They are little cheap switch-mode PSUs of reasonably poor design. I have one of these Powertech switch modes and I use it just for dew heater control and heat...nothing else. It it blows...I'll just toss it.
A far better and better and cheaper option is to use
a computer ATX power supply as a +12VDC supply. They offer far better protection & control of voltage rails...in this case +12V, are reliable and need little in the way of parts to modify and use. However I have noticed most computer PSU's you sometimes need to modify the +12V DC out with a low pass LR or LC filter to stabilise inrush current spikes on switch on.
FWIW