Author Topic: how to turn DC to AC?  (Read 510 times)

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Offline jordan_me160

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how to turn DC to AC?
« on: May 08, 2019, 12:40:16 pm »
hello, im working on a project right now that used a VFD screen and it requires 2.4-5v AC on a filament, problem is im running it off a 7.2v battery. is there any small and easy (possibly dirty/crude) way of turning that 7.2VDC even to 7.2VAC then using a resistor to create the 2.4-5VAC? Ive looked just about everywher and all the guides i find are for 12/24VDC to 110/220VAC only. could i just use the same parts of a simple one that makes a square wave and just take the massive transformer out?
thanks for the help in advance

Offline Psi

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Re: how to turn DC to AC?
« Reply #1 on: May 08, 2019, 01:22:40 pm »
How much current do you need at 2.4-5V AC?

You can generate square wave AC using two push pull outputs that are the invert of each other.
So a microcontroller with a couple lines of code, or a couple of logic gates.
But normally that will only get you 30mA or so.

A 2 channel mosfet gate driver IC can get you more current, if you have one.
Some of those will do up to 2A peak.
« Last Edit: May 08, 2019, 01:32:17 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)

Online bdunham7

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Re: how to turn DC to AC?
« Reply #2 on: May 08, 2019, 03:44:03 pm »
You can read here for OEM guidelines:

In short, the VFD filament circuit needs to do a lot more than heat the filament.  It must bias the filament and maintain a constant average voltage over the length of the filament.  The bias voltages are very small compared to normal vacuum tube circuitry and are so close to the filament voltages that an imbalance in filament voltage will severely upset the bias, resulting in uneven segment illumination and segments that won't turn off.  On a VFD circuit I recently worked on, the filament voltage was 3.3VAC and the bias voltage was -5.0VDC.  The way this is typically done is by using an isolated center tapped transformer and then applying the bias voltage to the center tap.  The simplest recommended way to do this in a DC system is to use a small HF transformer with a CT output.  You can then drive the transformer directly with a single-ended 50% square wave, with some capacitive coupling if you like.  In addition, since you will need the negative bias voltage and the higher grid/segment voltages  (typically -5 V and + 20-30V), you can use another small transformer or two driven off the same circuit, followed by rectifiers and filters, to get those voltages.  The filament voltage can be at any reasonable frequency, but it should be quite far apart from the VFD multiplexer frequency, if you are using a multiplex drive (almost all commercial products do this).


Offline David Cutcher CEG

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Re: how to turn DC to AC?
« Reply #3 on: May 08, 2019, 06:15:14 pm »
 bjdunahm7 answers your question in detail.
However,  I spend a good deal of time explaining how to get an AC signal working in a DC environment in my text.
The attached is from my text "Electronic Circuits for the Evil Genius" 2nd ed, McGraw Hill, 2010
It's using an lm741.  The AC signal is isolated. In this setup R1 and R2 offset chops the input signal.
The best AC output you can expect from this simple setup in a 7.5 VDC environment is 3.75 VAC. So you could use this output as the input to a transformer, and boost from there.
Good Luck
David Cutcher "Certified Evil Genius"
« Last Edit: May 08, 2019, 06:17:05 pm by David Cutcher CEG »

Offline jordan_me160

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Re: how to turn DC to AC?
« Reply #4 on: May 09, 2019, 01:12:14 pm »
thank you all for the help in this

Online Ian.M

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Re: how to turn DC to AC?
« Reply #5 on: May 09, 2019, 02:15:48 pm »
Back in the '80s, many consumer products with VFDs used single transistor blocking oscillators (similar to a 'Joule Thief'), with a small transformer with secondaries for the filament supply and to provide the HT for the VFD driver.   Its difficult to stabilise the output of such a blocking oscillator circuit, so run it from a regulated supply and use a shunt regulator for the VFD HT supply, so the load on the transformer remains constant no matter how many segments are lit.

Its possible to do without a filament supply centre tap by using a resistive divider across the filament.   Two resistors of five times the 'hot' resistance of the filament is a good place to start.       Connect a Zener from the divider or secondary centre tap to ground to provide cathode bias to get sufficient negative Vgk to get full cutoff even at the negative peak of the filament end drive waveforms.   If the required bias turns out to be close to 5V, you can also simply return the centre tap to your 5V rail, if it can handle the max current with all segments on without rising (guaranteed if the HT supply inverter is powered from the 5V rail).

Offline Rolo

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Re: how to turn DC to AC?
« Reply #6 on: May 09, 2019, 04:51:43 pm »
Maybe this is of any help:
Takes care of the filament and high voltage. Is adaptable for different VFD display's.
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