12 to 5 AC Rectification - Page 3 - 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 10th December 2012, 07:23 AM   #21
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Maybe.

The filament doesn't mind the "spiky" supply because of its huge thermal inertia. (In CRT TV's it's common to run a 6.3 volt heater with from a high frequency pulse of around 22 volts peak).

Are we stressing the transformer too much though. More current over only half a cycle.

Just ideas
Attached Images
File Type: jpg Capture 3.JPG (148.0 KB, 52 views)
__________________
-------------------------------------------------------
A simulation free zone. Design it, build it, test it.
  Reply With Quote
Old 10th December 2012, 07:25 AM   #22
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
Quote:
Originally Posted by woodrough View Post
Step down 7 volts using power resistors? That doesn't seem like a lot of energy waste to me... But this theoretically should work right? If I know that the tube rectifier filiment runs at 1.9A, I can put a resistor that stops down the voltage to be at 5V upon load. No capacitor hassle. Can someone verify if I can make that jump safely? (12V to 5v using only resistors)
It is inded a waste, but it is simple and safe if you dimension the resistor(s) properly.


Quote:
Also another idea came to mind:
What If I constructed a half wave rectifier to get half the voltage and try to smooth it out with a capacitor. And then deal with the remainder few volts by resistors.
Or am I wasting time trying to deal with diodes. Any harm to the transformer doing half waves?
That would be an abomination: it would magnetize the transformer, and it would buzz like mad.
If you smooth with a capacitor, it would be worst: the peak rectified output of a half wave rectifier is the same as for a full wave, it would simply create more stress on the components, the capacitor in particular.
Half wave could be a solution if you can balance it somewhere else with an opposite VA half wave.

The only realistic options for a wattless dropper are:
a) Conduction angle control (but it wil send harmonics all over the the place)
b) Inductive dropping. It can be made to work, but the reactor will have ~the size of a 12~14VA transformer, and it has to be carefully calculated (the air gap in particular)

Quote:
Originally Posted by Mooly View Post
I have one thanks... a tan δ meter... me and my tanδ meter
Me and my Tan Theta Meter (Capacitor tester)
I think your tester only works at 1KHz. The picture at 50Hz is quite different.
Quote:
I still think this could be made to work. In a hazy blur this was partly one reason why I kept the series trim resistor, to "lighten the work" the caps do.
Everything can always be made to work, but this doesn't mean it is a good idea.
Stacking two dozens of caps is a possibility, but not a very attractive one.
Quote:
My concern last night was trying to visualise the reactive load in combination with the inductive transformer winding. The voltage source in LTspice isn't inductive. I concluded that wasn't an issue in practice with the "real" resistive load.
The leakage inductance of the transformer will make things slightly worse, because part of the capacitive VAR you install will have to compensate for it before any dropping can occur.
Mixing reactive and resistive dropping is not very effective either, because the two vectors are in quadrature
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪
  Reply With Quote
Old 10th December 2012, 07:55 AM   #23
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
Quote:
Originally Posted by Elvee View Post
The only realistic options for a wattless dropper are:
a) Conduction angle control (but it wil send harmonics all over the the place)
b) Inductive dropping. It can be made to work, but the reactor will have ~the size of a 12~14VA transformer, and it has to be carefully calculated (the air gap in particular)
Actually, if you accept some complexity, there are other options.
The most obvious one being an SMPS 16V-->5V, but there are also cleverer possibilities, like voltage dividers AC or DC.
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪
  Reply With Quote
Old 10th December 2012, 09:42 AM   #24
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
Here is an example of voltage divider.

The spare volt can be wasted in an adjustment resistor.
Attached Images
File Type: png Vdivider.png (113.1 KB, 49 views)
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪
  Reply With Quote
Old 10th December 2012, 11:48 AM   #25
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by Elvee View Post
Actually, if you accept some complexity, there are other options.................
Of course... but complexity is the problem.

As I type this I have a 30Vac transformer feeding a 20 year old 270uf/400v cap in series with a 5 ohm 50 watt resistor bolted to a heatsink (the resistor not the cap ).

All is well and the resistor has around 10 volts AC across it (measured). So we're pulling around 2 amps. The cap must have a ripple current rating of no more than 300ma or so. I would guess after 20 minutes that it has gone to about 10 degrees C above ambient.

I think with suitable choice of cap this idea would be OK.

Some more "on the fly thoughts". If caps were paralled then maybe include a small resistor in series with each one. Hazy thoughts again... differing E.S.R. values with identical caps and all that...

There are plenty of caps with ripple current ratings over 3 amp.

Worth investigating I would say.
__________________
-------------------------------------------------------
A simulation free zone. Design it, build it, test it.
  Reply With Quote
Old 10th December 2012, 02:05 PM   #26
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
Quote:
Originally Posted by Mooly View Post
As I type this I have a 30Vac transformer feeding a 20 year old 270uf/400v cap in series with a 5 ohm 50 watt resistor bolted to a heatsink (the resistor not the cap ).

All is well and the resistor has around 10 volts AC across it (measured). So we're pulling around 2 amps. The cap must have a ripple current rating of no more than 300ma or so.
More like 3A probably
Quote:
I would guess after 20 minutes that it has gone to about 10 degrees C above ambient..
Drawing general conclusions from anecdotal evidence can be risky.
For example, you will easily find people convinced that a 400VDC rating is all that is required from a cap for across the mains applications, and they will provide ample evidence that it works, and that X-rated caps are useless.

Quote:
There are plenty of caps with ripple current ratings over 3 amp.
Indeed, but they also need to have the right value to drop the required voltage, and meeting the two conditions simultaneously will seriously narrow down the choice.
It is undoubtedly possible, but I am not sure it is advantageous.
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪
  Reply With Quote
Old 10th December 2012, 02:52 PM   #27
diyAudio Member
 
KatieandDad's Avatar
 
Join Date: Nov 2011
Location: UK
If you have an alluminium chassis then power resistors as simple voltage droppers are the easy option.

Personally I would go for a simple diode and cap to give you DC and then feed that through an LM317 regulator with a series pass transistor. You could use the CCS or slow start circuits around the regulator to slow turn on the heaters giving them a degree of protection.

I've never used reactive power supplies above a few mA. Undoubtedly the theory still holds true but to get 2A would take a very robust capacitor.

Last edited by KatieandDad; 10th December 2012 at 02:55 PM.
  Reply With Quote
Old 10th December 2012, 03:31 PM   #28
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Bear in mind that whatever solution you adopt must be able to cope with high voltage, as the 5AR4 heater is attached to the cathode (like most octal rectifiers).

If you really want to run from a 12V secondary, then I would try a diode plus resistor. The half-wave unsmoothed rectification turns the 5V 1.9A heater into a 7.07V 2.7A heater (but for only half the time) so the resistor only has to drop 5V instead of 7V so less power is wasted and less heat generated. The transformer should be OK, as the primary current will balance the secondary current. You will be drawing a little DC from your mains supply, but people do that anyway.

This form of diode dropper was used for the heater chain in some transformerless AC radio/TV sets, although then only for a 300mA chain.
  Reply With Quote
Old 10th December 2012, 03:33 PM   #29
diyAudio Member
 
Join Date: Aug 2012
So correct me if I am wrong with the V=IR. I want to take 12vac to 5vac. that is a 7 volt drop. When the heater to the rectifier draws 1.9 Amps in the circuit, that 7volts divided by 1.9Amps, I would get a Resistor value of 3.7ohms.
So power dissipated? 7 Volts x 1.9Amps = 13.3 Watts. Not the worst right? I can source some sturdy power resistors with a 20+ watt rating. I would call this a medium-hot worst case scenario.

Could some sort of X rated capacitor help this scenario out? Or is the amp rating the capacitor killer. Any way to bias more load onto the resistor than the feeble capacitor?
  Reply With Quote
Old 10th December 2012, 03:42 PM   #30
diyAudio Member
 
KatieandDad's Avatar
 
Join Date: Nov 2011
Location: UK
A 3R9 20W Power Resistor would do the job. I think they are actually 25W but it would have to be mounted to the chassis in order to dissipate that amount of heat.

http://www.ebay.co.uk/itm/Resistor-2...item5ae9692ee3

Last edited by KatieandDad; 10th December 2012 at 03:46 PM.
  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
12 VAC for both AC heaters and DC B+ via voltage doubler? MarkER Tubes / Valves 1 26th June 2012 08:04 AM
Need schematic for AC to DC converter (12V AC in to 12-18V DC out) maurycy Power Supplies 6 6th January 2012 08:27 PM
12 DC to 220v AC converter redhorseaudio Power Supplies 1 5th November 2005 04:53 PM
ac voltage after rectification raif Chip Amps 15 3rd June 2005 11:10 AM
12 volt DC to 9 volt AC eRiCdWoNg Parts 10 18th April 2004 01:52 PM


New To Site? Need Help?

All times are GMT. The time now is 03:04 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