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Trying to build a step up transformer that takes in 0-120V pulsing dc and steps it up to 20kv with a max secondary current draw of 3mA. I will be converting Maine’s voltage into the DC using a full bridge rectifier which will then be fed into the transformer. also, if you’re going to sit here and lecture me about safety, then ignore this post because I really don’t care to hear it. All I inquire about are the math formulas used to build a transformer. I have only been able to find these two formulas shown below that allow me to calculate the minimum turns on the primary however, what am I missing to calculate the core I need. also anything I need to take into consider would be great such as calculating the frequency of pulsing DC needed to power the transformer. Also, if you’ve never had experience building, one don’t bother helping me. Too many people in life love to give advice on things they’ve never actually tested

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    \$\begingroup\$ you can't drive a transformer with 'pulsing DC' from a bridge rectifier. You need some electronics between the recitifier and transformer. Or you can just run the mains AC into the transformer. \$\endgroup\$ Commented Jul 19 at 7:19
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    \$\begingroup\$ There are a couple things missing here: there is a loose term "pulsed DC", but it can mean a biased time-varying voltage or current, or a varying impedance such as from a single switch (which by its nature can only draw unipolar current). However, directly applying a fixed DC voltage (regardless of AC component) to a transformer, is typically a recipe for disaster (bias current, saturation, power dissipation). And a sinusoidal, non-zero-touching waveform like \$10+\sin \omega t\$ isn't very "pulsed". \$\endgroup\$ Commented Jul 19 at 7:55
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    \$\begingroup\$ Check out how the coils on a car work to produce 20-30kV from 12v dc. \$\endgroup\$ Commented Jul 19 at 7:56
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    \$\begingroup\$ Now I had to look up what's so special with Maine's grid voltage. \$\endgroup\$ Commented Jul 19 at 8:56
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    \$\begingroup\$ @scriptkitty The first pulse will in fact produce a high voltage pulse from the output. The second and subsequent pulses will draw more and more current from the supply as the core saturates, and produce less and less high voltage output. Eventually the input fuse will blow or the coil burn as the input current becomes too high. That's what the electronics are there for, to reset the core flux between pulses. It's what AC does automatically, having an average of zero DC voltage. Maths formulas are great, when they are applicable. You need to understand when they're applicable. \$\endgroup\$ Commented Jul 19 at 9:02

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Drawing the schematic that should have been included in your question, to see if I understand what you're proposing - on the left we have ... Maine’s voltage into the DC using a full bridge rectifier which will then be fed into the transformer. If this is not the schematic you intended, please correct me.

schematic

simulate this circuit – Schematic created using CircuitLab

As the input to the transformer has a finite average DC component, the primary current will continue to rise, and will saturate any finite core in a short time. Once saturated, the core flux does not change significantly, so cannot generate any secondary voltage. The current will continue to rise until something stops it, ideally a breaker, if not then smoke.

On the right is the minimum modification needed to make the circuit work continuously. The switch will be driven on and off by some suitable circuit, with on time limited to avoid saturation. When the switch opens, the tendency of the transformer's inductance to make the current to 'want' to keep flowing means it will generate a large negative voltage across the switch. This allows the flux in the core to drop below saturation, so it's able to change and work like a transformer again. Another way to look at it is this negative voltage transient, averaged with the rectified DC input, gives you average zero DC on the input to the transformer's inductance, to keep the current finite.

In old car ignition systems, this was a mechanical switch. These days, you would use a FET or IGBT, and some additional components to prevent the 'large negative voltage' from frying it.

Using 'chopped DC' like this is using the transformer in 'flyback' mode. This needs a low permeability core for best efficiency, not the gapless iron core used in a conventional 'forward' transformer for which you have equations.

Before maths, we need to get a usable configuration.

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