Question for you Electrical Engeeners

(Joel W) #1

I am using the Samsung Multi Purpose Sensor it uses the following battery, with the following specs. What kind of power supply could I get to prevent frequent changing of battery. Constant voltage isn’t necessary as it is for an Attic exhaust fan and if there isn’t power then the fan isn’t running anyway. I just am not sure what would happen to the pairing if the power fails? Also what do you think the longest wire to the device I could use to avoid voltage drop?
Classification: "Lithium Coin"
Chemical System: Lithium / Manganese Dioxide (Li/MnO2
Designation: ANSI / NEDA-5029LC, IEC-CR2450
Nominal Voltage: 3.0 Volts
Typical Capacity: 620 mAh (to 2.0 volts)
(Rated at 7.5K ohms at 21°C)
Typical Weight: 6.8 grams (0.22 oz.)
Typical Volume: 2.4 cubic centimeters (0.14 cubic inch)
Max Rev Charge: 1 microampere
Energy Density: 183 milliwatt hr/g, 647 milliwatt hr/cc
Typical Li Content: < 0.3g
UL Recognized: MH29980
Operating Temp: -30C to 60C
Self Discharge: ~1% / year


You would need just about any 3v power supply, like:

It will re-pair after the power comes on. Just don’t leave the battery in if you wire up a power supply. The coin battery should last 9-12 months, if you don’t mind getting in your attic once a year to change it.

(Joel W) #3

The unit is about 35 foot in the rafters which is a good climb and replacing the battery while doing a balancing act is scary. How long a wire could I run without a huge voltage drop? It is about 35 foot from the unit to the floor.

(Michael Hess) #4

You could also take a AA holder and wire it in. I tested this and it worked great. One that holds 8 cells in a series/parallel configuration so you get the 3v’s you need, then use lithium AA’s and it should last for a LONG time.

I tested power draw on one of the multi sensors and when it was reporting, my meter went to about.3mw for a half second or so, draw under no notification was not measurable on my meter. Need better meter…

(Joel W) #5

I found a dual holder wired parallel which holds CR123 Lithium batteries which I have a bunch as that is what my other devices use. So I will do that two lithium in parallel 35 foot away should work just fine. Thanks.


If you are using anything over 20 AWG you should be fine. You would only see about .1 V drop over 35 feet. Sounds like you have a plan for dual CR123’s. That should give you some long life.

(Joel W) #7

I figured I would use two number 14 stranded wire.


There would be a negligible drop of like .02 V with 14 AWG.

(Joel W) #9

That is what I figure, now to gather the parts.

(Eric) #10

A plug-in USB power supply will probably work, that is 5vdc out and the current that matters is that of the sensor, not the battery. I did cut one up and soldered it to a car presence fob which is much more convenient than changing batteries every 3 months. 5v minus the wire drop, is unlikely to harm the sensor rated for a nominal 3v.

Get yourself a cheap multimeter from Harbor Freight or Ebay, to make sure you identify +/- . If you don’t want to solder it in, then you could cut a shim to battery-size and wedge the wires in place. Glue the wire jacket to the sensor-body as strain-relief. Maybe the wires will loose connection at some future time and you will have to re-seat them.

14gauge wire for this purpose, is overkill and much harder to work with.

(Joel W) #11

I am not so sure that the extra two volts won’t damage the sensor.

(Eric) #12

You can add a small resistance to drop the voltage (or even potentiometer to dial in exactly what you want), but if you are not familiar with a meter or don’t solder, then it’s too much hassle.

I’m a EE and I don’t mind frying a sensor, but I haven’t let any home automation smoke out yet. 2volts / $55 is nothing.

(Joel W) #13

I already have the battery holder and the wire is in my supply now so I just have to get up there and solder it. I have been soldering for years so not a problem. The sensor is using a coin battery and the negative contact is very small so I have to use a alligator clip to act as a heat sink as to not damage the plastic.

(Michael Hess) #14

I can kinda confirm this, when I wired my 8 AA holder to the sensor I flubbed my math and actually sent it 6v, nothing bad happened, though I didn’t leave it that way for more than the few seconds it took me to realize my error. The usb charger and a resistor is a great idea. I’ve had to do some crazy resistor strings for Dodge’s CANBus system, so this is NOTHING! :slight_smile: Thanks for the idea for in car use.

(Joel W) #15

I have done the same thing many times. Sometimes I just don’t think.


What is the current draw of the sensor?

Voltage drop in the wiring is V=RI
I = sensor current
R = wire resistance (look up wire spec, typically R/1,000 ft)
Or us online calculater (easier)


Hmmm, i would just use that 3V power supply that siuengr suggested…

Get one with a regulated output. Who knows what current that sensor draws. Usually an unregulated supply will have a high output, but it regulates down to it’s nominal operating voltage with a load. If the sensor doesn’t draw much current the power supply voltage maybe much higher than 3 volts using an unregulated supply.

(Joel W) #18

As it stands I have the battery box wired in Parallel and that will be my source of power. The box is near the Attic door.