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I am trying to operate an IVL2-7/5 vacuum-fluoresecence-display with an Arduino Uno. It is a multiplexed 4 digit clock display that requires a cathode voltage of 2.4V and an anode and grid voltage of 24V. The 24V will be supplied by a seperate voltage source, the Arduino shall simply switch this voltage. (I know that VFD driver ICs exist but I want to try to do this with discrete components)

I have created this circuit, a high side-switch from two MOSFETS, but am unsure weither it's suitable. Schematic

My question is basically: Is this circuit a working switch and would it work to switch the 24V supply of a single anode element?

Also: what resistances should I select for R1 and R2? Do I need another resistor between Arduino and M1? I am very inexperienced in electronics, but I vaguely recall that a MOSFET's gate acts kind of like a capacitor and can pull strong currents if switched quickly. I'd like to utilise stronger resistors for R1 and R2 to minimize the current they "consume", but don't know if that will still allow fast enough switching for multiplexing of the VFD.

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  • \$\begingroup\$ Check the permissible Vgs limits for your high-side PMOS. Few can take -24V. \$\endgroup\$
    – pericynthion
    Commented Sep 13, 2017 at 5:24

2 Answers 2

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Your schematic is totally wrong, this will not work. Do you know, that you also have to connect all of the Grids correctly and switch them also to make it work?

You should use 8 switches on anodes and 5 on grids, also you have 2 cathodes in this tube.

You can use MOSFETs or bipolar transistors as switching elements, actually, this doesn't matter.

I can show you schematics for similar tubes (not exactly IVL2-7/5), but you will see the principle. They are for tube with one digit, and there are three options: MOSFET and two bipolar.

Note that you also have to control grids by separate switches. enter image description here

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  • \$\begingroup\$ Obviously my schematic only showed a single switch for 1 anode element, not to control the entire VFD. I am aware that I need to switch the anode elements and grids separately and in a time-share multiplexed way. My question was if my circuit works as a single switch for a single element, I will make it clearer. \$\endgroup\$
    – JMC
    Commented Jan 6, 2017 at 17:18
  • \$\begingroup\$ @JMC Can you see the image, attached to my answer? You need JUST switch, nothing more. If you would like to use MOSFET, use just a single N-MOSFET for each anode, you don't need anything else. Also you can used bipolar transistor with resistor for this purpose (what is typically cheaper). Also from your picture it's impossible to understand, how you would connect you tube. You have onle "+" & "-", but you need to connect cathode to two power lines and also need to connect grids and switch them also. That's why I'm telling you this. \$\endgroup\$
    – cyclone125
    Commented Jan 6, 2017 at 18:15
  • \$\begingroup\$ I have the cathode part figured out already, that's why I ommitted that part from the circuit. In your first picture, the anode A is switched by a p-MOSFET, which required a gate voltage in the vicinity of VDD, that is impossible in my case as I need to control the 24V anode supply with only 5V from the microcontroller. The same problem arises from the pnp-transistors in the second image. \$\endgroup\$
    – JMC
    Commented Jan 6, 2017 at 18:46
  • \$\begingroup\$ In the third, g and a are controlled with npn transistors, but the VFD "load" will form a voltage divider with the pullup transistor on the VDD, which will require VDD to be higher than 24V, but I suppose that would be doable, thanks. \$\endgroup\$
    – JMC
    Commented Jan 6, 2017 at 19:46
  • \$\begingroup\$ @JMC All right, I gave you several possible options, so you can find the bast way for you. I mean, if you need 13 switching elements on discrete parts, you should try to make them as simple as possible. You can use two MOSFETs with several resistors for each switch, but it will make the whole schematics really complicated. Also I think, that bipolar transistors will work fine here (at the time when IVLs were developed, there were no MOSFETs). Note: for reliable switching off (disabling) of any segment you should put negative potential 3V (absolute). \$\endgroup\$
    – cyclone125
    Commented Jan 6, 2017 at 21:16
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Since MOSFET is voltage controlled device.But it need some current to turn on the gate. so you can choose resistor between m1 and arduino of 100 OHM.

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