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| bd139:
Disagree entirely. Crowbar is important when the following equation is true: cost_of_load > cost_of_power_supply A year or so ago I had what i consider to be a fairly decent quality supply (HP) blow the reference zener. This caused the output to bang itself at the collector supply voltage. You should plan for such failures. In such circumstances it doesn’t matter if the supply blows up. That’s what a warranty is for. |
| Fungus:
Moral: Use batteries with protection circuits on them. (...and (b) car battery chargers cost about $20) |
| bd139:
Or just stuff a diode in series. :-// Farnell E series supplies used to blow up if the load drove the supply. This happened when you were charging an SLA off one and the mains went out. Blew the driver transistor and the pass transistor. |
| bdunham7:
--- Quote from: Fungus on March 03, 2021, 09:18:47 pm ---Moral: Use batteries with protection circuits on them. (...and (b) car battery chargers cost about $20) --- End quote --- I can't agree when we are talking about general purpose lab PSU's, especially those that might be used in an educational setting. The one I have can be connected to a battery with the power on or off and in forward or reverse polarity without damage. If I hook up the battery backwards, turn the PSU on and turn up the voltage and current to max, it will simply discharge the battery at its rated current. No smoke and no fans. Of course it weighs and costs more than a Kaiweets model and doesn't put out as many amps. A proper, robust lab PSU shouldn't blow up, period. For more protection of an expensive load device, a separately adjustable crowbar with adequate fusing before and after it would be OK. |
| Kleinstein:
Some supplies have a fuse for the crow bar. So the fuse hopefully blows before the rest of the circuit gives smoke signs. As the crow bar is often a really beefy SCR this may actually work, as SCRs are relatively robust. Quite a few supplies also have a kind of weak sink capability - so if the voltage is higher than it is supposed to be, they would first sink a moderate current of a few 10 mA, usually with not much harm. This way the ouput capacitor would drain if you turn down the voltage with no load. Many designs have a diode, so that if backpowered, the source from the input would just provide power to the supply circuit. Unless to high nothing bad happens - draining the battry meant to charge. A reversed battery is usually problem, because there usually is a diode to prevent significant reverse voltage. This is especially needed with 2 supplies for a +- supply and than the current lmit engaging on one side. Usually the same fuse could engage as with the crow bar (if present). For the KS supply the point may be that the 1 A channels shoud have reverse diode good for at least 6 A (the other 2 channels combined) - not every 1 A supply has this. |
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