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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?

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    \$\begingroup\$ A crowbar circuit will do the job. The term comes from third-rail electrified railways: the linesman could switch off the supply by throwing his crowbar between the live and one of the running rails causing a short-circuit. \$\endgroup\$ – Transistor Dec 25 '17 at 9:29
  • \$\begingroup\$ That linked crowbar circuit doesn't work, it specifies a zener but claims it works like a diac, I'll go and fix it some time. \$\endgroup\$ – Neil_UK Dec 25 '17 at 9:43
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    \$\begingroup\$ I am pretty sure all laptops include such a protection circuit (they should). And I am pretty sure the PSU also include at least a zener at its output. \$\endgroup\$ – dim Dec 25 '17 at 10:11
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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.

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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.

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    \$\begingroup\$ MC3423 is also a handy crowbar chip. \$\endgroup\$ – filo Dec 25 '17 at 12:58
  • \$\begingroup\$ +1 but I have a feeling that triac/circuit would be a tad sensitive to spikes. \$\endgroup\$ – Trevor_G Dec 25 '17 at 13:26
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    \$\begingroup\$ @Trevor You'd want it to be at least as sensitive to spikes as the laptop power supply. \$\endgroup\$ – Spehro Pefhany Dec 25 '17 at 15:48
  • \$\begingroup\$ @SpehroPefhany yup and that's the catch.. \$\endgroup\$ – Trevor_G Dec 25 '17 at 15:50
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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.

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