zener diode question

[PEU]

tomorrow I'll need a 13v zener for a circuit I'm building ( http://feng3.cool.ne.jp/en/rcd.html ), its the only component I don't have, and I really dont want to go shopping for it... lazy me /ubbthreads/images/graemlins/smile.gif

my question is:

can I use a 12v zener and a diode in series (say a 1n4148) to aproximate its value? or a 8.2v + 5.1v zener in series?

thanks


Pablo

 

[Crux]

In my experience, yes either of these could work for the application. But measure what the Vpp turns out to be with your parts and check what Vpp should be for the PIC being programmed.

If I had a 13V zener I'd send it to you ... but I don't.

What are you programming???

 

[Doug Owen]

This should work fine, just be sure you put enough current through the forward biased diode to get 'well over the knee'.

FWIW, I've seen this done in high precision circuits, the temperature induced shifts in voltages are used to counter each other. Alternately, 5 Volt or so Zeners are put in series (TC is almost zero around 5 Volts IIRC).

Doug Owen

 

[PEU]

[ QUOTE ]
Crux said:
What are you programming???

[/ QUOTE ]

flashlights D'oh /ubbthreads/images/graemlins/smile.gif


jokes apart, I'm self learning pic programming.

My next NEOCA project will use one, not programmed by me this time, but maybe in the future /ubbthreads/images/graemlins/smile.gif


Thanks


Pablo

 

[MrAl]

Hi there Pablo and others,

Most zeners have a reverse characteristic that makes it look
like a forward biased diode. This means if you put a diode
in series with a zener you'll change the reverse character
to make it look like a near open circuit instead of a
foward biased diode. Connecting a second diode in parallel
with the series combo solves the problem quite easily however.
The cathode of the second diode goes to the anode of
the first, and the anode of the second goes to the
open end of the zener (once the series diode is connected).
If the voltage across the 'zener' reverses, the second
diode conducts.

This second diode is important when the reverse character is
used as a reverse voltage clamp. I was unable to determine
whether or not this characteristic is needed for this
particular circuit without studying it further.

Also, i'd use higher current diodes like 1N4004 or similar
type. Should be fast enough for this circuit.

Please let us know how you make out with it when it's
completed.


Take care,
Al

 

[Doug Owen]

Al,

I took PEU to mean he was running the zener in reverse breakdown (as a voltage reference), rather than a bipolar clamp. Therefore no third diode is needed, although watching polarity is as you point out important.

I'm also of the opinion that a switching diode (like the 4148) is a better choice than a 4000 series GP rectifier. The dozen or so mA likely easily fits the current range of the smaller part and the smaller, more precise, junction is sure to give us better stability at these ranges.

Doug Owen

 

[PEU]

I used a 12v zener + 1n4148 and it worked like a charm !

I lost almost an hour trying to figure what was wrong, and I found that the problem was a 1.5meter extension cable I was using, it seems the voltage drop didnt allow the pic to be programed.

I'm happy /ubbthreads/images/graemlins/smile.gif


Pablo

 

[MrAl]

Hi Doug and Pablo,

I didnt check the current of that circuit either,
so i went with the higher rated diode instead /ubbthreads/images/graemlins/smile.gif
If you checked it, that's great.

Pablo let us know what you do with the pic's when you're
ready to program some and use them...


Take care,
Al

 

[PEU]

1st thing is to use it for the CPF logger, then I'll start self learning PIC programming.

Will post my advances after I practice the basics /ubbthreads/images/graemlins/smile.gif


Thanks


Pablo