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zener question

Started by unk April 25, 2018
I have an early version of a ham radio mobile supply that can't take more 
than 14.5 volts.  Later versions used different transistors good to 16v  
that sell for big $.

Could I wire in a 14.something-small zener to a big power resistor to gnd  
- and if so

would it cost less
what size of zener and resistor would the panel suggest

On Wednesday, April 25, 2018 at 11:45:34 AM UTC-4, unk wrote:
> I have an early version of a ham radio mobile supply that can't take more > than 14.5 volts. Later versions used different transistors good to 16v > that sell for big $. > > Could I wire in a 14.something-small zener to a big power resistor to gnd > - and if so > > would it cost less > what size of zener and resistor would the panel suggest
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.
On Wednesday, April 25, 2018 at 10:45:34 AM UTC-5, unk wrote:
> I have an early version of a ham radio mobile supply that can't take more > than 14.5 volts. Later versions used different transistors good to 16v > that sell for big $. > > Could I wire in a 14.something-small zener to a big power resistor to gnd > - and if so > > would it cost less > what size of zener and resistor would the panel suggest
What are you dealing with here ? A DC/DC convertor ? If you are talking about using a Zener to clamp the voltage, that is impractical 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, not to mention a bunch of wasted power. If you really need to clamp the voltage an SCR crowbar circuit is much better. It is innocuous when not activated 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 all the electronics don't get destroyed by voltage overshoot of the alternator should a connection to the battery open up suddenly. Normal;l;y the battery 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 crowbar 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.
On Wednesday, 25 April 2018 16:45:34 UTC+1, unk  wrote:
> I have an early version of a ham radio mobile supply that can't take more > than 14.5 volts. Later versions used different transistors good to 16v > that sell for big $. > > Could I wire in a 14.something-small zener to a big power resistor to gnd > - and if so > > would it cost less > what size of zener and resistor would the panel suggest
you could. I wouldn't suggest doing it though. Exactly what do you want to achieve? NT
On Wed, 25 Apr 2018 09:38:40 -0700, jurb6006 wrote:

> On Wednesday, April 25, 2018 at 10:45:34 AM UTC-5, unk wrote: >> I have an early version of a ham radio mobile supply that can't take >> more than 14.5 volts. Later versions used different transistors good >> to 16v that sell for big $. >> >> Could I wire in a 14.something-small zener to a big power resistor to >> gnd - and if so >> >> would it cost less what size of zener and resistor would the panel >> suggest > > What are you dealing with here ? A DC/DC convertor ? >
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.
> If you are talking about using a Zener to clamp the voltage, that is > impractical 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, not to mention a bunch of wasted power. If you really need > to clamp the voltage an SCR crowbar circuit is much better. It is > innocuous when not activated 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 all the electronics don't get > destroyed by voltage overshoot of the alternator should a connection to > the battery open up suddenly. Normal;l;y the battery 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 crowbar circuit is activated it will not burn up the power > supply.
Thanks, this sounds simple and cheap and if I understand the term "clamp" correctly it will do what I want.
On Thursday, 26 April 2018 00:25:29 UTC+1, unk  wrote:
> On Wed, 25 Apr 2018 09:38:40 -0700, jurb6006 wrote: > > > On Wednesday, April 25, 2018 at 10:45:34 AM UTC-5, unk wrote: > >> I have an early version of a ham radio mobile supply that can't take > >> more than 14.5 volts. Later versions used different transistors good > >> to 16v that sell for big $. > >> > >> Could I wire in a 14.something-small zener to a big power resistor to > >> gnd - and if so > >> > >> would it cost less what size of zener and resistor would the panel > >> suggest > > > > What are you dealing with here ? A DC/DC convertor ? > > > > 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. > > > > If you are talking about using a Zener to clamp the voltage, that is > > impractical 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, not to mention a bunch of wasted power. If you really need > > to clamp the voltage an SCR crowbar circuit is much better. It is > > innocuous when not activated 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 all the electronics don't get > > destroyed by voltage overshoot of the alternator should a connection to > > the battery open up suddenly. Normal;l;y the battery 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 crowbar circuit is activated it will not burn up 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
On Wednesday, April 25, 2018 at 7:55:23 PM UTC-4, tabb...@gmail.com wrote:
> On Thursday, 26 April 2018 00:25:29 UTC+1, unk wrote: > > On Wed, 25 Apr 2018 09:38:40 -0700, jurb6006 wrote: > > > > > On Wednesday, April 25, 2018 at 10:45:34 AM UTC-5, unk wrote: > > >> I have an early version of a ham radio mobile supply that can't take > > >> more than 14.5 volts. Later versions used different transistors good > > >> to 16v that sell for big $. > > >> > > >> Could I wire in a 14.something-small zener to a big power resistor to > > >> gnd - and if so > > >> > > >> would it cost less what size of zener and resistor would the panel > > >> suggest > > > > > > What are you dealing with here ? A DC/DC convertor ? > > > > > > > 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. > > > > > > > If you are talking about using a Zener to clamp the voltage, that is > > > impractical 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, not to mention a bunch of wasted power. If you really need > > > to clamp the voltage an SCR crowbar circuit is much better. It is > > > innocuous when not activated 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 all the electronics don't get > > > destroyed by voltage overshoot of the alternator should a connection to > > > the battery open up suddenly. Normal;l;y the battery 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 crowbar circuit is activated it will not burn up 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.
On Thursday, 26 April 2018 01:38:06 UTC+1, George Herold  wrote:
> On Wednesday, April 25, 2018 at 7:55:23 PM UTC-4, tabby wrote: > > On Thursday, 26 April 2018 00:25:29 UTC+1, unk wrote: > > > On Wed, 25 Apr 2018 09:38:40 -0700, jurb6006 wrote: > > > > > > > On Wednesday, April 25, 2018 at 10:45:34 AM UTC-5, unk wrote: > > > >> I have an early version of a ham radio mobile supply that can't take > > > >> more than 14.5 volts. Later versions used different transistors good > > > >> to 16v that sell for big $. > > > >> > > > >> Could I wire in a 14.something-small zener to a big power resistor to > > > >> gnd - and if so > > > >> > > > >> would it cost less what size of zener and resistor would the panel > > > >> suggest > > > > > > > > What are you dealing with here ? A DC/DC convertor ? > > > > > > > > > > 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. > > > > > > > > > > If you are talking about using a Zener to clamp the voltage, that is > > > > impractical 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, not to mention a bunch of wasted power. If you really need > > > > to clamp the voltage an SCR crowbar circuit is much better. It is > > > > innocuous when not activated 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 all the electronics don't get > > > > destroyed by voltage overshoot of the alternator should a connection to > > > > the battery open up suddenly. Normal;l;y the battery 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 crowbar circuit is activated it will not burn up 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.
I've no idea what you're talking about there
> Well the first question is, > (I don't know the Heathkit HP13) > what's regulating the low voltage power supply now? > > George H.
>"I just want to keep anything over 14.something-small getting to the power supply. "
If you are talking in a car, bear in mind that the voltage can go as high as about 15.5. That is only when it is very cold out and the car is first started and the battery is in a certain state of charge, then it tapers off quickly. 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 drain. Then you need a cathode fired SCR that can handle at least twice the expected current drain. (will probsably be a TO-220 and will not require a heatsink) 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 the 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 other end of that Rx goes to the gate(G) of the SCR. Another Rx goes from the gate(G) to the cathode(K) of the SCR. The Zener should be a ¼ watt, a higher wattage part might have enough 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 course you could use a circuit breaker but most of those can only stand so many trips. Rx can be anything from about 220 ohm to 3 K or so, depends on what 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 calculate the leakage of the Zener and add it to the (A) to (G) leakage of the SCR (very low) and pick a value just low enough to prevent a nuisance trip. then you could calculate the power dissipating of the Zener for the uSecs. it will be conducting and set that resistor to limit to exactly its power dissipation. But none of that is necessary, 1 K, 2 K, 330, 470, it really doesn't matter. And none of them have to be high wattage either. Standard ¼ watt will be more than enough. Like falling off a log, and so simple it could be built right inside the unit. 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 shorting out. A XXXXXX rig for sure but manufacturers do just as bad. Zip ties, duct tape, goop, they do it all, so it doesn't have to be pretty. If external you need some sort of case for it.