Regulated 24VDC from 12VAC

[aflynt]

Hello,

I'm trying to figure out the best way to get regulated 24VDC out of the 12V 1Amp tap on my power transformer. Right now I have it going into a FWB rectifier for unregulated 16VDC going into a 9V linear regulator to power relays, etc... at 9V. I've got a circuit that I want to power at 24VDC for more headroom. I figured I'd just switch over to 24 volt relays and run it all at 24VDC but I'm not sure of the best approach. Could I switch the FWB for a voltage quadrupler circuit and use a 24V regulator or would I not have enough voltage to drive it. I think I'm only going to be drawing a couple hundred mA off it.

-Aaron

[JazzGuitarGimp]

A voltage doubler will reliably get you there. A quadrupler will cause two negative results:
- you will exceed the absolute maximum input voltage for most regulators, which is 37V. But more importantly;
- you'll be throwing a lot of power away. With a voltage doubler, your 1A transformer will only be good for 0.5A post-doubler, and a quadrupler will knock the available current down to 0.25A

[aflynt]

I am still learning about doubler, triplers, etc... so please humor me. I thought that the doubler would basically give you whatever the AC rating was I.e.: 12VAC from 6-0-6 winding in DC minus diode drop yielding less than 12VDC.

-Aaron

[JazzGuitarGimp]

The DC output of a voltage doubler circuit will be very close to twice the peak AC voltage applied to the input. You would want to ignore the center tap of your transformer, and use the full 12V RMS available. So 2 x 12 x 1.414 = ~34VDC.

https://www.allaboutcircuits.com/textbo ... ltipliers/

If you are going to power an amplifier circuit, such as a FET boost, you'll probably want to use a full wave doubler, which is the second set of example circuits in the article linked above, and use at least 470uF for the two caps. By the way, this is a circuit that can be very easilt modeled in Mouser's free Spice simulator (Multisim Blue). Doing so would allow you to add a resistor at the output to GND to simulate your load, then you can see if the caps are big enough to support the load.

[aflynt]

Awesome! Thanks! I'll take a look at Multisim Blue too.

-Aaron

[aflynt]

Looks like I can just replace two diodes with caps in my FWB to convert it to a full wave doubler. Do you think two 2200uF 25V caps would work (got a couple in my parts drawer), or am I pushing it on voltage rating?

-Aaron

[aflynt]

I put the 2200uF 25V caps in and so far so good. I'm also using ultra-fast diodes too so I'm guessing that helps with ripple. I'll probably re-do it later with higher voltage caps once I do a new PCB layout for the relay supply board.

-Aaron

[martin manning]

25V caps should be fine since peak voltage on each one is <17V. Depending upon the winding resistances the available current is around 1/4 of your transformer's secondary.