# Mobile charger circuit

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#### Diver300

##### Well-Known Member
Cellphone chargers are really just a power supply, and the charging control is invariably done inside the phone.

Most cellphones will charge fine when fed with 5 V, and that circuit will work just as well as most.

#### audioguru

##### Well-Known Member
The charger circuit for a cell phone's battery is in the phone. It uses a regulated 5V power supply as its input.

The circuit you found tries to limit the current wrongly. The amount of current depends on the current gain of the transistor which is from 50 to 400 and changes with temperature changes. A transistor with a gain of 50 will limit the current to about 383mA and if its current gain is 400 then the current will be limited to 3A! The little transistor has a max allowed current of only 100mA. I don't know how much current the charging circuit in the phone draws.

#### Dr.james

##### New Member
i built it today!! And works great!! U don't know anything audioguru! U know nothing!

#### Nigel Goodwin

##### Super Moderator
The charger circuit for a cell phone's battery is in the phone. It uses a regulated 5V power supply as its input.

The circuit you found tries to limit the current wrongly. The amount of current depends on the current gain of the transistor which is from 50 to 400 and changes with temperature changes. A transistor with a gain of 50 will limit the current to about 383mA and if its current gain is 400 then the current will be limited to 3A! The little transistor has a max allowed current of only 100mA. I don't know how much current the charging circuit in the phone draws.
Yes, it was a truely appalling attempt at a circuit, presumably 'made up' (not designed) by someone without the slightly clue about electronics.

#### lebevti

##### Member
but it used a linear regulator, meaning it is inefficient and loses power in the form of heat

#### Nigel Goodwin

##### Super Moderator
but it used a linear regulator, meaning it is inefficient and loses power in the form of heat
Many circuits do, it has the advantage of being simple, cheap and reliable.

#### Dr.james

##### New Member
it (7805)doesn't heat as it spends only 100mA

#### audioguru

##### Well-Known Member
it (7805)doesn't heat as it spends only 100mA
How do you know how much current the charger in your phone uses? Did you measure it?
What is the DC input voltage to your 7805?
Then subtract the 5V output from the DC input voltage and multiply by the current to calculate the amount of heat.

I bet the overloaded useless little transistor gets very hot.

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#### Dr.james

##### New Member
How do you know how much current the charger in your phone uses? Did you measure it?
What is the DC input voltage to your 7805?
Then subtract the 5V output from the DC input voltage and multiply by the current to calculate the amount of heat.

I bet the overloaded useless little transistor gets very hot.
No, the transistor doesnt get hot and i could full charge my phone in 2 hours!! Its an amazing circuit!! I think some people in here(like audioguru) have to learn electronics well before posting wrong statements.

#### Nigel Goodwin

##### Super Moderator
No, the transistor doesnt get hot and i could full charge my phone in 2 hours!! Its an amazing circuit!! I think some people in here(like audioguru) have to learn electronics well before posting wrong statements.
Sorry, it's an utterly crap circuit - throw the transistor and resistor on the output away, they do nothing.

#### audioguru

##### Well-Known Member
Maybe your overloaded little transistor is shorted so it does not get hot. The transistor is not needed since the charging circuit in the phone limits the current. As I calculated, the little transistor does not limit the current anyway.

You can charge your phone IN A FEW MINUTES if its battery is not completely discharged.
I have a Nokia BL-4C battery. It is only 720mAh. My Nokia charger supplies up to 350mA and charges it in 2 hours and 12 minutes while powering the standby circuit in the phone.

#### RCinFLA

##### Well-Known Member
The PNP transistor is just a gate that gets turned on when 5v from regulator is present. It won't get hot because it is in saturation with very low Vce. It is not necessary as phone won't backwash current out charging connection when AC power is removed.

With a 12v AC transformer, the DC input to 7805 will be close to 17 vdc. For 5v output and 100 mA draw the 7805 will be dissipating 17v-5v = 12v, x 0.1 amps = 1.2 watts. It will get a little warm, it might roll back its current limit to keep temp under control which is probably why you are only getting 100 mA. You would be better off using a 6 vac transformer and increasing the filter capacitance. A 7805 properly heat sunk should be able to put out more then 1 amp.

It is unlikely that any present day cellphone only needs 100 mA for charging. For a small GSM phone with a 550 mAH battery it should take about 300 mA's. For a smart phone with 1500 mAH battery it should take 700 to 1000 mA. You will charge with 100 mA but it will take a lot longer to fully recharge the battery.

Many cellphone charger wall supplies have current limiters that helps reduce the actual charge regulator heating in the phone by allowing phone's regulator (series pass transistor) to operate as a fully saturated switch. When operated off a fixed 5v supply the cellphone's charge regulator is forced to operate in linear mode increasing its heat dissipation. Most smartphones, when forced to operate in linear mode from a 5v source will cycle on and off to keep from generating too much temp rise in the phone. This increases the total recharge time a little bit. We are only talking about 1 watt of heating but in a confined, enclosed space of a cellphone it will heat things up.

#### Ziddik

##### Member
ppl,

the simple cellphone charger circuit is designed by me so i want your opinion to improve the quality of the charger circuit, and should i remove the pnp transistor? I think the 100ma current is better to charge batteries slowly because fast charging will decrease efficiency of the battery, what should i do to make it better?

#### audioguru

##### Well-Known Member
The transistor has a base current of 4.3V/560 ohms = 7.7mA. At its max allowed collector current of 100mA its current gain is from about 50 to about 400 so it will try to limit the current to 385mA to 3A! The transistor is not needed since the charger circuit in the phone limits the current.

Modern cell phones use batteries that charge with a much higher current than the tiny old cell phone you have.

The transformer voltage is much too high that makes the regulator get too hot. The 12V transformer has a peak voltage of 17V and the rectifier bridge and filter capacitor reduce the input voltage for the regulator to 15V. The regulator needs a minimum input of only 7V so a 7V or 8V transformer should have been used.

#### Ziddik

##### Member
the maximum allowed current by a 7805 is only 1 amp so the 7805 gets hot only when a circuit draws more than the allowed (1amp) current, i use a smartphone from nokia and i charge my mobilephone using this circuit since 3 months, although it takes about 4 hours to fully charge my battery. And the regulator inside a phone will get very hot and will be destroyed if i didn't add that pass transistor! See 7805's datasheet first

#### Nigel Goodwin

##### Super Moderator
ppl,

the simple cellphone charger circuit is designed by me so i want your opinion to improve the quality of the charger circuit, and should i remove the pnp transistor? I think the 100ma current is better to charge batteries slowly because fast charging will decrease efficiency of the battery, what should i do to make it better?
So what is the transistor supposed to do? - it does nothing.

#### audioguru

##### Well-Known Member
the maximum allowed current by a 7805 is only 1 amp so the 7805 gets hot only when a circuit draws more than the allowed (1amp) current
NO!
Heat is caused by power dissipation. Power dissipation is caused by the current times the voltage from input to output.
You use a 12V transformer so the peak voltage is 17V and the voltage at the input of the 7805 is 15VDC. Then if the current is 1A the 7805 must dissipate 10W! Without a heatsink it will be extremely hot if it dissipates only 1.9W. A 10W heatsink is pretty big and must be outside the box.

If your transformer is 7V then the peak voltage is 9.9V and the input to the 7805 is 7.8VDC. Then with 1A it dissipates only 2.8W and it will need a medium size heatsink.

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#### Ziddik

##### Member
no!
heat is caused by power dissipation. Power dissipation is caused by the current times the voltage from input to output.
You use a 12v transformer so the peak voltage is 17v and the voltage at the input of the 7805 is 15vdc. Then if the current is 1a the 7805 must dissipate 10w! Without a heatsink it will be extremely hot if it dissipates only 1.9w. A 10w heatsink is pretty big and must be outside the box.

If your transformer is 7v then the peak voltage is 9.9v and the input to the 7805 is 7.8vdc. Then with 1a it dissipates only 2.8w and it will need a medium size heatsink.
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