# Need Help w/ Inverter Design & Biasing

#### athandpr

Joined Dec 3, 2010
11
Hey Everyone,

I hope someone can help . . .

I built the following inverter (see image) using 2N3055 transistors and a 60 watt transformer (12-0-12 VDC, 120VAC). The problem that I ran into was that on the transformer's output, it was able to power a small 120Vac 0.025 A neon light bulb. But if I used a 120Vac 15 Watt Incandescent light bulb, nothing would happen. (The entire circuit would continue and it did not shut down.)

I figure with 2N3055 transistors, the astable oscillator would be powerful enough to drive a simple light bulb. So my solution then was to use the same type of transistor as a current amplifier. Which I have all the parts arranged like the following to bias the transistor:

http://www.electronics-tutorials.ws/amplifier/amp_2.html

Unfortunately, when I connected the collector to the transformer, the transformer didn't output anything. What am I doing wrong here?

Also, I'm using a 2n3055 transistor on each of the yellow lines going to the transformer and arranged in the same way as in the link provided.

Lastly, I am using a 12 Vdc 200 amp hour car battery . . . so I have a good power source. And I tested each transistor . . . they are working. If I can't get this to work, I wonder if perhaps I should just use op amps, but I would like to make this deign work

In the end, I look forward to your replies.

PS. I tried a biasing calculator . . . the results from it were no luck.

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#### Audioguru

Joined Dec 20, 2007
11,248
The circuit is much too simple:
1) It has nothing to set its frequency so the 60Hz transformer is trying to work at a frequency that it doesn't work well at.
The 220 ohm resistors limit the base current to only 100mA so the output of the transistors is only 1A which is a power output of only 12W if the transformer works well at that frequency.

#### athandpr

Joined Dec 3, 2010
11
The circuit is much too simple:
1) It has nothing to set its frequency so the 60Hz transformer is trying to work at a frequency that it doesn't work well at.
The 220 ohm resistors limit the base current to only 100mA so the output of the transistors is only 1A which is a power output of only 12W if the transformer works well at that frequency.
So if I increase the base resistors to maybe 10k ohm and add a low pass filter that should solve the problem?

#### Audioguru

Joined Dec 20, 2007
11,248
So if I increase the base resistors to maybe 10k ohm and add a low pass filter that should solve the problem?
If you increase the value of the base resistors to 10k ohms then the base current will be so low that the output power will be alnost nothing.
If you decrease the value of the resistors then they will get very hot and probably drain the battery very quickly and since nothing controls the frequency (a lowpass filter won't), then the circuit probably still won't work.

Maybe you didn't hear me: The circuit is much too simple.

#### athandpr

Joined Dec 3, 2010
11
Maybe you didn't hear me: The circuit is much too simple.
Well thanks for the reply, but I did READ you before. See my thinking is to start with something that kind of works and learn to build upon it.

#### Audioguru

Joined Dec 20, 2007
11,248
To make a simple square-wave output inverter you need an accurate frequency push-pull oscillator to drive the output transistors. The square-wave can drive incandescent light bulbs or heaters but most electronic items depend on the higher peak voltage of a sine-wave (as used with mains electricity). The simple inverter will be missing voltage regulation so its voltage will be too high when the load is light and the battery is fully charged then its voltage will be too low when it is heavily loaded and when its battery has run down.

A CD4047 IC makes a good accurate push-pull oscillator. It has two opposed outputs that can drive driver transistors that can drive the output transistors. Later The IC can drive Mosfets that work much better than transistors. Even later the IC can drive gates to make a modified square-wave inverter that uses Mosfets. Even later again voltage regulation can be added to the circuit. Sometime in the future you can use PWM ICs or a microcontroller to make a pure sine-wave inverter.