# RMS transformer voltage.

Discussion in 'General Electronics Discussion' started by davelectronic, Sep 17, 2012.

1. ### davelectronic

1,087
12
Dec 13, 2010
Ive been meaning to build a big linear power supply for a while but not got there yet, anyway i think i know the answer to this question, but want to run it past the more experienced here.

I have a good source of toroidal transformers of varying secondary voltages 12 / 18 / 22 / 35 volts RMS AC, i am after a target voltage of 13.80 volts DC.

My problem is most of the suitable units are 12 volts AC, so after smoothing and rectification i would get about 16 volts DC, but as this is border line for linear voltage regulators is it to low, i know a transformer of 15 volts RMS secondary is fine as it allows that good margin of a few volts over for the rectifier and regulator.

So would a 12 volts RMS secondary after rectifying and filtering allow an output of 13.80 volts DC under load conditions, i don't think it will as there would be not enough over head to allow the regulator to function properly, only wanted the voltage low for maximum current, and the supplier has a good stock of 12 volts units.

The higher voltage units would create to much heat in the regulator i think, any thoughts appreciated.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
It's really the sort of thing where you need to suck it and see. It's hard to tell you what the regulation characteristics of the transformer are.

A lot depends on its DC resistance as when it is used with (say) a bridge rectifier and a filter cap, you're going to see very large currents for a very small part of the AC waveform. This gives you the worst effect of IR losses

Remember to ensure that your VA rating of your transformer is the output current times the (almost unloaded) voltage on your filter capacitors (+ 1.4V)

So if you see 18V on your filter capacitor at a (say) 10% load, you want your transformer's VA rating to be at least your max current * 19.4

3. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
I agree with Steve. Trying to jack up the voltage with lots of smoothing will just mess up the power factor even more than necessary, and put stress on everything. I would use a higher secondary voltage with less smoothing, producing say 30~35 VDC, and a switching regulator, with a linear post-regulator if necessary. Or a switching regulator that works directly off mains voltage.

Your post is quite detailed, but you didn't mention any figures for current...

4. ### davelectronic

1,087
12
Dec 13, 2010
Thanks for the input guys, i kind of thought that might be the case, just have to find slightly higher secondary voltage, figures yes sorry, about 300 VA or about 25 amps for 12 volts AC RMS, but expect it will be more VA for the higher voltage to get the same current output or as close as possible, thanks again.

5. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
How about modifying a PC power supply? Google modify pc power supply 13.8V

6. ### davelectronic

1,087
12
Dec 13, 2010
Thanks its a linear i am after doing this time, already done the ATX conversions.

#### Attached Files:

• ###### Photo0058.jpg C.jpg
File size:
143.1 KB
Views:
160
Last edited: Sep 17, 2012
7. ### Tuned

1
0
Sep 19, 2012
Hi, I'm new on this forum and this is my first post.

I have some experience with 12,15,24,50amps 13.8volts power supply. In my hobby as a ham radio(amateur radio) operator, most of our equipment can be powered by 13.8 volts but needs to be clean almost as clean as that from a battery cell. Instead of purchasing ready made units we hams prefer DIYing.

A group of 2n3055s or MJ2955s connected to a reference coming from a 3-terminal TO-220 package regulator will make you a cheap and stable regulated power supply. These parts are chosen for their availability and low cost. Be glad to share, I'll just have to dig in to my old notes of previous years to look for the schematic.

You will use the 18volts tap from the transformer.

Last edited: Sep 19, 2012
8. ### davelectronic

1,087
12
Dec 13, 2010
Thanks Tuned.
I have a lot of schematics of the type of circuits you refer to, and here in the UK no shortage of suppliers, the question in my opening post was something boarder line, so wasn't 100% sure, but i will use a 15 volt unit if i can find one, with how clean they are the linear supply's ive made are fine, the converted ATX units put out a little more noise but its with in exceptable limits, thanks again much appreciated for the thought.