Author Topic: How much voltage do VFDs need  (Read 1078 times)

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Offline Grayfox

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How much voltage do VFDs need
« on: April 05, 2017, 09:14:57 pm »
How much voltage do VFDs need to have a display?

The clock from my car looks like it is a VFD due to the silk screen having VFD on it and the glass top.

Do VFDs just die without warning or do they get progressively dimmer as they die?
As this just stopped working.

Voltage is going to it from the harness.

This is what is on the underside but I cant find a datasheet for it

I also take it that black spot on the top left is not good as it looks like a burn mark from inside.

The pins boxed in red are all ground.

Offline bktemp

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Re: How much voltage do VFDs need
« Reply #1 on: April 05, 2017, 09:26:55 pm »
The voltage depends on the multiplex ratio used. For static driving 12V provides sufficent brightness. When being multiplexed the voltages increases upto maybe 60V. For 1/10 multiplex around 20-30V are common.
If you go to high, the segements and the cathode will wear out faster.

The marked pins are the filament wire/cathode. They need a low ac voltage (1-6V depending on the display size) at a couple of 100mA.
All other pins are either segments or grids.

Offline Gyro

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Re: How much voltage do VFDs need
« Reply #2 on: April 05, 2017, 09:34:13 pm »
The "black spot" in the corner is a good sign - it is the getter, a metal (usually barium) sputtered onto the glass to mop up gasses. If it turns white it means that the vacuum seal has gone. It does look a bit faded round the edge, keep an eye on it for a few hours. Loss of vacuum would be a reason for it to suddenly stop working.

P.S. The filaments are all intact, so that's one failure mode eliminated.

P.P.S. The joined pins at the end are the filament connections - the filaments are most likely driven with AC to keep the brightness constant over the width of the display. Try measuring with your DMM set to AC.
« Last Edit: April 05, 2017, 09:46:31 pm by Gyro »

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Offline danadak

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Re: How much voltage do VFDs need
« Reply #3 on: April 05, 2017, 10:30:58 pm »
This answers your question about VFD lifetime -

Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer

Offline macboy

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Re: How much voltage do VFDs need
« Reply #4 on: April 07, 2017, 01:27:11 am »
VFD do age and dim over time, but they will not fade away until not visible. Only less bright.

They will suddenly die mostly due to loss of vacuum or loss of filament current (so no emission of electrons) due to some power supply failure. As Gyro said, the getter looks viable still, so vacuum should be intact. Check for low AC voltage (~3 VAC) on the filament. If no AC, check for DC. If neither, then you have a problem. The filaments are often driven through a series resistor; I'm very curious about that resistor (or two?) marked "270" (27 ohm?) under the glass. Check it/them. One may have failed open due to years of hot stressful work.

With the small number of segments and large number of pins, this may be driven with only 1/3 multiplex. Then, the 13.8 V electrical system voltage might be sufficient to be used directly as the grid/anode voltage. That would be good since there would be no need for a boost converter to create a higher voltage (with more components to fail). Look for some voltage between either side of the filament and some of the other pins, some of which will be connected to grids, some to anodes.

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