Power supply advice

OK, aside from the usual taping-LEDs-to-batteries antics, this is my first serious foray into electronics and circuit design. I'm looking to be educated, so don't feel like you need to be gentle ;-)

I've got an LCD panel I'm interested in tinkering with. According to the spec sheet it draws 4.3W at 8V DC (peak). My first challenge is to actually get ahold of a reliable 8V DC power supply. I already have access to a perfectly adequate 12V supply (specifically, a 12V 1A wall adapter); my thought is to start out with that and hack up something that will give me 8V, and worry about fabricating a more direct power supply later. My eventual goal is a more-or-less self-contained unit, so variable workbench supplies etc. are out of the question.

What I'm not sure about is getting clean power at the required voltage. My back-of-the-envelope work indicates that a simple 2-resistor parallel voltage divider would produce enough heat to flame broil my apartment, so unless I really goofed the math, I'm figuring there's a better solution out there


I'm working through my copy of "The Art of Electronics" and trying to sort this out as I go, but to be honest I'm running out of patience and itching to get my hands dirty.

So, the heart of the question - what's a good way to get the voltage I'm after? Some kind of zener setup? A regulator IC? Some other bit of wizardry I've not yet learned of?

Any/all input greatly appreciated

You are on the right path by using AoE - fantastic book.

Some points:
- your "12V" wall wart is probably not really 12V. It's unregulated so as the load increases, the voltage will drop. It *should* give 12V at what ever current it is rated for but don't count on it.
- you are correct about a resistive divider to get the voltage down. 4W at 8V is .5A. Get a voltage regulator and use your wall wart to feed it. 7808 fixed or a 317 variable will do the trick. datasheet has the schematics. it's just a couple of conmponents plus the Vreg. You will need a heatsink with that kind of current.
- I would oversize the WW current by at least 50% over the projected system draw.

good luck.

First, by starting with a wall wart or other pre-manufactured DC supply you are saving yourself some trouble as well as reducing the risk to yourself and others - in terms of electrical shock or fire.

If you have 12 volts and need only 8 volts then you need to do something to result in a 4 volt drop at your current requirements. Whether you use resistors or linear regulators (7808, LM317, etc) the result, in terms of heat, is the same. The regulator will keep the voltage relatively constant where the resistor offers no control. A switching regulator might be useful here but it's more complicated.

Whether 7808 or LM317, look at the datasheets to see what minimum voltage is required for a given output - for the 317 it's probably 2.5 volts or less. That suggests you need at least 10.5 volts input minimum at all times for the regulator to work. You might test your supply with some kind of load - possibly light bulbs from a car - so you can simulate 1/2 amp or so draw - then measure the output of your wall wart. You might also build the regulator circuit and just try it - measure the output of the wall wart before the regulator once you have your LCD panel connected. What you won't see are any momentary dips below 10.5 volts - though if the LCD works, that's what matters.

I forgot to mention that a fuse following the wall wart, rated for the current limit of the wall wart or possibly 25% more might be a good idea. If the wall wart is rated for 1 amp and your load is 1/2 amp then fuse it for 3/4 or 1 amp.

Keep in mind that the current rating of the wall wart might be conditional - based on some duty cycle and ambient temperature.

If your AC adaptor is can output more than 1A then I wouldn't worry about a fuse since the LM7808 will shut down if short circuited.

The fuse will protect from mistakes or failures between the regulator and the wall wart including any wiring. Now, some of the wall warts are protected as well however it is cheap insurance, in my opinion.

his 12V WW, if rated for more than 500 mA will probably output something like 15V unloaded and 12V at the rated current. So, I suspect it will be fine.

If using a fuse, use a slow-blow one. The transformer will handle momentary overloads (it takes some time to get hot) and as hero pointed out, shut down happens at 1A or so. a fast blow fuse will be more of a nusiance than anything else.

stevez said:

The fuse will protect from mistakes or failures between the regulator and the wall wart including any wiring. Now, some of the wall warts are protected as well however it is cheap insurance, in my opinion.

Click to expand...

If you want to play the "What if?" game then what would protect you if the wiring between the adaptor and fuse fails?

Hero999 said:

If you want to play the "What if?" game then what would protect you if the wiring between the adaptor and fuse fails?

Click to expand...

The 'protection' that's already incorporated inside the wallwart, as with any use of a wallwart.

Have you considered modifying the 12V wall adapter ?

Inside you can probably trace the circuit and find a resistor divider between the 12+ output and ground. It will probably be fed into the 'Sense' pin of a linear regulator or switcher.

Try monitoring the voltage output, and bridging one of the reisistors with a 100k resistor to see if it affects the output.

You might be able to replace 1 resistor and turn your 12V into 8V.
.. but do try not to electrocute yourself.

Nigel Goodwin said:

The 'protection' that's already incorporated inside the wallwart, as with any use of a wallwart.

Click to expand...

But the protection often isn't resetable, once you've blown that thermal fuse you need to buy a new adaptor. Some use resetable bi-metal strips but not many I've seen.

Cool, this is pretty much exactly what I was looking for.

Looks like I can pick up a 7808 for around 50 cents so I'll go ahead and pursue that route first. I've already set fire (literally) to a couple of wall warts with accidental shorts so I'll definitely be putting a fuse in the circuit as well


As for modifying the adapter itself - I'd really rather not mess with it, at least not at this phase. Just getting the casing open would pretty much ruin the adapter (it's a cheap one) and even then my fine-detail soldering skills are pretty horrid, so I'm not comfortable trying to screw around with replacing resistors in there.

If this project goes where I want it to I'll need the ability to draw significantly more than the 550mA or so needed for the LCD, so I'll likely end up acquiring/building a switching supply to handle that. In any event though that's quite a ways in the future.

If you're so concerned about losing the wall wart, and wasting money on blown fuses, use a Polyswitch rated for your application soldered in series with your transformer secondary leadout and wherever it was connected to in the wall wart, you will however, need to open it up to do that, something I would not recommend. As Nigel stated, in response to Hero999's comment about the wiring between the wall wart and the application, you're either at the mercy of the protection inside the wall wart, or sh1t outta luck, especially if it's a cheap unregulated supply. If it's a half decent regulated supply, there's a very good chance it already has thermal protection built in and should save it going in the bin in case of an accident, more so if it's one of the newer switching supplies.

StopGo said:

Have you considered modifying the 12V wall adapter ?

Inside you can probably trace the circuit and find a resistor divider between the 12+ output and ground. It will probably be fed into the 'Sense' pin of a linear regulator or switcher.

Try monitoring the voltage output, and bridging one of the reisistors with a 100k resistor to see if it affects the output.

You might be able to replace 1 resistor and turn your 12V into 8V.
.. but do try not to electrocute yourself.

Click to expand...

Maybe for a regulated wall wart but most aren't. A typical ww is just a transformer, diode (bridge, probably) and a filter cap. There isn't any resistor to change. If the WW doesn't say "regulated", it isn't

You might be able to do it with a regulated wall wart IF you can get it open with out ruining it. I don't think it's worth it. By the way, you can get used WWs for a dollar or so from thrift stores.

ApochPiQ said:

Cool, this is pretty much exactly what I was looking for.

Looks like I can pick up a 7808 for around 50 cents so I'll go ahead and pursue that route first. I've already set fire (literally) to a couple of wall warts with accidental shorts so I'll definitely be putting a fuse in the circuit as well


As for modifying the adapter itself - I'd really rather not mess with it, at least not at this phase. Just getting the casing open would pretty much ruin the adapter (it's a cheap one) and even then my fine-detail soldering skills are pretty horrid, so I'm not comfortable trying to screw around with replacing resistors in there.

If this project goes where I want it to I'll need the ability to draw significantly more than the 550mA or so needed for the LCD, so I'll likely end up acquiring/building a switching supply to handle that. In any event though that's quite a ways in the future.

Click to expand...

Why don't u buy a used PC power supply? Something like 2-3$ (I bought one for that price). Even an old AT/200W will give u 7-10A/12V. It has quite eficient overload protection. U can find info about wiring the source on diferent threads.

see this:
http://web2.murraystate.edu/andy.batts/ps/POWERSUPPLY.HTM
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply
If you'r interested

I've got literally half a dozen ways to get 12V power, including an AT power supply and a couple ATX supplies. I mentioned the wall wart because it happens to be what I've been doing most of my tinkering with (as it's cheap and expendable).

The real concern for me is getting from 12V to 8V cleanly, and now that's solved (I've got a handful of 7808s due to arrive Monday).

ApochPiQ said:

I've got literally half a dozen ways to get 12V power, including an AT power supply and a couple ATX supplies. I mentioned the wall wart because it happens to be what I've been doing most of my tinkering with (as it's cheap and expendable).

The real concern for me is getting from 12V to 8V cleanly, and now that's solved (I've got a handful of 7808s due to arrive Monday).

Click to expand...

Wel..yes, but the entire discusion was revolving around fuses, 500mA rating etc. so I joined.
Btw....I'l rather use a LM317 for regulating the 8V. It's more reliable. But I gues it's to late for that.

Tarsil said:

I'l rather use a LM317 for regulating the 8V. It's more reliable.

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Oh? A 7808 lasts one million years. An LM317 lasts two million years. Does it matter?

audioguru said:

Oh? A 7808 lasts one million years. An LM317 lasts two million years. Does it matter?

Click to expand...

lol: Good point lol: What could I say...I'm a 317 fan
All thought the 317 has several advantages like lower riple (not important for this app). Anyway, due to being a variable regulator the circuit can be easily modified for a diferent voltage, curent sources etc., whitch offers solutions for future projects. And I fried enought 78xx for not trusting them. lol: