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Stevezila
02-25-2010, 10:39 PM
O.K. at the risk of sounding totally stupid. I'm looking for a way to power a BS2 using 120 VAC without a transformer. Isn't there a way to run the line voltage through a full wave bridge and then and then feeding it through a regulator to power the stamp without involving a transformer? What am I missing? Thanks for replies in advance.

Mike Green
02-25-2010, 11:05 PM
The problem with what you want to do is that it's very very dangerous unless your entire circuit including the Stamp and anything connected to it is insulated from any contact with people, things, water, etc. It's also wasteful of energy in that most of the 120+V has to be dissipated as heat. You have to have transient suppressors to prevent voltage spikes on the AC line from getting through the power supply and frying your Stamp or at least causing it to reset. Basically, you need a really really good reason to skip having at least a simple transformer in your power supply.

Ron Czapala
02-26-2010, 01:16 AM
You can use a switching type power supply. They have many advantages - much smaller·transfomer (see below), they weigh less, less heat.
They are a little more expensive...

http://computer.howstuffworks.com/power-supply1.htm

The switching power supplies used today are much smaller and lighter. They convert the 60-Hertz (Hz, or cycles per second) current to a much higher frequency, meaning more cycles per second. This conversion enables a small, lightweight transformer in the power supply to do the actual voltage step-down from 110 volts (or 220 in certain countries) to the voltage needed by the particular computer component. The higher-frequency AC current provided by a switcher supply is also easier to rectify and filter compared to the original 60-Hz AC line voltage, reducing the variances in voltage for the sensitive electronic components in the computer.

A switcher power supply draws only the power it needs from the AC line.

erco
02-26-2010, 01:22 AM
All new wall warts are transformerless. Buy one that outputs 6-12VDC and avoid a good tingle.

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·"If you build it, they will come."

Phil Pilgrim (PhiPi)
02-26-2010, 03:09 AM
erco said...
All new wall warts are transformerless.

...meaning that they lack large 60 Hz transformers, right? They still have transformers; but, as ronczap points out, the transformers operate at higher frequencies and are much smaller. You can still get traditional wall warts, and they're still a little cheaper than the switching variety, but that's changing.

-Phil

erco
02-26-2010, 05:10 AM
Right, it's actually a new US requirement to delete the large, heavy 60-hz stepdown transformers in favor of more energy-efficient and smaller switching types. There are plenty of good examples out there already, but the iRobot Roomba battery charger is NOT among them. Its underspec'ed regulator is well-documented for blowing up, leaving an overvoltage condition that destroys the $60 NiMH battery shortly thereafter. Usually the day after the warranty expires.

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·"If you build it, they will come."

C-Bob
02-27-2010, 07:03 AM
Stevezila said...
O.K. at the risk of sounding totally stupid. I'm looking for a way to power a BS2 using 120 VAC without a transformer. Isn't there a way to run the line voltage through a full wave bridge and then and then feeding it through a regulator to power the stamp without involving a transformer? What am I missing? Thanks for replies in advance.
If you need linear regulator for this example, calculate:
dU = 120V - 5V = 115V
I = constant -·select 100mA peak 1A
you must neutralize 11,5 W of heating, for 1A 115W! You need·massive heatshink, bigger than transformer.
115W peak power consumption in stamp·device - new record.


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C-Bob

Every problem·have at least one·nice, simply and·wrong solution.

Peter KG6LSE
02-27-2010, 07:47 AM
You could use a "capacitive voltage divider " .

http://en.wikipedia.org/wiki/Voltage_divider

http://www.aikenamps.com/VoltageDiv3.gif

www.aikenamps.com/VoltageDividerRule.htm (http://www.aikenamps.com/VoltageDividerRule.htm)
its how my Kill-O-Watt Meter I have works . but its only good for isolated devices .


I would drop the voltage to 16V AC the rectify to DC then across the DC line put a TVS (transient voltage suppressor) rated at 20V to for the LM chips Safety and then drop it again with a LM REG to 5V to run said Stamp .


Note your load must not vary much for this to work well ..
I have used it for a many projects .


KG6LSE

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"Carpe Ducktum" "seize the tape!!"
peterthethinker.com/tesla/Venom/Venom.html (http://peterthethinker.com/tesla/Venom/Venom.html)
Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway. —Tanenbaum, Andrew S.
LOL

Post Edited (Peter KG6LSE) : 2/27/2010 12:53:56 AM GMT

logan996
02-27-2010, 10:29 AM
why would you want to do that?

Stevezila
03-06-2010, 05:34 AM
Thanks so much for the replies, My reason for doing this is that I need to build an occupency sensor that I can mount in a light fixture to turn the light on/off in a call center cubicle, I will then need to replicate it many many times to control all the fixtures·in all of the cubicles so that when a call center operator leaves their cubie for say lunch, break, or for the day, the light turns off automatically only in that cubie, and when they return, the light comes on automatically. So you see that I want to get the power from the fixture rather than having to run seperate wires for the OCC. I don't need to use a stamp, just need a PIR, a 555 timer and DIP switch to make it selectable for 1,3,5 minute time out. Just need to find a cheap way to get 5 volts to power the darn thing. I can mount all the circuitry inside the fixture so that only the PIR is outside and visable/touchable. This will·help to keep my lighting energy usage to a minimum while giving my the best flexibility in lighting options. Any idea where I can find a miniature power supply?·· Thanks in advance for the repiles..