[sdiy] Best way to get a +3.3V rail from +15V?
osalas at electronic-sea.net
Thu Mar 7 18:05:24 CET 2013
This is possible to be done with a linear standard regulator like the
UA78M33 without heat sink. It has a Junction to Ambient Thermal Resistance
of 29 C/W and a maximum absolute junction temperature of 150C. Recommended
For an ambient temperature of 40C and a maximum junction temperature of
The power to dissipate: (15-3.3)*0.2=~2.3W
> A similar board I did on stripboard (the new one is getting a PCB) draws
> 101mA on the 3.3V rail. This board will have a 74HC594 driving some LEDs
> too, so I need perhaps 200mA. Does that change the picture any?
> On 7 Mar 2013, at 14:26, blacet at blacet.com wrote:
>> You can always use a series low value resistor to drop the 15V a bit and
>> take some of the heat before the regulator. It depends on how much
>> you need and this will effect resistor value/wattage and regulator type/
>> heatsink requirements.
>>> Hi all,
>>> I'm putting a little dsPIC prototype dev board together for mucking
>>> on. I've done such things before, but I've always had a +5V supply
>>> available, so when I've needed a 3.3V rail for the dsPIC, I've stuck a
>>> regulator on the 5V supply. Which was fine, but that's not the
>>> this time.
>>> If I've only got +/-15V available, what's the best way to derive a 3.3V
>>> I've used 3.3V regulators on the 15V rail before and found they got
>>> which wasn't a surprise seeing how much voltage they've got to lose
>>> somewhere. But perhaps I was doing something wrong?
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