are you sure it's thyristors you're interested in? thyristors are for switching AC voltages.
You may want to use a MOSFET - these can switch DC. I recently used a N-channel MOSFET, a P30N06LE worked well for me... these can handle 30 amps at 60 volts with a proper heatsink! They only drop 1 volt across the source/drain which is quite nice.
Have a look on google for simple MOSFET circuits! there are millions
And if you DO want to switch AC voltage... use a triac