View Full Version : 12v power supply to prop

10-16-2011, 03:32 PM
I need to power the 44pin QFP prop from a 12v 2A power supply.
I have the 5v and 3.3v regulators that came in the PE kit... and I have a pile of LM317 regulators lying around.
So I was wondering if it's safe to swap the 5v regulator with the LM317, and run 12VDC 2A in.
Should I change any caps?

Mike Green
10-16-2011, 04:08 PM
The existing regulators that come with the PE kit can be used as-is with a 12V input as long as you don't draw much current through them. The issue is that the regulators convert any excess power into heat and they have limited capability to get rid of heat. When they get too hot, they turn off to prevent damage. The 3.3V regulator isn't much of a problem since its input is 5V. The 5V regulator has to drop 12V - 5V = 7V at whatever current is being drawn from the regulator. If you're just using the Propeller and maybe two LEDs, that could be as much as 100mA or more. That's nearly 3/4W of heat (7V * 0.1A) that has to be gotten rid of, a lot for a little IC package with no external heatsink. Note that, with a 7.5V power supply, you're dissipating only 7.5V - 5V = 2.5V * 0.1A = 1/4W ... much more manageable.

If you have more stuff drawing current from either of the regulators, you have a bigger problem with heat.

There are several solutions:

1) Use a 7.5V power supply instead of the 12V one

2) Use one of your LM317 regulators to take the 12V and produce 7.5V or 7V for the PE kit regulators to use as input. You could put an external heatsink on the LM317 easily. Read the LM317 datasheet for sample circuits and the formula for calculating the resistor values needed for a 7.5V or 7V output voltage.

3) Buy a 5V switching regulator like the ones from Dimension Engineering (http://www.dimensionengineering.com/de-sw050.htm)and use one instead of the 5V regulator for the PE kit. These don't turn the excess power into heat.

Different regulators require different input and output capacitor values. Read the datasheet for the regulator for recommended minimum capacitor values and follow the recommendations

10-16-2011, 04:30 PM
There are several components drawing off the 110vac to 12VDC switching power supply.
the prop has just a few pushbuttons, LEDs and a ULN2803
2x Stepper motors
Laser diode
12v 150ma fan
Total power consumption excluding the prop = 1.5amp
So there is 500mw left over for the prop.

This is the switching supply I plan to use:

Mike Green
10-16-2011, 05:30 PM
It matters very little what the other stuff draws (as far as the Propeller's regulators are concerned) as long as the current draw of the Propeller itself and the things drawing current from it is low enough. The LEDs, the pushbutton's pull up or pull down resistors, and the ULN2803 itself contribute to the total draw from the +5V regulator (via the 3.3V regulator). I assume the laser diode is also running off the 12V supply and not drawing from the regulated +5V or +3.3V supplies.

Similarly, using an AC input switching 12V supply will save weight and heat load. It won't affect any of the things I've mentioned though. You could be using a old-style 60Hz transformer supply for your 12VDC and it wouldn't change what I've discussed.

10-16-2011, 05:51 PM
Not sure about your experience with high voltage, for your safety I would rather recommend a wall wart, something like this one (http://search.digikey.com/us/en/products/EPS120125UH-P5P-SZ/T991-P5P-ND/1129490).

frank freedman
10-16-2011, 06:03 PM
Is there a big price difference in price between one w/ only 12V or one that can also give out 5.0 and/or 3.3V? If this is a one off or experimental that one day will get recycled into another project, it may be worth considering going with one that will provide the 5 and 3 volt outputs as well. Save another trip to the (online)store. Also, there are threads on the net for turning old PC supplies into dedicated bench supplies. Not variable, and seldom with -12 to go with the +12, but may be all ya need. (Keeps the controller happy here, whatever I want she lets me have if it meets the price point.....free......)


WBA Consulting
10-16-2011, 08:41 PM
Would one of my PowerTwigs (http://powertwig.com)give you what you are looking for? There is a picture on this thread (http://forums.parallax.com/showthread.php?131867)showing it connected to the PE Kit circuit setup.

10-17-2011, 03:10 AM
Hows about this! Lucky find, normally these multi-out supplies are very costly

5v current is a bit high at 3.5A... but the prop will only draw what it needs correct?
The only thing that I would connect to the 5v output is the 3.3v regulator for the prop.
Everything else will be running off the 12v.

By the way, I have never seen a device that runs off -12v.
Anyone have any idea what a -12vdc output would be used for?

10-17-2011, 03:44 AM

To answer your last question first, having a -12V power supply was fairly common at one time. Generally it was used for powering things like RS232 drivers and op amps. With the advent of USB and op amps that only need a single supply the -12V supply has become less common.

As for using 12V as input to your PE, it might possible to use the 12V directly if the current draw is low enough. Another option would be to use the 317 as a pre-regulator to provide something close to 8 or 9V to the 5V regulator. And, yes, the prop will only draw the current it requires from the 5V supply.

frank freedman
10-17-2011, 05:02 AM
Hey JB,

Prop will only take what it is spec'd for unless you feed it too much V, then it will only take enough to let the smoke out or shut down the switching power supplies safety circuit whichever comes first. 5V@3.5A is the max output current rating for the supply. +/-12 v was used for rs232 in PCs, terminals and DSU/CSUs, but many systems using analog would and still do have typically a +/-15 volt supply, though you could have used +/- 12 while some chips can have as high as +/- 18V. I have seen some systems that did use +/-12V for rs232 system interconnections derived from the +/-15 dual supply for op amps; from the +/-15, a reference voltage would also be taken for things like ADCs and DACs. Many times the Vref would be passed through an inverting stage to provide the opposite voltage reference. There is a good PDF out there from TI http://www.ti.com/lit/an/slod006b/slod006b.pdf that is a very good guide to using op amps in systems. Some things are better split supply, others on a single supply; depends on requirements/restrictions of your design. Battery powered/single supply (i.e. wall wart) is the main user of single supply from what I can tell from reading and observation.


I use a 12V wall wart to feed my GG USB board. I take the +5 off to the breadboard to feed ADCs and some logic and LCD display. No problems yet. Just watch total current draw and heatsink if you need to.

10-17-2011, 07:44 AM
Cool, I'm going to order that dual supply I listed above, I cannot find anything better suited for providing 5v to the prop and 12v to external components (and have sufficient current)... well actually I can, but at a minimum of 5x the cost
I found a few 'through-hole' chips that will convert 110vac to 12vdc, but they are max rated at 300ma... these could be useful in the future for smaller projects though. I have part# if anyone is interested... decently small chip perfect for prototyping.

10-17-2011, 09:17 AM
Say I cannot find a spec sheet for the QFP prop chip... I need to design a PCB but do not know the pin widths, lengths and spacing.
Can someone point me to this?

10-17-2011, 10:21 AM
The qfp specs are on the prop data sheet near the last pages..

10-17-2011, 02:18 PM
lol cluso99
Q: "I need pin measurements but cannot find the spec sheet "
A: "The measurements are in the spec sheet"
Gee thanks much!

I did finally find the spec sheet buried in the 'manuals' section...
I was looking in the 'diagrams and schematics' section

10-17-2011, 03:41 PM
If you are using diptrace check gadget Gangser's pages. There are diptrace layouts with QFP propellers too..

Peter Jakacki
10-17-2011, 03:52 PM
See JB, this thread is no longer about 12V power supplies and is now about PCB footprints. It's always better to start a new thread for the benefit of all forum members.

To keep it on thread here is a post that I did about switch-mode supplies (http://forums.parallax.com/showthread.php?133717-Switch-mode-power-supplies) which I use to power the Prop from any voltage up to 50V or more.

10-17-2011, 10:50 PM
Say I cannot find a spec sheet for the QFP prop chip... I need to design a PCB but do not know the pin widths, lengths and spacing.
Can someone point me to this?

You did say you could not find "a spec sheet" and I did say it's in the "prop datasheet" near the end. This shows the recommended footprint sizes and they work fine, even for hand soldering.

10-22-2011, 04:12 AM
By the way, that dual power supply I listed above arrived (12vdc & 5vdc output).... ITS GINORMOUS! almost 6" x 4"
I am going to have to go with a single 12vdc supply as size is a big concern.

One thing I need to clear up.
If I am running 2x devices off of one 12vdc1A output... lets just say for simplicity 2x PC fans, one rated at 200ma and the other at 500ma.
If I connect them both to the same 12vdc source, this will burn out the 200ma fan will it not? I would need to current regulate the input to the 200ma fan yes?

Mike Green
10-22-2011, 04:53 AM
Motors generally do their own current limiting. The exact mechanism depends on the type of motor involved. The ratings you mention are the maximum current drawn under normal operation. Those ratings are given so you can provide a power supply with the right current rating.

frank freedman
10-22-2011, 06:36 AM
By the way, that dual power supply I listed above arrived (12vdc & 5vdc output).... ITS GINORMOUS! almost 6" x 4"
I am going to have to go with a single 12vdc supply as size is a big concern.

One thing I need to clear up.
If I am running 2x devices off of one 12vdc1A output... lets just say for simplicity 2x PC fans, one rated at 200ma and the other at 500ma.
If I connect them both to the same 12vdc source, this will burn out the 200ma fan will it not? I would need to current regulate the input to the 200ma fan yes?

I may be off on what you are asking, but a power supply rated for 12V 1A assuming that it's given ratings were what you posted means that you can power any number of devices rated for 12V provided the total load does not exceed the max availabe output from the supply (1A in this case). At that point the output may do multiple things. It may shutdown the chopper drive wait and resume. If the load is still exceeding the max, then it will do this over and over until the overload is resolved. This is generally accompanied by a chirping sound every time the switcher pulses the chopper. Another posibility is a fuse blows. Lastly on an older unprotected straight linear supply, overload long enough and the supply will fail. Random odds as to whether the failure destroys the downstream circuits.

As to your fans, their rating plates give either the current and/or power requirement. If you run the fans at their rated voltage, they will run about the current specified. Just make sure the total of all current loads does not exceed the power source rating. Add up all the sources and then multiply by 1.20 so that you allow a bit of a safety margin. Yes some supplies may have done this in their design, others may not; trust them to if you dare.

Heve fun,


11-05-2011, 12:35 AM
Ok so I have decided to go with a 12v 1.25a wall transformer and run it in directly to all components & prop. Found them for about $5/ea, by far the cheapest solution... also beneficial is the xformer works with 110v-240v, so EU and USA are compatible.
Works good, heat isnt a serious issue. 12vdc is connected to one rail, using the 3.3 & 5v regulators for the prop, components are all connected to the rail.
There should be about 100-200ma leftover.

11-05-2011, 12:37 AM
I am still thinking about replacing the 5v LM2940 with an LM317.
I use the 317 for so many projects, it would be nice to be able to use a part I have a regular supply of.

I'm showing if I use a 100ohm & 300ohm resistor I would get 5vdc from 12vdc with the LM317, correct?
So I could use that directly in place of the LM2940? Or would I also need to current limit it?

Duane C. Johnson
11-05-2011, 04:14 AM
I just ran it through my spreadsheet and you should be just fine.
With 5% resistors (100 and 300 Ohms) the voltage is 5.015V nominal, 5.410V Max, 4.657V Min.
With 1% resistors (100 and 301 Ohms) the voltage is 5.028V nominal, 5.104V Max, 4.953V Min.

The LM317 has both self current and temperature limiting.


11-05-2011, 07:30 AM
The diagram I have only has a partial schematic... It shows R1 between Vout and ADJ, but R2 shows between ADJ and ???.
I'm assuming R2 goes between ADJ and cathode?

Toby Seckshund
11-06-2011, 09:03 AM
The circuit shows the input side, + IN and "ground" and the output side + OUT and "ground" obviously the "grounds" are common.

On a '317 the circuit maintains a voltage across the top resistor of 1.25 Volts, apart from a small quiesent current, from the reg chip, the bottom resistor sets the current through it. The specs state that about 240 Ohms is to be used and so that means that about 5 mA has to be drawn through it and the bottom resistor (this is high enough to swamp out the quiesent current).

So if the bottom resistor is 1K Ohms you will have 5 mA through it (5 Volts), and the 240 Ohms (1.25, which would give you an output of 6.25 Volts (approx).

For 3.3 Volts the bottom resistor will have to have 3.3 - 1.25 = 2.05 Volts across it so it would be about 400 Ohms. If you want the output to be exact then a 330 ohm fixed resistor and a 100 Ohm preset in series would be better.

The 240 Ohm resistor should be as close as possible to the '317's pins to prevent voltage drops across the sense pins from varying the output voltage. Remember the decoupling capacitors as mentioned in the application sheets too.