hello,
i have bunch of smarthome devices as amazon blink cameras (5v) , aqara motion sensors, hue motion sensors (3V)

I plan house reconstruction so i was thinking to have separate lines in the wall for 3V and 5V - so i can attach these battery powered devices to that.

Was thinking to use this PSU https://www.meanwelldirect.co.uk/products/25w-dual-output-open-frame-power-supply-2/
but it looks like most of these PSUs are 3.3V instead of 3V. Is there any reason behind it?

Can I power 3V sensors with that PSU?

My thinking was to simultaneously power 3V and 5V devices with it.


Not sure if my design / thinking makes sense.


Thank you for ideas.

 

hi thor,
Do you have any part numbers for those two 3V devices?

E

 

@ericgibbs
hello, thanks yes for example some of them are these

https://eu.aqara.com/products/aqara-motion-and-light-sensor-p2
https://www.philips-hue.com/en-gb/p/hue-hue-motion-sensor/8719514342125

thanks

 

hi thor,
Are you sure about a 3V device and not 6v,?

Clip from 1st link,
[Easy to Install & Long Battery Life] Thanks to its compact wireless design and adjustable stand, the Sensor P2 can easily be installed on a wall, ceiling or a particular zone, and set up with ease. The Motion Detector and Light Sensor P2 has a powerful 2*CR2450 Lithium battery and provides up to 2 years of power for round-the-clock security, one of the longest battery life among Matter over Thread sensors. (NOTE:The battery life depends on the particular Matter ecosystem and the number of Matter ecosystems the device is connected to)

 

yes, its 3V
as the batteries inside has connected togeter + and - ie in parallel.

 

hi thor,
OK,
You could use a Schottky diode in series with your 3.3V supply,
The voltage drop across a Schottky diode is usually between 0.15V and 0.45V

E

 

as most of the devices are 3V just wondering why they are producing 3.3V PSU?
Is it a common or a good idea to use that diode? What is the type? So i connect it from PSU in series on the plus pole and then to the device?


thanks

 

hi thor,
Most micro -controllers use either 5V or 3.3V supplies
E

 

Not a battery expert, but I DO know that a CR2032 has high internal resistance. Would imagine a CR2450 has a similar internal resistance. The 3.3V PSU would likely not have as high an internal resistance and could therefore possibly pose a problem for the devices under power.

And given that the "TWO" CR2450's are in parallel, I don't know how that would affect the internal resistance.

{side note (off topic)} Been looking for a while now trying to find CR2450 battery holders. Have had no luck.

 

@ericgibbs
thanks, but is it safe and fine to do it that way? No idea how other ppl solving automation/smart devices using PSU

so the final setup is



@Tonyr1084
what do you mean by high an internal resistance - i thought every device running on batteries can have it replaced by PSU?

thanks

 

The CR2032 battery (have little experience with it) can directly power an LED without a resistor. The reason is that the battery itself has a high internal resistance. That means it's capable of only a small current. No resistors needed when powered directly from a coin cell.

A power supply may be capable of much higher amperage - depending on what you have in hand. Its internal resistance will likely be much lower. Ohm's law predicts that at a given voltage and load of a certain resistance will draw a specific amperage. If that amperage is restricted by the power source then it might not have built in protection. I'M PURELY GUESSING HERE, but I think it might be worth while considering. Others like Eric are far more versed and knowledgeable about the topic. I only bring it up just to make sure all things are considered.

 

hi thor,
Yes, it is a commonly used method for dropping a low current, low voltage source.
E

 

@ericgibbs
thanks and any idea regarding that high internal resistance of the batteries? Is it something that needs to be considered somehow?

I mean - does it make a sense to replace battery sensors with PSU? Just wondering... maybe i am trying to achieve something which has no meaning in reality

thanks

 

hi thor,
If the units are supplied with the two CR batteries fitted, I would run them using those batteries.
Then it would not invalid any warrantee of the units.
E

 

@ericgibbs i dont care about warranty that much ... just wanted to "elegantly" get rid of the batteries.

 

Looking at the manual for the Hue sensor it appears the batteries are in series, 6 volt operation.
View attachment 337321

Would like to see the manual myself.
Found this on the Hue link:

Two AAA batteries. Not coin cells. That's what I think I saw on sghioto's post.

 

hi just to clear out confusion
1) philips motion 2x AAA (each 1.5V) in series = 3V
2) aqara motion 2x cr2032 (each 3v) in paralel = 3V
3) other devices as amazon blink camera = 5V

 

In a design for production use a 3.00V regulator or reference with sufficient supply current. Using diode drop is usually not a problem.
Using a diode drop characteristics like a Schottky diode just be aware that a small capacitance may be introduced.
It may not amount to much, if it is significant then Ltspice .noise command is an option to analyze the diode's noise contribution
and how to problem solve this using simulation. In this simulation isolating the resistor voltage noise contribution is shown.
In circuit board design I sometimes need to mess with noise reduction using simulation also reposition or move the diode.

 

Using a diode drop characteristics like a Schottky diode just be aware that a small capacitance may be introduced.
It may not amount to much

hi.
How do consider a Diode Capacitance of 7pF is going to effect the flow of current through the diode?
There is more capacitance in circuit wiring.
E

 

I wouldn't worry about the supply at 3.3 volts as a fresh cr2450 reads appx 3.2 volts and aaa alkaline read 1.6 volts