High so recently iv been experimenting with afew different inverter designs. Mainly old blocking ossilators. Iv also experimented with multivibrators.

However I want to light about 10bulbs. Yes I could buy a inverter or 12v bulbs but where the fun in that. So I went for a ir2153 design, image below. Only changes are the timing resistor is 1k and the capacitor is 47nf for about 1.4khz. As that's what I have to hand.


so my idea is to step up the 12v DC to 240v squarewave. Send that down a thin wire, then use a bridge rectifier and a high ripple capacitor to smooth the DC square wave and any harmonics to DC. Iv 240v dc tested several bulbs with different circuitry, they don't seem to care about the DC unless they use a capacitive dropper. To be honest they don't care about the high frequency but I would imagine Its going to ruin there internal bridge rectifier and capacitors.


So my question is as iv only used off the shelf center tapped transformers. On my secondary I have 2 layers of 277 turns @ 0.53mm. So my secondary is about 555 turns in total. So for my primary would I need two pairs of 27 turns. As 555/20 (12*20=240) =just over 27.



also I understand with AC you need to times the rectified smoothed output by 1.4. obviously when the square wave goes into the primary it's not going to stay very square. Would this have any effect on the DC wave after rectified and smoothed.

kind regards
Rick.

 

What is the core of the transformer made of - is it suitable for 1.4kHz?

 

Two Manganese-zinc ferrite rods mate.

 

That rod will make a poor transformer has the magnetic field has to transverse the large air gap from one end of the rod to the other.
That's why power transformers always have a closed-loop magnetic circuit with little or no air gap.

 

Well i will give it a test. Also had a backup if it didn't work. Toroid photo below. Plus as iv experimented with large air core transformers first. So I can use the wire off them.

 

Well i will give it a test. Also had a backup if it didn't work. Toroid photo below. Plus as iv experimented with large air core transformers first. So I can use the wire off them.
View attachment 235844
View attachment 235853

Wow that's a big toroid. What kind of power output are you looking for, or what output current?
That toroid might be good for 100 watts maybe more.

There is a bit more science that goes into winding a transformer than with other types of circuit where you just grab some caps and resistors and maybe an inductor or two and connect it all together and it works. A transformer core has to have the right characteristics and you have to know what they are in order to know the minimum number of turns you have to wind on the primary.

The core material has a property called permeability. You have to know what that is. You can do some tests but it might take some time and you need some measuring equipment ideally a scope.
The dimensions are also important, especially the cross sectional area. You have to know that too at least.

Once you get the primary turns on driving the core is also a little different than some more usual circuits. If you pulse it with a square wave (center tapped or not) you have to be able to maintain a volt seconds balance. If you dont do that no matter how many turns you wind on the primary it could very well saturate and then the whole thing doesnt work right or blows the drive transistors.

So there are a few things to think about which are probably the most important:
1. Core permeability
2. Core dimensions (OD, ID, Height)
3. Input voltage (peak of square/rectangular wave) and frequency
4. Rectangular wave volt seconds balance

If you dont know #1 then you have to do some experiments with some decent measurements. To do this you wind some turns on the core and drive it with a proper square wave and look for saturation. Looking for saturation is best done with an oscilloscope with a current probe or a very small resistor in series. You can use a meter but it will take a little guesswork to go with that.
The problem with #1 though is that some cores will not be very good for creating a transformer. If the permeability is too high it saturates easy. Some toroids are made for reactive circuits so their permeability is high. A lower permeability takes more turns to get to work but it 's easier to get along with when it comes time to driving it.

The volt seconds balance is a little tricky. If you drive the core with a square wave the volts times the time 'on' positive has to be the same as the volt seconds 'on' negative. If not, you end up with some DC current in the primary which causes excess current flow and could blow the transistors or make the converter very inefficient. It can also cause a lot of unwanted audio noise.
To test for this look for an average DC current in the primary. If done right, the average will be zero or very close to that. Keep in mind that if the primary resistance is 0.1 Ohms and the undesired voltage offset is just 0.1v, Ohms Law tells us that the average DC current is 1 amp which would not be good if the converter only put out 1 amp for example.

Think about this and then you will probably have more questions.

Also, ditch the two rods unless you can find a way to magnetically couple the four ends together, with two ends on the top and the other two ends at the bottom. The coupling material has be highly magnetically active so a steel bar for example wont work that well (permeability too low even though it sticks to a magnet).

Lastly, if you look into a magnetic material BH loop you will gain some insight as to how transformer cores work. You can look at one of the curves that does not have any hysteresis to avoid dealing with that issue at first. This kind of curve looks like a slanted line going through the origin and then flattening horizontally as it gets higher. The permeability is the slope of that line.

 

Thankyou mrAl for your detailed response, pc40 material. I assume il have to bump it up to 25kh. Which I should be able to do with the previous circuit. Info in image below.

Well about 100w, however if I can get more that's fine. I also have another core as it was 2 for 18quid. So I may use that for another inverter.

Any help on understanding how many turns for 100watts would be amazing, I have 0.31, 0.56,0.62,0.95 & 1.3mm wire.

 

So there are a few things to think about which are probably the most important:
1. Core permeability
2. Core dimensions (OD, ID, Height)
3. Input voltage (peak of square/rectangular wave) and frequency
4. Rectangular wave volt seconds balance

Okay the size of the toroid is 118mm/80mm/20mm

Iv tried afew calculators but they have been confusing. Iv managed to get my surface area in cm2 but alittle lost how it relates to the number of turns at 25khz 100w.

 

Thankyou mrAl for your detailed response, pc40 material. I assume il have to bump it up to 25kh. Which I should be able to do with the previous circuit. Info in image below.

Well about 100w, however if I can get more that's fine. I also have another core as it was 2 for 18quid. So I may use that for another inverter.

Any help on understanding how many turns for 100watts would be amazing, I have 0.31, 0.56,0.62,0.95 & 1.3mm wire.

View attachment 235923

Hello again,

Did you post the toroid core dimensions yet?
We need:
inside diameter (ID)
outside diameter (OD)
height (H) with it laying flat on a smooth flat surface.

That info is important to know what to do next.
It is nice that you can vary the frequency.

 

Okay the size of the toroid is 118mm/80mm/20mm

Iv tried afew calculators but they have been confusing. Iv managed to get my surface area in cm2 but alittle lost how it relates to the number of turns at 25khz 100w.

View attachment 236023

You posted just before i did.
Is that 118mm/80mm/20mm as:
OD 118mm
ID 80mm
Height 20mm

Is that info correct?

One of the transformer equations is this:
Bmax=E*1e8/(4*F*A*N)
with:
Bmax the maximum flux density in Gauss
E peak square wave voltage in volts
F frequency in Hertz
A area of core in square centimeters
N number of turns

To use this set the variables except for N, then calculate N.
Bmax will be found on the data sheet but be careful with this because a temperature rise lowers the saturation flux density of the core.

Since you can vary the frequency you have the option to measure the input DC current and vary the frequency until you get a minimum input current reading. You may have to experiment a little more.

As noted previously, you have to check for volt second balance because that will mess things up really bad if there is an imbalance in the primary.

 

You posted just before i did.
Is that 118mm/80mm/20mm as:
OD 118mm
ID 80mm
Height 20mm

One of the transformer equations is this:
Bmax=E*1e8/(4*F*A*N)
with:
Bmax the maximum flux density in Gauss
E peak square wave voltage in volts
F frequency in Hertz
A area of core in square centimeters
N number of turns

Okay BODMAS. The E*1e8 has me alittle confused. Also I think I did everything right.

E peak square wave voltage in volts 240-250v

F frequency in Hertz 25khz 25000hz

A area of core in square centimeters ,
outer surface area L1
L1 = 74.141586624719 cm2
inner surface area L2
L2 = 49.008845396001 cm2
end surface area
A = 61.57521601036 cm2

74.14+ 49+ (61.57*2) = 246cm2

N number of turns 240 on the secondary. 12x2 on the primary.

 

Okay BODMAS. The E*1e8 has me alittle confused. Also I think I did everything right.

E peak square wave voltage in volts 240-250v

F frequency in Hertz 25khz 25000hz

A area of core in square centimeters ,
outer surface area L1
L1 = 74.141586624719 cm2
inner surface area L2
L2 = 49.008845396001 cm2
end surface area
A = 61.57521601036 cm2

74.14+ 49+ (61.57*2) = 246cm2

N number of turns 240 on the secondary. 12x2 on the primary.

View attachment 236083

Ok slight problem there. The 'Area' in magnetic calculations related to the flux is the cross sectional area not the surface area.
The cross sectional area A for a circular toroid is:
A=H*(OD-ID)/2

Since OD=11.8cm and ID=8cm and H=2cm we get:
A=2*(11.8-8)/2=11.8-8=3.8 square centimeters.

The 1e8 in the formula comes from including the magnetic space constant in the derivation.
To repeat here with the improved description of the area A:

Bmax=E*1e8/(4*F*A*N)
with:
Bmax the maximum flux density in Gauss
E peak square wave voltage in volts
F frequency in Hertz
A cross sectional area of core in square centimeters
N number of turns

Note that sometimes when we say "square wave" we really mean "rectangular wave" but that is not the case here the square wave here means an actual symmetrical square wave symmetrical about zero and equal pulse width above zero and below zero. If the duty cycle (pulse width) decreases so there is some true 'off' time (dwell at zero volts) then the max flux density B will be reduced..

For a sine wave we change the '4' in that formula to '4.44' and we use the RMS value for E.

You should use the maximum voltage that will appear as the input with this formula. If the input is really 240 to 250 volts positive and the same negative then you should use 250 volts.

For your core then with 250v square wave applied and 25kHz frequency comes out to:
N=65790/Bmax
with Bmax in Gauss.
It looks like for your core material you should use 2500 Gauss for Bmax but double check that yourself. You want to use the lowest value you can find to allow for some temperature rise. That leads to 26 turns minimum on the primary. Thus 26 turns wound bifilar which would mean 26 turns of two wires in hand so if you add all the turns it would come out to 52. Of course you cross two of the ends to form the center tap.
You can test it for saturation without a secondary at first then later wind the secondary.
You should use high temperature tape between winding layers and maybe even some insulating material over the primary before winding the secondary.
For a higher safety class transformer wind the primary on one side and the secondary on the other side. So the primary may take up the core from say 0 degrees to 170 degrees and the secondary from 180 degrees to 350 degrees, leaving a 10 degree gap between the ends of the primary and secondary windings. That ensures that the windings will not ever touch. This brings in some extra leakage inductance but that may not be too bad. Testing is required of course.
Anything built for consumer applications has to have this type of separated winding construction. Industrial stuff does not always have to.

 

Testing is required of course.
Anything built for consumer applications has to have this type of separated winding construction. Industrial stuff does not always have to.

Okay will do, thankyou for all your help it's really appreciated. Well the square wave is push pulling the primary from a 12v battery, so it's 11.5v to 13v on half the primary as it's push pull and the out between 240 /250. If that clears up any confusion.

When you say bifilar, is that just paralleling up the windings. So it's more evenly distributed around the toroid. Also does wire thickness play a roll. I'm using 0.63 on the secondary @ 240turn. Do I just parralel bunches of wires together of the same thickness or use some 1.3mm wire instead.

Kind regards
rick

 

Okay will do, thankyou for all your help it's really appreciated. Well the square wave is push pulling the primary from a 12v battery, so it's 11.5v to 13v on half the primary as it's push pull and the out between 240 /250. If that clears up any confusion.

When you say bifilar, is that just paralleling up the windings. So it's more evenly distributed around the toroid. Also does wire thickness play a roll. I'm using 0.63 on the secondary @ 240turn. Do I just parralel bunches of wires together of the same thickness or use some 1.3mm wire instead.

Kind regards
rick

Yes "two in hand" means two individual wires held together and wound on the core at the same time.

Oh so it's a nominal 12v (probably more like up to 15v) on the primary.
So that means less turns on the primary.
Let me see if i got this right now...

12v nominal on the primary (such as from a car)
Max 250v peak square wave on the output at max 4 amps.
25kHz operating frequency.

100/12 is roughly 10 amps so you need primary wire that can handle 10 amps.

 

Okay.
Input
Leisure 12v battery. Full charge 12.6 , Low charge 11.9.

Output 240v. The bulbs I am using have a operating range of 165v to 265v. A very comman and easy to find bulb on the Chinese market. So I'd say Min 230 mean 240 and max 250v. But there quite abit f wiggle room. Aiming for 100watts of output. But can be more.


Removing the need for any complex electronics or feed back to regulate the voltage. Also one of my pet hates with inverters are there auto low voltage cutoff. In the case of lighting, I'd prefer a battery indicator, with a battery indicator led, for full and low. But with a extra led for emergencys, where a third led goes on an at 11.75v.

 

Okay.
Input
Leisure 12v battery. Full charge 12.6 , Low charge 11.9.

Output 240v. The bulbs I am using have a operating range of 165v to 265v. A very comman and easy to find bulb on the Chinese market. So I'd say Min 230 mean 240 and max 250v. But there quite abit f wiggle room. Aiming for 100watts of output. But can be more.


Removing the need for any complex electronics or feed back to regulate the voltage. Also one of my pet hates with inverters are there auto low voltage cutoff. In the case of lighting, I'd prefer a battery indicator, with a battery indicator led, for full and low. But with a extra led for emergencys, where a third led goes on an at 11.75v.View attachment 236300

Hello again,

Well with 12v input and that big toroid i am not sure how this will work. If i calculated the turns right, you only need 2 (yes two) turns on the primary. But that means it takes up a very narrow angle on the toroid which means you may run into some leakage inductance issues. Maybe you can start with 10 turns and see how that goes. You may have to go to a smaller toroid though.
With more turns you get slightly higher resistance which could be good in this case because you dont seem to have any active volt seconds balancer circuit. The leakage inductance would play a part in the filtering of the output if you rectify it into DC first.

 

I decided to do a simulation in LTspice. I have figured out the primary windings. I took a non-linear core with parameters for 100 degrees Celsius. As a result, I had to take transistors with a breakdown voltage of 300 V because of the leakage inductances, but the short-term breakdown occurs with these transistors as well.

 

I decided to do a simulation in LTspice. I have figured out the primary windings. I took a non-linear core with parameters for 100 degrees Celsius. As a result, I had to take transistors with a breakdown voltage of 300 V because of the leakage inductances, but the short-term breakdown occurs with these transistors as well./QUOTE]

Hello again,

Well with 12v input and that big toroid i am not sure how this will work.

putting 2 diodes across each primary May help
Thankyou both for your replies, I figure there's no point in flogging a dead house. So i looked at more suitable cores and downloaded a app for £1.30.

Price wise the EE35 for 10 ferrite halfs, 5 bobbins is just over £10. What you think.

 

I decided to do a simulation in LTspice. I have figured out the primary windings. I took a non-linear core with parameters for 100 degrees Celsius. As a result, I had to take transistors with a breakdown voltage of 300 V because of the leakage inductances, but the short-term breakdown occurs with these transistors as well.View attachment 236394View attachment 236398View attachment 236399

Hi,

Do those mosfets have internal reverse diodes? Those should be included.

Also, usually some kind of snubbers are employed to snub out the fast back emf.

 

Not sure what 'app' you are using.
Do you have the specs on the core you are thinking about using?
Also, snubbers are usually used to catch the fast rising back emf from the primary as a transistor turns off. The reverse diode of the other transistor will catch part of it but that depends on goo coupling which is not as tight when there is leakage inductance. The snubbers protect the transistors.