Inverter circuit design

Thread Starter

gunghey

Joined Jul 19, 2020
7
Hi all,
New to the forms and looking for assistance.
I did some searching and was not able to find anything closely applicable to my needs, so here goes-

I am looking to make a power inverter which will utilize a 12V battery to produce 120V AC at around 10kHz.

Any insight, links, designs etc would be extremely helpful; I've found schematics before like this one:
inverter

...but they are 'stable' at around 60Hz and I don't know if they can be modified to still function at orders of magnitude higher frequencies despite being able to modify the time constant.
 

bertus

Joined Apr 5, 2008
20,773
Hello,

Why would you want to use the inverter at 10 kHz?
For 10 kHz, you will need an other kind of transformer, the kind that is used in SMPS power supplies.

Bertus
 

Papabravo

Joined Feb 24, 2006
14,435
What output current will you require? I hope you are aware that power out will be less than power in. In some cases it will be much l;ess.
 

Thread Starter

gunghey

Joined Jul 19, 2020
7
Hello,

Why would you want to use the inverter at 10 kHz?
For 10 kHz, you will need an other kind of transformer, the kind that is used in SMPS power supplies.

Bertus
I don't need anything with high power, if I could drive continuously around 1A that may be sufficient, maybe less. A 12V battery bank may be suitable if the draw is too high.
 

Papabravo

Joined Feb 24, 2006
14,435
Hi, I am looking to drive 1A or less, but probably need at least 250mA.
OK. So the output power you require is 120VAC(rms) X 1 Ampere is 120 watts. For a "back of the envelope" calculation, let's say you have an inverter circuit that is 80% efficient. That means the required input power will be 120 watts / 0.80 = 150 watts of input power. Then 150 watts / 12 VDC = 12.5 Amperes. Based on the capacity of your 12 VDC source, how long can it supply 12.5 amps continuous? This assumes the voltage is at or above 12 VDC during this discharge period. If the battery voltage drops below 12 VDC, the converter will demand even high currents to maintain the power output. You are well and truly up against it.
 

Thread Starter

gunghey

Joined Jul 19, 2020
7
OK. So the output power you require is 120VAC(rms) X 1 Ampere is 120 watts. For a "back of the envelope" calculation, let's say you have an inverter circuit that is 80% efficient. That means the required input power will be 120 watts / 0.80 = 150 watts of input power. Then 150 watts / 12 VDC = 12.5 Amperes. Based on the capacity of your 12 VDC source, how long can it supply 12.5 amps continuous? This assumes the voltage is at or above 12 VDC during this discharge period. If the battery voltage drops below 12 VDC, the converter will demand even high currents to maintain the power output. You are well and truly up against it.
Thank you for the calculations! People like you are why I love forums like these!

I could sacrifice both voltage and amperage if the battery life looks like it is going to be too short.

The application is a metal detecting transmit coil. Normally these are lower voltage and low current, but I am effectively trying to drive for improved 'signal strength' on any metal objects in range for analytical reasons. Strictly speaking, the inverter circuit would be used as a black-box AC source which will feed directly into an LC resonator consisting of a capacitor and the search coil (inductor) tuned to resonate at 10kHz or whatever frequency I end up at.
 

Papabravo

Joined Feb 24, 2006
14,435
Rather than swinging for the fences on the first go-around, do you have the ability to take a series of intermediate steps to see what factors have the most effect on "detectability". What I mean is, could it be possible that raw power is not what you need. Is it possible you get better results with some other combination of voltage and current that has not occurred to you. I am an Amateur Radio Operator. When I put my station together I went for the best receiver I could buy and the best antennas I could construct. Transmitter power was the last thing on my mind. If you can't hear them -- you can't work them.
 

Thread Starter

gunghey

Joined Jul 19, 2020
7
Not sure what your knowledge level is with metal detectors but I'll wager your experience in radio operation is sufficient to understand it as they are analogous.

My initial design started with an AC source which was around 10V peak to peak supplying the transmit signal. Very low current, it was my buddy's bench top signal generator. My receive coil was connected to an oscilloscope. I could see the transmit signal on the transmit coil, and I could see the receive signal with the oscilloscope on the receive coil enough to 'null' it, but it would not show any signal when a metal object was passed over it as it should. I attribute this to a weak transmission, it needs to be powerful enough to induce a detectable field in the metal object such that it 'un-nulls' the receive coil. Furthermore I need higher power as I intend to reduce the receive coil diameter, which requires increased magnetic flux to achieve the same signal 'strength', thereby requiring a stronger transmit pulse.

Maybe I should restate my own needs; an AC source with 50-100V peak to peak operating at 5kHz-10kHz.

Does anyone have thoughts or suggestions for such a device?
 

Papabravo

Joined Feb 24, 2006
14,435
Not sure what your knowledge level is with metal detectors but I'll wager your experience in radio operation is sufficient to understand it as they are analogous.

My initial design started with an AC source which was around 10V peak to peak supplying the transmit signal. Very low current, it was my buddy's bench top signal generator. My receive coil was connected to an oscilloscope. I could see the transmit signal on the transmit coil, and I could see the receive signal with the oscilloscope on the receive coil enough to 'null' it, but it would not show any signal when a metal object was passed over it as it should. I attribute this to a weak transmission, it needs to be powerful enough to induce a detectable field in the metal object such that it 'un-nulls' the receive coil. Furthermore I need higher power as I intend to reduce the receive coil diameter, which requires increased magnetic flux to achieve the same signal 'strength', thereby requiring a stronger transmit pulse.

Maybe I should restate my own needs; an AC source with 50-100V peak to peak operating at 5kHz-10kHz.

Does anyone have thoughts or suggestions for such a device?
What you are talking about is near field behavior. In radio we deal with the far field -- more than 2 wave lengths from the antenna. The wavelength at 10 kHz. is incredibly long, like 30,000 meters. At 5 kHz it is twice as long. I question your notion that a strong field will induce a brand new field in an object. The object might alter or distort the near field, but the near field cannot create something out of nothing. So tell me more about this induced field.
 

Thread Starter

gunghey

Joined Jul 19, 2020
7
What you are talking about is near field behavior. In radio we deal with the far field -- more than 2 wave lengths from the antenna. The wavelength at 10 kHz. is incredibly long, like 30,000 meters. At 5 kHz it is twice as long. I question your notion that a strong field will induce a brand new field in an object. The object might alter or distort the near field, but the near field cannot create something out of nothing. So tell me more about this induced field.
An alternating or moving electrical field induces a current in a metal object within that field. The induced current in that object produces it's own field which is detected by the receive coil. This is why metal detectors only work on electrically conductive materials.
 

Papabravo

Joined Feb 24, 2006
14,435
An alternating or moving electrical field induces a current in a metal object within that field. The induced current in that object produces it's own field which is detected by the receive coil. This is why metal detectors only work on electrically conductive materials.
That field does not exist without the stimulus of the transmitter, just like in radio. It is not independent of the transmitter. I get what you are saying. I see no evidence among available circuits that more power in the transmitter is much of a consideration. I go back to my original suggestion that you concentrate on improving the receiver and the receiving coil. I think you will get more bang for the buck that way.
 

Thread Starter

gunghey

Joined Jul 19, 2020
7
That field does not exist without the stimulus of the transmitter, just like in radio. It is not independent of the transmitter. I get what you are saying. I see no evidence among available circuits that more power in the transmitter is much of a consideration. I go back to my original suggestion that you concentrate on improving the receiver and the receiving coil. I think you will get more bang for the buck that way.
I definitely need to improve the receive functionality. Either that or there is some aspect I am missing. I added an audio amplifier to the receive coil and was still not getting signal from the metal object (was getting signal from the transmit coil when not nulled.)

Perhaps I will create a new post about the function of the detector instead of continuing here with that part.

Here is another proposal- for the transmitters I need a better solution than a benchtop device anyway... How about an oscillator feeding into an op amp to boost the voltage instead of trying to use an inverter? Understandable that I may need multiple stages to get high enough voltage and will still likely encounter battery life issues ...
 
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