Hi, I am about to start on a mini project to convert a car phone charger into a charger for my electric shaver that I keep in my car, so I can have a shave at work if I forget. I want to be able to charge it in the car so that I don't have to take it out to charge it. I have an old Nokia phone charger that currently outputs 5.95 volts dc. The shaver says on the back 5v 400ma, so I have added a resistor to the shaver to drop the voltage down to approx 4.85v. Is this way to do this or should I use a voltage regulator instead ?. Also how is the output current of a power supply calculated / measured ?.

 

You won't be able to use a 3 terminal regulator like an LM7805 to drop from 5.9 to 5.0 volts. A linear regulator requires a minimum voltage across it for it to work. Many are around 3 volts or so.

You could build a switching regulator to do this.

Can your charger put out 400+ ma. Load the charger with a bunch of fixed resistors and plot E vs I. See where you want to be then see what you've got.

A resistor will only work if you have a constant load current otherwise the voltage to the shaver will not be constant. E=I x R. as I varies, so will E. Shavers are not a constant load.

I would skip the Nokia part and feed a cheap LM2940 hardened 5v regulator direct from the cigar lighter outlet.

 

Hi, I am about to start on a mini project to convert a car phone charger into a charger for my electric shaver that I keep in my car, so I can have a shave at work if I forget. I want to be able to charge it in the car so that I don't have to take it out to charge it. I have an old Nokia phone charger that currently outputs 5.95 volts dc. The shaver says on the back 5v 400ma, so I have added a resistor to the shaver to drop the voltage down to approx 4.85v. Is this way to do this or should I use a voltage regulator instead ?. Also how is the output current of a power supply calculated / measured ?.

Try to measure the voltage from the charger then it is connected to the shaver. If it is in the range of 5 volt +/- 10% you should be ok. Another solution is to use a 7805 voltage regulator with a small heat sink. And connect it to the 12 volt cigarette lighter plug

 

The battery in your shaver might be a "4.8V" Ni-Cad that is 6V to 6.4V when fully charged.
Then the "5V" charger is not regulated so its voltage increases as the battery charging current decreases as it approaches a full charge.

 

You will be able to use a 3 terminal regulator like an MIC5237-5.0YT to drop from 5.9 to 5.0 volts. This linear regulator requires a minimum voltage of just 0.3V across it for it to work. Many are around 3 volts or so so don't use those.

You don't need to build a switching regulator to do this.

However, just taking the output from the cig lighter and converting this directly to 5V would be the simplest way to go with the least parts. You will need to let about 4 watts burn off, which can get a little warm. An LM7805 with a heatsink should work for that.

 

A series rectifier diode (e.g., 1N4001) will drop the voltage by about 850mV (typ.) at 400mA.

 

Thanks for everyone's help , I think im going to go with voltage regulator and rip out the board of the charger and replace it with a regulator on a board instead.

 

When an appliance is rated for 5V input this is not rigid and there is some leeway. I would go with the diode solution myself.

 

Thanks for everyone's help , I think im going to go with voltage regulator and rip out the board of the charger and replace it with a regulator on a board instead.

hope you have enough heatsinking to handle the 3.6W of power dissipation.

 

Having dismantled several shavers over the years to replace the batteries since I have been shaving, there is no fancy electronics inside to charge the Nicads. Just a simple dropper resistor. It may limit the battery life but you are talking about mass produced items that may be replaced before the batts fail due to the foils getting blunt.

Charge the cells at 100mA and you should be fine. (13.8-4.8)/0.1 resistor ~100 ohm in series with a car charger. 1W will be OK.