24v to 12v project guidelines

[Dannyx]

Good day folks. A friend of mine recently asked me, thereby giving me a "mental challenge", to come up with a solution for converting a 24v supply from a crappy old transformer to a 12v supply which he'd use to power a car headunit he intends to install in his basement. To clarify, where I live, many apartment blocks don't have a mains supply running into their basement due to safety regulations regarding electric hazards caused by floods from leaky pipes or water used for putting out potential fires, so instead there's a beefy 24v transformer somewhere at the top levels feeding some crappy bulbs down there for light only and that's it, so if there ever IS water down there, the supply is isolated and harmless to those who might be standing in it.

The idea is to tap into the 24v bus, put a rectifier on it and regulate it to run a headunit...from what I understand, the headunit in question is a junker he got from a friend, so we're not talking amps and 1000w subs here

Come to think of it, many headunits run perfectly fine on 24v as well, since they're designed for use on trucks or vehicles which operate on 24v, so technically no conversion should be done at all, but without first seeing the thing I can't know for sure. Even then, I'm thinking of all the spikes going on on the mains side which would translate to possible overvoltage on the secondary as well, so I need at least some minimal regulation.

Obviously a switching regulator would be best suited for maximum efficiency, but that's a bit overkill and hard to design, especially since it would probably cost more than the headunit itself and would have to be fairly beefy, since automotive equipment tends to draw quite some amps (again, this is a number I haven't got at the moment, since I haven't seen the thing he's trying to power, but I WILL tomorrow hopefully....some rusty hunk of junk he found at the wreckers from what I understand )

I HAVE simulated a linear regulator one using an op-amp and basic feedback and it works (in theory at least), though it's going to be highly inefficient, because it would have to shed 12v (24v-12v) and even at 1-2 amps, we're still talking 12-24w for the pass element. Cooling and pricing becomes an issue at this point - again, would probably outweigh the radio itself. What would you use ? Thoughts ? Ideas ? A battery radio or a phone, I know what you'll say, but that's not the point of the discussion

[stj]

Re: 24v to 12v project guidelines

if the guy wants to cover the postage, i have a 24 to 12v convertor designed to power radio's in buses or trucks.

if your going to make one, you need a switching buck regulator.

[Dannyx]

Re: 24v to 12v project guidelines

if the guy wants to cover the postage, i have a 24 to 12v convertor designed to power radio's in buses or trucks.

if your going to make one, you need a switching buck regulator.

No, he'd definitely not be willing to pay too much for it, plus the idea is to make one

Out of curiosity nonetheless: what's the one you've got like ? Is it homemade or is it commercial ?

[mariushm]

Re: 24v to 12v project guidelines

You could just buy a second 230v..240v -> 24v AC transformer and wire it the other way, so you'd get 230v -ish into a mains socket ... then use a regular 12v laptop adapter or wall wart or something to produce 12v.


if it's 24v AC being sent ...


Bridge rectifier, 35v or higher rated capacitor to smooth out the voltage
then just buy a dc-dc converter to produce 12v :

Links : http://uk.farnell.com/c/power-line-p...-dc-converters

non-isolated dc-dc converters : link

isolated dc-dc converters (min 12w , that support at least 36v input voltage or more) : link

[stj]

Re: 24v to 12v project guidelines

No, he'd definitely not be willing to pay too much for it, plus the idea is to make one

Out of curiosity nonetheless: what's the one you've got like ? Is it homemade or is it commercial ?

it's a comercial unit made by ASTEC from a london bus!!

[eccerr0r]

Re: 24v to 12v project guidelines

picture of the 24V transformer? does it have a center tap? Can you MAKE a center tap? heh heh heh...

[Dannyx]

Re: 24v to 12v project guidelines

picture of the 24V transformer? does it have a center tap? Can you MAKE a center tap? heh heh heh...

I don't have access to the Xformer and neither does him, so we can only ASSUME it can supply the current required. Worst case scenario the lights will flicker to the bass - you can call that a feature

Just out of curiosity, is it possible to MAKE a center tap ? Of course it is if you rewind the thing, but just how exactly ? Might prove to be useful information

[eccerr0r]

Re: 24v to 12v project guidelines

Yes, that's what I meant, either rewind the secondary or somehow find the center of the winding and cut it there... Neither are pleasant but do what you need to do...

[momaka]

Re: 24v to 12v project guidelines

Here is another solution: find a UPS with a center-tapped output on its main transformer. I have an APC BackUPS 560 or something like that, and it has a 12V output with a center tap. Thus, if you wire 24V to a transformer like that, you will get 12V between one transformer leg and the center tap. That said, if the transformer is rated for 12V and you run 24V through it, it is possible that the transformer may burn out (due to self-heating).

Another option: get a 220/230V to 110/120V transformer and put the 24V supply on the 220/230V winding. This will give you about 12V on the 110/120V winding.

Apart from those options, you could also get a buck-regulator circuit from eBay or Ali Express for cheap (I think they are under $5 shipped). The really cheap ones will do up to 2-3 Amps.

Last option, and probably the cheapest and easiest: connect a simple single-diode rectifier + capacitor on the 24V supply. This will give you *roughly* 17V DC output. NOTE: I say roughly, because at the peak of the voltage wave, the DC power bus could spike up to 35V with no load - obviously not what you want! But with a load, that voltage will drop. If the 24V AC supply is 50 Hz (from what I know, Romania is like the rest of Europe, with 230V, 50 Hz mains), you can expect a very loud 50 Hz buzz to be heard through the radio's speakers - again, something you *won't* like too much.

Thus, if you really want the above simple solution to work, then add a 14-15V linear regulator after the rectifier circuit. This will smooth out the 16-17V output from rectifier circuit and remove the nasty 50 Hz ripple. Also, because you're only dropping a Volt or two with the linear regulator, the heat generated from it won't be that much, even at 1-3 Amps of current. To keep the current draw even lower, use 8 Ohm speakers instead of 4 or 2 Ohm speakers with the amp... and preferably something very efficient. A pair of old 8-Ohm floor-standing speakers with a high efficiency will give you good sound with minimum loading on the amp (and your power supply).

[budm]

Re: 24v to 12v project guidelines

One info you did not provide is the current rating of this 24VAC power source.

[momaka]

Re: 24v to 12v project guidelines

One info you did not provide is the current rating of this 24VAC power source.

If it's used for lightning in the basement, you can pretty much assume it's capable of more than a few amps. After all, with incandescent bulbs, you need a lot of power to produce decent lightning, so most likely each basement cell was calculated to draw 2-3 Amps. Of course, that's ASSUMING the power source was designed for incandescent bulbs and not LEDs. If it's an old building, though (more than 10 years old), then forget about the LEDs part.

[Dannyx]

Re: 24v to 12v project guidelines

I do not know any specs of said transformer outside the voltage and neither does my buddy who actually lives there. He's not too tech savvy for one thing and secondly the transformer is not installed somewhere accessible to look at it. Even if it were, I assume it was probably built somewhere in the dinosaur area and may not even have a label on it , so yes, it's almost certain it's running incandescent bulbs since LEDs weren't a thing when that system was installed.

If it didn't violate rules and stuff, I think it's safe to assume that at this point it would be easier to just run a 230v wire down there

[zx8401]

Re: 24v to 12v project guidelines

What about using an old mains to 13.8v linear supply but lopping off the transformer and feeding the 24vac into the supply bridge?
You might need to add a bigger heatsink and pass transistor depending on the current.
You need to define the current required really.

[Dannyx]

Re: 24v to 12v project guidelines

Not sure how much a crappy old car radio could possibly draw, even at full volume which will never happen to begin with :| Guess I have to take some measurements.

[eccerr0r]

Re: 24v to 12v project guidelines

Probably a rough guesstimate is take speaker watts, add them up, multiply by 3, and divide by volts...
Just a rough guesstimate to get in the ballpark...

[budm]

Re: 24v to 12v project guidelines

It will not be big constant current draw any way when listening to music.
BTW, you can also look at that lamp Wattage being used right now to see what the transformer can handle at this moment.
BTW, if you have spare 24V transformer with center tab then you can use it as buck step down to 12V.

[momaka]

Re: 24v to 12v project guidelines

I do not know any specs of said transformer outside the voltage and neither does my buddy who actually lives there. He's not too tech savvy for one thing....

In that case, you may definitely want to bring a multimeter and measure the voltage again. After all, you don't want any surprises with the voltage (let's say for some reason it was 230 V AC ).

Even if it were, I assume it was probably built somewhere in the dinosaur area and may not even have a label on it , so yes, it's almost certain it's running incandescent bulbs since LEDs weren't a thing when that system was installed.

As I thought.

If it didn't violate rules and stuff, I think it's safe to assume that at this point it would be easier to just run a 230v wire down there

I am surprised you guys have a voltage code requirement for your basements there. In Bulgaria (your southern neighbor) there's no such thing - or at least definitely not in the old Communist-era apartment buildings. And even if there ever was any code, I don't think anyone has ever followed it. People often used to run "mini-shops" in there with refrigerators, ovens, and everything.

Not sure how much a crappy old car radio could possibly draw, even at full volume which will never happen to begin with :| Guess I have to take some measurements.

Depends on the radio really and how many channels it has. Most average aftermarket radios (probably more powerful than an old crappy radio) usually have one or two amp ICs to drive 2-4 speaker channels. The better ones can typically do about 20-30W max per channel with 2-Ohm speakers. That translates to about 4 Amps peak per speaker channel, so you're looking at about 16 Amps absolute max to drive 4 speaker channels with 2-Ohm speakers.... which I don't think you're ever going to pull with music, even if you tried. If you use 8-Ohm speakers instead, you're looking at only about 1.1 Amps per channel - and again, that is absolute maximum at maximum (RMS) volume (after that, you'll get ugly distortion). So if you drive two 8-Ohm floor speakers, your current won't be more than 2 Amps.... which is approximately the same load as ONE 50W 24V incandescent bulb or two 25W 24V bulbs... and again, I want to imply that this is at MAXIMUM volume.

In reality, even 1 W of output power will be very loud, especially with enclosed speakers. So at an average listening volume (a few hundred mW with efficient 4-8-Ohm speakers), your radio won't draw more than 200-500 mA of current.

Probably a rough guesstimate is take speaker watts, add them up, multiply by 3, and divide by volts...
Just a rough guesstimate to get in the ballpark...

Interesting guesstimate, but where do those numbers come from?

The speaker power doesn't really matter here. Only the impedance does, as well as the amplifier's input voltage (and whether the amplifier is set to run in BTL or single-ended mode). This is what will determine the maximum power that can be fed in a speaker.

It will not be big constant current draw any way when listening to music.

Exactly!

BTW, you can also look at that lamp Wattage being used right now to see what the transformer can handle at this moment.

+1

BTW, if you have spare 24V transformer with center tab then you can use it as buck step down to 12V.

Yup.
That's why I suggested a UPS transformer. The bigger ones sometimes have a center-tap.

[eccerr0r]

Re: 24v to 12v project guidelines

Interesting guesstimate, but where do those numbers come from?

Again it's only a guess if you don't have any other information... just making the generalization that audio amplifiers have less than 33% efficiency converting power to speaker power. Of course some are better, some are worse, and the volume matters; so take it with a grain of salt.

Also there's the assumption that the speakers are matched with the amplifier (as well as people not using the PMPO number.)

[Dannyx]

Re: 24v to 12v project guidelines

Guess what: this is all fine and dandy except something just came up: the radio he wanted to use is locked, so the project is on hold until he gets a new one

With all seriousness: it's a "panasonic music 5" out of a Mercedes benz....any way of cracking radio codes ? Anybody got any sources/ideas/whatnot ? I can provide the exact label.

[momaka]

Re: 24v to 12v project guidelines

Again it's only a guess if you don't have any other information... just making the generalization that audio amplifiers have less than 33% efficiency converting power to speaker power. Of course some are better, some are worse, and the volume matters; so take it with a grain of salt.

Class AB has about 50-60% peak efficiency (usually near maximum output power), and class B about 65%, IIRC.

A better assumption for peak power consumption is speaker output power times two times the number of speakers... but again, the maximum speaker output power will depend on the speaker's rated impedance. (With a single-ended amp configuration @ 13V, you won't be able to push more than approximately 2.5 Watts RMS into an 8-Ohm speaker. And with a BTL/bridge setup and same voltage, the limit is about 10 Watts RMS for 8-Ohm speaker).

Also there's the assumption that the speakers are matched with the amplifier (as well as people not using the PMPO number.)

Speaker matching doesn't matter that much at low power levels (1-2 Watts or less).

Guess what: this is all fine and dandy except something just came up: the radio he wanted to use is locked, so the project is on hold until he gets a new one

Hmmm... you mean stolen?