Electronics > Beginners
Voltage multiplier
innkeeper:
hi sureshot.
Some random thoughts here..
transformers are typically (but not always) rated for their output voltage at 100% load
so its not unusual for a 12V transformer to be outputting much higher with low and 0 loads.
you can use this to your advantage and is something to keep in mind.
rectified output voltage - fill bridge that's gonna be roughly 1.414 x the ac voltage - the diode loss so 12V ~= 15.7 and the lm7812 dropout is ~14.5 depending on the model, so your close but should be ok. there are lower dropout voltage models of 7812's. by the time you get under 12v on the transformer under load, you likely passed the transfomers VA rating. that being said, i prefer to use 15V transformers for 12V regulated circuits.
doubling the voltage through a doubler is gonna get u around 30V and higher unloaded that is close to, if not over(unloaded) for the working voltage of an LM7812
even if you are under, its gonna get really hot. not sure id feel safe with that myself.
voltage multipliers with heavy load need a significant capacitance in each stage to handle the current draw to maintain voltage and have low ripple
sureshot:
As a hobbyist I glean as much info as I can find, I'm much more proficient at putting circuits together than working out the maths. With the maths I scrape by just. Thank you for the info you've all posted, much appreciated. I've got reams of data sheets, circuit schematics stored for references and such. The lm723 is the one I've not done yet, although I have collected a few variants of schematics from good sources. I'm mainly interested in power electronics, AC and DC motor control, and some relay stuff. Short story...
A friend asked me to repair a clay pigeon machine some years ago. It's wiring was a mess, it needed stripping right out and rewiring. I was going to see if I could work some of the 5 motors circuits out, that was before another friend tore out the wiring with side cutters whilst I went to get the tea. Started from scratch, took me two weeks to rebuild it. The owner then as me to adapt it to RC control for remote firing. Gained a few free trips to the range with my Daughter as a thanks for repairing it.
Very interested in anything power electronics. I'd like to attempt a smps in the near future, but will tackle the lm723 circuit first. Thanks again for the help and information you've all posted. :)
Zero999:
--- Quote from: innkeeper on July 03, 2018, 10:30:32 pm ---rectified output voltage - fill bridge that's gonna be roughly 1.414 x the ac voltage - the diode loss so 12V ~= 15.7 and the lm7812 dropout is ~14.5 depending on the model, so your close but should be ok. there are lower dropout voltage models of 7812's. by the time you get under 12v on the transformer under load, you likely passed the transfomers VA rating. that being said, i prefer to use 15V transformers for 12V regulated circuits.
--- End quote ---
For 1A, with Schottky diodes and monster filtering capacitors, you might just about get away with a 12V transformer and the LM7812. However, I think you've forgotten that the original poster wants to add a booster stage to get 3A, which will require an extra 1V to 2V of headroom.
sureshot:
For 3 Amps continuous output i would use 6 Amp diodes, and 4 x capacitors of 35 Volt 22000uf each. Two parallel pairs, then put them in series. I think the capacitance would be 22000uf total of the two parallel series pair capacitors. With a 12 Volt 5 Amp transformer that should work.... I think. With the use of a full wave voltage doubler there's enough headroom to not worry about drops across the components.
Zero999:
It makes no sense to double the voltage, with enormous capacitors, only to burn it off again in a power transistor. Here's a simplified schematic, showing what I was saying before about powering only the bias circuitry off the voltage doubler. This configuration has a lower drop-out, than the classic LM723 circuit, because Tr2's base voltage can go higher, than the output voltage, so the drop-out voltage can be as low as Tr2's saturation voltage, which will be about 1V, for a typical power transistor. D1 should use Schottky diodes, for a lower voltage loss.
The same principle can be applied to other circuits, such as the LM7812 with a booster transistor or an LM431 and a pass transistor. It will not work with the plain LM7812 or a fully integrated 3A regulator. The pass transistor must be accessible for it to work.
The original circuit can be found here, which is for a variable 3V to 30V, 2.5A PSU and will need some of the component values changing for a fixed 12V, 3A output.
http://powersupply33.com/adjustable-regulated-power-supply-3-30-v-2-5-a.html
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