9V Supply for Propeller kit

RogerInHawaii

The Propeller Education Kit comes with a 9V battery connector. But I'm burning through battery after battery with all the testing and diagnostics that I'm doing with my project. Is it safe to substitute a plug-in 9V power supply (wall wart?) instead of using batteries? Any particular cautions I'd need to take?

Beau Schwabe

RogerInHawaii,

Yes Roger by all means, move to a steady power supply, especially for testing and debugging.· 9V are notoriously low in the amount of mAH that they can pack.· On average this is about 400mA, but I have seen their rating as low as 150mA.· So a Propeller requiring·100mA with a 400mAH pack would only give you about 4 hours.· If you want to go with batteries, use a 4 pack of AA's feeding the 3.3V regulator, or a 6 pack of AA's feeding the 5V regulator.· Using the AA's would give you roughly 5 to 6 times the run-time as a single 9V battery.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 5/6/2009 5:05:32 AM GMT

ElectricAye

I've used a wall wart on mine and never had any problems with the Propeller. Just be sure you get the polarity hooked up correctly - test it with a multimeter to be sure. Some wall warts have little in-line plugs that allow you to switch polarity, so be sure what you get in the end is the same polarity as the 9v battery would be.

One word of caution, however: other components you place on your breadboard might not respond to a wall wart as well as the Propeller. I never figured out why, but a Real Time CLock I had on the same board was never able to keep time when using a wall wart - the seconds would always get messed up on power up.

I hope that helps. More experienced people here on the forum might have other tips, too. For example, depending on what you're running all at once, you might want to check that your wall wart can give you all the current you need.

good luck,
Mark

SRLM

I use the wall warts most of the time for powering things, but sometimes it's handy to have a benchtop power supply. I have the one that I link to last on this thread (here), and I really like it. It's handy for quickly getting the right voltage and being able to measure current at the same time. Any benchtop power supply will do, that's just the one that met my specs.

kwinn

You can pull the top off a dead 9V battery to make a connector for the wall wart. Just be careful to get the polarity right.

Nick McClick

There's always Vaati's Adjustable Power Supply! Just another option if you're tired of cutting the tops off your wall warts. Not as nice as srlm's power supply, but a lot cheaper.

I've always been confused about this - If I run a 9V battery thru a 3.3v regulator that's attached to a 100mA load, how many watts does the regulator dissipate / how long will my 9v battery last?

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kwinn

If it is a linear regulator it would be (9V - 3.3V) x 0.1A = 0.57Watts. For a switcher running at 75% efficiency it would be 0.1425Watts ( actually slightly less since some power is dissipated by the inductor).

How long it lasts would depend on the mAhr rating. For a best case scenario a battery with a 500 mAhr rating and the linear regulator circuit would last 5 hours. The switching regulator would increase that to 15 hours.

RogerInHawaii

I have a wall wart that says on its label that it generates 9V, 500mA. I figure it should work just fine. I tested it with my voltmeter and it shows 13 volts. How can that be? I also have a 12V unit, and that measures out at almost 19 volts. A 15V wall wart measures at close to the proper 15 volts and a brand new 9V battery measures at very nearly 9V. So what's wrong with the 9V wall wart? Is this typical? I can't imagine that it's safe to use this 9V-but-actually-12V unit with my propeller chip.

jazzed

Roger, 9V on your wall wart is the rating under load. When you plug it in to your hardware with the power switch on, the voltage should drop to 9v.

Make sure the polarity is correct. If you don't know what this means, ask someone in your neighborhood who knows to help you.

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--Steve


Propalyzer: Propeller PC Logic Analyzer
http://forums.parallax.com/showthread.php?p=788230

RogerInHawaii

jazzed said...
Roger, 9V on your wall wart is the rating under load. When you plug it in to your hardware with the power switch on, the voltage should drop to 9v.

OK, I guess that makes sense. But then why would the 15V unit measure at 15V and the 9V battery measure at 9V? If the ratings are for "under load" then shouldn't all of them measure high when just being measured by a voltmeter?

jazzed said...
Make sure the polarity is correct.

Yup, that much I can handle. [noparse]:)[/noparse]

jazzed

A battery is DC pile with an internal resistance. The 15V adapter may be a different design than the 9V adapter. I would bet the 15V adapter cost more than the 9V one. Maybe a hardware guy can explain all this better.

Glad you're ok with polarity. Many people north of me on the peninsula are really confused with that [noparse]:)[/noparse]

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--Steve


Propalyzer: Propeller PC Logic Analyzer
http://forums.parallax.com/showthread.php?p=788230

Toby Seckshund

The "wallwarts" are so unregulated because they have cheap components inside, a small usually underated and lossy transformer, a bridge rectifier (as it is usually the cheapest way) and an electrolytic capacitor for smoothing which is usually too small in value. Off load the voltage will be the secondary voltage multiplied by 1.4 (because of peak voltage, not the RMS voltage that is quoted) minus the 1 Volt or so dropped across the bridge. Hence the nominal 9V becomes (9 x 1.4) - 1 = 13 Volts approx.

When the supply is loaded the losses through the transformer and bridge increase and the smoothing becomes laboured and the output volage is reduced, ultimatly arriving at the quoted voltage at the quoted current. They are usually bloody hot at this point which eventually dries out the capacitor. A lot of them use the frail wiring of the primary as both the current and thermal fuse.

The only answer to these problems are to build heavier rated supplies with regulation, not so cheap or small or so easy to get in the Pound/Dollar Shop.

Batteries generate their volts from chemical reactions and are better on regulation, depending on which chemistry. Cheap old Zinc_Carbon exaust and give poor regulation quickly. Better batteries give better performance at a cost. To use a small 9V battery to drop straight down to 3V if throwing away most of its stored energy, which cost you money. On my boards ( home made Demos) I have used 5 Volt regulators (which req 2Volts headroom) feeding the 3.3V reg. This means that 7 Volts is the minimum that can be used but if the 5V rail could be ditched then 3.3V (+1V headroom) then 4.5V could be used ie just 3 x AA cells which would give about 5+ times the Amp-Hourage of dinky 9V. Not so cute/convinient though.

I must confess to hating cheap PSUs and batteries as I an fortunate to get/recycle/repair Switch mode PSUs and lithium cells free.

jazzed

Great answers Toby!

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--Steve


Propalyzer: Propeller PC Logic Analyzer
http://forums.parallax.com/showthread.php?p=788230

RogerInHawaii

I've attached the 9V wall art. The propeller kit is working. I can program it and run it and everything's fine. But the two voltage regulators (3V and 5V) on the development board are now running hot. I never noticed that before. Is that normal for them to be hot?

jazzed

The 5V regulator does get hot; the 3.3V regulator does not ... it may get a little warm.

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--Steve


Propalyzer: Propeller PC Logic Analyzer
http://forums.parallax.com/showthread.php?p=788230

localroger

@Roger it is normal for the 3.3V regulator to be a little warm because whatever the Prop is dissipating at the voltage it's using, 3.3V, the regulator is dissipating the difference between that and the supply which might be say around 10V at the same current. The 5V regulator might be getting warm if you're using a keyboard that needs the 5V, otherwise it shouldn't be in use at all. Warm is normal and OK for these parts -- they can withstand temperatures that will burn your skin without damage, so it's a safe bet that if you can hold your finger on the part for a few seconds, you're not damaging it.

Toby Seckshund

Try putting on a ethernet chip!! (do not touch anything that hasn't got a yard of heatsink bolted to it)

Ron Sutcliffe

Thats right ! the NIC chip draws up to 250ma. So that is another 250ma X 4 volts drop, or an additional 1watt to dissipate, thats assuming a 9 V supply

I use a 6 volts 1500ma supply, it is rock solid and the 5 volt REG is cool, 7.5 V might be a better choice if the supply cannot source 1 amp without ripple.

Ole Man Earl

A good and handy shop PS is the PS's from an old dead computer. The 5 VDC supply is usually enough for anything you want to program. Just hook it up to the 5VDC anywhere on the prop board. No need to remove the 5VDC regulator on the prop board. On the PS, short the green wire to the black wire on the PS header. That is to turn on the PS.
Works for me.

Toby Seckshund

Just make sure that no shorts happen, there are a lot of Amps available to cause mischeif. Also some switch modes like a minimum current on one (or more) rails to behave. In the good old days there was no problem in having large but predictable supplies now everything is expected to be the size of a matchbox and stuff out perfect volts.

I like the results of bad PSUs, the proceeds feed the family.