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Adjustable AC voltage

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Hi,

Yes check all your connections, and what kind of load do you have there? What value resistor?
Also, what frequency are you using and what value output cap and inductor?

You can look across the cap for the output voltage, and across the transistor for the switching waveform.
 
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well that resistor what is before fet, (R3) is 330 ohm/1w, but seems i forgot totally load...
 
Hi,

He he, no problem, but you'll need to add a load to see this work :)
 
so little chancge helped, i added only load
https://i48.tinypic.com/r1x7op.jpg

and this is now coils voltage now.

thanks again for your help. so, what i need now to adjust to continue testing? probadly bigger cap, as now its only 0.3uf, but id like to hear your opinion before i hassle anything up....
 
Hi,

Well, you might want to be able to adjust the frequency from say 20kHz up to 100kHz or something like that. You may want to go higher sometime in the future.

The output cap can probably be 100uf for now. That is there to provide a smooth output to the load, but for testing we dont need a super smooth output.

Your scope pic shows the voltage across the inductor? So it goes high, then swoops down, then goes negative?
Can you show the current through the inductor at the same time (dual channel scope perhaps)?

We can look for inductor saturation here but we need to know the inductor value and operating frequency. If you dont know the inductor value we'd have to do a test for that first.

The voltage across the inductor should really be more or less a square wave, but that's with the proper load and output capacitance too.
 
Hi,

Nice pics. The second one is a little harder to interpret though.

If you look at the current through the coil you can determine the inductance. We have to know the scope calibration though, as to what the divisions represent. For example, the horizontal might be 1ms per major division, the vertical might be 1 volt per division or 1 amp per division, or whatever you have it set for. We have to know these things in order to determine what the inductance is.
Looking at the current vs time and the voltage across the coil we can then use v=L*di/dt to determine the inductance. Probably have to look at the voltage across the coil with one scope shot, then the current with another scope shot, unless you use a resistor in series with the inductor then you can connect the common to the junction of the resistor and coil, and one scope probe on the other side of the resistor and the other scope probe on the other side of the inductor.
It's also a good idea to separate the traces on the scope vertically a little so they are not hard to interpret.
 
yeah, the second is harder to intercept , but i thought if it would help :D

the settings are 5volts/div and 10us/div

https://i49.tinypic.com/34huqo5.png
did you mean this cind of connection to determine coils current via scope? i newer used scope to determine current with scope before, this is pretty new to me too, but i heard it is possible however
 
Hi,

Yes that is the idea. This way you can view the current and the voltage across the coil at the same time. The extra resistance has to be kept small though, according to the current level, and does change the circuit a little bit, but it helps to understand these circuits by viewing the scope pics.

For a current of say 1 amp peak, you might use a 0.1 ohm resistor for example. The voltage across this resistor shows up on the scope and this represents the current as:
i(t)=v(t)/R
so that's pretty simple. For a 0.1 ohm resistor, this means that the voltage shown on the scope is really the current divided by 10. Multiplying that voltage by 10 then gives us the current. For example, if we see 0.1 volts peak on the scope, then that means that we actually have 0.1/0.1 which is 1 amp peak.
The voltage across the inductor, because of the way we connect the probe, shows up as the inverted voltage. No problem there if you have 'invert' function on your scope, but if you dont then we just invert it in our minds as we look at the scope pic. Graphically once you have a pic on the computer you can invert it by doing a graphical vertical flip.
Sometimes we get a lot of noise using the resistor method but it works when the budget is low. A current probe is nice to use but they are not that cheap to purchase.

Just to recap, what we want to do is view the voltage across the inductor and the current through it and then we can determine the inductance. The load can be varied to see how the inductor behaves with more load, as long as the transistor does not overheat.
 
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Hi,

What value output cap are you using now?
Also, what value output load are you using and what power supply voltage?
Also, we need to know the frequency.

The resistor in series with the inductor must be much lower than the load resistance. If the load resistance is 10 ohms 0.1 ohms would be good for the series resistor. 1 ohm might work but we'll see.
 
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output cap is 100uf, load is 20ohm, 12v supply
 
Hi,

Ok, what is the frequency, is it still 10us per division on the scope?
Might have to increase the frequency for this coil. If the frequency is too low the coil will not be able to work correctly.
 
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yes, i havent touched frequency at all, still 10us, about
 
yes, i havent touched frequency at all, still 10us, about

Hi,

Ok good, but can you increase the frequency to about double of what it is now?

Also, what kind of diode is that black diode in the center of you previous breadboard picture?
 
diode is basic 1n4007, i have in4148n, and some other faster type
 
Hi again,

When you say chan 2 is inverted, do you mean that it is inverted because of the way the scope probe is connected or do you mean that you have the button on the scope pulled out so that it inverts the wave form?

Yes you can not use a simple diode like 1N4007 it must be high speed. A Schottky is a good idea. 1N4148 works for lower level currents.

It looks like there could be a problem with the drive circuit. We might need more drive for the MOSFET. We'll have to see here.
What are you using for a power supply?
 
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ok, just for sure, i put power schottky in place of 1n4148, should i now increase current to coil or what you think?
 
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