Battery to Mains conversion

[desertsyd]

Hi Community,

 

I've got battery operated device that I'll like to convert to use mains power, I don't have an electrical engineer background

 

Have successfully powered the device off a bench power supply feeding in a voltage of 3.6v for a few days (no battery present). 

The device will fully operate in a voltage range of 3.6-3.8v.

Using a standard micro USB plug pack, could someone draw up a circuit that drops the input 5v to output 3.7v?

 

Read the comments from I HATE Batteries! - Converting Wireless to Wired (a cheapskate’s guide) there's talk about using 1uf capacitors, but this is beyond my current knowledge  

 

Thanks in advance.

 

[OhioYJ]

Does your bench power supply have a current meter or do you have any idea how much current this item draws?  

 

I wouldn't try and reinvent the wheel here. I honestly many times just use an existing power adapter I have for projects whenever possible. Many times I can find a spare adapter from an old device that supplies the correct voltage, and enough current. Just remember, get the voltage right, more current is fine (lower is not, it will only draw what it needs)

[desertsyd]

Require 500mA for stable operation.

 

When removing the battery, as it's an internal Lithium Ion on a daughter board. Also removes the charging circuit and proprietary power plug. The plan is to provide power via a micro usb by getting a custom board made from PCBway.

[desertsyd]

so after a bunch of research, the best answer would be a back-convertor and aliexpress has it covered.

 

https://www.aliexpress.com/item/1005005084434791.html?spm=a2g0o.order_list.order_list_main.10.56961802uM3npy

 

https://www.aliexpress.com/item/1005001597589461.html?gps-id=pcStoreJustForYou&scm=1007.23125.137358.0&scm_id=1007.23125.137358.0&scm-url=1007.23125.137358.0&pvid=99a786aa-e5a8-4351-b9df-f8e4cd523ba4&_t=gps-id:pcStoreJustForYou,scm-url:1007.23125.137358.0,pvid:99a786aa-e5a8-4351-b9df-f8e4cd523ba4,tpp_buckets:668%232846%238116%232002&pdp_npi=3%40dis!AUD!1.55!1.37!!!!!%402101ead816825892322224871ee305!12000016728249488!rec!AU!142991025&spm=a2g0o.store_pc_home.smartJustForYou_2004859917745.0

[Heats with Nvidia]

On 4/27/2023 at 11:59 AM, desertsyd said:

so after a bunch of research, the best answer would be a back-convertor and aliexpress has it covered.

What kind of device is it? These Power adapters are really OK, but for some devices their output voltage is not good enough and has too much ripple, but this is usually quite easy to mitigate.

[mariushm]

Get a 5v power adapter / phone charger you don't care about.   Connect two generic diodes in series with the voltage (ex 1n400x as in 1n4001 .. 1n4007, doesn't matter)  and you'll get your voltage range.  A single diode will have a voltage drop of around 0.7v, so two diodes will drop around 1.4v, and you'll be left with 3.6v.

 

There are diodes with lower voltage drop like 1n581x (1n5817 to 1n5819) - these will have a voltage drop as low as 0.35v at 100mA , and around 0.6v at 1A  - so you could combine a 1n4007 with a 1n5819 to get around 0.8v + 0.5v = ~ 1.3v drop at 500mA

 

Cut the USB or barrel jack connector at the end and you can solder the wires directly.

Or you could buy a barrel jack connector to maintain the ability to disconnect.

 

Example power supply with standard barrel jack connector : https://www.digikey.com/en/products/detail/xp-power/VEL05US050-US-JA/5023713

Diodes  .. 1n4007 :

https://www.digikey.com/en/products/detail/micro-commercial-co/1N4007-TP/773644

https://www.digikey.com/en/products/detail/diodes-incorporated/1N4007G-T/111822

1n5817

https://www.digikey.com/en/products/detail/diodes-incorporated/1N5817-T/22052

 

barrel jacks ( 2.1 and 2.5mm inner diameter jacks, the power adapter above has 2.1 inner / 5.5 outer diameter ) : https://www.digikey.com/short/v937jcj5

 

If you want a more fixed/guaranteed output voltage, they make linear regulators with fixed output voltage, but 3.6v is less common so not so many regulators are made with this voltage.

 

You can use an adjustable linear regulators and configure two resistors to set the voltage to 3.6v or 3.8v

A 1117 regulator would work . LM317 would also work.

1117 : https://www.digikey.com/en/products/detail/texas-instruments/LM1117T-ADJ-NOPB/363595

317 : https://www.digikey.com/en/products/detail/stmicroelectronics/LM317T-DG/2832895

 

 

You need a couple resistors and a couple capacitors, no big deal

For 1117 :

 

Cadj is not needed  in your case, ignore it. You can use 120 ohm or 100 ohm for R1,  whatever works better for you. Replace R2 potentiometer in the schematic with the resistor you want to get the right voltage. 

For example, to get 3.8v on output with 100 ohm for R1, you'd have :

 

3.8v  = 1.25 ( 1 + R2/100) =>   1 + R2/100 =  3.8v / 1.25  =>  R2/100 = 3.8v / 1.25 - 1 =>  R2 = 100 * (3.8/1.25 -1) = 204 ohm so you could use 200 ohm or 210 ohm and you'll get a bit less or a bit more than 3.8v - I would just use 2 100 ohm resistors in series for a total of 200

That would give the output voltage  V out = 1.25 ( 1+ 200/100) = 1.25 x 3 = 3.75v

 

You can use only 10uF on the output just like on input, to make ordering parts cheaper (ex get two 10uF 16v.. 50v rated electrolytic capacitors)

Or you could use any value between 10 and 100uF, it's not that important, 47uF is another very common value

 

Example of capacitor :  https://www.digikey.com/en/products/detail/rubycon/50YXF47MEFC6-3X11/3563091

 

LM317 works exactly the same, same schematic, same values, same everything.