What kills solid state PN junctions is excessive power dissipation (heat). You can abuse the breakdown voltage of a PN junction as long as the power dissipated there is within the power dissipation range of the PN junction. That's how Zener diodes work and it is why they have a power dissipation rating. All diodes will function as low-current Zener diodes if you manage to provide a high enough reverse voltage across them to break them down and limit the current so the power they must dissipate won't destroy the PN junction. An ordinary 2N2222 can be used as a low-current 7V Zener if you reverse bias its base-emitter junction. Relay "kickback" energy is a function of the relay coil size, applied voltage, coil resistance, and relay coil core permeability (determines how fast the field can collapse thus the max voltage the thing can make with no coil current flowing). A very large coil will have a lot of energy stored and all of that must be dissipated when the coil is de-energized. A forward-biased diode must burn off that energy that is not dissipated in the coil winding resistance (and core losses). If there is too much power applied to the diode for too long, the diode junction will melt and then it electrically shorts or opens, depending on luck. The "schematic" of a diode is more than what we all see in application schematics. It has a tiny amount of resistance in series with it a capacitance in parallel with what's left. When conducting the tiny resistance is what allows the forward voltage drop to be greater than the otherwise fixed forward drop we see at DC. The junction capacitance is also drawing current until it's charged so that makes the small voltage drop across that unwanted tiny resistance a bit greater, as you can see in the scope shots in the Clifton Labs pics. The bottom line is that a clunky power diode is fine as a snubber for small 12 and 24VDC relay applications found in amateur radio gear. The coil energy is not large enough to melt the diode PN junction and the tiny series resistance of the diode is generally inconsequential. Placing a 1N4148 across a large relay coil is not wise since the diode is "fast" and that implies a tiny junction area that is easily melted. The PIV rating of the diode need only be greater than the DC voltage applied to the relay coil when it is energized since that is all the diode will see when it is reversed biased. The small differences in max forward voltages seen between various diodes is due to the tiny series resistance each diode has. Rick KC0OV (for now-just upgraded!) ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net
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