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View Full Version : How to use a higher voltage power supply with a proto board?



hinv
11-01-2010, 04:26 AM
Hi,

I would like to use a 24V power supply to drive my protoboard, but obviously, I can't use it as is.
I was wondering if it were possible to pull up the input lead on the 5V regulator and solder a resistor between
the input of the regulator and the board. If this is doable, to drop some of the voltage through the resistor before it gets to the regulator, how would I figure out what value of resistor to put in there.
I am guessing there is a better way to do this on the cheap, but I don't know. Any suggestions would be great.
I use the 24V as Vcc2 on a L293D as an output amplifier for a "zapper".

Thanks,
Doug

Leon
11-01-2010, 08:39 AM
Build a buck converter using a suitable switcher chip, like those made by Nat Semi.

Dr_Acula
11-01-2010, 09:14 AM
I'll second Leon's suggestion. eg the National Simple Switchers and an inductor that will cost maybe $2. You probably won't even need a heatsink. Total cost would be under $5.

Or you could go old-school and use a pre regulator, eg a LM7812. Or LM7812 then a LM7805. Or a LM7815 and then a LM7805.

Or a big zener as preregulator.

Or many many 4001 diodes (each drops 0.6V)

max72
11-01-2010, 12:52 PM
With linear regulators you'll need a heatsink, while a switcher regulator requires more components, but you won't have overheating issues.
Regulate to any voltage between 6 and 9, use it as a power supply for your protoboard, and you are happy.

Massimo

hinv
11-01-2010, 06:40 PM
Thanks for your help.

Does anyone know of a 3 legged voltage regulator to replace the 5V one that is there that will take 20 or so Volts?

hinv
11-01-2010, 06:55 PM
@Leon & DrAcula

Is this what you mean?

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM2574N-5.0-ND

If I use one of these to bypass the built in 5V regulator, would 500mA be enough?
I notice that there are different switching frequencies offered. What difference does that make to me?

Can these frequencies make it onto my output? through interference?

Thanks,
Doug

Mike Green
11-01-2010, 06:57 PM
Use an external LM317 adjustable voltage regulator to drop the 24V to about 7V which you can use for input to your protoboard. The LM317 can handle inputs up to about 40V and comes in a TO-3 package which can handle heat when heatsunk better than the usual TO-220 package.

You'd still be better off using a switching type pre-regulator, but, if you really want to use a linear regulator, use the TO-3 LM317. Remember that you're dealing with a 17V drop. Any significant amount of current drawn will result in a lot of heat. Four LEDs running at 10mA each plus about the same for the Propeller itself would result in well over a Watt of heat. That's not huge, but it's not much current either.

Leon
11-01-2010, 07:14 PM
hinv:

You could use the adjustable version of that switcher to output 9V, and put that straight into the Proto board.

hinv
11-01-2010, 07:48 PM
@Leon

Like this?

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM2594N-ADJ-ND

Leon
11-01-2010, 07:53 PM
That should be OK.

Dr_Acula
11-02-2010, 01:36 AM
500mA should be more than enough for a propeller. Even running a keyboard, LCD display and SD card I don't think mine goes much over 100mA.

(Unless you are running servos or solenoids etc).

In terms of extra components, I think the Simple Switchers only need one - the inductor. Consider, linear vs switching, you need an input capacitor for both, and you need an output capacitor too, and you always need bypass capacitors for each chip.

I've not had any problems with interference. The switching frequency is way above human hearing for instance.

Actually, come to think of it, I think a switcher may use the same number of components, because you don't need a heatsink, so you swap the heatsink on a linear reg for an inductor. Looking quickly through a catalogue, I think an inductor might be cheaper than a heatsink. And smaller too.

The hardest part is finding an inductor rated for 500mA. (Or 1A if you want a bit of a margin).

If you are buying some switchers, maybe get a 5V, 3.3 and an adjustable one. I started off with a switching pre-reg and a linear, then went straight to the switcher only.

The 3.3V switcher I am using is in a DIP8 package, and the propeller chip gets warmer than the switcher.

As you can see, I'm hooked on switchers. The cool thing with the new chips is you don't even have to know how they work.

hinv
11-02-2010, 02:06 PM
I think I will go with this:
http://search.digikey.com/scripts/Dk...LM2574N-5.0-ND

I want to be able to plug my wall wart directly into the protoboard since the plug is there. That way I can keep anything I build on the inside of the case. I will just pull up 2 of the leads on the 5V regulator and solder in leads to my own switcher circuit. Is it OK to leave ground of the 5V regulator there, or should I remove it completely?

@DrAcula: Thanks for the tip on the inductor. I haven't messed with them before, and trouble is, if I knew what I was looking for, I should be able to find it on a scrap computer board around here. How do you measure those things?

Thanks
Doug

hinv
11-02-2010, 02:16 PM
Just curious, how does the efficiency of these compare with the linears that are already there?

Rayman
11-02-2010, 02:25 PM
Here's a circuit I've used to go from 24 to 12 and then 5 VDC: