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Power Supply

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robmitch

New Member
Hi,
What is the cheapest option for supplying 5v (Zilog mcu @ 20mAmax) AND 3.3(compass sensor @ 20mA max) for my project? I have access to a 10V-12V battery supply, and/or a 6V regulated supply.

I was thinking of getting 2x LM317 (or LM7805) and setting each up individually for 5V and 3.3V (from the 12v Supply)?

Then I thought I could use just one LM317 (or LM7805) to get to 5V, then use a resistor divider to get down to 3.3V? Suppose this wouldnt give a very constant voltage though, with varying loads etc?

What is the cheapest/reliable solution. I suppose I should get rid of the Zilog mcu and use a 5V Atmal Mega...

Look forward to replies,
Cheers,
Rob
 
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robmitch

New Member
Oh,
while I'm at it, what diode do I need for reverse polarity protection (with these currents)?
Cheers,
Rob
 

geko

Active Member
LM2931-5.0 100mA 5V LDO regulator has built in reverse polarity protection so no didode needed and will work with an input voltage of 6 volts.

Cascade a LP2950-3.3 from the LM2931 to get 3.3 volts from that.

There are other Low-Drop-Regulators that you could choose from.
 

robmitch

New Member
geko,
thanks for that. Are these more efficient than the old 7805 (eg taking down from 12v)?

My preference is to come from the 12v supply, so low dropout is not a major concern.

LM317 is a lot cheaper (from what i can see) than the one you mentioned. is there a cheaper alternative out there?
cheers,
Rob
 

geko

Active Member
Not really, just use a 7805 and a either a 3.3V regulator also off the 12V supply or an LDO 3.3V regulator from the output of the 7805. Put a 1N4001 diode in series with the input to the regulator(s)

If you've aleady got some LM317 use them but I wouldn't go out and buy them to do it, just use a fixed regulator.
 
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