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


#2

You would need just about any 3v power supply, like: http://www.amazon.com/ZJchao-Adapter-Power-5-5-2-1mm/dp/B009SNGQXU/ref=sr_1_1?ie=UTF8&qid=1456764634&sr=8-1&keywords=3v+adapter

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.


#6

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.


#8

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.


#16

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)
http://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table


#17

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.