Can I use the outputs on a PCI 6503 to switch a 24VDC relay?

[reg123]

I am using outputs 22 and 23 on a PCI 6503 to switch 24VDC relays with no problem. When I try to use outputs 8 and 11 to also switch 24VDC relays the relays do not fully drop out. I'm not sure why this is?

[Hal_L]

Hi,

The 6503 will output a 5V signal from the digital lines. I am curious as to what is happening when you use outputs 8 and 11 versus when you use outputs 22 and 23. Could you please provide some additional information about what is exactly happening? Also, could you try to drive the relays using other digital lines and let me know what happens?

Regards,
Hal L.

[Buechsenschuetz]

I do not know the output circuitry of the 6503. But on most, if not all, boards with +5V TTL level output it is dangerous to apply any higher voltage in any form. It is much better practice to use an extra driver transistor to drive a high-voltage relay, And it is even better to use a photo-MOS relay like the AQV212 from Matsushita.

Don't forget to switch a diode in parallel with the relay coil (of course in the reverse direction) to snub of high voltage spikes generated by the coil when switched off.