120V to 1.5V

Hi, I recently bought a remote thermometer but it all runs off of watch batteries and I don't want to have to keep replacing them whenever it dies. I was wondering if there is anyway I could take the 120V from my house and convert it down to 1.5v then permenently wire the remote temperature sensor to the house power. I am not totaly in the dark, could I use a plug transformer and get it down to 3V and then use a resistor to kill the rest?

Kill the rest?
Of what the amps?
Go to radio shack and buy one of those adjustable transformers they make.
Then go back a week later when the piece of shit burns up and buy another.

>>2
Well thank you for your valuable wealth of information.

First off, which watch battery is it using? If it's a CR2000-series coin battery, those are 3 volts and you should be able to connect an AC adapter directly to it. Only the old LR44-ish button batteries are 1.5v.

If you need 1.5v, your best bet would be to get a LM317T linear regulator IC from RatShack or whatever and use that to do the dropping instead of a resistor; it's more work, but it won't get as hot and it'll provide rudimentary surge protection.

Haha, sweet. This will be easier than I thought it would be. Thanks for your help.

Whats the voltage and the amperage of the battery? You could also just buy a couple diodes and make a whetstone bridge to get the same effect of the ic, but all that really wont matter if you get a transformer at the right amp/voltage output.

>>6
The watch battery is a Maxwell CR2032, 3V at 0.2mA How would I control the amperage?

Wait lemme get this strait your saying you already have a transformer that has the 3 volt output but its amperage is higher than .2ma?
Honestly I don't think you have much to worry about most transformers with that low a voltage only put out a couple milliamps anyways(cdplayer amirite?) and your watch batteries would probably peak allot more than that when they are new, so id say anywheres from .1 to .9 is safe, but if your really worried about to much amperage just get a variable resister and turn it down to where you want it, Id explain how to get the right resister but I'm really trying to forget about pie right now.

>>7
You dont need to control the amperage. The circuit will only draw as much as 3V will let it draw. Amperage ratings are there to tell you the limit that it can do, not what it WILL do.

The specs, 3v and 0.2mA tell you what voltage is required, and how many amps it pulls. According to ye olde Ohm's Law, unless you somehow alter the resistance of your circuit (ie by short circuiting it), a 3v input to the circuit will always be pulling 0.2 milliamps from end to end. Most devices are required to list an amp rating for safety reasons; houses have fuses that carry 15 amps on average, and some big appliances that need a lot of juice might blow the fuse if used together on the same circuit.

As for your problem, if 3v is the lowest setting you have, just get the resistance rating of the device by plugging in an ohmmeter to both ends. Then match the resistance using resistors, and hook it up in series with the power supply. This doubles the circuit resistance, and since it's in series the 3v will be distributed evenly, giving you the 1.5v you need to run the device within specifications. Hope that helps (ps. not responsible if your place burns down due to shitty Mexican soldering job).

>>10
I am German, and I can solder. :P

But thanks from everyone :D

If you're at radioshack, get a 3.3v zener diode. This will be your "clamp". Get a simple NPN transistor too (2n2222 or 2n3904). grab a 10uF capacitor, and a few 1k resistors. Also get a small potentiometer (10kohm). The use of an LM317 requires a few volts (~2) above the output rail, so it becomes a problem. The BJT will drop ~0.7v from base to emitter too. tie your BJT's collector to the +v supply voltage. Tie 2 1k resistors (in parallel) from +v voltage to the cathode of the 3.3v zener. Tie the anode of the zener to ground. Tie the 10uF capacitor in parallel to the zener diode. This gives you some supply stability and transient regulation. Now, tie the pot to the cathode node of the zener diode and the other end to ground. Tie the wiper to the base of the BJT. The output will be tunable from ~0v to 2.6v. Tie another 1k resistor to ground from the emitter in a temporary manner as its your test load. Measure the output voltage on the emitter and tune the pot until it reads 1.5v, or whatever you need. This will act as a pretty good voltage source and can source a few mA.

>>12
Thats WAY more complicated than this needs to be.

Who cares, it would be more stable over time and temp than most other solutions and it would be precise. The OP isn't going to keet a '317 active with such a light load on it anyway.

>>14
A better and by far easier solution would be a voltage regulator and a capacitor. Then you can give it just about any DC power supply you have hanging around.