> But there is still minor problem with your suggestion Mr. McMahon.

If any given notional solution ever has only one minor problem then 
the engineer is as good as home free :-)

> The led lighted products will be connected serially.  If i assume 
> that each
> board sink 300mA from 150W/6A smps power supply there will 20 parts 
> in one
> serie. But they will be seperated 1 meters apart from each other. So
> probably the input voltage will drop under 7805's rated min input 
> voltage
> 7.2V near the end of each serie..

Either

reduce the resistor value for the end units,

reduce the resistor value for all units (still vast power reduction in 
7805s),

use two resistors in series with a shorting link when required,

increase feed cable size,

feed in middle of run,

...  ... ... .

You COULD add a switch which drops out one resistor when voltage is 
too low (could be a relay or solid state) or numerous other solutions.

My  'GSR'*  buck converter would do very well here at very low cost. 
Inductor would be the greatest cost item. Ask more if/when you want to 
use a switcher again.



        Russell


* GSR = "God's Switching Regulator", as He designed it.
Craziest design I've ever seen - but happens to work superbly in the 
application it was intended for (as you'd expect), and in quite a few 
others. Installed volume is presently only about 10,000 units but it 
may be used in a 100,000 volume product shortly.





>
> 2006/5/6, Russell McMahon <apptech@paradise.net.nz>:
>>
>> > Power Wasted:  (24V - 5V) * (load current)
>> >
>> > load current = 4x22mA (for blue leds) + 4x22mA (for green leds) +
>> > 4x21.5mA(for red leds)  + 1mA (for PIC12F675) +
>> > 0.5mA (for MAX485)
>> >
>> > (I hope i've had interpreted the dc electrical characteristics of
>> > two ICs..)
>> >
>> > load current is about 264mA.
>> >
>> > PW = 19 x 264mA
>> > PW = 5.016 W
>>
>> Add a series resistor before the regulator dimensioned to drop most 
>> of
>> the voltage.
>> Let's allow for 300 mA.
>> Vin = 24V
>> Allow 7805 3 volts "headroom" = 8V input
>>
>> Vresistor = 24-8 = 16 V.
>> Resistor = V/I = 16/0.3 = 53.3R.
>> Use 56R
>> At max 300 mA resistor will drop IR = 0.3 * 56 = 16.8V
>> V Regulator in = 7.2V = OK
>>
>> Power in resistor = V^2/ = 16.8^2/56 = 5W
>> Power in regulator = (Vin-Vout)*Imax
>> = (7.2-5)*.3 = 0.66 Watt.
>>
>> TO220 Regulator with a smidgeon of heat sinking is fine.
>> Even without in free air is OK.
>> In a sealed package you need to make sure you can handle the 0.7 
>> odd
>> Watt but it won't be too hard.
>> The 5W in the resistor would ideally be outside the package if you 
>> can
>> manage this.
>>
>> As actual max current is (by your calculation) < 300 mA worst case
>> dissipation will be even less.
>>
>> Ensure that the worst case current does not put regulator into
>> "dropout". I've allowed 2.2 V min. AFAIR 7805 can exceed this in 
>> some
>> cases depending on temperature etc.
>>
>>
>>         RM
>>
>>
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