Electronics Newbie looking to buy an AC/DC Power Supply for testing LEDs of Unknown specs..

[ToddB66]

Hi All !

I have a grab bag of electronic components w/o specs and would like to test the LEDs to find the safe maximum
volt/amps that would give the brightest light possible without a burnout.

I have an application now to which I would like to add one of the red LEDs. I found a test method on another website that requires a variable power supply. I searched the Internet and found the following unit that sells for under $12.00. Do you think it would be a good one for starters ? Comments and advice needed.

Thanks for your reply !

ToddB66

RS-25-5
AC/DC 25W 5V Single Output Power Supply, Enclosed.
Manufacturer:	Mean Well USA
Status:	Standard
Datasheet:	RS-25-5 Data Sheet
RoHS Compliance:	Yes

Total Power: 25 W

% typ. Efficiency: 78.5

Number of Outputs: 1 Output

V(in) Min: 88 VAC

V(in) Max: 264 VAC

Output 1: 5 VDC

Output 1: 5 A

Cooling: Convection

Operating Temp. Min.: -20 °C

Operating Temp. Max.: 70 °C

Length: 3.11 in

Width: 2.01 in

Height: 1.1 in

Safety: UL, C-UL, TUV, CE, CB

[Skippy]

Sounds over-complicated to me. If you don't know what the parts are, all you can really do is assume they are 20 mA max devices. And for simple tests like go/no-go, checking the colour (of clear-packaged devices) and determining the pin polarity, a PP3 9V battery and a resistor is all you need, or some surplus "wall-wart" mains adaptor from your junk collection, with 6V + output.

[ToddB66]

Electronics Newbie looking to buy an AC/DC Power Supply for testing LEDs of Unknown specs..

Skippy,

Thanks for your advice, however, keeping in mind my goal in the first paragraph of my thread-starter post, i.e. "would like to test the LEDs to find the safe maximum volt/amps that would give the brightest light possible without a burnout.", below is the test method I referred to.
__________________________________________________ ____________________________________________

You need an adjustable power supply, voltmeter, ammeter, a variable (fixed) resistor, and test LED.
Repeat with different resistance values the following until a suitable range of values is achieved.

1. Hook up power supply, variable resistor , LED, voltmeter(across the LED) and ammeter(in series with the LED).
BTW, "variable resistors" are also called Potentiometers.

2. Set the variable resistor at a known resistance.

3. Set the power supply at zero volts or some reasonable value known to be less than the forward voltage of the LED. 2.2 V is a typical forward voltage for a RED LED.

4. Turn the power supply on.

5. Advance the power supply voltage making note of the voltage and current reading

6.Use LED brightness as an indicator of the maximum current. 20 mA is a good rule of thumb for maximum current, and 10 mA is a typical value.
__________________________________________________ ____________________________________________

Back to the Mean Well AC/DC 25W 5V Single Output Power Supply no. RS-25-5, do you think it would be a good one for starters ? Comments and advice needed. Others can feel free to jump in with their views too.

Thanks for replies !

ToddB66

[Skippy]

I still think it's a waste of money buying a PSU.
A few guides:
You don't need to know what the forwards drop of an LED is, in general. As a designer, you would just use a typical figure, and rely on the series resistor to soak up the variation.
And you can't judge how near to maximum output you are by just observing the thing operating. Some modern LED's will leave spots on your retina at 20 mA, yet very plain/low-tech/old ones will be quite dim even at 20mA. And many modern ones will run at much higher currents, eg. white ones for lighting. 1 Watt is becoming quite common, ie.250mA at 3.6 V.
The limit is often determined by thermal factors. A nominal 20mA max LED could probably be run at 50 mA if you went to some trouble heatsinking the legs. All the high-power ones need heatsinking, it's often a key part of their package.
A basic fixed output supply (like the 5V and 12V you find inside a PC), a multimeter, a selection of fixed resistors, and a pocket calculator will sort out most of your needs.
Don't forget that LED's have differing beam angles - ones that are super-bright often achieve this through being narrow-beam. The exact same LED with a different shaped lens will be less bright, yet has the same total output.
If you want to measure the actual output, a simple method is to use a large area photodiode, like a BPW34, with a carefully chosen load resistor, and a voltmeter. The load resistor is chosen such that there's no more than about 100 mV across it. That way, it acts pretty much like a short, the voltage on it is proportional to the illumination on the diode. The photodiode datasheet will give you the calibration figures (in terms of mA per watt / lumen etc). This useful for testing infrared LED's as well.

[DavidB]

I would make a simple adjustable constant current source with a LM317 regulator and a pot to adjust the current limit. The info is on the data sheet. Safest way to do it. Measure the voltage across the led when the brightness appears normal.

[ToddB66]

Electronics Newbie looking to buy an AC/DC Power Supply for testing LEDs of Unknown specs..

I would make a simple adjustable constant current source with a LM317 regulator and a pot to adjust the current limit. The info is on the data sheet. Safest way to do it. Measure the voltage across the led when the brightness appears normal.

DavidB,

See the attachment below. Is that what you are suggesting ?
Sorry the picture isn't focused better; hope you can make out the details.

ToddB

[ToddB66]

Electronics Newbie looking to buy an AC/DC Power Supply for testing LEDs of Unknown specs..

I still think it's a waste of money buying a PSU.
A few guides:
You don't need to know what the forwards drop of an LED is, in general. As a designer, you would just use a typical figure, and rely on the series resistor to soak up the variation.
And you can't judge how near to maximum output you are by just observing the thing operating. Some modern LED's will leave spots on your retina at 20 mA, yet very plain/low-tech/old ones will be quite dim even at 20mA. And many modern ones will run at much higher currents, eg. white ones for lighting. 1 Watt is becoming quite common, ie.250mA at 3.6 V.
The limit is often determined by thermal factors. A nominal 20mA max LED could probably be run at 50 mA if you went to some trouble heatsinking the legs. All the high-power ones need heatsinking, it's often a key part of their package.
A basic fixed output supply (like the 5V and 12V you find inside a PC), a multimeter, a selection of fixed resistors, and a pocket calculator will sort out most of your needs.
Don't forget that LED's have differing beam angles - ones that are super-bright often achieve this through being narrow-beam. The exact same LED with a different shaped lens will be less bright, yet has the same total output.
If you want to measure the actual output, a simple method is to use a large area photodiode, like a BPW34, with a carefully chosen load resistor, and a voltmeter. The load resistor is chosen such that there's no more than about 100 mV across it. That way, it acts pretty much like a short, the voltage on it is proportional to the illumination on the diode. The photodiode datasheet will give you the calibration figures (in terms of mA per watt / lumen etc). This useful for testing infrared LED's as well.

Skippy,

Attached are two pictures showing Top and Bottom views of a small pcb I scavenged from a trashed transformer.

I went to Mouser Electronics @ http://www.mouser.com/catalog/catalogusd/647/170.pdf and looking at the selection of LEDs I noticed that leds come in various diameters, like 3mm/0.118", 4mm/0.158" and 5mm/0.197".

I also noticed that the following leds listed on the mouse chart had Vf (forward volts) almost identical, regardless of the variation in diameter.

3mm/0.118", Red/Grn, Diff, 2.0/2.05 Vf
4mm/0.158", Red, Diff, 2.1 Vf
5mm/0.197", Red, Diff, 2.1 Vf

I'm not saying that the above comparison would hold true for Vf in every case if group comparisons were made with different types of leds , as I haven't done a thorough review. But the above example indicates that the diameter of leds has little to no affect on the Vf , at least for this particular group. In your experience, have you found this to be true ?

I ran across the following tutorial document on the Internet and it includes formulas, such as R series = (V - V f ) / If, where Rseries is the resistor value in ohms, V is the supply voltage, Vf is the voltage drop across the LED and If is the current the LED should see. I thought this might help someone who needs details on LEDs.

Title: Notes on LEDs, by W.E. Johns @ http://www.gizmology.net/LEDs.htm

Thanks for contributing to my knowledge !

ToddB66


......................

[CAS]

This is the circuit for your board. I cant see the resistor colours clearly on your photo but was it was a 5v supply?

I just noticed that even though it says 1N4001 on the board for the diodes, they have used a higher voltage diode (1N4002)

circuit.pdf

[CAS]

I wouldnt get too hung up on voltage drops for LED's. There is no hard and fast rule as LEDs all seem to be a little different. From memory it was around 470 ohms for 12v supply but you could use a 560 and it would be ok.

[Skippy]

Quote:"But the above example indicates that the diameter of leds has little to no effect on the Vf"
No effect whatsoever, it's just a box the led is put in. Semiconductor physics determines Vf, it's basically the 'band-gap voltage' of the particular alloy used - gallium arsenide, gallium arsenide phosphide, gallium aluminium phosphide, silicon carbide etc etc. Infra-red ones are the lowest drop, maybe 1.6V, red, orange,yellow,green are 1.8 - 2.2V, blue and ultra-violet ones are 2.8 - 3.8V.
Just play around, have fun, learn stuff. LED's are everywhere, get hold of some junk cellphones, they have some good LED's for backlighting the display/keypad. If you can get hold of a broken LCD TV with LED backlight, you'll find loads of very high brightness white LED's along the edge of the screen.
Don't forget infra-red: slotted opto switches contain an LED and phototransistor together in a box. They are common in things like computer printers, floppy disk drives, flatbed scanners.
Ripping things to bits in general is educational. There's so much stuff discarded these days, like PC's and all their bits - HDD's, CD drives, floppy disk drives, the PSU module, cooling fans. Learning what makes things tick, and salvaging a few bits on the way, is fun. Ask around your friends and family.

[ToddB66]

Thread: Electronics Newbie looking for AC/DC Power Supply for testing LEDs of Unknown specs..

This is the circuit for your board. I cant see the resistor colours clearly on your photo but was it was a 5v supply?

I just noticed that even though it says 1N4001 on the board for the diodes, they have used a higher voltage diode (1N4002)

[ATTACH]32824[/ATTACH]

Hi CAS !

I can't say what the supply voltage was, as I put the torn-apart adapter in the trash some time ago, but I did jot down the adapter make and number and kept this information. It was a Black & Decker no. 200635HW. I tried to look up the supply voltage on the Internet using this description, but Google and Yahoo didn't provide anything, nor did the B&D Corporate website. I guess adapter 200635HW is obsolete; it was probably about 20 years old.


Attached below are photos of (a) your circuit sketch, the salvaged PCB from the B&D adapter 200635HW (b)Top and (c)Bottom and (d) a sketch I made of the circuit in my Portable Outlet Box, for reference. Concerning the note on your sketch "(can't see colours clearly), I had the same problem, especially with the third color to the right; even using my 10X eye loupe, I couldn't make a conclusive color choice on that band. So I checked both of the resistors on the salvaged PCB, the larger and the smaller one, with my DMM set in the Ohms position and below are the readings I got. Sorry I didn't get all of the attachments arranged properly to match this paragraph...I was having trouble with uploading (no fault of this website.).


Larger (and longer) resistor........ 99.5 Ohms Smaller and (shorter resistor)........ 8.3 Ohms

If possible, I would like to connect the PCB to my Portable Outlet Box circuit, so that the LED comes ON when the switch-knob is turned ON. Up until now, when I get the P.O.B. out I have to use a test light to check if it's ON. Could you please advise me on how to make the connections ?

Thanks much !

ToddB66

[bklein]

Throw the LED's in the trash and go to Digikey and buy what you want.
if you don't understand LED's enough you are wasting your time with this. Get a book on what you really are interested in.

[ToddB66]

Thread: Electronics Newbie looking for AC/DC Power Supply for testing LEDs of Unknown specs..

Throw the LED's in the trash and go to Digikey and buy what you want.
if you don't understand LED's enough you are wasting your time with this. Get a book on what you really are interested in.

Hi Barry !

You might remember me. We had several email communications between February 08 and June 02, 2010 while you were teaching me how to test an AC-DC Power Adapter !

As for LEDs, I have two books that explain how Light-Emitting-Diod which appears es operate and have studied them, so that's not my problem. What I don't understand is how to connect the two wires at the LED-end of my PCB, shown in the following attachment...... ......PCB salvaged from B&D ac-dc adapter 200635HW 001.jpg listed in my previous post to CAS. See my last paragraph in that post, copied, pasted and colored below. There are two other wires at the opposite end of my PCB and on one of these wires you can see a white stripe to indicate the neutral, so I'm assuming these two wires connect to the mains supply, i.e. the power cord of my Portable Outlet Box, a picture of which is also attached to my previous post.

If possible, I would like to connect the PCB to my Portable Outlet Box circuit, so that the LED comes ON when the switch-knob is turned ON. Up until now, when I get the P.O.B. out I have to use a test light to check if it's ON . Could you please advise me on how to make the connections ?

Thanks again for the help you gave me on the adapter in 2010 !

ToddB66

[ToddB66]

Thread:Electronics Newbie looking for an AC/DC Power Supply to test LEDs of Unknown specs..

Barry,

You might have noticed that the first sentence in the second paragraph of my previous post to you was messed-up in one area. I don't know how this happened, but I retyped and corrected the sentence in color below. (The Edit button was no longer available.)

As for LEDs, I have two books that explain how Light-Emitting-Diodes operate and have studied them, so that's not my problem. What I don't understand is how to connect the two wires at the LED-end of my PCB, shown in the following attachment listed in my previous post to CAS....PCB salvaged from B&D ac-dc adapter 200635HW 001.jpg . There are two other wires at the opposite end of my PCB and on one of these wires you can see a white stripe to indicate the neutral, so I'm assuming these two wires connect to the mains supply, i.e. the power cord of my Portable Outlet Box, a picture of which is also attached to my previous post to CAS.

ToddB66

[ToddB66]

Thread: Electronics Newbie looking for AC/DC Power Supply for testing LEDs of Unknown specs..

Hi CAS !

I can't say what the supply voltage was, as I put the torn-apart adapter in the trash some time ago, but I did jot down the adapter make and number and kept this information. It was a Black & Decker no. 200635HW. I tried to look up the supply voltage on the Internet using this description, but Google and Yahoo didn't provide anything, nor did the B&D Corporate website. I guess adapter 200635HW is obsolete; it was probably about 20 years old.


Attached below are photos of (a) your circuit sketch, the salvaged PCB from the B&D adapter 200635HW (b)Top and (c)Bottom and (d) a sketch I made of the circuit in my Portable Outlet Box, for reference. Concerning the note on your sketch "(can't see colours clearly), I had the same problem, especially with the third color to the right; even using my 10X eye loupe, I couldn't make a conclusive color choice on that band. So I checked both of the resistors on the salvaged PCB, the larger and the smaller one, with my DMM set in the Ohms position and below are the readings I got. Sorry I didn't get all of the attachments arranged properly to match this paragraph...I was having trouble with uploading (no fault of this website.).


Larger (and longer) resistor........ 99.5 Ohms Smaller and (shorter resistor)........ 8.3 Ohms

If possible, I would like to connect the PCB to my Portable Outlet Box circuit, so that the LED comes ON when the switch-knob is turned ON. Up until now, when I get the P.O.B. out I have to use a test light to check if it's ON. Could you please advise me on how to make the connections ?

Thanks much !

ToddB66

CAS,

This is an addendum to my post #11 (in quotes above).

With reference to the attached picture below showing the Top side of my PCB, I tested the rectifier diodes with my DMM set in diode mode and got the voltage readings listed below, while keeping the red positive test probe always touching the Anode lead of the diode. When I reversed the tester leads I always got O.L. on the meter display.

Starting with the diode at far-left and going from one diode to another toward the right, the readings were as listed below.

567 mv (0.567 Volts) at far-left
579 mv (0.579 Volts)
562 mv (0.562 Volts)
565 mv (0.565 Volts) at far-right

So with this information, i.e. my meter readings on both resistors plus the four rectifier diodes, can you advise me on making the connections as requested in the following paragraph (copied & pasted from my previous post.) ?

If possible, I would like to connect the PCB to my Portable Outlet Box circuit, so that the LED comes ON when the switch-knob is turned ON. Up until now, when I get the P.O.B. out I have to use a test light to check if it's ON . Could you please advise me on how to make the connections ?

Thanks again for your help !

ToddB66