Propeller power supply.
In the schematic for the Propeller Education Kit Lab Series (PE Platform & Setup) on page 4, which shows the voltage regulators power connections.
I am wondering why the 3.3V regulator is being fed from the output of the 5V regulator?
Would it be better to just connect each regulator to the (+)9V?
Also, why do we need such a large electrolytic cap (1000 uf) on each regulators output?
Any info would be appreciated, thanks.
Tony
(I had to edit the above, they use (2) 1000 microfarads electrolytic caps, not millifarads)
Post Edited (TonyA) : 7/24/2007 7:07:57 PM GMT
I am wondering why the 3.3V regulator is being fed from the output of the 5V regulator?
Would it be better to just connect each regulator to the (+)9V?
Also, why do we need such a large electrolytic cap (1000 uf) on each regulators output?
Any info would be appreciated, thanks.
Tony
(I had to edit the above, they use (2) 1000 microfarads electrolytic caps, not millifarads)
Post Edited (TonyA) : 7/24/2007 7:07:57 PM GMT
Comments
2.5 of that will be used in the 5V devices and 1.65 in the 3.3 V devices, leaving 4.85 W to warm up the regulators.
Using regulators is series will distrubute this heat between them (at the possible cost of a stronger 5V-regulator). Parallel regulators might require an additional heat sink at the 3.3 V regulator. The choice depends on how much power you need from one voltage and the other. E.g. it COULD have been slightly less expensive to choose a parrallel configuration with a heat sink at the 3.3. V regulator and a low power type for the 5 V regulator...
10,000 mF sound a little bit overdone indeed, but I think it's 10 mF only. You add these capacities when in doubt whether your regulators are fast enough to react to fast changing current request from the connected devices. 10mF would allow you to deliver additional 1A for 10 msec which is rather uncommon. In general you choose a capacity of at most 1 mF.
Post Edited (deSilva) : 7/24/2007 6:39:33 PM GMT
EDIT: Mike Green got there first.
In my original post I meant to say "1000 uf" not "1000 mf"
So in series the 3.3V might get too hot?
I thought it might be more practical to use the "parallel" design, each regulator connected separately to the 9V.
(But I'm not an engineer, and just learning about this stuff.)
So, the 1000uf caps (connected to the output of each regulator) would be good if using a servo, but not necessary for normal input/output stuff?
Thanks,
Tony
The minimum amount of capacitance for a regulator input and output depends on the regulator. Some regulators also have maximum capacitance limits (mostly very old types). Check the datasheet for recommendations. The amount you "need" depends on what you're going to do with the power, what the instantaneous power requirements of the parts of the circuit are. Motors are often the most power hungry, but, if you're driving lots of LEDs, that might be the controlling factor. The capacitor helps the regulator supply large amounts of current for a couple of milliseconds until the regulator (and its power source) can catch up. You also need small capacitors (like 0.1uF ceramic) across the Vdd/Vss pins of nearly every IC in your circuit to supply current at the device for a fraction of a microsecond until the current can get from the regulator across a few inches of PC board and/or wiring to the IC. Typically, if you have several small PC boards, there's a 1-10uF tantalum capacitor across the power leads where they come into the board.
I'm using a 3.3V, 500 mA reg. (which I know is good), and then a 5.0V, 1A reg.
Is this 1A ok? Or should I use a 500mA 5.0V reg.?
Thanks,
Tony
If you want to draw anywhere near the maximum current rating of the regulator, you will need to limit the input voltage and provide a good heatsink. A low dropout regulator will typically let you use an unregulated supply only 0.5-1.0V above the output voltage. For a 500ma regulator, that's 1/4 to 1/2W of power that has to be dissipated. If you were using a 7805 regulator with a 2.5V minimum input voltage drop at 1A, that would be 2.5W of power to be dissipated. That's a lot of heat for a small circuit board.
Thanks again, I appreciate it.
Tony
It helps - a little bit - to screw the regulator to a metalic area on the PC board, but not much - the main effect comes by the screws I think
You can use a 3A type in a TO3 case giving you all safety you might need..
I'll check the datasheet for the 7805.
Thanks again.
Tony
The reason most new circuits are being designed with low dropout regulators is that the minimum Vin - Vout is much lower than in the older regulators like the 7805 where it's a minimum of 2.5V (which all gets turned into heat).
Very interesting. Thanks again. The equation cleared it up for me.
(About the Hydra Book, yes page 79. That's what got me thinking. The Hydra uses a completely different type ("parallel") power design. The reasons explained in the book seemed to make sense to me (only on a very practical level, since I am just learning about this power supply stuff).
He also made a point I could immediately understand, saying if one regulator (in the·"parallel" design)·failed the other reg.·would keep working in this case. In the "series" design they would both stop working if one failed.
I thought maybe there are reasons for such different power supply designs, since one is a complete system with peripherals and an expansion slot, etc. and the other just a chip and some test circuits (i.e. the Propeller Edu Lab stuff).
Tony
Post Edited (TonyA) : 7/24/2007 9:48:18 PM GMT
There is always the same amount of heat, proportional to your request of current (@TonyA !)! You can - a little bit - control where the dissipation takes place.
(1) Most of it in the 3.3 regulator (parallel solution )
(2) Less of it in the 3.3 regulator and the rest in the 5 V regulator (serial solution)
Of course this makes no sense at all when you just use the 5 V regulator to save a heat sink!
When you are in need for a little bit of 5V and you are within the range of inexpensive regulators (sub 1 A) this might be an option
When you mainly draw 5V - maybe MANY Amperes- an additional (LP?) 3.3 regulator after the 5 V regulator will certainly do no harm.
But I know of no really convincing reason for the serial approch...
Thanks again. I appreciate it.
Tony