12v Phone Charger  - Page 1
Forum section: electronics / projects

author: Chet T16
author_rank: Frequent Contributor
timestamp: January 15, 2012, 10:53:21 pm
content: I found a couple of phone car chargers dating back to my phone unlocking/repairing days and and i was wondering if it you be possible to modify the circuit to allow for different voltage outputsQuick sketch of the circuit, i done this ages ago and only found it today and took a quick picThe IC has no markings on it and i don't know the inductor valueCan anyone shed some light on the workings of the circuit? I guess its something to do with the relationship between the 1k and 3k9 resistors? Maybe i'll put a pot in place of the 3k9 and see what happens

author: Psi
author_rank: Super Contributor
timestamp: January 15, 2012, 11:16:46 pm
content: Are you sure that the circuit is correct (has no external feedback)?

author: Chet T16
author_rank: Frequent Contributor
timestamp: January 15, 2012, 11:22:50 pm
content: I was sure it was right when i done it but i haven't double checked since finding it again!

author: amspire
author_rank: Super Contributor
timestamp: January 16, 2012, 12:09:08 am
content: I suspect the junction of the 3K9 resistor and the 4.7uF cap is connected to the output. In that case, you adjust the output voltage by varying the 3k9 resistor.When it is regulating, there will be a fixed voltage on pin 5 like 1.2 V.It can only output voltages below the supply voltage, so you are not going to get 15V out with 12V in, but you may be able to adjust from the pin5 voltage up to 9V or 10V.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 16, 2012, 12:15:25 am
content: It looks like an MC34063 or compatible with a transcription error. You can look at the data sheet to get the schematic and formulas, but you should be able to change the output voltage by changing R1 and R2.

author: IanB
author_rank: Super Contributor
timestamp: January 16, 2012, 12:38:27 am
content: Your circuit doesn't look quite right.You should expect it to be a buck-mode switching converter. Such a circuit basically works by switching the 12 V supply on and off through an inductor to charge up an output capacitor. A feedback signal from the voltage on the output capacitor allows the control IC to adjust the charging pulses and regulate the voltage.The inductor is there to store energy and allow the circuit to be efficient. If the IC just tried to charge up a capacitor by applying 12 V pulses to it the capacitor would appear as a short circuit and cause enormous currents to flow resulting in big resistance losses. By putting the inductor in the way it impedes the flow of current by storing energy and not wasting it. When the current is switched off the inductor then releases its energy into the capacitor. The diode is needed to provide a current path between the inductor and the capacitor during the off period of the input pulses.

author: joelby
author_rank: Frequent Contributor
timestamp: January 16, 2012, 01:13:07 am
content: I suspect the 0.4 R is actually be an inductor. You'd need to measure it with an L meter.

author: amspire
author_rank: Super Contributor
timestamp: January 16, 2012, 01:38:47 am
content: Quote from: joelby on January 16, 2012, 01:13:07 amNo, it is the current sense resistor.See fig 11http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF

author: joelby
author_rank: Frequent Contributor
timestamp: January 16, 2012, 01:45:28 am
content: Ah, I retract my statement and will not comment further on anything at all until after I've had my cup of tea. I'm so used to seeing this circuit drawn the other way around that I didn't even look at the left hand side

author: Chet T16
author_rank: Frequent Contributor
timestamp: January 16, 2012, 08:46:50 am
content: The circuit is a bit arse about face, i never tried to draw a circuit from a board before.Seems i may have messed up somewhere then, i'll check it out this evening

author: Chet T16
author_rank: Frequent Contributor
timestamp: January 16, 2012, 08:04:06 pm
content: Quote from: amspire on January 16, 2012, 12:09:08 amYep, that looks right now that I look at it again!

author: kripton2035
author_rank: Super Contributor
timestamp: January 16, 2012, 10:49:37 pm
content: to play with the components values ...http://www.nomad.ee/micros/mc34063a/index.shtml