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

Oscar Salas osalas at electronic-sea.net
Thu Mar 7 18:38:12 CET 2013


Looking now the LM317 datasheets from National and from Texas instruments,
National Specifies the j-a thermal resistance 50C/W for the TO-220 package
and Texas Instruments specifies 29C/W. The same than specifies for the
UA78M33.
So I would test my suggestion or use a UA78M09 before the UA78M33.

Oscar


> 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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>>
>>
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>>
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