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Basic Electronics » YAWWT (Yet Another Wall Wart Thread)

January 22, 2010
by treymd
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I'm trying to find a batteryless solution to my kit, and while running off the usb port is convenient, the doodads I intend to create with the MCUs won't be tethered to a machine. I have a couple old warts sitting around, all basically within the 7.5v to maybe 12v range. the first one I tried reads 7.5v 500ma output The actual unloaded voltage appears to be 11.5 or so.

using only the LM7805 a 1k resistor and an LED just for kicks, I hooked this guy up and the 7805 gets HOT... I mean real hot.

Am I just hooking this test circuit up wrong, IE. not putting enough resistance in it, or do I just keep throwing out wall-warts until I find one that doesn't make the voltage regulator scream?

the basic layout is + to pin1, pin2 to -, pin3 to 1k to LED to -

January 22, 2010
by Phrank916
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I've just been going regulator free myself since I found an old LG cellphone charger that is rated 5v 1A. Multimeter said 5.2 unloaded and was rock steady so I went with it. No problems since. I actually had about 4 diff 5v old cellphone chargers to choose from. Hehe.

January 22, 2010
by treymd
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I think I actually read somewhere that all of the cell phone manufacturers agreed to use usb in one form or another to charge every phone moving forward so that chargers would be somewhat universal... good news for nerdkit owners!

January 22, 2010
by Rick_S
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Some regulators will heat up more than others. That is not completely uncommon. I have two regulator boards both with 7805 regulators that I use frequently on my solderless breadboards. One will heat up noticably more than the other one using the same 12vdc adapter (Actual no load voltage is about 14).

Good sources of regulated wall-warts besides the cell phone chargers include poser supplies for routers and network switches. A lot of them are 5vdc regulated with 1 - 2 Amp output.

Rick

January 22, 2010
by Phrank916
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I actually work for one of the BIG national cell phone providers and what you say is true. The manufacturers have finally agreed upon micro USB.

January 22, 2010
by Rick_S
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LOL, I just re-read my post and I said POSER supplies instead of POWER supplies...

:)

Rick

January 23, 2010
by carlhako
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Check the voltage before assuming. I learnt that one today, bought 5x 22000mcd leds for lighting, worked out which resistors i needed to test 4 at a time off 12v. After a few mins 1 led went off, i put the multimeter across all 4 leds and resistor was over 16v for a 12v power pack. leds were rated at 4v they were getting an extra volt each!

so just a warning even tho the power pack says 12v or whatever rating, it may be a fair bit higher.

January 23, 2010
by treymd
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My father had an old cell phone adapter... it already has alligator clips on it because his cord went bad.. convenient!

Anyhow this thing seems to put out a steady 5.3v, should I be ok with that? I know I can't use the regulator with it, as the voltage is not high enough, haw bad could that extra 0.3 volts be?

patiently awaiting advice since I don't want to burn up my MCU!

January 23, 2010
by Rick_S
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Under a load that will probably drop a bit, that said, the datasheet for the atmega168 states that the operating voltage for the mcu is up to 5.5V typ with a max of 6V. It should have no problem at 5.3v as long as it is clean 5.3vDC.

Rick

January 23, 2010
by treymd
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OK, thanks Rick!

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