Zener Follower - what did i do wrong? - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 2nd March 2014, 11:48 AM   #11
diyAudio Member
 
Join Date: Nov 2013
Quote:
Originally Posted by Mooly View Post
Why not use a power FET instead of a bjt ? More rugged, the gate drive draws essentially no current.
Mainly due to bad experiences with them..
I would argue about that "rugged" part, lol. Blew a few from fiddling and went over the max Vgs threshold. Should've put that gate-source protection zener..

Looking at my sand stash, i have IRFZ44N. Good thing is that it's TO220 and with G-D-S pin means it's easier to put in the TIP122 place (B-C-E pins). I'm just worried about the Vdss rating of 55 volt. I can imagine easily going over this at start up: Vd = 180v, Vg = 0v (waiting for the capacitor to charge) which means Vs also 0V.. so Vds = 180v. Clearly not OK? Or is it? I guess i have to get at least IRFP250?

Quote:
Originally Posted by Mooly View Post
a true regulator with error correction.
Thanks for the circuit suggestion. I'm not sure i follow how it works, especially the values of the base voltage divider. The 2N5550 will start to conduct at 12v + Vbe = 12.7v and this combined with voltage divider of 14.7K and 120K mean it takes about 116v at the mosfet source for 2N5550 to conduct and limit Vgate? Quite far from 150V.. Or am i doing it wrong?

Also, i guess i can get away with skipping R2 and C2? The PCB is already crammed as it is. Adding another bulky high voltage capacitor is not what i want.

Last edited by ballpencil; 2nd March 2014 at 11:54 AM.
  Reply With Quote
Old 2nd March 2014, 11:57 AM   #12
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
You need a much higher rated device than a 55v part.

The regulator is a classic "text book" implementation. Your description is spot on its me that somehow had it in mind it was 120 volts output you wanted. You just alter the divider ratio to get the output voltage you want or replace R4 with a 22k preset and set it exactly.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 2nd March 2014, 01:15 PM   #13
esgigt is offline esgigt  Netherlands
diyAudio Member
 
Join Date: Dec 2013
Quote:
Originally Posted by Mooly View Post
a true regulator with error correction. The addition of three components, the extra transistor and two feedback resistors create a proper regulator. The zener becomes a 12 volt one. The two resistors and caps to the left reduce the ripple component. The bits to the right are just for testing.

You can see how much ripple I've added to the unregulated input and how well the circuit reduces this under load.
If I where you, this is the schematic I'd go for
  Reply With Quote
Old 2nd March 2014, 01:32 PM   #14
diyAudio Member
 
Join Date: Nov 2013
Might as well ask about the zener. Am i really that way off for the zener current?

180v unregulated - 156 zener voltage = 24 volt over the limiting resistor.
24 volt divided by 2K2 resistor gives about 11mA.
The 120V zener is ZY 120, a 2watt type which means i should aim for 2/120 = 16mA.
The 36V zener is C36PH, a 1 watt type giving Iz-max = 1/36 = 27mA.
Am i that far off? Do i really need to aim close to the maximum zener power dissipation to get good regulation?

Oh by the way, i haven't tested the circuit using full load. I use 100K resistor for the load. So it's only about 1.5mA of load current.. but i don't think this would cause the zener regulation to overshoot?

Quote:
Originally Posted by esgigt View Post
If I were you, this is the schematic I'd go for
I would too, if i can find 2N5550 tomorrow.
  Reply With Quote
Old 2nd March 2014, 01:37 PM   #15
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by ballpencil View Post
Might as well ask about the zener. Am i really that way off for the zener current?

180v unregulated - 156 zener voltage = 24 volt over the limiting resistor.
24 volt divided by 2K2 resistor gives about 11mA.
The 120V zener is ZY 120, a 2watt type which means i should aim for 2/120 = 16mA.
The 36V zener is C36PH, a 1 watt type giving Iz-max = 1/36 = 27mA.
Am i that far off? Do i really need to aim close to the maximum zener power dissipation to get good regulation?



I would too, if i can find 2N5550 tomorrow.
Listen

That circuit is a basic workable example. The FET type is fine, the transistor just happens to be in the LTspice library. You can use any general purpose type as long as the ratings are OK, particularly Vce. A 2n5551 is a common device, and an MJE340 should be fine too.

The zener isn't actually critical at all because you can just alter the divider ratio to suit. So 8.2volt zener, 12 volt, 30 volt even. All can be made to work. Four or five milliamps is usually plenty for a typical small zener.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 2nd March 2014, 01:40 PM   #16
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
No you should look at the zener test current, not its power dissipation.
  Reply With Quote
Old 2nd March 2014, 01:59 PM   #17
diyAudio Member
 
Join Date: Nov 2013
Quote:
Originally Posted by Mooly View Post
Listen
I believe you from the start
I just want to know why my 120V + 36V series zeners give such bad voltage regulation so i know what NOT to do if i ever need to do something similar. Are zeners really that sensitive about the current?

Quote:
Originally Posted by paulb View Post
No you should look at the zener test current, not its power dissipation.
Hi paul.. it's not that i don't believe you.. but what you're saying contradicts what i read here.
Here's from the article
Quote:
One strategy for designing a Zener regulator circuit is to design for a maximum power dissipated in the zener diode when the regulator is open circuited.
It's actually the 120V zener that is quite far. I measured it gave approx 130v.. while the supposedly 36V zener gave 38-39 volt for a total of 168-169 volt.
  Reply With Quote
Old 2nd March 2014, 02:09 PM   #18
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
I seem to recall from the dim and distant past, that for best stability diodes that were around 3.9 to 4.7 volts were preferred this being the critical voltage point at which zener diodes became more correctly "avalanche" diodes. We use the term zener for all of them but its not strictly speaking correct. Devices have improved over the years though.

A problem with your original circuit is that there is no error correction, its just a simple series pass regulator that sees the output voltage vary a little under different load conditions... which isn't necessarily a problem at all.

Why not test some zeners sorry, avalanche diodes yourself. Rig up a DVM up and try them from a cold start, testing with different currents up to their rating and write down the cold and hot voltage. Try it with several different voltage devices.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 2nd March 2014, 03:16 PM   #19
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
Quote:
Originally Posted by ballpencil View Post

Hi paul.. it's not that i don't believe you.. but what you're saying contradicts what i read here.
No it doesn't. Your article just describes how to calculate a minimum resistor value based on maximum zener power when the load is removed. There's a maximum resistor value too, based on ensuring there is enough current through the zener for it to regulate. A good start for "enough current" is to use the device's test current.
  Reply With Quote
Old 3rd March 2014, 06:39 AM   #20
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Post 10 has the right idea. This is for a headphone amp so low noise should be a priority, in which case you should definitely scrap the noisy Zener. IMO
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Hot follower/cool follower webercarbmann Solid State 13 2nd November 2010 02:23 PM
Zener + Emitter Follower voltage regulator Alcaid Solid State 90 1st June 2010 11:12 PM
Wrong Voltage / current on BoZ... or Wrong Measuring ? gionag Pass Labs 8 14th May 2008 11:24 AM
Question about direct coupling a anode follower into a cathode follower. G Tubes / Valves 45 29th July 2004 06:47 PM


New To Site? Need Help?

All times are GMT. The time now is 03:35 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2