I built a very simple circuit: a digital AC wall timer that I plugged in a RadioShack 6V 1800mah transformer/adapter, then the adapter DC wires connect to a 6V DC motor.

Simple, right?

BUT. When the wall timer switches on the motor won't start. If I open the circuit (ie disconnect a wire), then close it (reconnect the wire), then the DC motor starts.

I've switched in a different transformer and the problem does not occur, so clearly it's the RadioShack transformer.

BUT WHY would this happen? Is it some kind of capacitor type or relay issue in the transformer? It makes no sense to me, just a simple, humble, mechanical engineer

The reason I strongly prefer the RadioShack transformer is it is 50% smaller than any transformer I've found for sale stock off the shelf. So if there is an easy fix to correct this non-start open/close circuit issue so I can use the RadioShack transformer that would be ideal.

Thanks for any help or advice!

 

Switched Mode wall plugs are poor in supplying surge currents required for starting a motor.
Try connecting a 1000uF 16 V capacitor across the wall plug output BEFORE connecting the motor.

 

It looks like you need to switch the output of the Radio Shack supply, not the input, for it to start the motor.
Once the supply is powered it apparently has enough capacitance on the output to start the motor, but it doesn't have enough oomph to start up the motor and also charge the capacitor from a cold start.