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3.6 -4.5v

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J Reidy

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Hi guys, playing around with phones and powering them direct from the mains. Any easy way of making up a 3.6-4.5v supply of about 300mA. ALternitavly an easy way to reduce a 5v supply. I would like to keep it small,


Regards

James
 
sorry i wasnt clear, i meant an easy way to reduce 5v to ~4v..... the phones i am playing with come with a 5v charging adapter. on a previous model i shorted out the power pins on the phone and put 5v direct to battery pins, i was then able to power up by just adding power to the p suppy. Now on the current phone im using the 5v is too much and the phone isnt working properly
 
J Reidy said:
sorry i wasnt clear, i meant an easy way to reduce 5v to ~4v..... the phones i am playing with come with a 5v charging adapter. on a previous model i shorted out the power pins on the phone and put 5v direct to battery pins, i was then able to power up by just adding power to the p suppy. Now on the current phone im using the 5v is too much and the phone isnt working properly

hi,
You could connect a diode in series with the +V wire, drops about 0.7Vfwd.
A 1N4001 diode has a rating of 1 Amp.
 
If you are using a GSM phone, a 300 mA supply is hopelessly small. It will not work.

When GSM phones are transmitting, which they have to do briefly to register on the network, they take up to 1.9A, for 1/8th of the time, 216 times a second. So if your power supply isn't up to supplying 1.9A, the GSM phone won't work at all.

The average current is about 240mA when transmiting

The pulse is 579us long at 1.9A. If you need a capacitor to drop less than 1/2 volt, it needs to be at least 2200 uF, and more if you are running from the mains and there will be gaps of 1/2 cycle of the mains.

GSM phone supplies are hard work to get right, because of the pulsed nature of the load. That is why ordinary mobile phones won't work from their chargers alone. They need a battery fitted to work, while, for example, laptops don't.
 
I hate been told things i don't want to hear! i reckon it was just luck that it worked for me before so?
 
The charger could have had a large capacitor, or if you are close to a transmitter which would make the phone transmit with less power. 5V will destroy phones if you are unlucky, but there could have been more room to drop to 3.3V or so because of that. The 5V is to charge the battery, not to run the phone.
 
Diver300 said:
If you are using a GSM phone, a 300 mA supply is hopelessly small. It will not work.

When GSM phones are transmitting, which they have to do briefly to register on the network, they take up to 1.9A, for 1/8th of the time, 216 times a second. So if your power supply isn't up to supplying 1.9A, the GSM phone won't work at all.

The average current is about 240mA when transmiting

The pulse is 579us long at 1.9A. If you need a capacitor to drop less than 1/2 volt, it needs to be at least 2200 uF, and more if you are running from the mains and there will be gaps of 1/2 cycle of the mains.

GSM phone supplies are hard work to get right, because of the pulsed nature of the load. That is why ordinary mobile phones won't work from their chargers alone. They need a battery fitted to work, while, for example, laptops don't.
Wow.. how do you know all these?
 
Cheers for your input diver, a bit of knowledge goes a long way, so if you dont mind myself annoying you further, in theory, a 1.9A supply with ~4v should work ok?
 
or another idea.....
leave the battery + charger connected to the phone, short the power button contacts again, on the v+ line from battery to phone add a switching circuit (or is there a relay that'll do that job) that will detect ac? whats the best kind of circuit for this? or will i just leave ye all alone till ive finished uni.... :)
 
Last edited:
J Reidy said:
or another idea.....
leave the battery + charger connected to the phone, short the power button contacts again, on the v+ line from battery to phone add a switching circuit that will detect ac? whats the best kind of circuit for this? or will i just leave ye all alone till ive finished uni.... :)

I don't understand what detecting AC will do for you. If there is AC above 1/2 V you have a problem, but it will only be there while the phone is transmitting.

A 1.9A supply at 4V is OK, but make sure you have plenty of capacitors, including ceramic ones, to smooth the supply. The current starts and stops very quickly, so you need capacitors that deliver the current very quickly, and the big electrolytics are fairly inductive.
 
bananasiong said:
Wow.. how do you know all these?

because I make vehicle tracking devices that use mobile phone modules.

The power supplies have to run the phones at 3.8V from car or lorry batteries. It is all too easy to let the voltage dip and the phone module turns off.

The modules we use are from www.telit.com

I work for On-Trak.
 
sorry, i wasnt clear, i mean to detect the ac voltage when it is applied to the charger, i want to be able to switch it on when i plug it in.
 
J Reidy said:
sorry, i wasnt clear, i mean to detect the ac voltage when it is applied to the charger, i want to be able to switch it on when i plug it in.

If you can get to the insided of the charger, you can connect an extra diode and as small capacitor the output of the transformer. That will give you a signal that is high (maybe slightly higher than 5V) when the mains is on and low when the mains is off.

The bigger problem is how you get that to turn the phone on and off. You can't just connect a battery to a phone and expect it to turn on, because it won't. You usually need to short the "on" button for about 1 second and then release. You will need a timer for that.

Also, you turn the phone off in exactly the same way, so you need some circuit to detect whether the phone is on before you start. You can just use a relay to remove the power to turn it off, and then when you restore the power, short the on button.

Getting phones to turn on and off took me far too long. The phone modules behave just like real phones and it would be much easier if they had an enable line like voltage regulators.
 

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sorry, i missed your reply and played around in the mean time.
My working solution at the moment is as follows
Short out the power button indefinatly, this works on some phones, not on others.
I have a 5v minature relay detecting the output from the charger, when it is powered it connects, that then closes the switch from the battery to the phone.

Not sure how reliable in the long run but seems to work ok. Thanks for your above solution too diver
 
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