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Which regulator is best suited for 5V @ 5uA to 75mA ?

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blueroomelectronics

Well-Known Member
I'm a little stuck with 5V power supply selection for a PIC that can draw between a few uA sleeping to 75mA peak (RS485, Relay, XBee etc...) with an average draw of 0.5-2mA
I would like it to generate as little heat as possible, considering the following

  • 3xAA batteries maybe but prefer external mains power
  • 5V regulated wall adapter
  • Roman Blacks SMS design
  • LM2675
  • LM2940
  • LM78L05
Any thoughts, other suggestions?
 

Sceadwian

Banned
5V switch modes are pretty good nowdays, many cell phones use them. They're regulated, plenty of current, small, light, and cheap. Shouldn't be too hard to find a supplier for bulk purchases, and you can cannibalize the charger from an old phone if it's a one off. I have at least a half dozen floating around here.
 
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blueroomelectronics

Well-Known Member
Thanks, I have been leaning towards the wall warts, they are common and cheap.

It's for a thermostat and since the sensor is on the same PCB it will pick up any heat created by the other circuitry especially the regulator and relays. The relays I can handle by using PWM or a latching relay (common in battery operated thermostats)
 
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Sceadwian

Banned
Depends on if you really need regulated or not, if you don't, any old wall wart around your acceptable input range will do. If you need regulated the 5V SMPS from a cell phone charger like device is perfect, all you need is a cap right at the device and you're all set, if it's cheaper you could pick a wall wart with a high enough overhead and use a linear regulator, but I prefer the SMPS one's myself, all the works done, and I get family members that give me one or two a month.
 
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Sceadwian

Banned
Then it comes down to cost. If you can find a supplier for surplus or new cell phone chargers use them. If you can find a wall wart and linear regulator that will be cheap enough for you to bother adding the regulator to your circuit then do that.
 

Sceadwian

Banned
Ahh, gotcha.
 

tcmtech

Banned
Most Helpful Member
If your switching a 24VAC signal like most heating and cooling thermostats are based on you may get by with simple capacitive coupled power supply. It would generate no heat and if properly built it will easily handle the micro amp to milli amp loads while maintaining a good voltage regulation.

I rebuilt my digital wall thermostats to work that way several years ago and have not had any problems so far.
 

MikeMl

Well-Known Member
Most Helpful Member
Just an idea, but recently I was looking for a very efficient regulator to run a 5V LCD DPM which is powered from the same 12V battery that it is displaying the voltage of. I found that a series resistor-LM431 shunt regulator was more efficient than an LM78L05 or LM317. This is because the load was relatively constant, and I could size the series resistor so that it just barely supplied the current used by the DPM at the lowest expected battery voltage.

Could you do something similar, where you spec an unregulated power supply (wall wart), regulate only the PIC power, and don't regulate the power feeding the relay...
 

Mr RB

Well-Known Member
I bought a few sticks of Micrel MC5209 5v regulators, these are a SMD device but large (SOT-223) with the pins still close to 0.1" pitch, so I can solder them on veroboard ok. They are easy enough to hand solder for kit use.

I love them! Very low drop out, 500mA max, max Vin-Vout drop is 0.5v I think at the max output of 500mA. Normally you only need about 5.1v in to get a nice regulated 5v out. (So you can get a reliable regulated 5v from 4 AA cells, for the life of the battery).


They only waste a few microamps of quiescent current, from memory I think about 10 uA? Also the +5v out is more accurate than 7805 which is about 5.08v or worse, the MC5209 I have used are always within 2mV of 5v.

You can often get them for pennies from electronic surplus agents. When my sticks get used up I will definitely be getting more! :)
 
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