Heat Bed not operating as I thought it would, electronics help needed.

I'm in the testing phase of building a new printer based on a RAMPS 1.5 / Arduino 2560 board set.

Firmware is based on the Marlin RC-6

I wanted a little larger than normal print area so I decided on a 8 1/2 x 12 1/2 heat bed.
So I made a separate PCB that contains a circuit that I though would control the heat bed
using an external relay. The heat bed should draw about 18/19 amps when it first starts
then settle down to 14/15a while heating (according to the manufacture).

I'm doing a step by step check as I go. In general everything seems to be working ok until
I get the heat bed circuit (Please see attached). As soon as I connect the D8 terminals to
the external circuit the RAMPS applies 14V to D8. This is without even the relay being
connected (it uses a socket). It only happens when the external circuit is connected.
I've checked the voltage at D8 both ways, with and without the circuit connected.
So at this point the only component in the external circuit is a diode. The circuit
showing 19A IN/Out is circuit in question.

Any suggestions, or things to try?

Is there something in the Firmware I could have missed that might cause this?

thanks

Don

Possibly solved.

It appears like there may be a conflict between the RAMPS board ground and the accessory board ground.
I still don't know how or why it turns the bed voltage on, but it does. So I will not be using that part of the accessory board.

Don