Author Topic: 24v to 5v power supply help (Read 10547 times)

Treehouse · « **on:** March 28, 2011, 05:41:20 PM »

Hi, all I was hoping you could help me out I'm trying to build a 5v power supply for use with my 24V system. It needs to be stable at 5v and max current no more than 500ma. I have tried using a LM7812 to a LM7805 so each regulator was sharing the heat loss. Even with heat sinks they are still running very hot when the system hits 28.8V or more input. I have checked that they are wired and have correct capasitors where needed. It's driving me nuts. I have an LM7805 with no heatsink running the same load, working just fine on a 12V system that gets up to 14.4V and it never feels warm. Well I hope the Electronics guys here can help me out. Thanks..

Madscientist267 · « **Reply #1 on:** March 28, 2011, 06:13:21 PM »

Hate to spoil the fun here, but your best bet is just to use a cell phone charger, rather than building one.

They are very efficient (buck converter) and can take a very wide range of input (from like 9 to 30V) and will put out the 500mA that you're looking for.

They can be got for cheap at wally world. Go for the USB port type to make your life easy.

Steve

ghurd · « **Reply #2 on:** March 28, 2011, 06:34:10 PM »

Is the powered device something USB powerable?

Many of the car lighter adapter plug USB things are switchers.
More efficient, about no heat.

Not sure, but I believe everyone I ever had said "12~24VDC input" on the sticker.

Can get them on ebay for $0.99 with free S&H. "car USB"

Guts,

In use,
(that cell phone wore out, and the new one is charged via a factory supplied standard USB to micro-USB cable with no problems on any adaptor I tried so far)

I was very concerned about connecting some of the expensive items to a 99-cent free-S&H doohickey from China, but never had a problem with about 8 of them.
Knock on wood.
G-

Madscientist267 · « **Reply #3 on:** March 28, 2011, 11:30:03 PM »

That's the one!

They all do about the same thing, provided they can dump enough power.

There's generally no issue with charging a phone (or anything else USB) for that matter from any of these units, Except - the trend among phone manufactures here lately has been to make various forms of 'authentication' mechanisms so that you have to use their charger with it. The phones that do this will simply refuse to charge with the counterfeit charger, and some are so bold as to tell you about it.

The methods vary, some are as simple as a specific value resistor across the data pins that the phone 'accepts' in leu of an actual USB connection with a computer. Others get fancy and go as far as to use a damn chip with an ID in it.

Either way, as long as the polarity is good, you won't kill the phone, it will just ignore it.

Steve

ruddycrazy · « **Reply #4 on:** March 29, 2011, 04:33:05 AM »

G'day treehouse,
It does seem ya pretty handy with a soldering iron so go checkout Romans site where he has designed switchmode step down circuits using minimal parts.

http://www.romanblack.com/smps/smps.htm

Lower on the page he describes how to use them for higher voltages

Cheers Bryan

SparWeb · « **Reply #5 on:** March 29, 2011, 02:41:30 PM »

Forgive me for dumbing this down but-

Can you use two 7805's in parallel with each other to split the load in half?

The datasheet has a switching regulator circuit (FIgure 17): http://www.datasheetcatalog.org/datasheet/fairchild/LM7805.pdf

DamonHD · « **Reply #6 on:** March 29, 2011, 03:24:45 PM »

In general I would not try to do that because one of them is likely to have a slightly higher set voltage or whatever, try to carry too much of the load, and overheat.

It might be possible with (very) small resistances in the ground and output lines to ensure better balancing, but I don't know what they'd be, and regulation won't be as good/stiff.

All IMHO.

Rgds

Damon

joestue · « **Reply #7 on:** March 29, 2011, 06:25:22 PM »

can it be done? yes.

should it be done? meh

the datasheet shows the voltage stays within .1% between 0C and 50C so you're good for avoiding thermal run away.

but... these are not lazer trimmed zenars, you could have a 4.9v and a 5.1v regulator.

how much regulation you will lose depends on how equally you want to share the current
much easier just to split it up betwen a 12v and 5v.

madlabs · « **Reply #8 on:** March 29, 2011, 07:10:05 PM »

I just got a couple Murata 7805SR 's. They are little switch supplies, can take up to 40V in and put out 500mA. Or so says the spec sheet, but I haven't pushed it past about 100mA. So I don't have a wealth of experience with this part but so far it's working for me. They are a little pricey, especially compared to an LM7805. Howeve, they are a direct, pin compatible replacement, so you can just plug it into exsisting projects.

Jonathan

Madscientist267 · « **Reply #9 on:** March 29, 2011, 11:26:33 PM »

Mad-

That's friggin awesome dude! How'd you end up stumbling across those?

I can think of several uses for some of those where low current and tiny packaging are the need.

Traditional 78xx has slowly died as a species in my toy box just because they are so inefficient. I only use them now to cut back on power consumption for small fans, and even then, they're not ideal. I haven't given up on them completely tho because they are so quick and dirty, and for fans, I don't even need a heatsink.

Might have to give these things a go for myself...

Steve

DamonHD · « **Reply #10 on:** March 30, 2011, 01:51:45 AM »

I'm using a 5V switcher 7805 drop-in replacement to power my main server from my 12V nominal, with a rather higher output current than that (maybe 2A?), though its max input is ~27V so no good for you.

The point is that there seem to be many devices intended as 7805 substitutes with varying efficiencies (eg switching vs linear), input voltage range, max current and so on.

Rgds

Damonb

ruddycrazy · « **Reply #11 on:** March 30, 2011, 02:39:37 AM »

Well since my first thread advising Treehouse who did ask for an alternative to a 7805 which is a waste of power going from 24 to 5 volts as 19 volts is lost in heat and the risk of overheating when the 7805 hit above 60% of it's max current isn't the best way to go. Every post since has still gone about those lousy 7805's and even dumbing it down a bit is still a wastefull solution. That link I provided is for a simple 2 transistor and few other parts for a 85-91% efficient switch mode power supply in the region Treehouse was asking. Everything is explained on the page and overall all of the circuits are cheaper to build and more efficient than the 7805. If you guys are that challenged at even simple circuits it does me wonder whether anything said in the future on electronic's will hold any credence.

DamonHD · « **Reply #12 on:** March 30, 2011, 10:00:03 AM »

My 7805 switcher drop-in substitute is ~90% efficient with an input range of 10V--25V, so it can be done, easily.

Rgds

Damon

ghurd · « **Reply #13 on:** March 30, 2011, 12:47:57 PM »

Quote from: Madscientist267 on March 29, 2011, 11:26:33 PM

That's friggin awesome dude! How'd you end up stumbling across those?

They have been around a Long time now.
The 1st ones I bought were $15, never found a use for them worth the cost! By late '06 they were down to $10.
( http://fieldlines.com/board/index.php/topic,142468.msg955759.html#msg955759 )

Quote from: ruddycrazy on March 30, 2011, 02:39:37 AM

If you guys are that challenged at even simple circuits it does me wonder whether anything said in the future on electronic's will hold any credence.

I agree, and disagree. Depends on what the objectives are.

The 99 cent free S&H adapter is a great example. It did exactly what I needed to do. If I built it myself, I would have $8 in S&H minimum. Plus the parts. The lighter plug is $1, the female USB is $1, taking the total (so far) to 10X more than it cost to buy one.

The future of component level DIY electronics is bleak because of the selection of available parts. TO-3 is almost an antique size. Many of the newer parts were never offered in through hole like TO-92, released as SMD only. Many less newer parts are discontinued in TO-92, but still available in SMD.

SMD packages get smaller and smaller, while my eyes get older and older.
G-

SparWeb · « **Reply #14 on:** March 30, 2011, 12:53:55 PM »

Bryan,
I see your point: your suggestion is definitely way better than mine. But: On Roman Black's webpages I see no circuit diagram that satisfies all 3 conditions for Vin=30V and Vout=5V and 500mA, unless I missed it somewhere. Hiding in the text are some instructions on how to modify the circuit for such purposes, but it's a lot of "substitute this for that" to wade through. The alternative is to stack the 24-12 circuit on top of the 12-5 circuit, reducing the efficiency from 80% to 64% and the parts count goes over two dozen. After all that it still doesn't kick out 500mA like the OP needs, so modification is still necessary. The website is a good one, and I plan to read through in more detail because the explanations and tests are excellent. Maybe a higher-power circuit is there, but I haven't found it yet.

Madscientist267 · « **Reply #15 on:** March 30, 2011, 02:16:28 PM »

Quote

through hole like TO-92

Don'tcha mean TO-220? TO-92 is the small black 3 pin with no tab (like the components in the dump controller)...

For everything else:

I don't see the problem here.

A MC35063 (and I'm sure others) based 'cell phone' buck converter for the cigarette lighter is capable of 40V input, and the tranny on the chip can sink 1.5A!

Most converters designed around this chip however utilize the current limiting function, and are set to start backing off at 500mA as a general rule. If you tweak the sense resistor, you can get whatever you need out of it (within reason of course) - well within the requirements of the OP's request.

The only thing you may need to do for input voltages higher than 25 or so is change out the input side filter cap (a small 47uF typically, rated at 25 or 35V). Swap it out for a 50V and be done with it.

I've done it. It works. They are very efficient little converters.

Heat? Goes bye-bye.

Steve

ghurd · « **Reply #16 on:** March 30, 2011, 02:53:41 PM »

You mean MC34063?

I meant TO-92.

Either way, same idea- By a quick count, MC34063, Mouser has 13 in SMD. 6 in through hole dip.
G-

Madscientist267 · « **Reply #17 on:** March 30, 2011, 07:01:08 PM »

LOL yeah thats it heheh

I'm a little off today. Wondered if you were too. Takes one to know one? WORK WITH ME HERE!

It's a rather versatile chip considering its only an 8 pin.

Its the basis for the buck converter for the solar panels ("Commanda's Design")

Steve

ghurd · « **Reply #18 on:** March 31, 2011, 12:52:46 AM »

I had an super extra very extraordinarily rough day, and week too, trust me on that one.

"only an 8 pin chip"? It will be a lot less DIY versatile when the largest physical size can fit 50 on a grain of rice!

Part of my point was the least expensive MC34063 was $0.61. A stamp is $0.44. That is $1.05.
Can have it dropped in my mail box in 4 weeks for $0.99, and ready to use.

Part of my point relates to "Swap it out for a 50V and be done with it." Why bother?

Most of my point was Treehouse is looking for less than 500ma output, 24V to a stable 5V, without linear 78XXs, and anybody, almost anywhere in the world, can get one ready to use for $0.99. And it probably works better than a DIY unit.

Many of 'us' could build a 78xx or 555 or 741 from parts. That doesn't make it a great idea to persue that option in the interest of the pure resulting function, right?
That's all I intended to imply.
G-

ruddycrazy · « **Reply #19 on:** March 31, 2011, 05:35:11 AM »

Well guys this weekend I'll finally be finishing Bruces Shunt controllers now I have all the parts and while I'm there I'll make up one of Romans 2 transistor circuits that will suit a 24 volt battery array. As Treehouse only wanted a 500mA output thats what I'll aim for and show just how easy it is to make. I am a mechanical fitter/machinist/welder by trade and I started learning electronics when I got my first 4wd which needed a total rewind. Instead of going down the pub everynight I sat at home reading the net and learning as I went. I'll do a design on the circuit on veroboard and post a pic here to show just how easy it is too.

Cheers Bryan

Madscientist267 · « **Reply #20 on:** March 31, 2011, 09:11:09 AM »

G -

Just that 24V is awfully 'next to' 25WVDC and I generally don't like to push them that close.

Aging aside even, if there's a manufacturing defect that inadvertently 'tweaks' the cap's abilities in the wrong direction, poof, magic smoke.

Most that I've seen are 35V anyway, so this probably isn't an issue, but was worth mentioning just in case (mainly for other applications 'later' by other readers). I let the smoke out of mine when I wasn't thinking about it during early tests for the solar stuff.

Steve

ghurd · « **Reply #21 on:** April 01, 2011, 07:51:26 PM »

24, 25, 35?
Considering it is a lighter plug, with a sticker that says 12~24V input, I'd be inclined to think that meant about 29V nominal on the high end, and capable of dealing with more.
EU vehicles use 24V in the lighter socket semi-commonly, if I understand it correctly.

The open unit was built into something long ago. Not in a hurry to open another one for a look.
They were not intended to be opened.

G-

MagicValleyHPV · « **Reply #22 on:** April 02, 2011, 11:01:23 AM »

IMO, ghurd makes a valid suggestion.

I bought 3 of these 24v=>5v auto converters at my local secondhand - $0.50 each. If I smoke one or two in the hunt for just one that'll reliably tolerate 28-29v... I'm still $ ahead.

Be nice if I could land a cheap & efficient 24v=>19v (50w) to feed my laptop. Cheapest production units I've seen are $15 each.

News:

Author Topic: 24v to 5v power supply help (Read 10547 times)