Hi,
I wanted to make my electric razor rechargeable with a 18650. It originally used 2 AA batteries. The specs for the motor used are hidden, there is no model name whatsoever (probably for commercial purpose). The motor is bigger than a normal 3v dc motor though. I have tested it with my power supply and it can withstand 4v+. So I added a classic DW01 TP4056 protection circuit with a 18650 battery, plus added a diode and capacitor in series with the motor in case it cooks the ICs (Ive waited a bit before adding the diode and cap and turned on the circuit a couple times before adding them, so maybe that plays a role in my problem).
Once finished, I turn on the motor and half the times, it spins slowly for like 1 revolution and then stops even though the switch is on. This appened half the time or so. When i screw everything together, adding the razor mecanism to the motor (adding load on the motor) its even worse, it doesnt move at all most of the time.
What do you think is happening? Is the motor stalling?? that would be wierd since it's the same mecanism the motor was bought with, and everything was working fine with AA batteries. Is it due to the protection circuit? The 18650 battery can supply enough watts for this job, so I really don't know what's up. I continuity checked my connections and everything seems soldered properly. maybe one of the wires has too many broken strands so it creates resistance?
jremington:
The protection circuit probably has an overcurrent limit. Post a link to the datasheet.
DW01 has 2 overcurrent protections. p 6 talks about it, but I'm not sure I understand how it works. It talks about mV for overcurrent, but no where does it mention max amps. I used a multimeter and the motor uses around 0.25 amp with the 18650 battery.
Test the razor with AA batteries to check whether you have damaged it.
jremington:
A powerful, low voltage motor could easily draw more than 3A.
Don't forget that the startup/stall current of a brushed DC motor is 5 to 10 times higher than the running current.
I measured it 2 minutes ago, while it was running (sometimes it does, looks like leaving it alone for a few hours helps) and it was drawing 1.9 A. Do you think the power draw can fluctuate that much, up to over 3 A sometimes?
jremington:
As stated, the start/stall current is typically 5 to 10 times the running current.
You can estimate the start/stall current by measuring the motor winding resistance, and divide that into 3.7V.
Hoooo so when it starts, it uses the same amount of current as when it is stalled?? So how the hell can I make the motor start with the protection circuit on if it inhibits discharge as soon as i turn it on.
Hoooo so when it starts, it uses the same amount of current as when it is stalled?
Yes, briefly, until the armature starts to spin and a back EMF is developed.
You can add a large capacitor (say, 5000 uF, 6V) to buffer the high starting current draw, but it has to be charged to be effective, i.e. across the output of the battery protection circuit.
Another approach would be to find a protection circuit that does not have an overcurrent shutoff, just an overdischarge (low voltage) cutoff.
jremington:
Yes, briefly, until the armature starts to spin and a back EMF is developed.
You can add a large capacitor (say, 5000 uF, 6V) to buffer the high starting current draw, but it has to be charged to be effective, i.e. across the output of the battery protection circuit.
Another approach would be to find a protection circuit that does not have an overcurrent shutoff, just an overdischarge (low voltage) cutoff.
Im willing to try the capacitor option. How would I hook it up on the circuit output exactly? I have done some research and I cant find anything that makes sense. In parallel would short the cap, and in series would make an open circuit?
I don't understand your comment about "in parallel would short the cap".
Connect the cap between the protection circuit output and GND (battery negative), obeying cap polarity. This may not work if the protection circuit is latching, because the capacitor charging current behaves like a dead short, the instant the battery is connected.
Did adding a large Capacitor worked for you, I had a 2200uF 25V Cap, it didn't help.
TP4056 is rated to give over-current protection at 3Amp. but it still doesn't work.
I am sure that the small motor is not drawing more than 2A at start or stall... at constant running when directly connected to 18650 without protection it is drawing 650mA.
I am thinking of not using protection at all directly run the motor from battery and add a low voltage indicator to hint me not to use the device but to charge the battery.
As the original trimmer circuit was using a single NiMh battery (1.2V-1.35V) to power this motor.
As this motor is designed to work at low voltages its internal resistance is very low, ranging between 3ohm to 6ohm (do not know exact value as I didn't open the motor to test the coil directly).
with this small resistance a smaller voltage will allow smaller current, higher voltage will allow higher Current. Voltage and current are directly proportional as resistance is constant.
Solution: To fix the issue I added 2 diodes IN4007 in series with the motor. And it started working.
P.S. I am on a electronic expert, just a hobbyist, I learn by experimenting.