I need a simple 120/240 Vac to DC power supply design to illuminate a single LED.

Any suggestions?

 

Transformer, diode, resistor.

 

Transformer buddy I highly suggest it for you.

 

A wall wart is premade, dirt cheap, and works well. You can even get them is really small sizes.

We get a lot of posts trying to make it simpler, you can not eliminate a transformer safely (a wall wart has one included). Without a transformer the circuit is dangerous, and the threads trying to go this route get closed very fast.

 

An old cell phone adapter\charger will be ideal for the job...ask around. If not check out item # G17395 at goldmine-elec.com. The item is only $2.29 but you will get hit with a shipping charge.

I bet that you will almost certainly find a friend that has had a cell phone and upgraded or gotten a new one and they have an old one lying around. I don't know what the Forward current is for the single LED you intend to use, but these small cell phone adapters will drive even a Luxeon variety.

 

Given the small current requirements for a typical LED you could probably drive 50 or so off a phone charger (5V @ 1A).

 

Drop by a few thrift shops, most have "orphaned" AC adapters laying around that they've received in boxes of stuff and haven't a clue what they fit. I've picked up quite an assortment for 50 cents each, some are even regulated 12V @ 2A output.

Just remember to put the proper resistance in series with your LED.

Ah, but you want 120/240V? Autoselecting or can you flip a switch when you change input voltage?

Is size a consideration?

 

Thanks for all the replies, however I guess I didn't explain myself very well.

For the lack of better terms, I am designing a custom power strip on a circuit board for my own hobby bench. I am making 2 different ones, one for 120Vac and the second for 240Vac. They will plug into the appropriate wall outlet and have a switch to power up. What I am looking for is a simple design to illuminate an indicator so I know they are energized.

I have been using a design based on the description in Volume 3 Chapter 3 titled "Special-purpose diodes" http://www.allaboutcircuits.com/vol_3/chpt_3/12.html for my AC power indicator. It is a simple RC in series with an LED. I designed it exactly as described and it works, but I have had LED's fail when I plug things into the energized power strip. I was also advised against using that design because there is no isolation from the primary and it can be dangerous.

Neon pilot lamps or a transformer/diode/resistor design has been suggested. I haven't had much luck finding neon lamps and my enclosure doesn't have much room I am pressed to find a transformer that will fit. I only need 20mA off of 120Vac or 240Vac and thought it would be simpler than I am finding it to be.

Any new thoughts now? - Thanks in advance!

 

Uhhh, neons are common and dirt cheap. What part of the world are you in?

Neon lights (all types, but especially indicators) were the LEDs of the world before there were LEDs. Unless you are in Antarctica, I bet I can find a source in under 2 minutes using Google. Hardware stores usually sell screwdrivers with neon indicators to show whether a wire is hot or not.

Newark, McMaster Carr, Granger, Allied Electronics, any industrial outlet would have them.

 

We don't know where you live, so we can't be sure to tell you where to look.

Please edit your Profile using the "User CP" link near the top, and at least fill in your Country of residence in the Location box. Province/state and city are optional, but if your country is quite large, state/province are quite helpful.

If you live in the USA, there are many distributors who carry power switches that have integrated neon lamps w/resistors that work very well on 120v or 240v. They are perfectly safe to use. The wiring to the neon bulb is completely enclosed in the switch housing.

The diagram that you are referring to in Volume 3, Chapter 3 has been determined to be unsafe, and a request has been put in to remove it from the E-book; it was somehow put in by mistake. Capacitors frequently fail as shorted, which would cause full mains power to be applied to the circuit, which would be very unsafe. It cannot be made safe unless there is some form of transformer to isolate the circuit from mains power.

 

SgtWookie I'm sorry I am fairly new to this forum, or any forum for that matter, so I hadn't entered much in my profile. I live in Montana, USA.

I would love to find a Neon replacement that I can just solder into the LED holes in my circuit board. I could remove and jumper the series capacitor and I do have a series resistor that I could use or jumper if not needed.

You help is greatly appreciated!

 

Capacitors frequently fail as shorted, which would cause full mains power to be applied to the circuit, which would be very unsafe. It cannot be made safe unless there is some form of transformer to isolate the circuit from mains power.

They make specialized capacitors that are meant to deal with AC mains often called "AC suppression capacitors" - usually rated at 400 or 630 VAC and are constructed with an integral safety "fuse" inside so to speak.

 

If in a sealed unit said capacitive LED power supplies are safe, but they are not usually safe if constructed by a hobbyist or if expected to be opened.

 

Yes my project will be in an enclosed, glued permanently, enclosure when I get it finished. However I am dealing with the LED burning out on me so I am trying to solve that problem. I currently I am using R=1.2k, C=0.22uf in series with LED and I have a 1N4002 parallel to the LED in the opposite polarity.
I have also seen a design that uses a Zener in parallel to the Resistor + LED in reverse polarity, I wonder if that may solve the problem?
Any thoughts?

 

Please don't attempt to do that.

We have to be extra careful here. Mains power is nothing to fool around with. We want for our members to have long, happy and productive lives. Electrocution can end a life in moments.

Neon indicator lamps work well because they take a fairly high voltage (55v-60v) to ionize the neon gas in the tube. Once the gas is ionized, a high-value resistor in series with the lamp keeps the current flow very low; on the order of a milliamp.

Radio Shack stores typically carry several neon indicators; there is likely a store near you. Have a look at this link:
http://www.radioshack.com/family/in...120+Volt&filterName=Type&filterValue=120+Volt

They also carry lighted switches rated for 120v:
http://www.radioshack.com/product/index.jsp?productId=2062522
http://www.radioshack.com/product/index.jsp?productId=2062519

You should not use a power strip that is rated for 120v with 240v. The outlets would not be rated for the higher voltage.

Your strip should have a circuit breaker or fuse that is rated at or below the lowest current rating of any item in the strip; ie: if the outlets are rated for 120v 10A, then use a 10A 250V fuse or circuit breaker.

A few words on insulation:
Use heat shrink tubing on your connections after you have soldered them using 63/37 (tin/lead) or 60/40 solder and rosin flux. Once properly applied, it will not come off unless you cut it off.

Do not use "electrical tape" or "electrician's tape", as the adhesives used on this kind of tape will eventually get "gooey" and the tape will come off. There is a type of RTV silicone tape which is rather expensive, available in some auto supply stores marketed as "Extreme Tape". It adheres to itself when wrapped under tension. However, it's somewhat difficult for n00bs to apply correctly the first few times they try it, and it has a fairly short shelf life. If you go that route, plan on using up a roll just learning how to wind it on properly. However, if properly applied, it's about as good as shrink tubing, if a bit more bulky.

Don't use twist-on "wire nuts" either, unless your finished project will be permanently mounted somewhere. Wire nuts can loosen up under vibration, so are not really suitable for portable devices.

I do not like the crimp-on style terminals that are available commercially. The crimp that a typical inexpensive crimper makes is really not all that good, and copper stranded wire inside the crimp will eventually corrode, making a high-resistance joint. The 4-point crimping tools used for connectors used on military specification projects work very well, but they are prohibitively expensive. If for some reason I need to use terminals, whether ring, spade, or whatever, I remove the factory insulation, slip a piece of shrink tubing over the wire, crimp it a bit, then solder the wire to the terminal, and shrink the tubing around the connection. That way I know it will not fail, and will be well-insulated.

For sealing the enclosure, consider using RTV silicone. If you epoxy the thing together, you'll have a very hard time getting it apart if you ever need to do so. RTV silicone will provide a watertight seal that is reasonably strong and electrically insulative. However, remember that most RTV silicone emits ascetic acid until it cures, which is somewhat corrosive. GE makes a red RTV silicone which is specifically designed for such projects; can't remember the number offhand.

 

For 110v, Radio Shack carries a number of neon indicators (272-707 for example). You can use it with a 68k series resistor for 220v. Mouser sells 607-1051A3 (220V) and 607-1050A3 (110V); both are orange neon indicators with wire leads.

 

Ok, I found Mouser has some Neon bulbs I would like to try.
http://www.mouser.com/Catalog/catalogusd/642/112.Pdf

Can anyone tell me what kind of a circuit I need to drive them? I have never worked with a neon bulb before so I don't know what they take.

 

Hello,

Better have a look at the neon lamps on this page (see PDF) of mouser.
They are ready to use on 120 Volts.
(there are also 230 Volts versions).

Bertus

 

Ok, I found Mouser has some Neon bulbs I would like to try.
http://www.mouser.com/Catalog/catalogusd/642/112.Pdf

Can anyone tell me what kind of a circuit I need to drive them? I have never worked with a neon bulb before so I don't know what they take.

It would be preferable for you to use a neon indicator that contains the appropriate resistor already, as they will be UL approved.

If you feel that you must use one of the neon bulbs on that page, I suggest that you use one of the the Type C bulbs.

Let's take part# 506-B1A as an example.
Breakdown voltage is 65 min, 90 max - so it'll work fine on 120v.
Driving current is 0.300mA, or 300uA (microamperes)
Once the neon breaks down, there will be practically no voltage drop across the bulb. So, a resistor needs to be selected to limit the current to 0.3mA

120v / 0.3mA = 120/0.0003 = 400k Ohms.
A table of standard resistance values is here:
http://www.logwell.com/tech/components/resistor_values.html

Use the E24 columns. 390 and 430 are standard E24 values. 390 would allow a bit too much current, so select 430k Ohms for maximum bulb life.

If you wish, you could opt for the E48 402k value, but when you compare prices, you'll find the 402k resistor will cost somewhat more than the E24 value 430k.

Next, you need to calculate the power rating requirement of the resistor.
P=EI, or Power in Watts = Voltage * Current(Amperes)
120*.0003 = 0.036 Watts, or 36mW. We double that for reliability, so 0.072W.

You can use a 1/10W or higher rated resistor. It should be completely enclosed in shrink tubing. The resistor goes on the hot (line) side; the black wire; the narrower of the two flat prongs on the cord's plug.

The breaker/fuse must be between the cord's line wire (black) and the switch.

 

I would love to find a neon bulb with resistor all in one that I could just solder into my circuit board. Do you know a source I can find them?