In this post we are going to construct a simplest possible power inverter whose size is not more than a match box. This miniature inverter circuit can operate from 1.5V to 9V DC and can be used for powering small loads like 0.5 to 6 watt (120/220V) LED lamps. This inverter comprises of just 3 components and even a beginner can accomplish this project with ease. It can be a good project for school science fair or as an emergency light for your room.
We will see:
- Circuit diagram of 1.5V to 220V inverter.
- Circuit description.
- Where to obtain a ferrite core transformer and its pin diagram.
- Working prototype images.
- Testing the inverter circuit at different voltages.
- How this circuit works?
NOTE: There are lot of fake inverter projects online where they claim to convert 1.5V from an AA battery to 220VAC and there are real inverter projects where they do light up a 220V LED lamp using a 1.5V battery, but unfortunately there are no clear explanation about its practicality and reliability in real life circumstances and there are no explanation how the circuit works. So, we are here to explain all the aspects of one such inverter, so keep reading on….
Circuit diagram: TESTED
The proposed inverter circuit is very simple and there are only 3 components to be collected to build one: a 470 ohm resistor, a medium power NPN transistor (BD139/BD137/BD135/D882) and a ferrite core transformer which can be salvaged from a DC adapter. The other two components are the source and load i.e. the battery and a LED lamp (0.5 watt to 6 watt).
The above circuit is essentially a ferrite core transformer based inverter. If you don’t know what ferrite core transformer based inverter is, please let us explain……..
As the name suggests it utilizes a ferrite core transformer instead of an iron core transformer, traditionally inverter’s step-up transformers are made using iron core where it operates at 50/60 Hz. The iron core transformers are bulky, expensive and produce more energy loss.
Ferrite core transformer based inverters on the other hand are very light weight when compared to iron core, compact in size, offers superior efficiency and costs less to manufacture.
The ferrite core transformers are operated at high frequencies like tens of KHz range which cannot directly utilized by all AC appliances, so the high voltage high frequency output of ferrite core transformer is rectified and converted to standard 50/60Hz AC output.
We have designed a 12V ferrite core inverter which can deliver 500W power; you can find the circuit and detailed explanation about this inverter here.
By now you should have an idea that we are fundamentally building a crude ferrite core transformer based inverter that is operated at lower input voltage.
Where can I find a ferrite core transformer?
Ferrite core transformers are NOT readily available at retail stores or e-commerce sites, but instead we can salvage one from a DC adapter, and surprisingly we can find a ferrite core transformer easily on most commonly available DC adapters.
Here is a ferrite core transformer which we salvaged from a USB 5V / 0.5A adapter. This is a step-down transformer but we are going to use it as a step-up transformer by using its primary as high voltage output and it’s secondary as low voltage input.
You can too salvage a ferrite core transformer from any DC adapter that is lying on your junk box and you don’t need it anymore. We recommend you to salvage it from an adapter whose DC output voltage is rated less than 15V and its current ratings don’t matter.
Ferrite core transformer’s pin diagram:
On most DC adapters its ferrite core transformer’s terminals are most probably the same as shown above.
You can identify its correct terminals by holding the transformer’s four terminals towards you and the two terminals on the opposite side away from you, as illustrated in the above image.
Most probably the couple of terminals at the right side are the primary winding which consist of large number of turns. You can confirm this by measuring its winding resistance using a multimeter, it will be in the order of few ohms, we measured its resistance and it was approximately 8 ohm, which was highest of the three windings.
The couple of terminals at the left side are the auxiliary winding and it will be utilized as feedback.
The two terminals on the other side are the secondary winding through which we are going to apply low voltage.
Note: On some transformers the primary and auxiliary terminals could have its sides switched. You can always find the correct terminals by measuring its winding resistance. Always the primary winding will have highest resistance of the three and secondary winding is at opposite side.
Transistor pin diagram:
What LED lamp to choose for this inverter?
This inverter has very limited application because of its limited power output and rich in HF noise, the only viable application is to light-up a 120/220V LED lamp whose wattage is rated less than 6W.
A very important point to be noted is that branded LED bulbs won’t work with this inverter.
Branded LED lamps have well designed LED driver which filters out noisy power input. We purchased a well-known reliable brand for testing purpose and it failed to light-up. Later we purchased a brand which was not so well known (it was also much cheaper than the reputed brand) and it lit-up immediately.
So dear readers, if you are building this inverter get a cheap LED bulb with wattage lower than 6W; also don’t connect a LED lamp that is dimmable.
Here is our prototype, we have tested this circuit using BD139 and D882 which are medium power transistors and you may also use BD137 or BD135 and it should work just fine.
We didn’t had a 470 ohm resistor at the time of testing this circuit, so instead we connected two 1k ohm resistors in parallel which gave us an effective resistance of 500 ohm which is close to 470 ohm.
The transistor is screwed with a suitable size heat sink; this is because the transistor gets hot and this inverter consumes around 500 mA when a 3 watt LED bulb is connected as load.
Testing at different voltage levels:
- 1.5V input: At 1.5V our inverter did not light-up the bulb; this could be because our transformer didn’t suit for 1.5V operation or 3 watt load is too much for 1.5V input. But it may work for you for the transformer you salvaged.
- 2.5V input: At 2.5V we could see a dim illumination of the LED bulb.
- 3.5V to 4V input: At 3.5V to 4V input using an 18650 li-ion cell, the bulb was bright enough to light up a small area in a dark room.
8V / 9V input: At around 8V input (using two li-ion cells in series) the 3 watt LED bulb was bright enough to read a book in a dark room if you hang the bulb above your head.
We even able glow a couple of 3 watt LED lamps in parallel at ~8V DC:
- Above 9V: The intensity of illumination did not increase past 9V. We recommend you not to increase the input beyond 9V. We did try raising the input voltage but the transistor got damaged after 10V – 12V and this could be because the base terminal was over biased / the transistor got too hot.
Now you know how to make this inverter and make it work properly, now let’s see how this inverter works.
Pro-tip: Use rechargeable batteries to power this inverter, non-rechargeable batteries get discharged in few minutes. With two li-ion cells we were able to light up a 3 watt bulb for more than 90 minutes.
How this inverter works?
You may refer the circuit diagram along with the below given explanation to understand its working better.
- When you connect the battery, the +Ve supply flows through the 470 ohm resistor and through the auxiliary winding and reaches the base of the transistor. The resistor prevents over biasing of transistor.
- Now the transistor turns ON partially which will weakly energize the secondary winding and induce a small magnetic field on the auxiliary winding.
- The magnetic field induced on the auxiliary winding generates current (stronger than initial current) which will again pass through the base of the transistor, which will turn ON the transistor more and energize the secondary winding even more.
- This higher intensity magnetic field from the secondary will induce even more current on the auxiliary winding which will turn ON the transistor even further.
- While the magnetic field is getting stronger at the core, not only the auxiliary winding is receiving secondary winding’s magnetic field but also the primary winding is receiving the magnetic field.
- At some point the magnetic field gets strong enough such that the primary winding can generate enough voltage to turn on the 3 watt LED lamp.
- The strength of the magnetic field cannot rise forever, once the transistor is fully turned ON and no further changing (rising) magnetic field occurs. At this point magnetic field collapses and transistor turns OFF and the cycle repeats from the beginning of the explanation.
- The rising and collapsing of magnetic field occurs at tens of KHz frequency.
If you have any questions regarding this project, feel free to ask us in the comment section, you will get a guaranteed reply from us.
Top comments by real people:
Wow it work very nice, thank you very muchsamuel (Reader)
Good day sir. Thanks for the response. I’ve built the inverter. It’s amazing, working like a charm…….Kamil (Reader)