Modifying a phone charger from 5V down to 3V&4V

[richjtf]

Hi. I relized today i have an old charger lying around that has no use anymore, so i wanted to do a little project with it, it indicates that the input is 100V-240V~50/60Hz 0,1A and the output is: 5.0V DC 350mA.

I want to add a feature to this charger, a switch, that changes the outpunt, and let's me choose between 3V 3,5V 4V 4,5V and 5V. Adding resistances to the end is a way i guess, but i would like to read more ideas!

Here is an album of about 9 pictures of said charger from every angle.

http://imgur.com/a/m0WEX#0

 

[Windadct]

You want a voltage regulator to do this. If you use an adjustable one, you can switch select the output setting. The one issue is 5V is relatively low for the input. Adding resistors will not work as this is making a voltage divider- as the connected load changes the ratio and thus the output voltage will change.

 

[donpacino]

you can use a voltage regulator like this http://www.ti.com/product/lm317

 

[berkeman]

you can use a voltage regulator like this http://www.ti.com/product/lm317

But as Windadct points out, the high input-output dropout voltage is pretty big for standard linear regulators like the LM317 (around 1.5V or more). The OP could use low-dropout linear regulators to get maybe to 4V. A better solution is a buck DC-DC regulator.

 

[berkeman]

Also note that if you want to go the low-dropout linear regulator route, many of them have a minimum output current requirement, and generally they need some minimum output capacitance in order to stay stable. Check their datasheet to see what-all you need at their input and output terminals.

 

[meBigGuy]

I would need to see a schematic in order to determine whether you could modify the charger to give different output voltages.

It looks like a bridge rectifier connected to the mains followed by a 1 transistor switcher of some sort.

This is typically what is done: from http://www.eetimes.com/document.asp?doc_id=1279469 In this example, resistors can be changed to affect output voltage, but this is not exactly what your charger looks like. It looks like you have a single transistor pwm circuit driving the primary of the transformer. It may be that the zener diode sets the output (you have a zener diode labeled ZD1), in which case it may be difficult to change.

The simplest thing is to use a low dropout linear regulator, or a bunch of them to produce the voltages you want.

 

[Jeff Rosenbury]

I'm skeptical of meBigGuy's take on this. He seems to be assuming the job was done right. I think the job was done cheap. (I don't see enough parts from the picture.) I think this is a simple linear power supply. I could be wrong though, I'm on limited bandwidth, so only looked at the first picture.

That transistor looking thing in the middle of your first picture is possibly a linear regulator. If you can find numbers on it anywhere, go to findchips.com and see what part it is.

It is possible you can adjust the voltage by changing the voltage applied to the reference pin. It's also possible you can't. If you can't, you could replace the regulator with one that you can adjust.

If you're going to fix it by building a buck regulator, it would likely be easier to start from scratch. (Although the parts would be useful in the new supply.) I would start from scratch anyway since a linear supply is much less efficient than a switcher. Wasting a couple of watts isn't going to break the bank, but every little bit helps.

BTW, Radio Shack used to sell a nice switcher with variable output. So I'm assuming this is a hobby.

 

[berkeman]

That transistor looking thing in the middle of your first picture is possibly a linear regulator. If you can find numbers on it anywhere, go to findchips.com and see what part it is.

Which part looks like a linear? It's a switcher IC with a built-in FET for doing the switching.

 

[Jeff Rosenbury]

I could be wrong. I've looked at the other pictures and the inductors don't look good for my theory. There's no need for Q2 on the back either in my theory. OTOH, I've never seen a three terminal switcher. Q1 looks a lot like a three terminal linear regulator, though it could easily be a FET switch.

If it's really fancy it could be a switcher followed by a linear regulator to cut down on switching noise. There's a lot of parts on the back I didn't see earlier. Too many for a cheap unit, so I'm likely wrong.

Is there any chance we could get part numbers off Q1 and Q2? Add Q3 to the list as well if there is one and I missed it.

 

[berkeman]

I could be wrong. I've looked at the other pictures and the inductors don't look good for my theory. There's no need for Q2 on the back either in my theory. OTOH, I've never seen a three terminal switcher. Q1 looks a lot like a three terminal linear regulator, though it could easily be a FET switch.

If it's really fancy it could be a switcher followed by a linear regulator to cut down on switching noise. There's a lot of parts on the back I didn't see earlier. Too many for a cheap unit, so I'm likely wrong.

Is there any chance we could get part numbers off Q1 and Q2? Add Q3 to the list as well if there is one and I missed it.

Jeff, my apologies. I was looking at the schematic in post #6, not what the OP posted in post #1. Sorry about that.

The OP pictures do appear to be of a linear supply.

 

[Jeff Rosenbury]

No problem. I could actually believe either type at this point.

 

[meBigGuy]

It is not a linear supply. The Line voltage comes directly into a bridge, and then is switched by Q1/Q2 somehow. It is a switcher with a transistor doing PWM switching.

 

[Jeff Rosenbury]

I've given this some more thought. It occurred to me there might be some sort of current limiter, crow bar, or such to match the charging characteristics of the battery. That might explain some of the extra circuitry.

 

[meBigGuy]

No way. It is the minimum number of components needed to get 5V @ Xma from the rectified AC line (through a switched transformer). You will never find a USB wall charger with any sort of battery protection mechanisms (just UL-like mandated safety mechanisms for smoke avoidance). And, they know NOTHING about what kind/size/etc of battery they are charging, or even if they are charging a battery. It is just a 5V supply. All of the battery related smarts are in the device with the battery. It has to be that way since devices have different batteries with different requirements and the charging requirements are complex.

 

[richjtf]

Hey there, have been quite busy, I'm sorry i didn't reply this long, I'm going to make a schematic for you guys, but for now, Q1 = T2095A
Q2 = Can't really tell, maybe you can?

 

[nsaspook]

How exact do the voltages have to be? I've used forward biased silicon and/or Schottky diodes in-line with the output wire to drop voltages if the load circuit can handle a few hundred millivolts of variation as current changes. It's a hack but can be helpful in a pinch.
http://www.cliftonlaboratories.com/1n400x_diode_family_forward_voltage.htm

 

[meBigGuy]

Its going to be something like what I posted in #6 except the IC is replaced by the 2 transistors and zener diode.

Here is 1 that uses 1 transistor http://www.homemade-circuits.com/2012/05/cheapest-smps-circuit-using-mje13005.html

There is a good chance the output voltage is determined by the transformer design. In that case your best bet is an LDO added to the output.

 

[davenn]

Q2 = Can't really tell, maybe you can?

Q2, a 1P is a 2N2222A type general purpose, NPN transistor