using zenners for a crowbar circuit

[m3vuv]

im building a crowbar over voltage circuit for my home brew psu,i want it to trip at about 15v,the issue is ive only got 18v zeners,am i right in thinking if i put 6 silicon junction diodes in series with the zener i should make it trip at about 14.4 volt as near as dam it?,cheers M3vuv.

[iMo]

If you put 6 Si diodes in series with your 18V zener it will trip at 18V+6*0.65V.
You have to fine tune the crowbar first, as the zener diodes do not have sharp voltage "knee". Also mind noise (some induced voltage peaks) may trip your crowbar as well.

[oPossum]

The TL431 "adjustable zener" is a really handy part to have on hand. You can make an adjustable crowbar with it. A schematic showing the fundamental concept is attached.

Many low cost small switching power supplies use them in the voltage regulator circuit. You may have something that you could salvage one out of.

[m3vuv]

sorry i just realised,its 12 volt zenners i have not 18,was wondering if diodes in series with the zenner will raise the zenner voltage,if so i suppose i can put a cap between the zenner and ground to absorb spikes but at the cost of speed of operation,also why im at it im using a 723 reg,can i use the voltage thats developed across the current sense resistor to turn on a transistor to light an led when its in current fold back mode,as a sort of cc indicator,as at when the voltage across the sense resistor reaches .6v is when fold back happens,im assuming when its in current foldback mode its constant current mode?.

[iMo]

The over-current protection will not save your radio from sudden overvoltage, which may happen when the power transistor you put on top of your 723 gets shorted.
I was experimenting with firing the crowbar via transil (unidirectional tvs) diode (14-18V) when messed with my radio power source, after some effort I got rid of the crowbar and I ended up with a single 1.5kW 15V transil (you may try with 1.5KE15A .. 1.5KE18A) wired across the 13.8V output connector..

[German_EE]

When a 723 regulator reaches the over-current stage the output voltage will start to fall, I don't think that this will be a true constant current supply.

I'm guessing from your username that this supply will be for a ham radio station, so make sure that you have 100n capacitors from both the positive and negative terminals to chassis and, if possible, use a filtered mains input. That way the RF has no chance of getting into the PSU. Depending on the frequency and power you will be using you may also need to pass the power cord and/or the output wires through a ferrite ring.

Finally, DO NOT test your power supply for the first few times using your transceiver as a dummy load. Wirewound resistors, car headlamp bulbs, even 230V electric heaters can be used in PSU testing but don't use anything expensive or the magic smoke will escape.

[m3vuv]

your totaly missing the point!!,the over voltage is for protecting my radio,the cc is for charging lithium ion batterys.