zener diode question

I need to come up with a zener diode that will handle 1.0 amp
of current at 12V. I currently am using two 6.8V 10 watt stud
mount zeners in series mounted with insulators on a large
heatsink but it's pushing it. When the current spikes to 1.0 amp the
diodes get pretty hot.
Since I have a bag of these diodes Would it be a problem to
employ four of them in a series/parallel arrangement to attain
13.60V at 20watts? I realize that the exact voltage that each
diode "clips" at might be a bit different but would that really
matter? It would be a simple job to simply add one diode in parallel
with each one presently there. Thanks, Lenny

 

Depends on exactly what the spec says, but...
You're asking a 10W diode to dissipate 6.8W. That shouldn't
be a problem if you keep the stud cool.
Mounting twice as many diodes on the same heat sink won't change
the heat sink
temperature much at all. You'll save half of the junction to case
thermal resistance in the package and the chip runs a little cooler.
But the package will still get "pretty hot" whatever that means.

At low current, a 6.8V zener has a slight positive voltage temp
coefficient, but the actual voltage may be all over the place.
I'd carefully match them at rated current.

Better solution would be a bigger heat sink or a fan.
Better insulators?
Two heat sinks and no insulators?

 

It is a lot easier to find and mount power transistor on a heat sink
than it is finding high power zeners. You can create a high current
zener this way.

But, I only use that type of circuit as an active clamp mostly for
protection, where the current sink will be high at times.

I was going to suggest this to Klem earlier but I seems from past
post and replies that he does not seem to always understand some
concepts and explanations of things. It's like quantum physics.

Jamie

 

"klem kedidelhopper"

** All you need is a NPN power transistor and a 6.2 V zener:

http://sound.westhost.com/appnotes/an007.htm

Got any old 2N3055s lying about ?

Easily make a 50W zener with one.



.... Phil

 

My question is why use shunt regulation in the first place.

Also true about the "pass" transistor, any transistor of suitable ratings will work and most likely help solve any thermal problems. It doesn't even matter if it is NPN or PNP. Works both ways.

The only reason to use shunt regulation is if you expect spikes which should be clamped. This can happen in an automotive application for example by just having loose or dirty battery cables, or any electrical fittings between the alternator (generator) and the load. This porblem is compunded these days by the fact that a modern fuel injected car with multiple coils (a coil pack)pulls a hell of alot of current to just run. Have any idea what a heated O2 sensor pulls ? what's more a car might have three or four of them. That is on top of an injector for every cylider maybe more.

The thing is I am am having a hard time imagining another situation where such a regulation scheme would be appropriate. If it's automotive then it is, but that has not been revealed.

 

Yes this is related to my post of several months ago. I employed
Arfa's suggestion of using two of my 6.8V 10 watt zeners in series to
make a 13.6 V diode. And from my previous discussion this was then
put in series with the rectified 24VAC, (39VDC) to drop the voltage to
the input of my 12V regulator to a safer level. These two diodes were
mounted with insulators and silicone grease on a large old black
aluminum heat sink that I pulled out of a 1960's Delco automobile
radio. With the diodes in the circuit the voltage to the input of the
regulator was 19VDC, and under full load it was 16.5VDC, which is just
about where I wanted it to be. The only problem is that the diodes as
well as the heat sink were getting extremely hot when the regulator
was fully loaded, (1.0Amp). In the interest of survivability
assessment for my test I ran the unit overnight. So I added more
aluminum and beefed up the contact of the heat sink to the mounting
surface. Now the heat sink runs cooler however the diodes are still
too hot to hold a finger on at full load. This surprises me. If I had
good contact with the heat sink shouldn't IT be warmer than the
semiconductor mounted to it? The thought now occurs to me that since I
had to use plastic insulators from the junk box between the diodes and
the heat sink they may not be providing suitable thermal conductivity,
and this could also be a problem. I tried to buy a couple of suitable
insulating diode mounting kits for DO-4 devices that would provide
good thermal contact with the heat sink but I couldn't find a source
for them. My distributor couldn't find any and NTE doesn't seem to
sell them separately. I remember in my previous post that you had
suggested a transistor arrangement last time Jamie, but you're correct
in that I didn't fully understand what you were referring to at the
time. Now that Phil has provided the schematic it seems quite clear.
Thanks Phil. I could certainly try that arrangement too but after
putting this all down on paper, (so to speak), now I guess I'm
wondering. What if thermal conductivity between my diodes and my now
massive heat sink is my problem? My junk box mounting hardware
certainly could be suspect. Perhaps I should try to obtain the correct
insulators and then reassess this heat issue. Does anyone have a
source for them? The diodes are 1N2970B stud mount. Thanks, Lenny

http://www.datasheetcatalog.org/datasheet2/a/0a1whup4cfhdaygr9zxyqtaki7yy.pdf

 

I've lost the original context.
If the transformer has the secondary on the outside, removing almost
half the turns is a better solution in several dimensions.

A heat sink that's actually sinking heat is always colder than
the device it's sinking.
Measuring the temperature is often more helpful than "extremely hot".
My calculator doesn't have a key for that.

Bottom line is that it appears that every part of your system
is unsuitable for the function it needs to perform. Hang them
together and you get something that sorta works. There is no
magic fix.

 

"klem kedidelhopper"

Now that Phil has provided the schematic it seems quite clear.
Thanks Phil. I could certainly try that arrangement too but after
putting this all down on paper, (so to speak), now I guess I'm
wondering. What if thermal conductivity between my diodes and my now
massive heat sink is my problem? My junk box mounting hardware
certainly could be suspect. Perhaps I should try to obtain the correct
insulators and then reassess this heat issue.


** WTF !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Wot sort of TOTAL BLOODY IDIOT tries to *insulate* the threaded stud
of a diode, zener or thyristor from the fucking heatsink ?

ROTFLMAO !!!!!!!!!!!!!!!!!!!!!



..... Phil

 

Probably the same sort of person who thinks about what he wrote
before pressing that send button. ;-)

 

"Arfa Daily"

** A TO3 device has a large surface area.

A DO4 stud device has almost none unless you include the thread.

So it is blindingly obvious that for best heatsinking, you screw them into a
threaded hole.

The heatsink may then need to be insulated from other metal work.

This is how all high power stud diodes zeners must be mounted, if you wish
to get anything like the maker's power ratings.

**** head.



.... Phil

 

"mike" <>


** **** off you BULLSHITTING bloody radio ham !!!!!!

 

"Wild_Bill"

** The difference is HUGE.



.... Phil

 

My god.

Jamie

 

He's clamping the supply to keep it just under detonation level.

I guess he isn't to concern about saving energy, the greenies would be
all over him if they found out!

At least he can keep himself warm in the winter!

Jamie

 

**The zeners are in SERIES with the load.

 

"Arfa Daily"

** No they wouldn't.

You fucking, pig ignorant moron.

" Ye canne change the laws of ffysiks... "


.... Phil

 

"No, they are in series with the load to act as a constant voltage dropper,
so they pass the full load current. The drop across them is slightly higher
when they are passing a high current.

Arfa "

Well excuse me, I am not familiar with the original thread so I didn't knowthat.

Now getting back to Lenny, there are so many better ways to do this it ain't funny. You know if you just take a regular old OPAMP to control a regulator transistor and put inductance at the output it is so easy to coax it into oscillation it ain't funny. That would be my actual solution. Shit, you can do it almost without math because you can design it to let it pick it's own frquency. What do you care the frequency ? Figure anything the devioes are comfortable with and be done with it. Hell, just eliminating the input filtering would allow you to make a quasi switcher out of it, with a littlebit of refinement you're done. Maybe I'll draw something up.

When you are as old as Lenny (and me) you have large inductors laying around so you do not need superfast switching MOSFETS, this shit can be done with a 2N3055.

Another thing is that this whole thing is wrong. There is too much head voltage and not enough balls (current) behind it. We are only talking one ampere here ? You want 12 volts just dig up an old AT type PC power supply. Nowthere's something I am sure you have laying around.

If you are dropping from 24 to 19 rectified, it must be one el cheapo transformer. Maybe get one out of a car battery charger. I know we like to hang on to our pipe dreams sometimes but you know that heat dissipated costs money.

Anyway, I should draw something up but that is a pain in the (_|_) these days becasue I have to go to another PC to scan it AFTER I draw it on paper. I can't do it on a PC, though I have only tried with regular graphics programs. I know there are some available for designing electronics and some areeven free, but I have thusfar been too stupid to figure out how to use them effectively.

It boils down to your basic OPAMP to be used as a comparator, that is easy.however what you have to do is to provide a positive AC feedback path frombefore the output inductor. You ALWAYS use an emitter resistor and readingacross that with a scope you set the frequency for the best efficiency. Actually with a large inductor and a cheap transistor that will be a rather low frequency. With a more expensive transistor and a cheaper inductor it will be a higher frequency.

You could even chop it unfiltered at the 60 or 120 Hz rate. (that's 50 or 100 for Phil because them people are 16 % slower down there). If you can design it you can even do it without an output inductor.

There are too many easier ways to do this which make this current design undesirable, along with the extremely shitty efficiency.

Damn, you could do it using a regular SCR and Zener diodes and then let thecurrent regulatior that comes after just act as an active filter. Old Sears/Warwick TVs did that. (so did others, at least hitachi that I know of)

Does anyone remember those ? The only difference here is that there is a power transformer.

 

On 4/16/2013 9:23 PM, wrote:


You want 12 volts just dig up an old AT type PC power supply. Now
there's something I am sure you have laying around.That can be made to work, but it's rarely the slam-dunk that you imply.
Think about how the regulation works. You'll figger it out.

 

"That can be made to work, but it's rarely the slam-dunk that you imply.
Think about how the regulation works. You'll figger it out. "

It would only be a slam dunk if the 12 volt line was the one being sensed by the regulation loop. Actually it would not be hard to modify it to make it so, but that would require taking it apart. I can think of no way to tie it in with the lower source to make it follow. The only other way would be for the error amp to cause a load on the sensed line, which is as bad as what we have now. I big mess. Inside the PS though, simple cutting a foil andadding the right resistor would do it.

But is it that critical ? a passive regulator runing like that is probably not all tha ttight anyway. Remember I missed the original thread so I don'teven know the purpose of this thing. All I know is it need about an amp. It certainly isn't for car stereos, what is it ?

 

"Years ago I found the easiest method for me to create schematics was using a
Paint (bmp) drawing program. "

I never figured out how to copy and paste in that damn thing. Same with Paintshop Pro. I didn't even try with Adobe, screw it.