zener question

How much current? Are you just trying to regulate some voltage to ~14V? How about a three terminal voltage regulator? (LM317) What's the input voltage?

George H.

What are you dealing with here ? A DC/DC convertor ?

If you are talking about using a Zener to clamp the voltage, that is imprac tical unless the current is very small. Any current not used by the load is dissipated by the Zener and then you get into a stud mount and heatsink, n ot to mention a bunch of wasted power. If you really need to clamp the volt age an SCR crowbar circuit is much better. It is innocuous when not activat ed and actually more effective, it is also more reliable. The only place a high current clamping Zener might be used these days is in a car so that al l the electronics don't get destroyed by voltage overshoot of the alternato r should a connection to the battery open up suddenly. Normal;l;y the batte ry does the clamping and the filtering.

A small Zener, two resistors and a suitable SCR will be cheaper than a huge Zener needed to directly clamp that source. Also, if you can get into the power supply and clamp the regulator to put out zero volts when when the cr owbar circuit is activated it will not burn up the power supply.

Yeah, I didn't consider that he actually might want to regulate with it. I figured it would be overvoltage protection.

Whatever the case, we agree that a shunt Zener is not the way to go. Let's see if he gets back with more information.

you could. I wouldn't suggest doing it though. Exactly what do you want to achieve?

NT

Heathkit HP13. It's a smps that makes 750v at 250ma, 300v at 150ma, and

-130v at 20ma. The 13A had the higher voltage transistors.

I just want to keep anything over 14.something-small getting to the power supply.

Thanks, this sounds simple and cheap and if I understand the term "clamp" correctly it will do what I want.

Sounds liike you want an LDO voltage regulator.

NT

But it's from ~1-300V. Well the first question is, (I don't know the Heathkit HP13) what's regulating the low voltage power supply now?

George H.

I've no idea what you're talking about there

supply. "

If you are talking in a car, bear in mind that the voltage can go as high a s about 15.5. That is only when it is very cold out and the car is first st arted and the battery is in a certain state of charge, then it tapers off q uickly.

I have no image hosting so I am going to have to just describe the crowbar ckt. First of all fuse the whole thing at higher than expected current drai n. Then you need a cathode fired SCR that can handle at least twice the exp ected current drain. (will probsably be a TO-220 and will not require a hea tsink) The voltage should be fine, it would be hard to find one that can't handle 20 volts or less. The anode(A) goes to + right after the fuse, and t he power to the unit also comes from there. The the Zener anode goes to the anode of the SCR and its cathode(banded end) goes to a resistor, Rx. The o ther end of that Rx goes to the gate(G) of the SCR. Another Rx goes from th e gate(G) to the cathode(K) of the SCR.

leakage to nuisance trigger the crowbar. a 15 volt will have it trigger at about 15.6, a 14 volt at 14.6, your call. You have to buy the fuses. Of cou rse you could use a circuit breaker but most of those can only stand so man y trips. Rx can be anything from about 220 ohm to 3 K or so, depends on wha t you got. That is one of the least critical values in the world. They don' t even have to be the same. If you want to get persnickety you could calcul ate the leakage of the Zener and add it to the (A) to (G) leakage of the SC R (very low) and pick a value just low enough to prevent a nuisance trip. t hen you could calculate the power dissipating of the Zener for the uSecs. i t will be conducting and set that resistor to limit to exactly its power di ssipation. But none of that is necessary, 1 K, 2 K, 330, 470, it really doe sn't matter. And none of them have to be high wattage either. Standard ? ? watt will be more than enough.

Like falling off a log, and so simple it could be built right inside the un it. With an all plastic TO-220 SCR all you need is a hole to screw it down, wire it up and use goop to keep the wires from shaking and breaking. Or sh orting out. A XXXXXX rig for sure but manufacturers do just as bad. Zip tie s, duct tape, goop, they do it all, so it doesn't have to be pretty. If ext ernal you need some sort of case for it.

Forgot to mention the cathode(K) of the SCR is common negative to everything.

Do you really mean that? Nothing gets to the load unless the battery is below 14v or do you mean regulate higher voltages down to a safe 14v?

A 300W switched mode DC to regulated 14v output DC converter would do what you want efficiently but would be expensive to buy.

An alternative depending on just how much current you need to draw is a zener diode with a series pass power transistor that regulates the battery voltage down to something less than 14.5v or an LDO regulator.

The other possibility which is very simple if you are prepared to do the measurement each time would be a chain of series diodes in the PSU supply lead each dropping about 0.6v. Connect it up manually according to the actual battery voltage on the day.

Diodes intended for solar panel bypass* might be adequate and cheap. Back of the envelope your PSU could draw around 300W on full load so the diodes will need to be rated for >25A and dissipate up to 20W each! eg

• I am slightly amazed that these tiny diodes can stand such treatment.

Mid 1960s germanium, 250W, self oscillating, inverter PSU.

.... Phil

Sorry, I was reading the voltage signal path backwards. GH.

Thanks Phil, stupid question; couldn't someone hack in some big-ass pnp silicon transistors?

George H.

The normal method was to connect the power supply feed directly to the battery terminals, with a proper fuse.

Another question is, why on the earth somebody wants to run a tube radio on car power nowadays?

big-ass pnp silicon transistors? "

Big-ass ? I think I still have a couple of 2N5436s kicking around. How about those ?

The 16volt ones for the later version are available - at something over $30 each before shipping and import duties.

IC(max) too low. .

Umm, 60 amps is too low ?

OK.