So, if you’re looking for the the simplest and cheapest shortwave transmitter circuit, it’s here.
This transmitter is very low power and very stable as well, uses a readily available 3 terminal ceramic resonator to set the frequency.
Though this shortwave transmitter circuit can reach barely around 5 meters, but you can be increase the range easily with a simple RF amplifier and a low pass filter stage. Lets get started…
Simplest shortwave transmitter circuit diagram
Lets have a look at the circuit diagram,
Just one transistor and a 10.7 MHz ceramic resonator with few other passive components !
Part list
Below the part list,
Q1 – BC548 or any quivalent NPN transistor
X1 – 10.7 MHz ceramic resonator, 3 terminal
C1 – 10 nF ceramic disc capacitor (103)
C2 – 100 nF ceramic disc capacitor (104)
C3 – 100 pF ceramic disc capacitor (101)
R1 – 10 kOhm
R2 – 150 kOhm
R3 – 1 kOhm
That’s all you need to make this shortwave transmitter circuit . Power up the circuit with a clean 3 V to 5 V DC supply, i.e with a single Li-Ion battery is.
You can also replace the 10.7 MHz ceramic resonator with any other between 2.3 MHz to 26 MHz range.
Construction and testing
It’s quite easy to build, but you shouldn’t use a breadboard. I’ve built a prototype on a little piece of strip board.
A little about the audio input, you cant just connect a microphone to the audio input. This transmitter circuit needs a amplified signal input, should be powerful enough to modulate the amplitude.
Antenna plays a vital role on the range of this shortwave transmitter, without an antenna it won’t go more than one meter. Antenna length is not critical just connect a piece of 1 meter wire.
The frequency can be changed a a little(0.1-0.2 MHz) by adding a capacitor in series with the ceramic resonator.
We often need some low value resistors, less than 1 Ohm. Mostly as current sensing resistors in power electronics projects. You can’t measure less then 1 Ohm with cheap multimeters. So here’s a low resistance measurement method without a expensive miliOhm meter.
There’s some other tricks to measure low resistance, here I’m going to use the constant current source method.
I’m using this method to measure homemade wire wound resistors with satisfactory results. Lets get started.
Low resistance measurement method
Basically the trick is to pass a stable known amount of current through the low value resistor and measure the voltage drop across it.
In the above picture, R is the unknown resistor and V is a multimeter, set to read millivolts. Obviously the multimeter is connected permanently as shown in picture.
After setting up the circuit like above, you’ve to measure the voltage across the unknown resistor and devide it by the constant current. That’s all you need to do to determine the resistor’s value.
Measuring tips, test and example
Though this method is fairly accurate but it’s accuracy directly depends on the current source and the multimeter.
So you’ve to check the accuracy of the current source and the calculate any possible offset of the multimeter by comparing with a known stable voltage source.
It’s accuracy also depends how you’re measuring the voltage drop across the resistor, and how the circuit is constructed. Few tips below.
Use thick and short connecting wires, be sure to make the joints tight.
After connecting everything, wait a minute before taking the measurements.
Measure the voltage drop across the resistor as close as possible, more about this later.
Don’t touch any conductive part of the multimeter probes while taking measurements.
And don’t place any wire conducting AC near the setup, interference could change readings.
Also be careful while measuring in millivolts, don’t touch the probes to any other high voltage(>0.2V) part of the circuit.
Now, why you should connect the multimeter probes as close as possible to the resistor ?
Simply because connecting probes to different location could dramatically change the readings !
In the above picture I was measuring a 10 mΩ current sensing resistor. The voltage drop between the resistor’s solder pad was exactly 10 mV at almost 1 Amp current, but it was around 15-18 mV between the clips ! Huge difference.
So how was my experience ? With the cheapest(presumably) multimeter, I can measure low value resistors with around 2% accuracy respective to their marked value. I think that’s great for such a inexpensive setup.
Finally, what do you think about this low resistance measurement method? Share you thoughts in the comment section.
A constant current source source can supply a fixed current to a load regardless of input voltage or load change. LM317 constant current source is one of the simplest design.
The LM317 IC is quite useful as a constant current source, works on a wide input voltage range, from 3 V up to 40 V, and cheap too, here’s the datasheet.
So, here’s the LM317 based constant current source, it’s design and a little about it’s working principle.
LM317 constant current source circuit diagram
First, have a look at the circuit diagram, it’s pretty basic.
Only three components, excluding the power source and connecting wires. It’s really simple, you can build the circuit on a bread board or just by soldering components to each other.
LM317 is said to be sensitive to reverse polarity, most designs uses a protection diode before the input. But that will add another voltage drop of around 0.7 V to 1 V, so I excluded it.
LM317 constant current calculator
The value of R sets the amount of constant current, use the formula below.
Where I is the amount of constant current and R is series resistor.
So R has to be precise enough and it should be capable of dissipating the generated heat.
Metal film resistors are good choice for their 1% tolerance. But you can’t find a 12.5 Ohm, 6.25 Ohm or 1.25 Ohm resistor for 100 mA, 200 mA and 1 Amp current respectively.
So the the trick is to use some higher value resistors in parallel combination. Like for a 12.5 Ohm resistor, you can use eight 100 Ohm resistor in parallel. Or for a 1.25 Ohm resistor, use eight 10 Ohm half watt resistor in parallel.
Using resistors in parallel has many advantage over a single resistor.
A resistor bank can be much more precise than a single high power resistor.
They can dissipate the heat more effectively than a single resistor.
It’s easier to get small resistors than a single high power resistor, cheaper too.
What about the heat dissipation on the LM317 regulator IC ? LM317 is available in various packages, the commonly available TO-220 package can dissipate up to 500mW without heatsink for few minutes.
Prototype and testing
I built a prototype 100 mA LM317 constant current source on a breadboard.
The resistor bank is eight 100 Ohm carbon film resistors in parallel, around 12.8 Ohm resistance, measured with a cheap multimeter.
The output constant current is around 99.4 mA, quite close to the estimated value. The input voltage was around 4.10 V, a single 18650 Li-Ion cell.
I’ve used this to measure few low value resistors, 1 Ohm, 0.33 Ohm, and 0.05 Ohm. It performed surprisingly well, around 1% to 2% difference compared to the marked values.
Possible applications
You can use this current source in many ways, few of them below.
So, that’s all about this constant current source. The next constant current source will be a variable type, 4 selectable ranges, 1 A, 500 mA, 100 mA, 10 mA, stay tuned ! If you have any suggestion, question, just drop a comment.
LEDs are DC devices, operating from a few volts of direct current. Where power is supplied by a DC battery like mobile phone, or torch, you can easily run LEDs.
But running LEDs by AC requires a driver. Appropriate LED driver is required for the LED to last longer.
While LED itself costs less, say Re.1/pcs the driver circuit is often costlier than the LED itself.
The good news is that you can run LEDs without any complicated driver circuit. A good maths will help you design your driver circuit such that it ensures your LEDs are safe.
Today I’m going to build one LED array that runs off mains i.e. 230V AC. Here, multiple LEDs are connected in series so that the voltage drop across the array equals the supply voltage.
Certainly it is not the right SAFE way to drive a LED array, but it’s the simplest out there. Since there are no converters used, the efficiency of this circuit is much higher than other drivers.
Construction of the 230V LED driver
First I convert the 230V AC to DC by using a bridge rectifier and a capacitor. 230V AC becomes 230x√2=325V DC. A typical white LED runs on ideal 3.6V, but can run from 3V to 4V(max).
Feeding 325V DC into a series of such LEDs will require 90 LEDs (3.6V each). In my prototype I’m using 12×8=96 LEDs in array, so each LED will get 3.38Volts which is a bit less and will cause my LED strip to be less bright. But in other hands, I’m protected from little over-voltage spikes that may happen in mains line.
This is my schematics. I have used a bridge rectifier built by 4x IN4007 diodes, and a 4.7uF/450V capacitor for smoothing the DC wave. Use of the capacitor removes flickers from the output light.
Now it’s turn to make the physical circuit. I’ve used a 4″x6″ PVC switch box as an enclosure. I used a thick needle to make the holes and arrange the LEDs in 8 rows and 12 columns.
After connecting LEDs in series I test the LEDs by adding the bridge rectifier/capacitor circuit. All LEDs glow up in even brightness and none burned.
The advantage of connecting LED in series is that only one LED will burn in case of burning. From the top, you will be able to see a brown/black dot in middle of the LED (yellowish area). This way replacing that particular LED is easy.
Now I place eveything inside the 4″x6″ PVC board and I add an AC wire. Right now it’s been running for 2+ hours and no heating or burning. It’s successful.
If you build one like this or get any inspiration to make yours by different way, let me know by dropping a comment below.
A dielectric resonator oscillator is basically a high frequency oscillator, operates within a range of few tens of gigahertz.
The main frequency determining component is piece of dielectric material, usually made of ceramic.
This piece of dielectric is called dielectric resonator, usually constructed in special cylindrical shapes.
The elements have a relatively larger dielectric constant, high quality factor and a very low dissipation factor in microwave range.
Dielectric resonators are widely used in microwave application, specially when extreme frequency stability is not the top priority.
As high frequency oscillators in microwave range (DRO).
Microwave antennas, called Dielectric Resonator Antenna (DRA).
High quality band pass filters in microwave range.
Satellite communication equipments.
Millimeter wave radars and so on.
Construction of a dielectric resonator oscillator
Here I’m not going to make one, rather I’ll tear down a satellite TV’s LNB to show you how it’s made. Dielectric resonator oscillators are widely used as local oscillator in low cost LNB.
There’s two DRO in this LNB, perhaps one for the vertically polarized waves and another for horizontally polarized wave.
These two dark grey button like things are the dielectric elements, mounted on little plastic risers.
The oscillator circuit is simple, there’s only one tiny RF transistor in M04 super minimold package, probably 2SC5508 or something similar. And there’s also few SMD passive component.
A DRO can’t oscillate without a metal cavity, the top aluminium cover of the LNB along with the PCB ground plane forms a cavity.
There’s a brass tuning slug in each cavity to set the oscillation frequency. Actually a DRO is quite sensitive, a very little change in anything will ruin the tuning. That’s why LNB’s are always sealed with some sort of resin.
Conclusion
So, that’s all for this short article about DROs, I hope you enjoyed it. I’ll add some more pictures of different types of DRO in future, stay connected, also don’t forget to share your thoughts in the comment section.
HH004F is a low power integrated boost converter IC in a TO-92 package, look exactly like a small transistor.
Though it’s quite a uncommon and undocumented IC, but you may find it occasionally in some LED torches. The IC is available in many different numbering and marking.
I got one of them, and decided to draw the schematic and build a prototype.
HH004F based LED driver
This IC is typically used as a low power LED driver with 1.5V input voltage, here’s the schematic.
So, it’s pretty simple, just the HH004F IC, a 3.3uH inductor, a LED, a switch and a 1.5V battery.
Here’s the pin out of the IC.
I decided to build a quick prototype after salvaging the IC from a torch, below the picture of my prototype.
The value of inductor is not so important, anything between 2.2uH to 10uH should work.
So, that’s all for this short post. Don’t forget to share your thought through the comments.
There’s no short of free PCB design software to get your very own PCB ready. But which one should you choose ?
Each of them have a fairly time consuming learning curve. So you’ve to pick one carefully, it should be worthy enough to invest some time.
So, in this article you’ll get a rough idea about many PCB design software, their features and more.
Classifying the free PCB design software packages
First of all, lets short them according to their usability and OS platform dependency.
Native apps or single platform: Natives apps are usually designed to run on a single operating system. Like most premium PCB design software are built to run on windows only.
Cross platform apps: A cross platform app runs on many OS platform, including Linux, BSD and macOS. They performs as well as a native app, it’s highly recommended to choose a cross platform software.
Web apps: Web apps could run on almost any OS platform with a descent web browser, including Chrome OS and Android. They have great portability, but lacks the performance compare to native apps, in most cases.
Now I’ll short them according to their licencing model.
Free and opensource software: The best choice if you’re going to use a free PCB design software. There’s many of them, just be sure to pick an active project with a large community base.
Free software: They are free to use, without any feature restriction or expire date, still the software source code is not avail.
Limited freeware or crippleware: You can use the software with some limitations, but usually don’t have a expire time. Some of them works on the fremium model.
Trialware: These are most the restricted type of freeware, you can’t use them after certain period of time. Many features are crippled often, I try my best to avoid them. Anyway there’s always some trick to use a trialware after their expire time.
Best Free PCB design software
So, you should not choose a crippleware or trialware if you’re going to use it in long run without being forced to pay.
There’s some opensource PCB design software as good as professional software, you’ve to just find it out. Let’s start exploring them one by one.
KiCad
Undoubtedly it’s the best free PCB design software till now, fully featured and opensource, backed by CERN.
KiCad is a truly cross platform and multilingual application, runs seamlessly on Windows, Linux, macOS and most BSD flavors.
Till now there’s five main tool in KiCad EDA suite,
The KiCad program is the project manager, takes care of everything else.
Eeschema is the schematic drawing tool, can also import schematics from other EDA software.
Pcbnew is the PCB layout designer part, it can also render a 3D model of the final product.
Gerbv is used to view the Gerber files, drill files and export to other formats.
Bitmap2Component, it’s used to create new component footprint, useful when you’re using some custom part.
Here’s the official website for more details and installation instruction, http://kicad-pcb.org .
Many opensource hardware project used KiCad to design their PCB. The best part is it’s under heavy development, I hope it will be as good as any premium PCB design suites like Xpedition PCB or Altium Designer within few years.
I think the only downside is the user interface, it needs some improvement. The whole package is a bit large too, around 900 MB.
Fritzing
Fritzing is another opensource multipurpose EDA software, if you like to design with Arduino and breadboards, you’re really going to love it.
Fritzng has four different tool set,
A schematic capture tool, you can import almost every component and lots of third party boards to get your schematic ready.
A PCB layout tool to create the PCB, though it’s not very feature rich compared to other counterparts.
A breadboard view tool, that’s really cool, with lots of third party boards to import.
A coding IDE, somewhat similar to the Arduino IDE, you can program an AVR or PICAXE Microcontroller.
It’s a cross platform application, runs on Windows, Linux and macOS, the sleek Qt based user interface is quite attractive.
Here’s the official website, http://fritzing.org/home/ , you can get more info, tutorial and installation instruction there.
Though it’s said to be the EDA software for non-engineers, but it’s much better than others if you want to design some graphics rich circuit diagram for presentation.
Eagle PCB design software
It’s one of the most popular circuit designer software, used by hobbyists to large corporations, runs on Windows, macOS and Linux.
Eagle is purchased by Autodesk from CadSoft, here’s the official website, to download the free version.
Though Eagle’s free version is limited to only 80 x 100 square mm board size and two layer PCB, but it’s definitely worthy to give it a try.
There’s thousands of free online tutorials available about how to get started with eagle.
EasyEDA
EasyEDA is a web based almost complete free PCB design software , you don’t need to install anything to use it, works well on Chromebooks. Though it’s relatively newer, but gaining popularity fast.
Here’s the official website, https://easyeda.com/ , there’s also a Chrome extension of EasyEDA.
You can do almost everything from schematic capture, PCB layout design, SPICE simulation, gerber file generation and optionally fabricate the PCB from them.
EasyEDA can import schematics generated by other some EDA software, like Eagle, KiCad, LTspice, Altium Designer and more, that’s very handy.
gEDA
gEDA is a opensource cross platform PCB design software, runs on Linux, BSD, macOS and Windows.
In It was close competitor of KiCad in past, but now the development process is somewhat slow.
It’s available in the software repository of most Linux distributions, here’s the official site, http://www.geda-project.org/
Anyway gEDA is a good tool for basic free PCB designing and schematic capture, but lacks the ease of workflow for large designs. Another advantage of gEDA is it’s somewhat scriptable like Eagle, helps if you’re really good at gEDA.
You can import schematics from gEDA to the opensource application GSpiceUI for simulation.
CircuitMaker
It was a discontinued product from Altium Limited, recently they’ve released it as a freeware for hobbyists and students.
CircuitMaker is particularly useful for mixed signal and RF PCB design, where track length and width plays a vital role.
Though it’s free, but requires sign up to download. Here’s the official website http://circuitmaker.com/ .
Sadly it runs only on Windows only, can’t run on Linux even through Wine, So I’ve not tested it personally. As it’s now being used by many, so it’s beleived to be a good one.
Pad2Pad
It’s a neat PCB design software by the company of same name, Pad2Pad.
Pad2Pad is in active development, runs on windows only, I’ve not tested it personally, but it’s believed to be a good one.
OrCAD
It’s a proprietary complete EDA software with many features including schematic drawing, PCB designing and SPICE circuit simulation.
A free version with limited features is available upon request, you can get it from here.
It’s considered as a professional PCB design software, suitable for complex PCB design and FPGA design. If you’re planning to use OrCAD in future, it’s definitely worthy to spend some time on the free version.
DesignSpark
This free PCB design software is developed by the electronics component distributor RS Components.
It’s free to download and use, without any feature lock, but you’ve to register yourself to RS components. You can download DesignSpark from here.
DesignSpark have almost everything you need to fabricate a PCB from your concept, built to run on Windows only, latest version doesn’t run under wine on Linux.
Conclusion
So that’s it, a pretty comprehensive list of best free PCB design software , hope it will help you to pick one. And here’s the round up.
Basically go with the KiCad if you’re looking for a long term free solution with some serious production goal.
Fritzing is good if you’ve to deal with a lot of breadboards and hobby projects.
On the other hand, EasyEDA offers a quite usable PCB design software also with their PCB manufacturing service, and you don’t need to install anything to use it.
If you’re using some other, let me know through the comments.
This circuit uses the C9018 high frequency transistor, based on a different spin of the common base Collpit’s oscillator.
Single transistor FM transmitter circuit diagram
The circuit is rather simple, uses only one transistor and few passive components.
Here the RF feedback is taken from the center tap of C2 and C3, close to original Collpit’s oscillator.
You can calculate the effective capacitance of the tuned LC circuit by determining the series capacitance of C2 and C3.
If both C2 and C3 are equal, the effective capacitance will be the half.
FM transmitter circuit Part list and construction
Here’s the component list,
Q1 – C9018 NPN transistor
R1 – 470 Ohm, 1/4 watt
R2 – 10 kOhm, 1/4 watt
C1 – 2.2 nF ceramic disc capacitor (222)
C2 – 30 pF ceramic disc capacitor, (30)
C3 – 30 pF ceramic disc capacitor, (30)
C4 – 10 pF 30 pF ceramic disc capacitor, (10)
C5 – 1 nF 30 pF ceramic disc capacitor, (102)
C6 – 100nF 30 pF ceramic disc capacitor, (104)
L1 – 16 turn on 3 mm diameter
This circuit uses only 10 parts including the coil. The coil is winded on a 3 mm drill bit, 16 turns of 0.4 mm (28 S.W.G) enameled copper wire.
It’s very easy to construct the circuit on a strip board, just few components to solder. Always try to minimize the track length.
My prototype without the audio source connected, small enough to fit on a matchbox. Fix the inductor with some wax for better frequency stability.
This circuit runs on a 3.3 Volt regulated power supply.
Testing the FM transmitter circuit
This FM transmitter circuit performed well in terms of frequency stability, almost zero drifting after about 4 hours of continuous operation.
But still digital FM receivers can’t tune to this transmitter, as it’s not a PLL based design.
There’s no antenna connected to the transmitter, but you can easily receive clear signals from 10-12 meter distance. After that, the signals starts fading rapidly.
Conclusion
So, that’s all for this FM transmitter circuit project. As the transistor supports very high frequency range, you could expect significant amount of harmonics along with the main frequency. That’s basically you can listen to this transmitter even in the commercial VHF/UHF TV frequency spectrum. Which is undesirable in this case.
If you have any suggestion or question, just drop a comment.
FM transmitter circuit projects are indeed quite popular among electronics hobbyists/students.
But the frustrating part is most transmitters refuses to work at all, and secondly the internet is full of crappy transmitter circuits.
Designing a stable FM transmitter circuit is rather a difficult job, many calculations are involved their. There are also some construction error and component value tolerance. Here you can find a reasonably stable and well tested transmitter that actually works.
Stable FM transmitter circuit diagram
First of all, have a look at the circuit diagram. It’s basically a common base collpits oscillator, like the previous simple FM transmitter.
It’s a bit complex than the previous one, but I think the complexity is fair for the sake of stability. Follow the article to know why this circuit is stable.
FM transmitter part list
Though I’ve mentioned the values of all parts in the diagram itself, but it’s good to have a list.
R1 – 100 Ohm, carbon film 1/4 watt
R2 – 10 kOhm, carbon film 1/4 watt
R3 – 22 kOhm, carbon film 1/4 watt
C1 – 68 pF, ceramic disc
C2 – 10 pF, ceramic disc
C3 – 68 pF, ceramic disc
C4 – 1 nF, ceramic disc
C5 – 1 nF, ceramic disc
C6 – 100 nF, ceramic disc
C7 – 470 uF, 10V electrolytic
C8 – 150 pF, ceramic disc
C9 – 150 pF, ceramic disc
L1 – 11.5 turn on 6 mm diameter, see text below
L2 – 3 turn on 3×2 mm ferrite bead, see text below
Q1 – BC548 transistor
U1 – AMS1117-3.3 LDO regulator
So, it uses 16 components excluding the wires, connectors and circuit board.
As always, you can replace all the components with their nearest value counterparts.
The antenna is just a peace of 75 cm single stranded wire, technically a quarter wave whip antenna.
The coil L2 is specially important, it’s winded on a ferrite bead. The bead MUST be a RF ferrite bead, else the circuit wont work. You can salvage them from old TV balun, TV tuner box, DVD RF box, radios and so on.
The coil L1 is winded over 6 mm diameter, 11.5 turns of single stranded hookup wire. It’s basically a RF choke coil.
You can use any 3.3 volt LDO regulator instead of the AMS1117-3.3, but definitely not a Zener diode and resistor combo to get the regulated voltage.
Construction
You’ve to pay a little attention while constructing RF related circuits.
Component leads should be trimmed to minimal.
There should be no or lowest possible capacitance between two PCB tracks, anyway this can’t be avoided.
Use as tittle solder as possible.
Clean the solder flux thoroughly after soldering, preferably with alcohol.
All components should be soldered tightly.
Finally, enclose it inside a little metal case if possible.
I had constructed the circuit on a strip board, which is not fit for this purpose.
Also messed up all the above rules, due to an extensive trial end error to find the right component values. But finally it worked, now the FM transmitter circuit is reasonably stable, without any frequency drifting.
Few pictures of my prototype.
You can see the little ferrite bead in the above picture.
So why this miniature FM transmitter circuit is reasonably stable ?
I did some research and calculations before making the final circuit and choosing the components.
I’m not going to extensive details, but these are main reasons why this transmitter is stable.
A simple common base Collpits oscillator is a voltage controlled linear harmonic oscillator. In fact the frequency modulation is achieved by the varying voltage at the transistor’s base, due to varying P-N junction capacitance. So for a stable operation I’ve to keep the supply voltage as stable as possible.
All oscillators generate some order harmonics along with the fundamental frequency. So I’ve to choose such a transistor which can operate at the fundamental frequency of the LC tank circuit, but not at the 2nd harmonic. BC548 is a good candidate for this purpose, which has a transition frequency of around 100 MHz, but also a larger noise figure.
Ferrite cores tends to absorb higher frequencies much more than lower frequencies. Thus by using a ferrite core inductor at the LC tank circuit, it it minimises the 2nd order and 3rd order harmonics further.
Ferrite core inductors also tends to have higher Q factor than air core inductors, so the coil also improves the transmitter’s quality.
Range test and future improvement plans
Undoubtedly it’s a very low power FM transmitter circuit, it’s intended for stability, not range.
In my tests it’s transmitting not more than 20 meters, but the audio quality is good enough, and no frequency drifts observed.
But increasing the range is rather simple than increasing stability, it just needs a buffer stage and RF amplifier.
It’s also transmitting AM waves, as the modulating signal is directly fed into the base of the transistor, though the AM modulation index is quite low. I’m planning to use a varactor diode for FM modulation in the future design.
First of all, lets have a look at the charger’s circuit diagram. As transformer is a bit odd, so I’ve also decided to draw it by hands.
Unfortunately every charger circuit is not same, some of them contains few extra capacitors or resistors.
But even though, you can get a clear overview of the mobile charger circuit from the above diagram.
The design is quite straight forward, built on a paper phenolic PCB, could be easily repaired.
Part list of the mobile charger circuit
Finally the part list, you can replace most of them by their closest alternative.
Q1 – 13001 transistor
D1 – 1N4007 diode
D2 – 6.2V Zener diode
D3 – 1N4148 diode
D4 – SB260 schottky diode
R1 – 6.8 Ohm – 1/2 watt
R2 – 1 MOhm – 1/4 watt
R3 – 6.8 kOhm – 1/8 watt
R4 – 330 Ohm -1/4 watt
C1 – 2.2uF – 450V
C2 – 4.77uF – 50V
C3- 680pF ceramic (681)
C4 – 470uF – 10V
As I’ve said before, this type of 13001 transistor charger circuit may vary in design and part number. But the basic circuit is same, few of them have a little LED as indicator.
Transformer details:
Primary: Around 250 turns of 36 to 40 SWG enamelled copper wire.
Secondary: 6 turns of 26 to 28 SWG enamelled copper wire.
Auxiliary feedback: 8 to 15 turns of 36 to 40 SWG copper wire.
If the transformer is broken, you can use the transformer from other broken charger of similar type.
Working of mobile charger circuit
Let’s discus about how this circuit works, first have a look at the picture below.
The first stage is a half wave rectifier, made with D1, R1 and C1. It rectifies and filters the AC input to high voltage DC. So, the voltage between point A and poing B is approximately 170 volt for 120V AC input and 311 volt for 220 volt AC input.
The Second step is a self oscillating(ringing choke converter, RCC) flyback oscillator, consisted of all the parts shown inside the red box and the primary+auxiliary winding of the transformer.
So how the flyback oscillator oscillates? When the AC power is connected, base of the transistor starts opening as it’s biased by the Resistor R2. The current through primary winding starts rising rapidly, and reaches to the threshold level within no time.
But at the same time, an opposite(but low) voltage starts rising across the auxiliary winding of the transformer. This opposite voltage starts charging the capacitor C3 negatively, much faster than charging it through R2, thus ultimately blocks current flow through primary winding.
As there’s no more current flow in the auxilary winding, C3 starts discharging through R3 and the current through R2 again starts opening the base of transistor Q1.
This process repeats itself again and again very fast. May be around 10,000 to 50,000 times a second, depending on various parameters. So ultimately we got the circuit oscillating.
As the circuit is oscillating, the energy stored in the primary winding is dumped in the secondary winding as well, when the transistor is in off state.
The Rectifier 2 stage is responsible for rectifying and filtering the induced current and voltage on the secondary. The rectified and smoothed voltage apears between poing C and D. Which could be as high as 8-9 volt under no load. But very quickly drops when a load is connected.
The resistance R4 ensures a little current flow, thus prevents the capacitor from being over charged.
As there’s no feedback mechanism between low voltage side and the oscillator, the voltage drops between point C and D when a load is connected.
Conclusion
Well, that’s certainly not the easiest explanation, but I think simple enough to understand what’s happening inside the mobile charger circuit.
If you have any question or suggestion, please feel free to ask through the comments.
