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

Oscar Salas osalas at electronic-sea.net
Thu Mar 7 18:21:35 CET 2013


Sorry, my previous mail was not finished.

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 the maximum power output is 2.9W (125-40)/29=2.9W
The power to dissipate in your application: (15-3.3)*0.2=~2.3W

Also, you could attach the regulator to the PCB to improve slightly the
power dissipation.
To output more current or to operate it at lower temperature, you could
add a UA78M09 before the UA78M33.

Regards,
Oscar.




>> 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
>>>>
>>>>
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>>>
>>>
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