Power for a minimal prop board?

Oldbitcollector (Jeff)

I'm putting together a minimal Propeller setup and am curious if
just feeding the Propeller current from a 7805 (+5 regulator) thru a
10K resistor (to bring it down to 3v) would sounds like it would
provide safe/decent power for the Propeller chip?

Thanks
OBC

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New to the Propeller?

Getting started with a Propeller Protoboard?
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Updates to the Cookbook are now posted to: Propeller.warrantyvoid.us
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Paul Baker

I would not recommend this, the voltage seen by the Propeller will be inversely proportional to the current drawn. If the Propeller is placed in a low power mode (all cogs in waitcnt or off) the voltage at Vdd would exceed the absolute maximum voltage of 4V. I think the wide range of current the Propeller can draw (a few uA·to 100 mA plus whatever is consumed by I/O pins) would result in too large of a Vdd swing to operate the Propeller over the entire current range.



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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 10/24/2008 5:33:08 PM GMT

Oldbitcollector (Jeff)

Thanks Paul

OBC

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New to the Propeller?

Getting started with a Propeller Protoboard?
Check out: Introduction to the Proboard & Propeller Cookbook 1.4
Updates to the Cookbook are now posted to: Propeller.warrantyvoid.us
Got an SD card connected? - PropDOS

Rayman

you could probably use a diode or two to drop the voltage though...

Leon

Just use a 3.3V regulator. I use an LM317 on one of my Propeller PCBs.

Leon

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Suzuki SV1000S motorcycle

Oldbitcollector (Jeff)

Yes, except the LM317 requires another two resistors to adjust depending on the voltage.
and I'm trying to keep this board very minimal. [noparse];)[/noparse]

I'm curious about the mention of using diodes? Can you elaborate?
(Sorry, I'm just not an EE. )

An LED/resistor might serve dual purpose of power indication as well as reducer?
OBC

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New to the Propeller?

Getting started with a Propeller Protoboard?
Check out: Introduction to the Proboard & Propeller Cookbook 1.4
Updates to the Cookbook are now posted to: Propeller.warrantyvoid.us
Got an SD card connected? - PropDOS

Post Edited (Oldbitcollector) : 10/24/2008 5:52:55 PM GMT

Paul Baker

Once you add up the cost of a couple diodes, you start to approach the cost of a cheap voltage regulator. I would recommend a diode setup only if your in a situation where you have to get something working right now and dont have a regulator on hand. If you are creating a design just use a low cost linear regulator like the PQ1L333M2SPQ which sells for $0.68 in single quantities from digikey.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Leon

Oldbitcollector said...
Yes, except the LM317 requires another two resistors to adjust depending on the voltage.
and I'm trying to keep this board very minimal. [noparse];)[/noparse]

I'm curious about the mention of using diodes? Can you elaborate?
(Sorry, I'm just not an EE. )

An LED/resistor might serve dual purpose of power indication as well as reducer?
OBC

The LM317 plus resistors is cheaper than a 3.3V fixed regulator, and can give better performance.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Paul Baker

The idea of diodes is that they have an associated forward voltage drop. Over the current range of the Propeller a diode like the MA3J142A will have a drop between 0.45V (for 10uA) to 0.9V (for 100mA) volt drop @ 25 C. You would have to use 2 (they are $0.33 apeice), and you would still run into trouble in certain regions of operation (extreme temperature, extremes of current draw (high or low)). With 2 diodes in series the voltage drop seen could be as low as 0.3 V (10uA draw at 150C) and as high as 2V (100mA draw at -20C). I can't fathom a setup (even with 3 diodes) where the voltage will be in the accepted range for all current draws and operating temperatures.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 10/24/2008 5:59:22 PM GMT

CarlosFandango

There are plenty of alternative fixed voltage regulators around that don't require adjustment. I'm using one of these (it's a 3V reg that I'm using for a clock standby supply, but it's available in multiple voltages inc. 3.3). Mind you this thing really IS minimal, it's only a couple of millimetres across! Good if you like fiddly soldering jobs.

parts-man73

On the subject of Diodes and Regulators....

I notice on the ProtoBoard, there seems to be no Diode to protect against possible damage if a Negative center wall wort is plugged in accidentally. Do the regulators have some built in protection in this regard? I'm designing a new version of SpinStudio, and if that diode is really not necessary, it'd be best to go without.

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Brian

uController.com - home of SpinStudio - the modular Development system for the Propeller

PropNIC - Add ethernet ability to your Propeller! PropJoy - Plug in a joystick and play some games!

SD card Adapter - mass storage for the masses Audio/Video adapter add composite video and sound to your Proto Board

Phil Pilgrim (PhiPi)

I would not recommend using an LM317 to go from 5V to 3.3V. Its dropout voltage is perilously close to the 1.7V differential, so regulation may be poor under these conditions. A fixed 3.3V low-dropout regulator that's stable with low-value ceramic filter caps will be your best bet. Don't even think about messing around with diodes to drop the voltage.

-Phil

Addendum: DigiKey carries the Microchip MCP1700-3302E/TO for $.53/1. This is in a TO-92 package and specs up to 250mA output current. I requires only a 1uF ceramic filter cap for stability.

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'Just a few PropSTICK Kit bare PCBs left!

Post Edited (Phil Pilgrim (PhiPi)) : 10/24/2008 8:05:09 PM GMT

Paul Baker

parts-man73 said...
On the subject of Diodes and Regulators....

I notice on the ProtoBoard, there seems to be no Diode to protect against possible damage if a Negative center wall wort is plugged in accidentally. Do the regulators have some built in protection in this regard? I'm designing a new version of SpinStudio, and if that diode is really not necessary, it'd be best to go without.

All of our power supplies have a + center, so we did not feel the need to include a reverse polarity diode.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Beau Schwabe

The LM2940 and the LM2937 provide reverse battery protection.

http://www.national.com/ds/LM/LM2940.pdf

http://www.national.com/ds/LM/LM2937.pdf

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Mike Green

The Protoboard uses an LM1086 regulator. It's not clear from the datasheet whether it provides reverse battery protection, but the electrolytic capacitor at the input of the regulator will probably explode if the supply voltage is reversed. In addition, if the servo supply jumper is set to Vin, that 1000uF electrolytic may pop and any servos or other devices plugged into the servo headers may be damaged as well.

Post Edited (Mike Green) : 10/24/2008 10:00:45 PM GMT

CannibalRobotics

I've used this one and I love it. Small foot print, cheap with rock steady output.

parts-man73

CannibalRobotics - That's exactly the one I chose, except I am using a different name brand, ST. Perfect for my application, not so great for the type of projects that OBC is known for. He'll need a non-surface mount package.



OBC, sorry I took the thread a little OT there. Don't forget the capacitors that the regulators need to provide a steady output. You mentioned the 7805, I'm sure because they are readily available, as most Radio Shacks carry them. Sounds like the general consensus is to use the right tool for the job. Either a fixed voltage 3.3V model, or a LM317 (which is also carried at most Radio Shacks) with a couple of extra resistors.

Since they've got to get the Propeller from somewhere online anyway.... They ought to just order a good fixed 3.3V regulator at the same time... Just my thoughts

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Brian

uController.com - home of SpinStudio - the modular Development system for the Propeller

PropNIC - Add ethernet ability to your Propeller! PropJoy - Plug in a joystick and play some games!

SD card Adapter - mass storage for the masses Audio/Video adapter add composite video and sound to your Proto Board

b.p.m.

speaking of powering the prop with minimal parts, what about using a 3.3V zener
diode with a suitable current limiting resistor?

blake

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CannibalRobotics

The Zener approach would probably work fine but you're just dumping current unnecessarily. The nice thing about a regulator is (for most) they don't waste power.

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