4.5V from 9V

[Capt Kremen]

I have a mains power supply, stepped down to 9VDC to supply a guitar effects pedal board 'daisy chain' lead of outputs. (The manufacturer quotes 9VDC @ 3000ma(27W) in their product specs). I have a music stand light consisting of four(4) LEDs. This runs off either three(3) AAA cells or a separate 4.5V source from a step-down wall-wart mains power plug transformer.

Wishing to simplify and dispense with the 4.5V transformer plug and utilise one of the spare 9V 'daisy-chain' power outputs, I understand I need to step down the current and voltage to avoid 'blowing' the LEDs.

I tried a 10Kohm linear taper resistor pot. This worked but became rather hot!

Electronic wizards out there, how, in the simplest, safest & most reliable way, can I achieve 'bright' but not blown LEDs using this 9V power source?

The 'electronic gizmo' must be capable of operating for up to 4 hours+ at a time. (Typically Sound Check then Performance itself!)

Thanks in advance for any advice folk may have to suggest.

[Phil Green]

Almost any model aircraft BEC will suffice. The LEDs will take very little current.

If you want posh, one of those £1.50 ebay servo testers driving a small brushed ESC will give you full dimming control.

Cheers

Phil

[Peter Beeney]

Capt K, If you step down the voltage the current takes care of itself, courtesy of Ohm’s Law. Your resistor was getting hot because it was not really up to the job in terms of watts, for that sort of caper a pot would probably need to be quite big. They’re generally not designed for power reduction.

What you really need is a regulator, perhaps. What is the current consumption of the leds? These can sometimes now be a bit juicy. It might be a bit difficult to source one with a specific 4.5 volt output, but there are adjustable output types. If you can find the right device, and there are a few around, once you switch it on it will run forever. Or maybe a standard 5 volt, as in BEC, with a series dropping resistor would be easier, you only have to lose half a volt. Depending on how you were to assemble this, and you had some longish wire connections to the leds, say, even this might be enough.

Is this any good as a starting point?

PB

[FastFlyer Smyth]

Or,

You could put a 4.5 volt zener diode accross the 5v output (Thats if you can find one)

[Phil Green]

The half a volt wont matter a jot, 5v will be fine. Any BEC will supply 5v from 9v no problem, thats their purpose!

The LEDs wont take much, even a 1W Cree would only be 200mA.

Cheers

Phil

[PatMc]

If the light stand is designed to run of 3 x AAA cells then there's no question that just about any BEC will do.

 

Edited By PatMc on 07/01/2014 00:16:31

[Rob43]

You could try this.
Its 5v output rated but i think that would be ok on 4.5v rated leds.
http://www.hobbyking.co.uk/hobbyking/store/__16663__HobbyKing_HKU5_5V_5A_UBEC.html

[Phil Green]

Theres no such animal as a 4.5v rated LED. They're current devices!

Whites have a Vf of about 3.6v so if its a 4.5v supply it will have internal current limiting. A 5v BEC will be fine. If you're not confident then just slip a 1N4001 in line.  One amp will be plenty, a 5A BEC is huge overkill!

Cheers

Phil

 

Edited By Phil Green on 07/01/2014 02:04:42

[Sparks]

You could just put a 4.3V Zener Diode in series with the 9V supply. That would drop the voltage to 4.7V (ish)

A 1N5336 would do the job. It is rated at 5W, so would handle a current draw up to 1A

Farnell Part No 9557938 - LINK

[Plummet]

You could always get another music stand light and just run the two in series.

Plummet

[Masher]

There are some "strange" responses here captain!

Just do what Phil suggests, he has it spot on

[Capt Kremen]

'Let There Be Light'

5V UBEC works perfectly!

(Pinched one off a 'big' ESC I have that's been awaiting a new project).

Thanks to all for their inputs.