[sdiy] Best way to get a +3.3V rail from +15V?

Oscar Salas 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
125C.

For an ambient temperature of 40C and a maximum junction temperature of
125C (125-40)/29=2.9W

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?
>
> Thanks,
> Tom
>
>
> 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
>> current
>> 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
>>> about
>>> 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
>>> situation
>>> this time.
>>>
>>> If I've only got +/-15V available, what's the best way to derive a 3.3V
>>> rail?
>>>
>>>
>>> I've used 3.3V regulators on the 15V rail before and found they got
>>> hot,
>>> which wasn't a surprise seeing how much voltage they've got to lose
>>> somewhere. But perhaps I was doing something wrong?
>>>
>>> Thanks,
>>> Tom
>>>
>>>
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at synth-diy.org
>>> http://synth-diy.org/mailman/listinfo/synth-diy
>>>
>>
>>
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