# Powering a 1V 500 mA Load

Hello,

I am currently working on a digital clock using 8segement VFD tubes. I am using ten DG10Q1 tubes.

For the filament / heater, each tube requires a 0.6 - 1 V supply with up to 50 mA consumption.

I want to be able to specify the voltage from the Arduino so I can dim the clock if it's dark.

So far, I've only stumbled across voltage regulators which can do 1.5 V (e.g. TL497A) but nothing less.

I think I have to come up with some transistor circuit? Can anyone give me some tips about this, I can't find anything on the internet.

There could be better ideas but try the FeatherS2 which has a second 700mA regulator that could power the whole project.

for controlling the voltage at a variable level you could use a PWM-DAC combined with a high current op-amp (tlv2464). Set the PWM frequency to 312.5KHz and the RC filter cutoff to 30Hz (ish) and that will comfortably give you 8-bit precision on your output with up to 80 mA for each tube.

The ESP32 series also has the advantage that the dimmer could be controlled by wifi or bluetooth.

I'm not sure that controlling the heater voltage is a particularly useful way of controlling the brightness. I would expect it to cause stripes on the display.

If you know the heater will draw 50 mA at 1 V, then supply 1.5 V to each through a separate 10 Ohm series resistor.

To dim, use PWM on the anodes.

The feather is out of my price range.

I'll look into the tlv2646 and report back, thanks for the idea.

Paul suggested that regulating the heater voltage isn't the best idea anyways, I'll test it and reconsider my idea.

then supply 1.5 V to each through a separate 10 Ohm series resistor.

But then I'll be burning 10 * (.5 V / 10 ohm) = 50 W, right?

scrouthtv:
But then I'll be burning 10 * (.5 V / 10 ohm) = 50 W, right?

Wrong.

Paul__B:
Wrong.

sure, but could you explain why 1.5V and 10 Ohms? Would that not be 150mA per tube? Why not 1V, 20 Ohms to give the max required 50mA?

Rry putting a diode in series with the output of 1.5V regulator or you can use an adjustable one. This will remain stable.

For dimming a VFD tube which contains multiple digits, simply insert some “blank” cycles in the multiplexing function or lower the digit “on” period so there is a time gap between one digit being lit and the next one being lit.
You could also alter the duty cycle of the entire high voltage supply, maybe using the blanking pin of the driver chip, without synchronizing it with any multiplexing, but then the frequency has to be several times the multiplexing frequency to avoid any visual effects (from beating).
Changing the heater voltage is not a standard method and definitely don’t exceed the maximum voltage specified.

the2ndtierney:
sure, but could you explain why 1.5V and 10 Ohms? Would that not be 150mA per tube? Why not 1V, 20 Ohms to give the max required 50mA?

It is understood you have a 1.5 V supply. You want 1 V. You want a resistor to drop the difference which is 0.5 V. You say each heater draws 50 mA (but other references put it at 31 mA) so you want a series resistor for each heater to drop 0.5 V. 0.5 V divided by 50 mA is 10 Ohms. But if you want to drop 0.7 V at 31 mA as per the other reference, it is 22 Ohms.

Paul__B:
It is understood you have a 1.5 V supply. You want 1 V. You want a resistor to drop the difference which is 0.5 V. You say each heater draws 50 mA (but other references put it at 31 mA) so you want a series resistor for each heater to drop 0.5 V. 0.5 V divided by 50 mA is 10 Ohms. But if you want to drop 0.7 V at 31 mA as per the other reference, it is 22 Ohms.

Thanks!

Keep in mind that VFD filaments are designed to be powered by 1.5V AC not DC. While it will work with DC... you will often see uneven illumination of the phosphor.

Actually, it is 1 V or less and given that both the grids and anodes are operated at about 12 V, the difference between the ends of the filament will not be much in proportion.

I published a design for a VFD clock using an Russian IV-18 here about a year ago IV-18 Vacuum Fluorescent Display NTP Clock - Exhibition / Gallery - Arduino Forum. This tube has 8 digits, a filament voltage of 5v and an anode/grig voltage of about 30 volts. I read a lot about a noticeable brightness gradient across the tubes so I used an LM9022 VFD filament driver chip (very similar in specification to a stereo amplifier, but the same effect could be achieved with an H bridge). I did some tests with it and the benefit of feeding the filament with AC was hardly noticeable. The brightness gradient, if you are using DC to drive the filaments in a multi digit tube, can also be overcome in software by adjusting PWM to give the digits at the "darker" end of the display a bit more time.

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