Power supply transformer connections - Page 1
Forum section: electronics / projects

author: MarkF
author_rank: Super Contributor
timestamp: May 17, 2019, 09:41:39 pm
content: Will this work? Less than 500mA for each output.Based on something I saw online but can't wrap my head around it.Seems like it needs diodes on the 5V winding.Can someone describe the operation?

author: Ian.M
author_rank: Super Contributor
timestamp: May 17, 2019, 10:15:23 pm
content: Nope. You'd need taps either side of 0V to feed the 5V regulator via a pair of diodes (or at a minimum, a single diode if the load on 5V was low enough for half-wave rectification to be acceptable). As-is the 5V regulator has AC on its Vin pin, and wont be happy. C3 will also probably blow messily.

author: mariush
author_rank: Super Contributor
timestamp: May 17, 2019, 11:57:02 pm
content: Something like this would work.Problem is if you want 12v at 500mA and 5v at 500mA, the U1 regulator needs to produce at least 1A of current, to give 500mA to the 5v regulator... so you must pock a suitable capacitor before the 12v regulator to keep the minimum voltage above the output voltage + voltage drop (1v..1.5v)Also keep in mind the 5v regulator will dissipate a lot of heat, you'd be dealing with (12v - 5v) x 0.5a = 3.5 watts.You could split the heat by adding a 9v regulator between the 12v and 5v one... a 7809 is cheap.Alternatively, you could just place a bunch of diodes in series after the output ... ex 1n400x diodes will have around 0.8v-1v drop per diode... but individual diodes would be harder to keep cool compared to a plain metal sheet or a heatsink on which you can screw a to220 regulator.

author: Ian.M
author_rank: Super Contributor
timestamp: May 18, 2019, 12:13:30 am
content: Its also a good case for using a switching 5V regulator, direct from the unreg V+ rail.

author: ArthurDent
author_rank: Super Contributor
timestamp: May 18, 2019, 12:35:32 am
content: 500 Ma isn't that much and heating shouldn't be a problem at all. However the input to the regulators must be at least 2.5 volts above the outputs. I would wire it this way which is based on the previous schematic.

author: MarkF
author_rank: Super Contributor
timestamp: May 18, 2019, 12:52:03 am
content: This is an old transformer I was going to use.My project needs +/-12V and 5V. The 500mA is probably twice the actual load. If that.Maybe I can just use two full wave bridges on a PCB I did.

author: MarkF
author_rank: Super Contributor
timestamp: May 18, 2019, 02:07:24 pm
content: Here is the original circuit I was trying to adapt.I thought it was using a full wave bridge.After closer inspection, it is not.

author: Ian.M
author_rank: Super Contributor
timestamp: May 18, 2019, 05:37:26 pm
content: Your transformer's 7.7V-0-7.7V winding + a bridge rectifier and two resevoir caps between bridge + and winding CT, and CT and - can feed your +5V (from CT) and +12V regulators reasonably efficiently, and use the 14.4V winding + another bridge and reservoir cap to feed the -12V regulator.

author: MarkF
author_rank: Super Contributor
timestamp: May 18, 2019, 06:32:49 pm
content: Will the 5V regulator input be halfwave rectified then?

author: Ian.M
author_rank: Super Contributor
timestamp: May 18, 2019, 08:55:36 pm
content: Nope. Full wave. Consider the CT as ground and you'll see that both the bridge + and - have equal and opposite full wave rectified voltages. Now consider output 0V (bridge -) as ground and you'll see that the +12V regulator gets the full wave rectified whole winding voltage and the CT provides the 5V regulator with half of that so neither should run excessively hot.