How to convert 7i77 outputs to sinking type?

Hello!

In my Gildemeister lathe all relay inputs need to be connected to ground for activation.

So, how to correctly convert 7i77 sourcing outputs to sinking type?

Maybe is it possible to do with a single powerful transistor for each output?

P.S. My relays takes about 26mA at 24-30V when activated, and have protection diodes.

just connect one side of the relays to +24v ( which they probably are ) and the other to the 7i77 gpio pin

The 7I77 outputs are sourcing only so its awkward to drive systems pre-wired for sinking outputs

Options are:

Re-wire your relays so they have a common ground
(If you just change common +24V to common ground the protection diodes will need to be reversed)

Use a external transistor/base resistor to make a sinking drive output (messy if you have many channels)

Use a 7I72 for sinking outputs (actually if you need many sinking outputs this is probably the best choice)

I don't want to change existing wiring on relay board (it is very complicated, and i need compatibility with old brains of machine).

Bying a new card will take too much time, and i don't have enough place in my connection box.

And i need to change only 16 outputs (or even 13).

So, it is not a problem for me to cut 16 wires and solder 16 mosfet's with resistors - is that correct idea?



If i'll use powerful N-chanel mosfet in TO-252 body and 10K pull-down resistor?

I would not cut any traces or bother with individual MOSFETS

I would do this: get a couple ULN2002As

Wire 7I77 24V output --> ULN2002A input, ULN2002A output --> relay
ULN2002A E --> field GND, ULN2002 COM to +24V (redundant if your relays have diodes)

one ULN2002A provides 7 channels so 2 will do your 13 relays

ULN2002As are available in DIP packages so can be socketed
for easy replacement and easy hand wiring

PCW wrote:

I would do this: get a couple ULN2002As

Thank you for advice!

But way with MOSFETs is also possible?

Yes MOSFETs will work but if you use 24V field power (suggested if inputs are 24V)
you will need a voltage divider on the MOSFET gate since 24V is too high:


7I77 output --> resistor1 pin1 --> MOSFET gate, resistor1 pin2, resistor2 pin1
MOSFET source --> resistor2 pin2

a 2/1 divider should be fine say 2x 4.7K

PCW wrote:

Yes MOSFETs will work but if you use 24V field power (suggested if inputs are 24V)
you will need a voltage divider on the MOSFET gate since 24V is too high:


7I77 output --> resistor1 pin1 --> MOSFET gate, resistor1 pin2, resistor2 pin1
MOSFET source --> resistor2 pin2

a 2/1 divider should be fine say 2x 4.7K

Thanks again!

Today i've tried IRF540 with two 1K resistors - works fine!