Problem driving 24V relays with P8055-1 USB card

Kits PC Related Projects
1

Hi everyone,

I’m trying to use a P8055-1 with 2 x 24V relays to control a large 24V motor to drive it either forward or backwards depending on 2 outputs from the board. I did this and blew up up the ULN2803 chip on the output side - but I can’t see why! My relays have got protective diodes in them. Any ideas much appreciated!

Neil

2

Do you know how high the coil current of the relay is?
Should be no problem if less than 100mA.

If the relays have internal protective diodes, then the polarity of the coil must be correct.
In other case the 24V is connected through the diode to the K8055 output.

3

Hi Vel255,

The coil is max 75ma, but it’s actually taking just 65ma.

I’ve checked the coil polarity, and yes it’s correct.

I can’t think what the problem is. I had a very similar circuit with 12V which worked fine. Is there any issue with 24V?

Many thanks,

Neil

4

The 24V should not be an issue. The ULN2803 is specified for 50V output voltage.
onsemi.com/pub_link/Collateral/ULN2803-D.PDF

You may try using the internal clamp diodes of the ULN2803.
Connect the relay coil between the CLAMP terminal and the digital output terminal.

799×410

5

Hi,

Thanks for the suggestion. I had another suggestion on another forum which seems to have solved the problem. I’ve put a couple of diodes to stop any current driving back up into the UNL2803 - but actually, it could also have been a problem the the particular motors I was using. It’s the first time I’ve experienced any problem with the board so I was quite surprised. Anyway, this seems to have solved the problem, but thank you for your help anyway.

Best regards,

Neil