Converting an Powersupply to a Lower voltage charger

Thread Starter

hardusb

Joined Apr 20, 2020
16
Hi,

I have an Wahl hair clipper to which I lost the charger, I am trying to build / convert an existing power supply to charger it:
It requires 3.5v 1A to charge, I followed some other guide on the internet to reduce the voltage of an 12v 2A Ac Adapter to the required voltage.
The device picks up the power input and shows its charging but I left it through the night and it didnt charge 1% even.
I measured the voltage while on charge with this circuit and it dropped 1V while charging to about 2.5v, not sure if its normal, I dont have any fancy measurement tools to use, only a basic multi meter.

Is there some other components needed to charge a device?

I did a little circuit diagram to illustrate what I did to accomplish this, this is all the components I had available at the moment.

I connected my device to charge on the bottom points.
 

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ericgibbs

Joined Jan 29, 2010
11,890
hi h,
Welcome to AAC,
That circuit is totally unsuitable as a charger.
Do you have a old mobile phone charger on the bench you could modify.?
E
 

Thread Starter

hardusb

Joined Apr 20, 2020
16
Just use any mobile phone charger, cut the plug off and solder it .
The device says it requires 3.5v 1A input and in clear writing to Only use the provided charger, so I wanted to get the same voltage at least of the original charger.

I opened up the device to see which battery it uses and its an 2.4V 1200mAh. 2 Long sells wrapped in green.
 

dendad

Joined Feb 20, 2016
3,720
Add a couple of diodes in series with the 5V out.
2 x 1N5004 diodes will work ok. Each diode will drop around 0.7V so the result will be pretty close.
 

ericgibbs

Joined Jan 29, 2010
11,890
hi,
I very crude charger using a 1R 3W resistor and two 1N5401 diodes in series could be made, powered from thr 5V power supply.
E
 

Thread Starter

hardusb

Joined Apr 20, 2020
16
Thanks, will see what I can get my hands on, under lockdown so my options are limited, going to check for appliances I can strip for parts,
 

Dodgydave

Joined Jun 22, 2012
9,911
2.4V @1200mA sounds like 2 Nicads or Ni_mhis in series, you could put a 10 Ohms resistor in series with the 5V supply to drop the voltage easy.

Over Voltage wise, 4V probably ok.
 

Thread Starter

hardusb

Joined Apr 20, 2020
16
I managed to get 6 diodes from an old set-top box and put them in series, it dropped the voltage to 3.5v from the 5v, its been about an hour and the device hasn't charged 1% even.
 

Thread Starter

hardusb

Joined Apr 20, 2020
16
Just to be sure, I put them in series on the positive end like follows:

+5v in -->|--->|--->|--->|--->|--->|-- (Load)
- (load)

Load goes at the end of the diodes and the negative.
 

BobTPH

Joined Jun 5, 2013
2,863
I suspect that the charger is just that: a charger as opposed to a power supply. A charger limits its current and monitors the battery, cutting off the charge when it is finished.

If you apply 3.5V to a two cell NiMH battery, it will overcharge and destroy the battery.

NiMH is the most difficult chemistry to charge correctly. It requires either monitoring the temperature and cutting off the charge when it starts increasing steeply, or detecting a drop on the battery voltage of only a few millivolts.

You can safely trickle charge it at 5% of the capacity, in this case, 60mA. To do this, take your 12V supply and put a resistor in series to limit the current to 60mV when the battery is at 2.4V. The calculation is, based on Ohm’s law:

12-2.4 = R * 0.060

R = 9.6 / 0.060 = 160 Ohms.

Edited to add: Or with 5V, use 47 Ohms.

This will take about 24 hours to fully charge a depleted battery, so just keep it on the charger whenever it is not in use.

Bob
 
Last edited:

BobTPH

Joined Jun 5, 2013
2,863
While the device is charging the voltage is 2.4V, without the device charging its 3.54v.
That is because the charger is connected directly to the battery, as I surmised. You are basically reading the battery voltage. Follow the advice in my previous post or you will destroy the battery.

Bob
 

Thread Starter

hardusb

Joined Apr 20, 2020
16
I see what you mean above, half of the mission here is to make a compatible charger so that in the future if I have a similar situation I know how.

The original Charger is rated 3.5v 1A, thus im attempting to replicate just that.

The original charge time was 16h. 24h is not that far off.

Closest I have now is 1 X 10 ohm, 1 X 11 Ohm resistors and one 23 Ohm, that makes 33 Ohms.

All other resistors I have is in Kilo Ohms.
 

Thread Starter

hardusb

Joined Apr 20, 2020
16
After putting all these resistors in series I get 42.3 Ohms, voltage hasnt dropped at all, still 5v.

Charger is an Samsung mobile phone Charger with a USB Port. 5v 2A, got the extra 9v fast charging feature also but I think the mobile phone triggers that., I took an old USB cable and getting my power that way.
 

Thread Starter

hardusb

Joined Apr 20, 2020
16
Its been on charge almost an hour now, the lcd display is just moved from 26% to 27%, so it seems to be working.
 

Thread Starter

hardusb

Joined Apr 20, 2020
16
Its now charging at an rate of 1% every 35 Minuites, its now 29% charged, at this rate it will take 35 Mins X 71% = 2485 Mins = 41 .42 Hours to complete charging to 100%. This is a bit long. With the diodes it didnt charge at all, maybe too much diodes taking away all the charging current?
 
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