Question about voltage multipliers

00940 · 11th November 2008, 10:04 AM

I know how to use basic voltage doublers, triplers and the like. But is there a way to multiply an ac voltage by 1.5 ? I need about 24-25VDC regulated out of a 18VAC xformer for a small load (about 50ma).

Thanks in advance.

Mooly · 11th November 2008, 10:29 AM

Best bet is a doubler and regulator for so small a current.
78L12 + 12 volt zener for example.

Frank Berry · 11th November 2008, 10:35 AM

Under such a light load, the transformer may put out somewhat more than 25 volts. If you're lucky, you may need nothing more than a bridge rectifier and a 7824 regulator. Measure the transformer secondary voltage with no load. Otherwise, I agree with Mooly.
Voltage doubler and a regulator.

00940 · 11th November 2008, 10:51 AM

Thanks. It seems like I have no choice...

- actually the transformer is also used for other stuff at 200ma/17VDC, so I don't expect it to put it a lot more voltage than what it's rated for. A quick check shows me about 25VDC after rectification and filtering.

- with a doubler, I can expect about 44VDC. That means about 20*0.05= 1W of dissipation for the regulator. That's why I asked about this.

Mooly · 11th November 2008, 11:05 AM

Could you perhaps play around with the value of the caps in the doubler --- using the reactance as a wattless dropper and so on --- and still have reasonable DC voltage for the regulator to work off without the ripple component being too high.
Would have to think about that.

Elvee · 11th November 2008, 03:41 PM

Hi,

Fractional voltage multiplier do exist, but they're probably too complicated for your application.
I would use a regular voltage doubler, but with a small input capacitor serving also as ballast, and a shunt regulator at the output.
A capacitive supply in fact.
The power that will be wasted is tolerable, and the complexity is minimal.
LV

lineup · 11th November 2008, 03:58 PM

One small 2x12 VAC,
<= 5 VA
transformer is not a big cost.

00940 · 11th November 2008, 09:29 PM

Quote:

Originally posted by lineup
One small 2x12 VAC,
<= 5 VA
transformer is not a big cost.

No, but space is an issue. The aim of this is to provide a higher regulated voltage for an (headphones) amplifier front end, using the existing power rails.

I think I will go for a voltage doubler but I'll stick a pair of 33R/2W resistor in serie before the voltage doubler.

lineup · 12th November 2008, 09:04 AM

i see.
can you please post your circuit used later
and tell us how well it works

thanks

11th November 2008, 10:04 AM	#1
00940 diyAudio Member Join Date: Sep 2003 Location: Paris	Question about voltage multipliers I know how to use basic voltage doublers, triplers and the like. But is there a way to multiply an ac voltage by 1.5 ? I need about 24-25VDC regulated out of a 18VAC xformer for a small load (about 50ma). Thanks in advance. __________________ Ben.

11th November 2008, 10:35 AM	#3
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	Under such a light load, the transformer may put out somewhat more than 25 volts. If you're lucky, you may need nothing more than a bridge rectifier and a 7824 regulator. Measure the transformer secondary voltage with no load. Otherwise, I agree with Mooly. Voltage doubler and a regulator. __________________ Frank

11th November 2008, 10:51 AM	#4
00940 diyAudio Member Join Date: Sep 2003 Location: Paris	Thanks. It seems like I have no choice... - actually the transformer is also used for other stuff at 200ma/17VDC, so I don't expect it to put it a lot more voltage than what it's rated for. A quick check shows me about 25VDC after rectification and filtering. - with a doubler, I can expect about 44VDC. That means about 20*0.05= 1W of dissipation for the regulator. That's why I asked about this. __________________ Ben.

11th November 2008, 03:58 PM	#7
lineup diyAudio Member Join Date: Dec 2005 Location: the north	One small 2x12 VAC, <= 5 VA transformer is not a big cost. __________________ lineup

12th November 2008, 09:04 AM	#9
lineup diyAudio Member Join Date: Dec 2005 Location: the north	i see. can you please post your circuit used later and tell us how well it works thanks __________________ lineup


	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

11th November 2008, 10:29 AM	#2
Mooly diyAudio Moderator Join Date: Sep 2007	Best bet is a doubler and regulator for so small a current. 78L12 + 12 volt zener for example.

11th November 2008, 11:05 AM	#5
Mooly diyAudio Moderator Join Date: Sep 2007	Could you perhaps play around with the value of the caps in the doubler --- using the reactance as a wattless dropper and so on --- and still have reasonable DC voltage for the regulator to work off without the ripple component being too high. Would have to think about that.

11th November 2008, 03:41 PM	#6
Elvee diyAudio Member Join Date: Sep 2006	Hi, Fractional voltage multiplier do exist, but they're probably too complicated for your application. I would use a regular voltage doubler, but with a small input capacitor serving also as ballast, and a shunt regulator at the output. A capacitive supply in fact. The power that will be wasted is tolerable, and the complexity is minimal. LV

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Differences between various vbe multipliers	Leolabs	Solid State	40	12th June 2011 01:10 PM
Voltage Multipliers	nhuwar	Tubes / Valves	10	6th August 2007 03:40 AM
Voltage Multipliers	Eli Duttman	Tubes / Valves	23	20th July 2006 02:56 PM
voltage question	Spev	Solid State	2	9th December 2005 07:52 AM
Capacitance Multipliers	Kinks	Solid State	3	25th October 2002 08:09 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read