Help with simple electronics question - convert 5vdc to 4.5vdc?


I know there are probably lots of groups and boards out there where a question like this would belong, but I’m not a member of any others…and most Wooters are pretty high-tech in general.

So, before I go digging through lots of electrical engineering webboards, maybe someone here can give me a clue.

On a scale of 1-10, 10 being a PhD in eletrical engineering and hands-on practical circuitry building, and 1 being Cletus the SJY who doesn’t know what a battery is, figure I’m about a 3.

I can solder, I can follow a basic circuit diagram with switches, resistors, capacitors and diodes, and I understand the CONCEPTS of Ohm’s law, the differences between volts and amps (current) and such…

But, in practical stuff, I’m pretty clueless. I have what I assume is a very simple question, but I’ve no idea how to do this.

Simply put – I want to do some geeky projects with laser pointers and LED pens that usually run on 3 1.5v “watch batteries.”

I figured the minor difference between 4.5v they need and the 5v coming from my computer power supply and/or USB ports wasn’t a big deal, and wired up some stuff to run off that supply.

After a few hours, the LED pens and laser pointers were dead, leading me to beleive that in fact that .5v is pretty important. :slight_smile: (this was junk from a dollar store, no big deal, but obviously I want it to last more than a few hours…)

What’s the simple way to drop a 5vdc power source to 4.5vdc? I don’t know how much current (amps) I’m using but figure it’s far, far under the 500ma total of a USB port…

Do I just wire in a resistor? If so, how does one compute that? Using Ohm’s law I’d need to know the “current” and I’m not sure what that means - current the load (laser pointer in this case) is going to draw, or max current over the wire, or what?

Googling to this point has lead me to using a simple “voltage regulator” to do this, but they seem to come in standard outputs of 5, 12, and 24 volts…which isn’t going to work, as I need to go from 5vdc to 4.5vdc.

I could probably get away with lower, such as tapping a 3.3v wire off the power supply, but seeing as that voltage is going to much more sensitive items on the motherboard than that which is coming out of the USB ports or the drive MOLEX connectors, I’m leery of touching those wires.

Sorry for the huge post. I’m hoping it makes sense, and someone can kindly and without TOO much pedantry and/or scorn for my ignorance help me out here. :slight_smile:

(Side question, that I would have thought I could figure out in 30 seconds on Google but can’t as all answers are either over my head or presume I’d already know this: How the heck does one use a multimeter to figure out how much a device will draw in amps? I’ve NEVER been able to figure that one out…I’ve got a darn fancy multimeter here but when it comes to such a simple thing I can’t figure it out, does it go inline with the power source (wired in series) or across the +/- (parallel) while the device is running, or is it hooked up in lieu of a power source, or what?!)

Thanks in advance!!



I was waiting on Blue to answer this, but it seems he is away. I could easily be wrong about this but…

A resistor will create more resistance (Ohms) and reduce the Current (amps). Most likely your problem is that you are running to much current through the LED’s. If they are 4.5 Volts, I doubt .5 volts would matter much. A comp power supply is WAY overkill for tiny LED’s. That being said, I believe most LED’s run on about 2-3 volts.

Do you have any idea how much current you need for the LED’s?


I would recommend using AA batteries to mess around with.

Blue is the one you need, I’m not even shure it matters what the voltage is, I think the Current is all that matters, provided you have enough voltage.

But like I said, I don’t know.


Well, thanks for the replies guys…I was starting to worry after 24 hours and no replies, thinking, “What, how can NOBODY on Woot at least have an opinion for this kinda question…” :slight_smile:

I have always been under the impression, though, that “too much current” isn’t really possible, in that a circuit will only draw what it needs. I get this from my automotive background, where as you know as long as a device is 12 volt, whether it’s a tiny little radar detector or a pumpin’ bass subwoofer, you can wire it directly to the battery and it’s no problem, it’ll draw only as many amps as it needs and no more.

But…I could be completely backwards on that. Could be it’s both, i guess, since I was overdoing the voltage a little - maybe that causes a corresponding over-draw on current, too?

That’s exactly my problem when it comes to this stuff, I only know the barest surface-level details…rules of thumb and common sense only go so far and often are wrong on deeper analysis, eh?


I think that you have got it fliped. Best bet is to wait for Blue or Qwerty to reply, but that could be some time.

Edit: Also, I don’t think I understand what you are saying.


Well, I shouldn’t say the voltage doesn’t matter at all, but I think it has a lot more give with most things. Again, I could be wrong.


Here is a link that might shed some light on the subject:


It’s not the voltage-it’s the current you’re feeding them.
LED’s have a wattage rating. You can have approximately the right voltage within a range of a volt or so, but if you’re pushing a tsumami of current through them they’ll burn out.


OK, but what about the reverse - ie, if I have the voltage dead-on (say, a 5v LED on a 5v power line) and I hook it up to a power line that may go all the way up to, say, 5 amps, but they only “need” 100ma, will that be a problem?

Because I still don’t understand, then, how you can wire up a simple 12v light bulb to a car battery, or a 500watt subwoofer (or the car’s starter, for that matter, which will draw 200-400 amps briefly) and not need anything special to inhibit the current flow.

I know I’m missing something but not sure what it is.

And, let me redirect the question a little bit anyway, back to the original:

If I want to run a little toy or something that usually takes 3v or 4.5v from the 5v @ 500ma USB power line, what do I need to put in my power circuit to make it “safe” - something to lower the volts, the amps, or both?


To reduce the voltage in a circuit you need a resistor. Devices will draw only as many amps as they need (as long as it is available to them) but it has to be delivered to them at the correct voltage or else they get fried. Think, would you rather have someone toss a handful of bullets to you or fire them out of a machine gun? You can run with that analogy as you’d like.

This is why they have transformers on your electrical poles. You could have AC electricity coming through the lines at 7,000 volts or whatever, but then that has to be taken down to 120 volts (in the US) to be used with your appliances. Most of the things you plug into your wall then have their own transformers to reduce the current to their specific voltage. This also explains why you need special converters or transformers when you travel to countries that use 240 volt systems. Plug in your 120 volt American appliance and boom.

Amps in a car stereo setup are designed to work with a specific voltage battery (with variance taken into account). Fuses are added to these systems to keep them from drawing too many amps if there’s a surge, the amps peak above what is available to them or something else goes wrong.

Check this out.


Hi, let me first say that I don’t know anything about electrons. Electrons bad! They have quite the negative aura about them. Anyhoo, everybody else that has already answered you knows much, much more about matters electronical than I do.

That said, I have a couple of questions, just because I’m curious about this mystifying topic…

What’s a battery? :tongue:

Dumb question: did you just hook up the power source to the battery leads in your pen, or did you take apart the pen and hook up the LEDs directly to the power source?

Another dumb question: did you check the voltages with your multimeter before hooking it up to your LED pen, just to make sure that you weren’t accidentally hooking up a 12 V lead?

Yet another dumb question: have you considered just buying an AC adapter that has, say, 4.5 V and max 1000 mA output? Here’s one on ebay for $10 (not including postage) rated at up to 1000 mA, selectable voltages of 3, 4.5, 6, 7.5, 9, and 12 VDC output.

Sorry, pedantry and scorn is the going price here. “JDFensty, you ignorant slut.” :slight_smile:

I’d guess in line with a DC power source, or clamp the AC current clamp around the AC line (if your multimeter is that fancy). Don’t you have a manual to go with that fancy multimeter? Read this.


OK, thanks again to Blue and Qwerty for contributing, lemme just sorta answer everything at once here.

Blue, I follow 100% and you’ve confirmed what I already thought.

So, a more simple, direct question: What ohmage (is that a word? :slight_smile: ) resistor do I pu in to step 5v down to 4.5v? I can’t quite figure that out…

Q: I could just use a DC converter like that (I’ve got at least two in my junk drawer at the moment) BUT I don’t want to add that much complication/extra “stuff” into my project when I’ve already got a power supply in there that happily provides 12v and 5v to work with…knowwhaddimean?

I’ll go ahead and explain the two projects I have been messin’ with that started this post…

#1 - at a dollar store, I bought 3 of those little “glitter lamps” that’s similiar to a lava lamp but just has some sillver glitter in it. Want to incorporate two of them into a case mod project. However, they CONSTANTLY blew out the 5v flashlight-style light bulb in them, probably from overheating, which is probably why they were in the dollar store to begin with. :slight_smile: They run off the USB port (normally) and hence have a 5v incanescent bulb inside…

At the same dollar store, I bought two of these goofy little keychain doodads that run on 3 “watch battery” button cells (4.5v total) and have a laser pointer in the center surrounded by 3 different colored LED’s that flash on/off in various patterns and sequences. No doubt you’ve seen similiar junk, right?

Well, what I did was took the little flashlight/laser pointer apart, stripped it down to the tiny little circuit board/LED/laser assembly, desoldered the switches and wired the laser to be “always on” and the LED light switch to a pair of long leads to the front panel of the case. Wired up the +/- of the USB power cord to where the battery leads had attached. (yes, polarity was correct…)

Device worked like a charm for several hours while I was tinkering with other things, then I left it on to burn in (literally as it turns out) overnight and next morning…the laser pointer no longer lit up at all and the LED circuit seemed fried - only the red LED would come on and only on/off, no longer flashed or faded in/out.

PRoject #2 was much simpler - instead of the standard green LED “power on” indicator, I took another dollar store laser pointer and wired it up to a 5v (red/black) pair on a spare MOLEX connector so it would turn on with the power supply, and shoot the laser dot onto the ceiling over the computer. (eventually, woud have done something more interesting with that, but this was just a feasibility test.) Same thing - ran fine for the first few hours, but durning the overnight burnin seems to have fried itself to the point where the laser no longer lights - there is a very, very faint red glimmer from the laser LED, but only if you look directly at it. :slight_smile:

I can only assume the extra .5v was the culprit, because as a control I left the other keychain like this “on” all night long the next day running on it’s 3 batteries and it did not burn out, so it wasn’t simply over-use or overheating from being left on for 12 hours.

So back to the original point – what resistor ohmage do I need to put in the power lead (the positive or negative lead? I assume positive?) to cut that 5v down to 4.5v?


Ahh, Blue, thanks for the link to the casemod page, from there there was a link to a LED resistance calculator which pretty much did what I needed.

I know that all that was doing was working out Ohm’s law for me, but what I wasn’t sure of in applying Ohm’s law was…when it asks for current, does it mean the value of the current that’s available (total current) or the value of current that the device needs to draw in normal operation. From dissecting the source on that LED calculator page I pretty much get it now, it’s the current draw that’s needed.

If anyone was curious, here’s pics of my last 2 casemods and one in development…not even sure where these laser/LED toys will get incorporated but it’s been fun playing with them anyway… :slight_smile:

(the laser pointer, I think, is going to go in the old 1940’s radio/phono so that when you open the lid, it turns on the laser and projects some kind of image - dragon, skull, spider, whatever i can find that looks cool - onto the inside of the open lid.)


Oh… I see what you where saying now.
I thought you just wanted to power the LED’s I didn’t understand you where making a comp case.
BTW, your cases look really cool. If you want a bunch of lazer image thingy go to fleamarkets in florida, they have tons.


Word to the wise, stock up on Victory Pig and Tastykakes before attempting any lengthy projects. :wink:


Glad you seem to have it figured out, but something that might help others in similar cases, the LED’s are rated for the current they can accept, not that they will draw. LED’s don’t really have much resistance so they must be combined with a resistor (of size determined by LED brightness, color, etc- should be written on package.) to keep it from essentially being a dead short. Not it does have SOME resistance, but unless you get one that says “built-in resistor” it will break soon after applying voltage.