looking for small 5v to 2.4v-3v regulator PCB with connection to 2x AA rechargeable batteries

Thread Starter

radiance32

Joined Mar 10, 2021
11
Hi,

I'm looking for a small PCB that accepts 5V (coming from a USB port, only used as a power supply),
and that provides 2.4v to 3.0v (the equiv of 2 alkaline or NI-MH rechargeable AA batteries)
to the system, and connects to a pair of rechargeable NI-MH batteries,
that it uses for power if the USB 5V power is not connected/available.
It should also recharge the batteries up to %100 when it is connected to the 5V USB power supply.
The current drawn by the device at the 2.4 to 3.0 voltage is approx ~100mah to ~250mah.

I'd like a ready-made, drop-in solution (not a kit that i need to solder together myself),
small size PCB board that does this. The board needs to be tiny, as small as possible,
I'm hoping for something that's just 25x20mm or similar, just the smallest possible...

Does anyone know of a suitable board like this that I can purchase online? (I need 14 of them in total)
I don't have much electronics skills, so if you have any questions, please be verbose,
however, I'm moderately proficient at soldering.

If anyone could drop a reply with a name and/or link for a suitable item like this,
I'd really appreciate it :)

Thanks,
Terrence
 

Thread Starter

radiance32

Joined Mar 10, 2021
11
How about something like this.
Hi Crutschow,

Thanks for your reply! :)

This looks like what i need :),
but, it has an input for the 5v coming from the USB power port,
and an output going to the batteries,
but what about the device ? is there a separate output for the device,
that powers the device while the battery is charging ?
Or, can I hook it up in such a way that is basically does the same ?

Terrence
 

Yaakov

Joined Jan 27, 2019
3,501
Sorry I made a mistake, it's not 100-300 mah (that's another project), but it's approx 150 to 220mah when powered with 2 AA alkaline or NI-MH batteries...
Do you mean milliamps? Milliamp-hours is not a measure of current rather total charge over time. Current would be in plain amps.
 

crutschow

Joined Mar 14, 2008
27,732
Do you need the voltage to the load be 5V when available?

If not, then you can just connect the charger and batteries directly to the load.

If you need 5V to the load, then you need isolation between the batteries and the 5V source.

Below is the LTspice simulation of an example isolation circuit.
It uses a Schottky diode to isolate the source and a P-MOSFET (for minimum voltage drop) to isolate the battery.
There is about a 0.4V drop through the diode from the 5V source, but I assume that's not a problem with your load.
Notice that when the 5V source (green trace) goes between 0V and 5V the output voltage (yellow trace) goes between 2.39V and 4.63V, while the battery current (red trace) goes from about -240mA (charging) to -240mA (discharging).

The Schottky diode can be just about any with a 1A rating or higher.
The P-MOSFET can be just about any logic-level type (Vth(max) of 2V or less) that has an on-resistance of no more than 0.1 ohm.

1615562366321.png
 

Thread Starter

radiance32

Joined Mar 10, 2021
11
Do you need the voltage to the load be 5V when available?

If not, then you can just connect the charger and batteries directly to the load.

If you need 5V to the load, then you need isolation between the batteries and the 5V source.

Below is the LTspice simulation of an example isolation circuit.
It uses a Schottky diode to isolate the source and a P-MOSFET (for minimum voltage drop) to isolate the battery.
There is about a 0.4V drop through the diode from the 5V source, but I assume that's not a problem with your load.
Notice that when the 5V source (green trace) goes between 0V and 5V the output voltage (yellow trace) goes between 2.39V and 4.63V, while the battery current (red trace) goes from about -240mA (charging) to -240mA (discharging).

The Schottky diode can be just about any with a 1A rating or higher.
The P-MOSFET can be just about any logic-level type (Vth(max) of 2V or less) that has an on-resistance of no more than 0.1 ohm.

View attachment 232568
Hi again Crutshow,

Thanks again for all the effort you've put into helping me on this forum,
I really appreciate it.

The device originally ran on 2x AA batteries before (2x alkaline = ~3v, or 2x NI-CD or NI-MH = ~2.4v), so the voltage it needs is the same as the batteries, so am I correct in understanding your reply that I can simply connect the device's power input to the battery charging terminals along with the battery ? So it can run when the batteries are empty/charging and the device needs to run off the power supplied through the board (from the battery terminals, together with the battery connected to them aswell) ?

If this is correct, then I have all I need to know and am ready to order a few of those boards and try them out :)

Thanks again,
Terrence

Thanks,
Terrence
 

Thread Starter

radiance32

Joined Mar 10, 2021
11
Do you mean milliamps? Milliamp-hours is not a measure of current rather total charge over time. Current would be in plain amps.
As said I don't know that much about electronics, but I assume it's just normal current and I should have used something else than
"mah", but I am unsure...

If the device runs for approx 10.5 hours on a pair of NI-MH AA batteries rated at 2300miliamps,
that would be 2300 / 10.5 = 219 milliamps usage. don't know if I should call it milliamps or milliamps/hour though... Maybe you can correct me :)

Terrence
 

Yaakov

Joined Jan 27, 2019
3,501
As said I don't know that much about electronics, but I assume it's just normal current and I should have used something else than
"mah", but I am unsure...

If the device runs for approx 10.5 hours on a pair of NI-MH AA batteries rated at 2300miliamps,
that would be 2300 / 10.5 = 219 milliamps usage. don't know if I should call it milliamps or milliamps/hour though... Maybe you can correct me :)

Terrence
If someone asks you “how much current does it use” they are asking about the amount of charge per second the device needs to operate. 1A is 1C (Coulomb) per second. That is an amount of charge per unit time. So 1 AH (amp hour) is 1 C/s for 1 hour. It’s a different measurement. Important but different.

It’s not necessarily clear without some study, but if you hear “how much current does it need” the answer with be in amps or fractional amps. If you hear “how much capacity does that battery have” it will be in amp hours or fractional amp hours.

[edited due to typos]
 
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Thread Starter

radiance32

Joined Mar 10, 2021
11
If someone asks you “how much current does it use” they are asking about the number of charge par second the device needs to operate. 1A is 1C (Coulomb) per second. That is an amount of charge per unit time. So 1 AH (amp hour) is 1 C/s for 1 hour. It’s a different measurement. Important but different.

It’s not necessarily clear without some study, but if you hear “how much current does it need” the answer with be in amps or fractional amps. If you hear “how much capacity does that battery have” it will be in amp hours or fractional amp hours.
Yeah, that's for explaining things in detail for me,
I do have some knowledge of amp/hour and coulombs, but quite vaguely...

As said, i'm not an electronics engineer, I've a small amount of knowledge of electronics, and moderate soldering skills... that's all...

Thanks!
Terrence
 

crutschow

Joined Mar 14, 2008
27,732
So it can run when the batteries are empty/charging and the device needs to run off the power supplied through the board (from the battery terminals, together with the battery connected to them aswell) ?
Yes, but the battery charging current will be limited to the difference between the current from the charger circuit and the load current taken by your circuit.
If there's only a small difference then it may take a long time to charge the battery.
Would that be okay?
 

Thread Starter

radiance32

Joined Mar 10, 2021
11
Yes, but the battery charging current will be limited to the difference between the current from the charger circuit and the load current taken by your circuit.
If there's only a small difference then it may take a long time to charge the battery.
Would that be okay?
Yeah, I don't think that would be an issue...
I've ordered a few of those boards and will give it a go ;)
Thanks for all your help :)

Terrence
 
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