarma]

you can use BC547 with a base resistor of 1K and other side of the relay can be tied to 12V with a series resistor, instead of 5V
this would help prevent back emf spikes created by relay not to spoil the 5V line.

Better to use a fast recovery diode like FR106 with 1 amp capacity would do. It is better than using 1N400x series

 

[usbcd36]

FR106 diodes seem to be obsolete parts now, and most of the places I've checked want me to buy a few thousand, minimum. I only need 16! Is there an alternative part that's more commonly stocked now?

 

[mvs sarma]

Google for any fast recovery diodes, May be there are many types Perhaps SR and SF series are current from ST microelectronics
you may end up using 1N4007 itself
if you don't get better ones

 

[MikeMl]

Or even a 1N4148 or 1N914

 

[tvtech]

Also, I know I need diodes backward across the coils, but what kind?

AFAIK a 1N4007 is good to go to handle a Relay coils back EMF. No specialized fast switching diodes necessary.

And they are cheap too. Correct??

Cheers

 

[MikeMl]

The PIV only needs to be about twice what the Supply voltage is, so a 1N4001 would do. The current rating only needs to be about 1/2 to 1/3 of the current that it takes to pull-in the relay coil. The current through a normal automotive relay (12V coil, 85Ω) is about 140mA, so even a 100mA diode (1N4148) will take a peak current of 140mA which decays to zero in ~100ms.