Power supply RC (ohm's law) question -NEWBIE ALERT - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 25th August 2011, 09:14 PM   #1
diyAudio Member
 
Join Date: Aug 2010
Default Power supply RC (ohm's law) question -NEWBIE ALERT

OK,

Trying to get my head around this. I want about 100VDC out from a simple bridge and RC filter network with a 100v zener diode across the output as a sort of regulator. I'm stuck with this topology, because that is all the PCB is setup for. I have 120 volt transformer winding feeding the bridge. I have room on the PCB for another resistor (maybe) on the anode of the zener.

I can't seem to calculate the value of the R to knock down the 160or so volts after teh diodes to 100v. The tube plate and screen is drawing maybe 10mA, and the plate resistor is 100k.

Caps are 220uF.
Attached Images
File Type: png psuquestion1.png (2.9 KB, 143 views)
  Reply With Quote
Old 25th August 2011, 09:44 PM   #2
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
If you want the zener to provide any regulation, you need some current running in it. You can look up the recommended current in the datasheet for the zener diode, or figure that you'll need a few mA. So if the circuit this supply is driving is drawing 10 mA, then you need to size the resistor for a current draw of 10 mA plus the zener current.

So let's assume you want 5 mA in the zener. Note that at 100 V, this equals 100*0.005 = 0.5 W dissipated in the zener. It needs to be able to handle this.

Then as outlined above, use ohm's law. R = (Vin-Vzener)/I --> R = 60/0.015 = 4000 ohm (= 4 kOhm). I'd use 3.9 kOhm as that's the nearest lower standard value. If you want slightly lower current in the zener, go with 4.3 kOhm or 4.7 kOhm.

~Tom
  Reply With Quote
Old 25th August 2011, 09:44 PM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
You need to allow for the current drawn by the zener diode too. This is often 5mA minimum, but check the zener datasheet. If 5mA then the resistor will be 4000 ohms.
  Reply With Quote
Old 25th August 2011, 09:48 PM   #4
diyAudio Member
 
Palustris's Avatar
 
Join Date: Aug 2006
Location: Cape Cod
I was going to answer this but the numbers don't add up: you can't have a B+ of 100V with a plate load of 100k and 10mA going through the tube. It has to be 1mA going through the tube.
  Reply With Quote
Old 26th August 2011, 03:50 AM   #5
dgta is offline dgta  United States
diyAudio Member
 
Join Date: May 2011
Yup, that set of numbers ain't gonna happen. That pesky Ohm guy again
  Reply With Quote
Old 26th August 2011, 03:00 PM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Oops! I should have spotted that.
  Reply With Quote
Old 26th August 2011, 09:03 PM   #7
diyAudio Member
 
Join Date: Aug 2010
Thanks to all. The tube is a 5840 pentode. Screen is connected to plate. Internal resistance is 260k and the plate resistor is 100k, I thought that you paralleled the two to obtain 72k (Or am I thinking of something else?)

I took the 10mA figure from the data sheet for screen and plate current at average operating conditions.
  Reply With Quote
Old 26th August 2011, 09:40 PM   #8
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
You parallel the anode/plate resistance and the external resistor to get the AC output impedance. For determining the working point you just use the 100k. You have to work things out at your working point, not the one given in the data sheet, unless they are the same.

I doubt if the plate resistance will be 260k with the screen grid connected to the plate. This is triode connection, which has a much lower impedance.

More reading needed?
  Reply With Quote
Old 29th August 2011, 05:26 PM   #9
diyAudio Member
 
Join Date: Aug 2010
More reading, and less posting while traveling....:-) To recap, I should be use the 100k plate resistor as the load. From there it should be pretty straightforward to figure the value of R as indicated above.
  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
Help understanding a few Ohm's law calculations jmillerdoc Tubes / Valves 6 2nd January 2010 05:42 PM
Where is the error with this ohm's law calc? riotubes Parts 19 24th November 2006 05:33 PM
need some help with Ohm's law-fast! mr mojo Tubes / Valves 4 30th September 2005 10:55 PM
Ohm's Law 101 Steve Eddy Everything Else 49 21st November 2003 12:11 PM
newbie gainclone/power supply question wotan2525 Chip Amps 7 26th February 2003 06:25 PM


New To Site? Need Help?

All times are GMT. The time now is 05:30 AM.


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