[Premium-Rx] lamp replacement

[Carcia, Francis A HS]

Why not use a LM317 voltage regulator with a series resistor on the output.
Same package and no drift over temperature. fc

-----Original Message-----
From: Ahmet Gundes [mailto:ahmet-m at usa.com]
Sent: Tuesday, March 01, 2005 6:38 PM
To: Dave; premium-rx at ml.skirrow.org
Subject: Re: [Premium-Rx] lamp replacement


It would be possible to control the current flow thru LEDs by
using a high power transistor ( TO-220 case is one example )
with the LEDs connected in series with the Emitter of the
transistor. Then connecto a Potentiometer to the base of the
transistor to control the current into the base which in turn
will regulate/control the Emitter current ( Ic=BxIb ).  Especially
white LEDs are current driven and adjusting the current flow 
thru the LEDs will control their brightness. 
In this case LEDs need to be connected in Parallel as a cluster and this
then gets connected between the Emitter and the ground.  If there is a lot
of LEDs then more than one transistor might be used to better handle higher
current drawn.  
Series resistors used with white LEDs do not give good results and
they will get very hot or burn out due to high current.

Regards,
A. Gundes

> 
> Don't do what I did...  I replaced the lamps with surface mount 
> white LED strings mounted on edge (so as to shine into the acrylic) 
> with a surface mount resistor for each string.  I used a good 
> amount of these LEDs but still was way under the current drawn by 
> the lamps.
> 
> Here's where the fun begins... The first time I turned it on, I was 
> nearly blinded by the amount of light coming from the LCDs.  The 
> radio is impossible to use right now unless you're wearing welding 
> goggles.  I'll try to post a photo since it's pretty funny now.
> 
> I'll need to put a series resistor in the 15V line to the PCB so 
> the receiver will become usable.
> 
> /Dave
> 
> > Jim Garrison wrote:
> >
> >> How easy?/How difficult?, and How cheap/How expensive?
> >>
> > If you are talking about leds which are found in the later version, they
> > should not ever need replacement. If you are talking about the
> > incandescent filament bulbs found in the early version then some or all
> > will almost certainly have died by now...
> >
> > You can buy a replacement led board kit for the old version, I'd guess
> > around $50, and this is fairly easy to install in the old version, but
> > may require some machining of the acrylic spacer round the windows for
> > the leds to fit.  I have found it easier to fit little surface mount
> > leds (side firing yellow ones by Panasonic) on 1/8th inch wide strips of
> > pcb board material, after removing the bulbs from the board.  They are
> > connected in series in groups of 4, each fed with a 180 Ohm R.  8 above
> > and 8 below each display.  Little slots are cut in the pcb material
> > under each led.   The original tracks are used as supply busses and the
> > strips can be glued, but also are held in place by the Rs and the ground
> > returns.
> >
> > I got the leds from digikey, buy a few extra, as they have a habit of
> > jumping away and getting lost...  This is actually more complicated to
> > describe than to do.
> >
> > I have also fitted led lighting to one of my RA1792s, using a similar
> > approach, a big improvement over the original which had no lighting...
> >
> > Good luck,
> > Dan
> > ac6ao / g3ncr
> 
> 


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