555 Timer voltage drop problem

[cteeto]

I am trying to build a power inverter using this circuit:


**broken link removed**

The transformer I have is actually 24v to 120v, it is just temporary right now. And I also dont have the capacitor connecting pin 5 to ground as I didnt think it was neccessary. The problem I am having is that the supply voltage is 12 volts, but the 555 is outputting only half of that (around 6) and I have no idea why because I have never had this problem when using a 555. I thought maybe it was bad so I switched it out for a few different ones, but the problem remains. As a result I am only getting about 5 volts across the transformer so only about 20 volts coming out the other end when it should be around 60. I also tried replacing the whole output with just a resistor and an led, then I was getting about 10.5 volts on the output which I think is about normal. So I think there is something wrong on the output side of the 555 but i just dont know what. Any feedback is greatly appreciated thank you!

 

[audioguru]

The output of the 555 with a 12v supply is only from +2.5V to +9.5V (a total of only 7V p-p) when it has a 200mA load but some are worse.
the 100 ohm resistor has voltage loss and the base-emitter voltages of the transistors also have voltage loss of about 3V.
The output capacitor also has a voltage loss.
The "12V" transformer has resistance so it is actually a 15V or 18V transformer and when used in reverse its output voltage might be only 150V or less when loaded.

So your inverter is hopeless. Many electronic products won't work from its square-wave output anyway because they rely on the much higher peak voltage of a sine-wave.

 

[cteeto]

Thank you for the quick and informative response. Would you recommend a different method of doing this to get a more sine-like wave? If I decide to stick with this circuit, do I just need to increase voltage to account for the drops? Or is there a way to decrease the voltage drops?

 

[audioguru]

In this extremely simple square-wave inverter, use a 9V transformer and a 12V battery to increase the output voltage.
It does not have voltage regulation. The output voltage will be too high when the battery is fully charged and/or the load current is low.
The output voltage will be too low when the battery is discharged and/or the load current is high.

Making a sine-wave inverter that does not get extremely hot is very complicated. Adding voltage regulation is also complicated.

 

[cteeto]

hmm I see..

So it would probably be a better idea to just go buy one? I have never seen any that run off of batteries that you can charge however.

What kinds of things would work with a square wave like this? Do you think phone chargers and CFLs would work?

Also I just have a question about that circuit up there, what is the purpose of the 2200 uF capacitor that is in series with the transformer windings?

 

[Blueteeth]

If you can produce a 'square wave' at the desired voltage/pwoer requirements, you might be able to use it for low power devices that use switched mode power supplies, such as mobile phone chargers, printers, small LCD tv's, DVD players etc.. since these first rectify the mains intput to DC anyway. And even though they rely on the peak value (~300v DC from 240 AC) most SMPS will accept a wide input voltage range, some from 90-250V - so peak voltage isn't always an issue. Of course, a square wave is significantly noisier than a pure sine wave, so I can't see it not interfering with audio.

There are pseudo-sine-wave inverters (again for LOW power) that produce the output via resonance rather than digitallly controlled wave shaping or PWM. A resonant royer converter generally has a fairly clean output, but using off-the-shelf mains transformers in reverse, the output frequency would be much higher - again, not always a big deal for appliaances which immediately rectify the mains input). They won't have square wave outputs, but not true sine either - just less noisey and easier to filter.

Oh and the purpose of the 2200uF cap is to block the DC offset, so the transformer see's +4V and -4V with reference to ground, as opposed to 2V and 8V. In the first case, the net/average magnetic current in the transformer is zero. In the second case, the magnetic field builds, and never colapses giving rise to saturation. - bit out of my depth on this one though

 

[audioguru]

I have never seen any that run off of batteries that you can charge however.

ALL inverters have rechageable batteries. Usually they are deep discharge lead-acid batteries.

What kinds of things would work with a square wave like this? Do you think phone chargers and CFLs would work?

Anything that makes heat and some AC motors that do not have an electronic speed control.
I don't think a phone charger will work. i don't know if CFLs will work.
Even a simple multimeter cannot measure the amplitude of a square-wave.

what is the purpose of the 2200 uF capacitor that is in series with the transformer windings?

It couples the signal from the transistors to the transformer but blocks the DC that is at the transistors.

 

[cteeto]

Thanks to both audioguru and blueteeth for all the useful information and the ultra quick responces! I am very interested in electronics but I haven't gone to school for it, I am actually a 2nd year electrical trade apprentice.

I am curious how this circuit even works. I can see how I am getting a positive flow through the windings (through the top transistor when the output is high, but not through the bottom, then through the cap and out the windings) but I can't see how negative current is flowing. I know that the bottom transistor activates when the output is low though, I just cant see it.

 

[audioguru]

I am curious how this circuit even works. I can see how I am getting a positive flow through the windings (through the top transistor when the output is high, but not through the bottom, then through the cap and out the windings) but I can't see how negative current is flowing. I know that the bottom transistor activates when the output is low though, I just cant see it.

The emitters of the output transistors average half the supply voltage so they can swing equally up and down.
The coupling capacitor quickly charges to half the supply voltage when the power is turned on so it drives the transformer winding with AC (no DC) from about -4V to +4V.

 

[Reloadron]

What was your interest in building the circuit? Did you have an application in mind or just building as a learning process project? Also, what voltage and current (power) did you want out?

The reason I ask is because as mentioned inverter systems can be purchased relatively inexpensive for example units like this have become very commonplace. While not a sine wave output they do produce what is known as a "modified sine wave" output. Not much like a sine wave but an improvement over a standard square wave.

Additionally there are units like a UPS (Uninteruptable Power Supply) which consist of an on board battery as well as a charge circuit to maintain the battery. Sort of an Inverter, battery and charger all rolled into a single package for power backup in the absence of mains power.

Inverters that produce a true sine wave are available also, however, as AudioGuru points out they are complex and get expensive making building one at home on a bench not very practical.

On another note something to consider is when using an inverter there is considerable battery drain. For example a small 300 watt inverter would draw about 25 amps under full load and that does not take into consideration loss due to inefficiency and heat. This means the inverter needs to be placed close to the power source (battery) and the 12 volt connection would require heavy gauge wire like AWG 10 here in the US. Even a small 100 watt unit would draw over 8 amps. This is why AudioGuru mentions:

ALL inverters have rechargeable batteries. Usually they are deep discharge lead-acid batteries.

So there are a few questions and considerations.

Ron