any disadvantage to using voltage doubler or tripler ?

M

mw

Guest
Is there any down side to using a doubler or tripler to get a higher
output voltage out of a standard AC power transformer?

I want to get about +80 and -80V at 0.1A from a power supply, but there
are very few transformers available in that output range. There are
some in the 36VAC and 24VAC CT range, so if I use a doubler like in "Art
of Electronics" (2nd edition, pages 47 and page 48), I can get my
desired voltage. This is an educational project so I am only building
one or two, so it doesn't matter at all that I'd have to buy more
diodes... they are easy to get compared to an odd transformer.

Is it better to use the one in Fig 1.78 or the one in Fig 1.79? If I
use the one in Fig 1.79 I can get both the + and - voltages from a
single transformer.

If someone can share their experiences I'd be grateful.
 
"mw" <mw@avoidspam.com> wrote in message
news:j%fbe.323$gd5.92@newssvr12.news.prodigy.com...
Is there any down side to using a doubler or tripler to get a higher
output voltage out of a standard AC power transformer?

I want to get about +80 and -80V at 0.1A from a power supply, but there
are very few transformers available in that output range. There are some
in the 36VAC and 24VAC CT range, so if I use a doubler like in "Art of
Electronics" (2nd edition, pages 47 and page 48), I can get my desired
voltage. This is an educational project so I am only building one or two,
so it doesn't matter at all that I'd have to buy more diodes... they are
easy to get compared to an odd transformer.

Is it better to use the one in Fig 1.78 or the one in Fig 1.79? If I use
the one in Fig 1.79 I can get both the + and - voltages from a single
transformer.

If someone can share their experiences I'd be grateful.


Assuming you want to run this off 120VAC, one way to get a transformer would
Click to expand...
be to buy a 220:120 transformer. NOT AUTOTRANSFORMER. Connect the 120VAC to
the 220 winding, and get abot half of that off the 120 secondary. If these
exist, they should be cheap at your power rating.

Tam
 
mw wrote:
Is there any down side to using a doubler or tripler to get a higher
output voltage out of a standard AC power transformer?

I want to get about +80 and -80V at 0.1A from a power supply, but there
are very few transformers available in that output range. There are
some in the 36VAC and 24VAC CT range, so if I use a doubler like in "Art
of Electronics" (2nd edition, pages 47 and page 48), I can get my
desired voltage. This is an educational project so I am only building
one or two, so it doesn't matter at all that I'd have to buy more
diodes... they are easy to get compared to an odd transformer.

Is it better to use the one in Fig 1.78 or the one in Fig 1.79? If I
use the one in Fig 1.79 I can get both the + and - voltages from a
single transformer.

If someone can share their experiences I'd be grateful.
Click to expand...
Starting at the beginning:
To get about 80 volts DC you need about 56 volts AC. There are 56
volt transformers available from Digikey.
MT2236, 56 volts@ 0.22 amp for example:
http://dkc3.digikey.com/PDF/T051/1374.pdf
Putting a full wave rectifier and capacitor across this would give you
about 80 volts. If you need .1 amp on both 80 volt supplies, you
should probably use two of these and stack the DC outputs.

You could use a 56 volt secondary and the doubler in fig 1.78 and use
the junction of the two caps as the zero volt point, but if the loads
on the + and - supplies are not equal, there will be DC in the
transformer secondary, and that will cause the transformer to get hot.
You would need a transformer rated for about .4 to .5 amps to
provide .1 amp across the full 160 volts.

You can also use a 28 volt transformer and the circuit from Figure
1.78 to provide one of the 80 volt outputs. This transformer should
also be rated for at least .4 amps. A second transformer and doubler
could provide the other 80 volt supply.

The circuit in figure 1.79a can provide both positive and negative
supplies from a single 28 volt winding (rated for at least .8 ampere)
if two copies of the capacitor diode network are connected to the
ungrounded end of the secondary. You will have to reverse the
connections on the capacitors and diodes for one of the copies.

The gotcha here is to find an input capacitor (the one connected to
the secondary) rated for a ripple current of at least twice the load
current (three times would be better. So no cheapies, here.

Keep in mind that doublers tend to have at least twice the ripple
voltage for a given capacitor size compared to normal full wave
rectifier filters do. So be generous on the capacitor values.
 
I read in sci.electronics.design that mw <mw@avoidspam.com> wrote (in
<j%fbe.323$gd5.92@newssvr12.news.prodigy.com>) about 'any disadvantage
to using voltage doubler or tripler ?', on Tue, 26 Apr 2005:

it doesn't matter at all that I'd have to buy more diodes... they are
easy to get compared to an odd transformer.
Click to expand...
The diodes are no problem but there are cost and size issues with the
capacitors.

Also, the source impedance of a multiplier is higher than for a normal
rectifier.

A 60 V secondary (loaded voltage) will give you nearly 83 V DC.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
Thanks, everyone, for all the good suggestions. I will probably use
either the "230V --> 115V" idea or else the "pair of 56V transformer"
idea, depending on availability. The multipliers might be more trouble
due to the complexity and poor regulation, as pointed out in the replies.

It would be nice if I could have used some of my junk box
transformers... I have several in the 26VAC CT size that weigh 3 to 5
pounds. Oh well... I'll save them for "later"...


John Woodgate wrote:
I read in sci.electronics.design that mw <mw@avoidspam.com> wrote (in
j%fbe.323$gd5.92@newssvr12.news.prodigy.com>) about 'any disadvantage
to using voltage doubler or tripler ?', on Tue, 26 Apr 2005:

it doesn't matter at all that I'd have to buy more diodes... they are
easy to get compared to an odd transformer.


The diodes are no problem but there are cost and size issues with the
capacitors.

Also, the source impedance of a multiplier is higher than for a normal
rectifier.

A 60 V secondary (loaded voltage) will give you nearly 83 V DC.
Click to expand...
 
Are you referring to capacitively coupled voltage 'doublers' ? I don't have
the book you mention.
Click to expand...
Yes. The same circuit is in my college textbook and I've seen it in the
ARRL Handbook, too.

The downside is limited output current and poor regulation.

Graham
Click to expand...
Thanks.
 
mw wrote...
Thanks, everyone, for all the good suggestions. I will probably use
either the "230V --> 115V" idea or else the "pair of 56V transformer"
idea, depending on availability. The multipliers might be more trouble
due to the complexity and poor regulation, as pointed out in the replies.
Click to expand...
That's a good approach, I'm sure you can find a good transformer.
BTW, Signal Transformers is a good place for general-purpose xfmr
inventory, 24-hour delivery. http://www.signaltransformer.com/

I often use voltage doublers or triplers for making low-power modest
high voltage supplies in the 250 to 600V range. These circuits are
simple enough and work well when the required current drain is under
20mA or so. The higher ac ripple John mentions isn't a problem when
the voltages are high to begin with.

In the grand old tube days you could get nice small 350 to 500V
instrument transformers rated at no more than 20mA, but I've not been
able to find anything like that in stock for decades now... Signal
has some with 230VCT secondaries, such as their LP-230-10 and -25
pcb-mounting types that are just right to feed a voltage doubler. A
simple hv MOSFET pass-element drops the voltage to the value needed.


--
Thanks,
- Win
 
mw wrote:
Is there any down side to using a doubler or tripler to get a higher
output voltage out of a standard AC power transformer?

I want to get about +80 and -80V at 0.1A from a power supply, but there
are very few transformers available in that output range. There are
some in the 36VAC and 24VAC CT range, so if I use a doubler like in "Art
of Electronics" (2nd edition, pages 47 and page 48), I can get my
desired voltage. This is an educational project so I am only building
one or two, so it doesn't matter at all that I'd have to buy more
diodes... they are easy to get compared to an odd transformer.

Is it better to use the one in Fig 1.78 or the one in Fig 1.79? If I
use the one in Fig 1.79 I can get both the + and - voltages from a
single transformer.

If someone can share their experiences I'd be grateful.
Click to expand...
There is no shortage of available standard transformer secondary
voltages in this range. Doublers are mostly used in circuits powered by
multiple voltages and the magnetics can be conserved in some cases by
using a doubler to eliminate the need for a second transformer. But
using a doubler as a single power source is always disadvantageous to a
direct transformer conversion at the required voltage. If you are
working with junkbox parts then the thing to do would be to use your low
voltage transformer, which should be rated at about 24-30VA and then
build a boost mode switching converter to arrive at the +/-80V.
 
"mw" <mw@avoidspam.com> wrote in message
news:vfqbe.1155$461.303@newssvr11.news.prodigy.com...
Thanks, everyone, for all the good suggestions. I will probably use
either the "230V --> 115V" idea or else the "pair of 56V transformer"
idea, depending on availability. The multipliers might be more trouble
due to the complexity and poor regulation, as pointed out in the replies.

It would be nice if I could have used some of my junk box transformers...
I have several in the 26VAC CT size that weigh 3 to 5 pounds. Oh well...
I'll save them for "later"...


Actually, voltage doublers aren't all that bad if you use the correct
Click to expand...
configuration. They are common as AC rectifiers that power switching power
supplies that can operate off 120 or 220VAC. The method is to configure the
rectifier as a bridge when operating off 220V and as a doublers when
operating off 120V. The rectified DC is then around 300V for either case. To
change configurations requires one jumper. These supplies can run hundreds
of Watts.

The preferred doubler is the kind where the filter consists of two
capacitors in series, with one side of the AC signal connected to the
junction of the capacitors. This is called a full wave doubler. Relative to
a bridge rectifier with the same output , the doubler requires two
capacitors each of twice the value of C and 1/2 the voltage rating. I don't
think there is any way of getting both (+) and (-) 2X the voltage off the
same transformer since the transformer secondary will be floating at 1/2 the
DC output. Note that the doubler requires 2 diodes, as opposed to the bridge
that requires 4, although this is moot when the diodes cost 10 cents .

Tam
 
On Thu, 28 Apr 2005 10:04:04 -0400, Tam/WB2TT wrote:
....
capacitors each of twice the value of C and 1/2 the voltage rating. I
don't think there is any way of getting both (+) and (-) 2X the voltage
off the same transformer since the transformer secondary will be
floating at 1/2 the DC output.
Click to expand...
As long as the tranny is floating, it's not that hard - just stack up
a positive and negative half-wave doubler and pick up ground in the middle.
Of course, ripple goes up, current goes down, and you need something like
4X the capacitance and a certain dose of Black Magic. ;-)

Cheers!
Rich
 
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