I just got a DC-DC step-up converter for my laptop, and I want to make double sure in case it malfunctions it will never affect my laptop. I am looking for an extra security device to prevent the voltage from getting higher than 19v. What would be the easiest and most efficient way to do that?
A crowbar circuit is one way- it's (essentially) a Zener plus a thyristor. Once the Zener voltage is exceeded enough current flows to the gate to turn it in, and it "shorts" the power supply blowing the fuse (or depending on inherent current limit of the supply). The Wikipedia entry shows a nice circuit that uses the synthetic Zener TL431 chip and a triac, which can have excellent unadjusted trigger point accuracy.
A power supply from a reputable supplier will have over voltage protection of some kind (OVP) built in, most likely. Using a garbagey one on an expensive laptop would be inadvisable, in my opinion. Personally, I use the OEM power supply and an inverter, which gives a lot more protection at the cost of some inefficiency.
I have seen in the past a hefty zener diode between plus and minus. It prevents the growth of the voltage by starting to take plenty of current. One volt marginal is enough. Have 2 diodes in series, if needed, for the right voltage. There must also be a fuse which breaks the circuit when the zener diode really starts to take current.