Using the ATtiny24A to drive 6 latching relays (spread out between pins 0 and 5 due to ATtiny's 40 mA limitation)
What kind of flyback should I use for this configuration?
I'm considering a bidirectional TVS diode and I would appreciate some tips about the right parameters.
(working voltage, clamping voltage, etc.)
Please post details of the hardware, ie datasheet for the relay(s).
You need 2 schottky diodes protecting every single pin, bolstering the tiny internal protection diodes up to
higher currents. Otherwise those protection diodes will probably burn out. I presume these are bidirectional
windings?
The easiest way to drive lots of relays is a chain of 74HC595 shift registers each driving an ULN2803.
For latching relays that means using two-winding versions, not single/bidirectional sort. Since ULN2803's
have inbuilt free-wheel diodes, this means 2 chips per 8 relay windings. Some variants on the 595 shift
register have built-in relay drivers and freewheel diodes, reducing that to 1 chip per 8 windings. You
also only need 3 Arduino pins to switch any number of relays.
relay datasheet attached. also here
https://www.digikey.at/product-detail/en/panasonic-electric-works/TQ2-L-5V/255-1004-5-ND/251776
I wired the whole thing up and it works just fine. turned the single-coil-relays back and forth for a million times and the ATtiny holding up ok. however, I know it is best practice to use flyback diodes, in order to protect the ATtiny from coil spikes.
mech_eng_tq-1299280.pdf (302 KB)
You are damaging your Arduino everytime you switch those relays. One day it will stop working and
you'll only have yourself to blame. Add those schottky diodes.
schottky means shift registers, right? Could I still switch the polarity?
but regardless, it would still be necessary to use flybacks parallel the coil relays.
I was just wondering if tvs diodes are the only option that I have (because of the polarity switching).
And how does one pick a tvs diode? (in terms of working voltage, clamping voltage, etc.)
"schottky diode" - have lower forward voltages than pn junction diodes, which is what is needed to
prevent overloading the on-chip protection diodes.
The shift register plus ULN2803 solution is different and the ULN2803 has its own free-wheel diodes,
but can only drive unipolar windings.
thanks for the tip!
I am aware of schottky diodes and their low voltage drop. but they do not let voltage in reverse. so my present relay circuit/code would not work.
I am looking however for a parallel diode (tvs diode?)
that will protect the ATtiny by shorting any voltage spike coming from the coil.
