Building an AC-2-AC PSU - Page 1
Forum section: electronics / beginners

author: Sebzy
author_rank: Contributor
timestamp: May 20, 2014, 10:37:17 am
content: Have a piece of audio kit I need to whip up a PSU forIt draws 1000mA and I'm planning on using a two resistor voltage dropper curcuit.Diagram hereMy question is how do I work out the wattage of R1 and R2 required.In : 230vACOut : 10.5vAC @ 1000mAR1: 47,000R2: 2,250

author: Psi
author_rank: Super Contributor
timestamp: May 20, 2014, 10:54:21 am
content: You should use a proper voltage regulator like a lm317 and set it for the voltage you need.Using resistors to do voltage regulation is a terrible way to do it.As the load current goes up and down you upset the balance between R1 and R2 (which is setting your output voltage)So, to reduce this effect you need a lot more current flowing through the resistors vs your load.This extra larger current makes the whole thing terribly inefficient.

author: han
author_rank: Frequent Contributor
timestamp: May 20, 2014, 11:23:46 am
content: Are you need AC voltage exactly 10.5vAC @ 1000mA ?One way is to hack/mod servo voltage regulator combined with your transformer + isolated feedback circuit.

author: David_AVD
author_rank: Super Contributor
timestamp: May 20, 2014, 11:30:25 am
content: Is the 10.5V AC value really that critical? Why can't you simply use a transformer with the required output voltage?

author: Fank1
author_rank: Regular Contributor
timestamp: May 20, 2014, 11:34:25 am
content: You can't do the calculation until you know the secondary voltage of the transformer.

author: retrolefty
author_rank: Super Contributor
timestamp: May 20, 2014, 11:51:35 am
content: Quote from: Psi on May 20, 2014, 10:54:21 amHow would using a DC voltage regulator like a LM317 result in a regulated AC output voltage?

author: N2IXK
author_rank: Frequent Contributor
timestamp: May 20, 2014, 12:28:25 pm
content: Let me get this straight. You want to pull 1 ampere load current through R1, which is 47,000 ohms. That would mean that the voltage drop across R1 would be 47 kilovolts!I assume that the transformer you show was stolen from an electrical substation somewhere? R1 would be dissipating 47 KILOWATTS of heat, so your schematic needs to be updated to show the blowers or water circulating pumps used for cooling it.R2, on the other hand, only sees the 10.5V across it, and dissipates about 50 milliwatts. A standard 1/4 W resistor would handle this just fine.I hope your 1A load current is VERY stable, because any variations will cause HUGE swings in output voltage.Maybe you need to go back and rethink your approach a bit? Like maybe a transformer with a 10.5 V secondary?

author: peter.mitchell
author_rank: Super Contributor
timestamp: May 20, 2014, 12:32:08 pm
content: buy 20 watt class AB amplifier, feed in your sine wave, adjust gain, forget.

author: Fank1
author_rank: Regular Contributor
timestamp: May 21, 2014, 08:22:27 pm
content: Resistor Power = (current)^2 * RResistor Power = (voltage)^2 / R

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 21, 2014, 08:55:55 pm
content: Wrong question. It is extraordinarily unlikely that your (undisclosed) load must have exactly 10.5VAC.It would be very surprising if the mystery gadget couldn't take something between 10 and 12VAC, or maybe even a wider tolerance than that.As others have said, trying to achieve the exact voltage with a simple resistor divider is just about the worst possible solution.And the values of your resistors are orders of magnitude too large for a 1A load, anyway.