any disadvantage to using voltage doubler or tripler ?

Assuming you want to run this off 120VAC, one way to get a transformer would 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

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:

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.

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.

I read in sci.electronics.design that mw wrote (in ) about 'any disadvantage to using voltage doubler or tripler ?', on Tue, 26 Apr 2005:

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

trouble

replies.

you can... unless Im missing something?

NT

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:

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.

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

Are you referring to capacitively coupled voltage 'doublers' ? I don't have the book you mention.

The downside is limited output current and poor regulation.

Graham

Yes. The same circuit is in my college textbook and I've seen it in the ARRL Handbook, too.

Thanks.

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.

On Thu, 28 Apr 2005 10:04:04 -0400, Tam/WB2TT wrote: ...

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

Actually, voltage doublers aren't all that bad if you use the correct 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