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
Closed Thread
 
Thread Tools Search this Thread
Old 17th February 2014, 07:05 PM   #1
HP8903B is offline HP8903B  United States
diyAudio Member
 
Join Date: Jun 2010
Default Capacitor Drain for Tube Bench Power Supply ?

I am building a 1200VDC 200mA bench power supply.

It has tube rectifier. I placed the variac (voltage adjuster) on the primary side of the power transformer.

What is the best way to automatically drain/discharge the capacitors ?
There are 4uf, 30uf, and 30uf caps.

If using a resistor between B+ and ground is the best way, what would be a good resistance value ?

Also, are there any problem with putting the variac on the primary of the power transformer ?

Last edited by HP8903B; 17th February 2014 at 07:15 PM.
 
Old 17th February 2014, 08:56 PM   #2
diyAudio Member
 
Join Date: Aug 2010
Location: Near Eindhoven, the sanctum of the pentode. and among that other things.
Send a message via Skype™ to v4lve lover
Beware that, as there are no rectifiers whit sufficient heater cathode voltage ratings, you will need an auxiliary transformer that has sufficiently high insulating properties.

Might I suggest insulating the transformer by putting a thin phenolic sheet between it and chassis.

I wouldn't go tube rectifier anyway. I would opt for multiple toroid s whit 230 vac out, you can find caps for 450V easily. Just stack the supplies. And add a choke on the low side of the supply.

George (Tubelab) wrote about that, I believe the transformers are called industrial control transformers.
 
Old 17th February 2014, 09:09 PM   #3
HP8903B is offline HP8903B  United States
diyAudio Member
 
Join Date: Jun 2010
I am using 2 6CJ3 damper diodes.
 
Old 17th February 2014, 10:10 PM   #4
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
You need to decide how quickly you want to discharge the caps, and how much power you are happy to waste in the resistor.

Be aware that a powerful PSU needs some serious design work if it is to be both safe and reliable.
 
Old 17th February 2014, 10:19 PM   #5
HP8903B is offline HP8903B  United States
diyAudio Member
 
Join Date: Jun 2010
I want a good balance between discharge speed and power waste.

Please make recomendation
 
Old 17th February 2014, 10:20 PM   #6
HP8903B is offline HP8903B  United States
diyAudio Member
 
Join Date: Jun 2010
Let's say around 10 seconds from 1000V to 0V
 
Old 17th February 2014, 11:27 PM   #7
diyAudio Member
 
aardvarkash10's Avatar
 
Join Date: Mar 2007
Location: Auckland, NZ
imo, if you have to ask this question, you shouldn't be fiddling with 1.2kv supplies.
__________________
"Folks, you can't prove truthiness with information. You prove truthiness with more truthiness. In a process known as truthinessiness." - Stephen Colbert, The Colbert Report
 
Old 17th February 2014, 11:38 PM   #8
Keit is offline Keit  Australia
diyAudio Member
 
Join Date: Feb 2014
Location: Australia
I concur with DF96 - a 1200 VDC supply @ 200mA DOES need some serious COMPETENT thought in order to be safe. DC is a heck of a lot safer than AC, but 1200V can still kill. More likely, the kick will cause your muscles to violently react, causing you to knock yourself and anything on the bench flying. Further, you need to think about the consequences of shorted rectifiers, wiring shorts, flasher-overs, and the like. If the capacitors are series electrolytic strings, you could have explosions. If the capacitors are series strings (to get the voltage rating high enough, AND the ripple current rating high enough)) you need shunt resistors anyway, to ensure equal distribution of voltage. The electros in any one series string need to be matched. Otherwise - explosions!

In fact, if you need to ask a question like this, you don't have sufficient competence.

Having said all that, firstly, the primary side is the best place for the variac - for a start it means a variac that you can actually buy.

Requesting a shunt resistor value to go from "1000 V to 0V in 10 seconds" is the wrong question. It doesn't work that way. The voltage in CR circuits decays exponentially - fast at first, then slower and then more slower. It is generallly considered that less than around 70 V DC is human safe, but to protect tools and semiconductors, you need it much lower. But start with 70 V anyway. To get 30 uF (which will give you approx 700V ripple at 200mA, full wave rectification of 50 Hz) from 1000 V down to 70 V, you need approx 3 time-constants of decay, CxR ~ 3 sec, i.e., R = 100 kohm. Power in the resistor at 1200V (ignoring ripple) will be 14.4 watts. This sensibly low compared to the power supply output of 1200x0.2 = 240 watts. Choose resistors 2 to 3 times bigger than the calculated dissipation, as if ever the resistor fails, you've got a SERIOUSLY dangerous power supply.

But the ripple means that you will need MUCH more than 30 uF, and that will mean intollerable dissipation in the ressitor. What I do in cases like this is choose a relatively high value of shunt resistor, to get from 1200 V down to 1000V in one second, plus a circuit that detects abnormally high ripple or voltage drop (ie a drop in voltage equal to just larger than the calculated ripple), and when it occurs, trip a shutdown that 1) turns off the AC supply, 2) puts a very low value resistor across the output, and 3) turns on a lamp to tell you it's now safe. Than protects humans, tools, and the supply itself from excessive loads and shorts, inside or external. It's important that the safe lamp turns on, not off, otherwise you could hurt yourself just because the lamp failed.

Keit

Last edited by Keit; 17th February 2014 at 11:54 PM.
 
Old 17th February 2014, 11:51 PM   #9
HP8903B is offline HP8903B  United States
diyAudio Member
 
Join Date: Jun 2010
The smooth stage is CLCLC.
All caps are rated at 1500VDV.
The 4uf is paper in oil.
The 30uf are oil with no pcb.
 
Old 17th February 2014, 11:52 PM   #10
HP8903B is offline HP8903B  United States
diyAudio Member
 
Join Date: Jun 2010
I am using an external variac.
 

Closed Thread


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
Bench Power Supply: What to look for ? Which one to get ? HP8903B Tubes / Valves 33 19th April 2013 03:31 PM
Bench Power Supply ilardi Power Supplies 1 14th September 2012 08:34 PM
capacitor voltage for tube amp power supply alecu7 Tubes / Valves 18 27th July 2012 09:31 AM
Great bench supply for tube work. gearheaddruid Tubes / Valves 25 21st August 2007 10:54 AM
bench power supply jethari Parts 1 14th March 2003 05:31 PM


New To Site? Need Help?

All times are GMT. The time now is 02:40 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