24VAC Power Supply - Page 1
Forum section: electronics / projects

author: lesabb
author_rank: Contributor
timestamp: December 30, 2014, 11:32:16 pm
content: Hello all,A bit of a noobie here (2nd year physics and electronics, bla, bla ,bla) mostly doing audio. I took apart a power module form a Mic pre-amp that uses a 24VAC wall adapter to run. After careful examination (it wasn't that complex actually) I came up with this schemo. Please let me know if it makes sense, did I miss anything? Looks ok to me but...Can I also replace the choke (circled) for a RC filter instead? This would supply +48V and +24 from the 24VAC. The 7824 hooks up to a 33v Zener. Values are not in the Eagle schem yet.ThanksL

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2014, 12:42:11 am
content: Looks like a reasonable transcription. Obviously I don't know what the original looks like, but you didn't make any dumb backwards errors or anything.Yes you can, but not very much. Resistance drops DC. So don't expect any big reduction in noise or hum relative to what the inductor may've been providing.If you aren't using it for sensitive audio (or other) applications anymore, you might not even need the pre/post regulators either.Tim

author: lesabb
author_rank: Contributor
timestamp: December 31, 2014, 12:58:36 am
content: Thanks Tim! I thought that the LC filter was for better ripple control. Could I just calculate what the LC filter ripple is and choose the proper value of RC for the same ripple correction?L

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2014, 01:10:31 am
content: Unless the L was very small to begin with, no, you cannot.If 1 / (2*pi*sqrt(L*C)) >> 120Hz (taking C = the smaller of the two capacitors), then R ~= 2*pi*L*120Hz. Obviously, you get a DC loss of V = I*R, which becomes prohibitive when V > 10% of Vsupply (or around 3V).Tim

author: lesabb
author_rank: Contributor
timestamp: December 31, 2014, 01:14:51 am
content: That's great. Thanks. BTW the L is 100mH, which is not that small I think?...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2014, 02:10:58 am
content: If the caps are like 100uF or more, then the cutoff frequency is around 50Hz, which is into the range of not being able to approach the filtering with a resistor. You can arbitrarily increase the value of the caps, but that takes up a lot of space.Tim

author: Seekonk
author_rank: Super Contributor
timestamp: December 31, 2014, 03:37:08 pm
content: I worked with a mechanical engineer many years ago who thought electrical engineering was an inexact science because every time we have a problem, we just add a capacitor. I thought it was humorous then, it seems more true now. Looking at schematics is a great way to learn electronics, if you will ask why did they do this. Ask for a circuit here and you will get ten different answers. Each will focus on a particular issue depending on the posters experience and because entire requirements were not provided.I sense you want to make copies of this circuit and I dont think this is a great example. At first look this circuit makes me want to gag. They use a voltage doubler to increase the voltage over 50V to provide power to a 24V regulator. That regulator can't take that voltage so the voltage is dropped by a transistor and resistors. More than half the power just turns into heat. The 24V AC could easily supply the voltage to the 24V regulator, but then there wouldn't be an easy way to to get 48V.The 48V section seems fairly normal, but possibly the heavy load on the 24V doesn't provide enough voltage for the transistor regulator to guarantee filtering over the entire line voltage range. The inductor is then needed to passively lower the ripple.This could be an old circuit or relatively new. It seems one could easily do better. Yet if you want to stay with a 24V wall wart and very common components, there is no way around this mess.

author: lesabb
author_rank: Contributor
timestamp: December 31, 2014, 05:23:38 pm
content: I'm not sure how old the design is. It's from a fairly recent commercial mic pre-amp. I've seen other design where ther is no choke at all. Like the one below?Thanks guys

author: Seekonk
author_rank: Super Contributor
timestamp: December 31, 2014, 06:15:18 pm
content: That is very similar. As you know, field sound reinforcement locations often have very low line voltage and some equipment has a reputation for working better in those conditions. The other design can probably work fine in most applications without an inductor, especially if the filter capacitor is made larger. It is also a function of how many units are powered off this supply.