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Strange PSU requirement #199214 02/16/11 08:31 PM
Joined: Jul 2002
Posts: 8,379
Trumpy Offline OP
Hi Guys,
I wondered if I could run a little project by you folks and try and get some feedback on how I'm going to do it?

First a bit of back-ground on what I'm up to:
A few years back when I was working in the dairy industry as a technician, I was sent to a job in a 10 year old dairy shed that had electronic cup-removers on each of the 40 bales.
What bought me to the job was that the owner of the shed had had a bit of a muck around with the electrical systems under the rotary platform and had cooked the circuit boards in all of the cup-remover units, by putting 230VAC through what should have been a 24V DC system crazy

I ended up replacing all of the boards in each unit and on each unit is a 10mm dual colour LED, to indicate various states of operation and programming status.
Being the sort of guy that doesn't like to see anything go to waste, I asked the guy if I could please keep the LED's for my own use, he said no worries.

Jump forward to now, I've had these LED's sitting in one of my draws here in the workshop and a bit of an epiphany struck me the other night.
I have the idea of making some sort of a lighting source for use in here at night when I'm operating my ham gear.
Making up some sort of a mount for the LED's is not a problem at all, what is troubling me is how am I going to power this.

I got one of the LED's and hooked it up (with no current limiting resistor, I might add) and something sounds a bit strange about the current draw results I got.
With the Red PN junction connected, I got a current draw of 230mA @ 3VDC.
With the Yellow PN junction connected, I got just over 100mA @ 3VDC.
These measurements were taken with a FET driven analouge meter.
I also tried a variable resistor in series with the LED on both junctions and while the brightness more or less fell off very quickly (almost exponentially), the current draw between full brightness and quite dim only reduced by about 40-50mA.

Now, if I am to use 30 of these LED's, we are talking quite a bit of current, 7 amps on Red and 3 amps on Yellow.

See folks, this is what I'm on about, if it was a common voltage, it wouldn't be an issue, but 3V power supplies are few and far between, especially at this sort of current capacity and also having (reasonably smooth) DC output.
Had this been a few years ago, I would probably have wound up a custom transformer, however, I've now run out of bits to make a decent transformer.

Any and all input will be taken into account, there is never any such thing as a silly idea when you are designing something. grin

Tools for Electricians:
Re: Strange PSU requirement [Re: Trumpy] #199224 02/16/11 10:38 PM
Joined: Nov 2002
Posts: 790
wa2ise Offline
You could connect some of the LEDs in series, and with a current limiting resistor, run them off say 12VDC. 3 LEDs and then a resistor, all in series circuit, to the 12VDC. LEDs are like Zener diodes, in that they really want to maintain the same voltage across their terminals despite whatever the supply voltage is. Exceed that voltage and the LED will draw lots of current, most likely enough current to burn it out. A lot like the need for a ballast for florescent light bulbs.

Last edited by wa2ise; 02/16/11 10:38 PM.
Re: Strange PSU requirement [Re: Trumpy] #199227 02/16/11 11:27 PM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline
Broom Pusher and
I was thinking the same as wa2ise..., except using a 24V or 48V Supply instead of 12V.

Connect several arrays of Series LED Strings, driven by a 24VDC 2 Amp supply.

For the Reds, (6) Diodes in Series, ballasted by a 26 Ohm Resistor... figuring:
  • FVD for each Diode is 3.0 Volts,
  • Peak Regulated Output DCV from the Supply is 24.0VDC,
  • 230ma is not excessive (renders no loss of smoke).

5 "Strings" of 6 Diodes in Series, with each "String" drawing 0.23A equals 1.15 Amps for the Reds.

The Yellows would be similar, except the Resistor would be 60 Ohms, so as to allow 100ma flow, and total draw from all 5 "Yellow Strings" would be 0.5A

Total Load Current for both Red and Yellow combined = 1.65 Amps at 24VDC

-- Scott

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: Strange PSU requirement [Re: Trumpy] #199228 02/16/11 11:42 PM
Joined: Feb 2011
Posts: 12
tomn Offline
Hi Trumpy,
Here are a couple of neat links for LED connections.
Hope they help your design.

Re: Strange PSU requirement [Re: tomn] #199229 02/17/11 12:40 AM
Joined: Jul 2004
Posts: 783
LarryC Offline
3 lead LEDs ?

Energizing both colors at the same time might lead to some heating problems.

To reduce heating issues and adding dimming ability, try Pulse Width Multiplexing them.

Re: Strange PSU requirement [Re: Trumpy] #199234 02/17/11 02:37 AM
Joined: Jul 2004
Posts: 9,569
gfretwell Offline

Indicator" leds are usually driven at around 15MA. They are also not really designed as a lighting source so the junctions are not that big (or whatever it is that gets big MCDs)

Greg Fretwell
Re: Strange PSU requirement [Re: LarryC] #199238 02/17/11 03:16 AM
Joined: Jul 2002
Posts: 8,379
Trumpy Offline OP
Originally Posted by LarryC
3 lead LEDs ?

Energizing both colors at the same time might lead to some heating problems.

To reduce heating issues and adding dimming ability, try Pulse Width Multiplexing them.

I won't be using both colours at the same time, it would be a situation of either Yellow or Red.

Re: Strange PSU requirement [Re: Trumpy] #199295 02/19/11 02:00 PM
Joined: Nov 2005
Posts: 823
JoeTestingEngr Offline
Aren't these 3 lead LEDs? If so, series connections are out. Are they CA or CC LEDs? The values you gave seem overdriven to me and I would worry about their longevity. I can't think of a single chip that would work with your specific requirement for 30 Dual color LEDs. You would probably need small value balancing resistors and would be best served by setting max current and PWMing for brightness. By any chance, were you able to find a data sheet for that LED?

Re: Strange PSU requirement [Re: Trumpy] #199302 02/19/11 05:20 PM
Joined: Feb 2002
Posts: 183
n1ist Offline
I would use a constant current source instead of a simple ballast resistor. You can make one from a LM317 regulator and a resistor.


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