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# Electronics and Electrical Engineering Design Forum

## How to convert high DC voltage into AC?

I need to convert high DC voltage to AC at certain freq. how this could be done?
I tested it with a transistor so that feeding the gate with the ac signal frequency and putting the high DC voltage to the drain but didn’t work. any suggestions?

High DC voltage = 100v
Frequency = variable (30k – 50kHz)
Signal type = square wave
AC signal output = 100v square wave

I’ll tell the idea to let you think with me. For my project, i need a square wave signal with frequency range (30k – 50kHz) and with amplitude values (low = 0 & high = 100).

For me to achieve this, i used a 555 to generate the square wave signal with intended frequency. The 555 output signal is (V=7.5v peak) & (I= 35mA). Then i needed to reach to 100v peak. I thought that i may use many stages of voltage doubler (will generate high(100v) but DC). Then i thought i may use a transistor and feed that high DC to the collector and the 555 output to the gate so that get the intended frequency out of the DC signal. Also i searched for transformers to achieve that but i failed. Any suggestion?

###### Asked By: Omar Hegazy
1 year ago

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What is value of high DC voltage & frequency?

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High DC voltage = 100v
Frequency = variable (30k – 50kHz)
Signal type = square wave
AC signal output = 100v square wave

I’ll tell the idea to let you think with me. For my project, i need a square wave signal with frequency range (30k – 50kHz) and with amplitude values (low = 0 & high = 100).

For me to achieve this, i used a 555 to generate the square wave signal with intended frequency. The 555 output signal is (V=7.5v peak) & (I= 35mA). Then i needed to reach to 100v peak. I thought that i may use many stages of voltage doubler (will generate high(100v) but DC). Then i thought i may use a transistor and feed that high DC to the collector and the 555 output to the gate so that get the intended frequency out of the DC signal. Also i searched for transformers to achieve that but i failed. Any suggestion?

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Also, what are the criteria for the ac wave? Do you want a sinusoidal wave, or is a square wave what you are looking for?

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High DC voltage = 100v
Frequency = variable (30k – 50kHz)
Signal type = square wave
AC signal output = 100v square wave

I’ll tell the idea to let you think with me. For my project, i need a square wave signal with frequency range (30k – 50kHz) and with amplitude values (low = 0 & high = 100).

For me to achieve this, i used a 555 to generate the square wave signal with intended frequency. The 555 output signal is (V=7.5v peak) & (I= 35mA). Then i needed to reach to 100v peak. I thought that i may use many stages of voltage doubler (will generate high(100v) but DC). Then i thought i may use a transistor and feed that high DC to the collector and the 555 output to the gate so that get the intended frequency out of the DC signal. Also i searched for transformers to achieve that but i failed. Any suggestion? .

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what you need is an inverter, however you have to supply the necessary infos like DC voltage input 12v, 24v, or maybe 36v), AC voltage output, (110v or 220v), at what frequency (50Hz or 60Hz), output power, (200w,500w or 1000w) current output, (2A, 6A, or 8A). once you supply this infos specific recommendations could now be suggested.

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High DC voltage = 100v
Frequency = variable (30k – 50kHz)
Signal type = square wave
AC signal output = 100v square wave

I’ll tell the idea to let you think with me. For my project, i need a square wave signal with frequency range (30k – 50kHz) and with amplitude values (low = 0 & high = 100)..

For me to achieve this, i used a 555 to generate the square wave signal with intended frequency. The 555 output signal is (V=7.5v peak) & (I= 35mA). Then i needed to reach to 100v peak. I thought that i may use many stages of voltage doubler (will generate high(100v) but DC). Then i thought i may use a transistor and feed that high DC to the collector and the 555 output to the gate so that get the intended frequency out of the DC signal. Also i searched for transformers to achieve that but i failed. Any suggestion?

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Here’s what I would try first: Use two transistors, a PMOS and an NMOS. Use the NMOS to pull the output to 0V when the driving signal is high, and use the PMOS to pull the output to 100V when the driving signal is low.

I’m away from my SPICE computer right now or I would draw and model it for you…

If you need greater switching speed try swapping the MOSs for darlington pairs. Also, make sure that the transistors you pick have the right threshold voltage and switching frequency to match the 555 output, that could’ve been your problem as well. If you would rather use FETs, make sure that 30-35 mA is enough gate current to fully switch them (it should be, but its good to check)

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I think the voltage of 100v is too high for 555 to handle, i suggest you to look for some power devices instead of 555. what is the output load or output current you want?

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The output voltage needed is 100v & output current is not important for me.

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Thank you for your answer. Would you please make a quick model for me with suggested components because i’m a little confused about that and i’m not good at electronics. That would be great help from you.

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I dont know when I’ll have time to model it, but if I get the chance ill upload something for you. Failing that, I’ll at least draw you a picture when I get home after work.

For sizing the componenets:

Vce Max >= 120V
Vthresh (Vt) ~= 2.5V (room to flex here, but that should give you a nice switching rate)
Vbc Max >= 120V

should do the trick. Note that some values will be negative for the PMOS compared to the NMOS. Also, 100V should be just fine for the 555… its not going to care what is on the other side of the transistor gate at these frequencies unless you have a supremely crappy transistor.

Still, its always prudent to pick a transistor with sufficiently small gate capacitances, just in case. Make sure that 1/Cbe is >> 50,000. Shouldnt be an issue, just something to keep in mind

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Thank you Mr. Cody. I’ll be waiting for your draw. If you look at Mr. Steve draw, this is exactly what i did but it didn’t work, i don’t know why.

I used transistor IRFP250N.

I made some measurements with the voltmeter & ammeter and noticed the following: – Vo (of 555) = 7.5v AC
-Io (of 555) = 35mA AC (between 555 o/p terminal and GND terminal)
-Io (of 555) = 0.5mA AC when applied to the gate of the IRFP250N (Is this logic? will it drive the transistor?) i’m confused about that.
-Io (of 555) = ~1.5mA AC when applied to 1k ohm grounded resistor.

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If output current drive is no issue, then the simple answer is the attached. Make sure the MOSFET can handle the high voltage—Vds > 100V [typically 150% greater or more].

Attached Files:

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Thank you Mr. Steve. This is exactly what i did however it didn’t work, i don’t know why.
I used transistor IRFP250N.

I made some measurements with the voltmeter & ammeter and noticed the following: – Vo (of 555) = 7.5v AC
-Io (of 555) = 35mA AC (between 555 o/p terminal and GND terminal)
-Io (of 555) = 0.5mA AC when applied to the gate of the IRFP250N (Is this logic? will it drive the transistor?) i’m confused about that.
-Io (of 555) = ~1.5mA AC when applied to 1k ohm grounded resistor.

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Hmm… that should have worked. Perhaps you didn’t get the pins right on the MOSFET, or perhaps you zapped it with ESD (did you use ESD precautions while handling it?)

With the IRFP250N laying in front of you with the pins pointing towards you and the flat face facedown, the gate is the pin on the left, the drain is the center pin, and the source is the right most pin.

MOSFETs are voltage driven devices so not much current is needed to drive them (only enough to charge the parasitic capacitance on the gate). The voltage threshold on that MOSFET is 4V, and at 7.5V it can handle upwards of 80A, so this device is overkill for this application, but it should still work.

Also, if you are measuring 0.5ma on the gate, then something is wrong. But, judging by the fact that you apparently connected your current meter, set to measure AC current, from the output of the 555 to ground, indicates, frankly, that you need some tutoring in that reqard—and, whew, I’m unable to devote that much time to this, sorry.

Perhaps you should try using a bipolar transistor. They are more forgiving, and for this “no current drive necessary” application, perhaps that would be better. Try using the circuit attached.

Also, it occurs to me to ask the question, how do you know it didn’t work? Do you have an oscilloscope so you can see what is going on at the drain of the transistor? I mean, what, exactly, are you observing?

Attached Files:

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The Transistor pin connection is as right as you said and as in the datasheet. I don’t understand how this transistor is overkill for this application?

Sure i measured AC component with the ammeter and i said to ground just to clarify the terminals as i measured it between two other terminals (555 o/p & transistor gate).

I know it doesn’t work as i measure the AC voltage on the drain terminal of the transistor and it shows (almost zero). No i don’t have oscilloscope but this is how i did to check.

Thank you for your time. This was generous from you. I’ll try the transistor you provided with the resistors as well and check again. I hope it works.

Thank you Mr. Steve

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Well, you said “output current is not important for me”, so to use a transistor that is designed for currents in the neighborhood of 50A or more, is overkill (or put differently, if you are using this to drive something with currents that high, then that IS important, as the techniques for doing that are quite different.

So, answer me this, what value resistor did you use in the drain of your transistor?

Also, there is another reason the selected transistor might be considered “overkill”: if you are using it to drive a very low current, then it might be that the transistor can never really turn off. If the 555 output is never quite reaching zero volts, then the transistor might still be “on” enough pull the drain down to near zero (this will depend on what value resistor you have on the drain).

Now, consider this: your “AC meter” is probably designed to read sinusoidal waveforms (probably at RMS), yet you are trying to read a square wave with it. Not sure what the expected result would be — especially since you haven’t divulged the frequency you are running this at. BTW: too high of a frequency could also be why your meter is reading nearly zero volts. Try dusting off the instructions for your meter and see what it’s actually specified to measure.

Also consider that a square wave is more correctly classified as intermittent DC rather than AC. But, your AC meter probably has a capacitor in it, and, thus, would, technically, be converting the “intermittent DC” into “AC” and it could be called “AC” because the polarity is, after all, alternating, but, within the context of measuring AC, a sinusoidal waveform is implied.

My suggestion is that you connect the gate of your transistor to the source of that same transistor (after you verify that it is, indeed, the “gate” and the “source”) and then measure the voltage on the drain with a DC voltmeter. If it’s working correctly, the voltage should be very close to 100V. Then connect the gate to 7.5 volts and see if the drain goes to near zero. Then, get your 555 timer going at around 3Hz and test the drain voltage with a DC volt meter. The voltage should shift from near zero to near 100V at whatever frequency the 555 is running at (presumably at or near 3 Hz). If you’re not getting that kind of voltage swing, then connect the DC volt meter to the output of the 555 and see if it is swinging between near 0 and nearly 7.5V.

That, my friend, is how you troubleshoot a circuit without an oscilloscope ;)

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I appreciate your electronics abilities :)

I used 1k ohm resistor for the drain.

Also the ammeter can measure the square signal as it already measures the o/p of the 555 (which is square). To make sure that the ammeter can measure up to 100v (which is already shown on its marking) i measured the o/p of the house normal electricity (220v).

Ok, i’ll try the transistor and the configuration that you provided and see how it will do. Thank you one more time for your time :)

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Well, I hope you didn’t hook an “ammeter” across the 220AC mains! ;)

Also, am I to assume that the frequency of the 555 signal is at or near 50Hz (or is it 60Hz from your wall?), because you are testing your meter with that frequency (or do you think the meter should work the same at all frequencies == but, you did say that you got a reading that you consider “correct” when measuring the 555 output.

Just thinking out loud ;)

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No i’m not so stupid to hook the ammeter to the wall AC terminals, it was just AC voltage measurement as all i care in this circuit o/p is voltage.

I think that no matter the frequency, the signal peaks remains the same. Which will be measured by the ammeter/voltametter.

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Sorry, You did say “Ammeter” — perhaps you meant “Multimeter” ;)

And, it’s not a matter of “stupidity”, it’s a matter of experience and knowledge. And, my only measure of that is what you write ;)

And, yes, the signal peaks will remain the same regardless of frequency, but every meter has it’s limits, and it’s important to know those limits, so you can properly interpret the reading (which may, or may not, reflect reality).

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Thank you Mr. Steve. It worked as per your proposed schematic. I shall get back to your for further assistance. Thanks

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Great, glad for your success. BTW: don’t just “get back to me“ , include everyone else. I don’t reply to emails from this forum.

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Also, when you say “worked as per your proposed schematic” are you referring to the bipolar transistor version or the MOSFET version?

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MOSFET transistor

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Wow, awesome!

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