Fun with Nixies : Arduino and Voltage Divider?

[Mach]

I'm working on my electronics skills and need some help. I'm trying to interface an old school nixie bar graph with an arduino. It's a fairly simple circuit but I can't seem to get there without expensive blue smoke.

Take from here http://www.die-wuestens.de/iz/IN9-2.pdf

I'm using an Arduino Uno for the "IN" but it causes the bar graph to only extend 1/4 of the way. When I swap in a constant 5v from my PSU, it extends 100%. I believe the problem is with the Arduino output (frequency maybe). Would a capacitor in the circuit help? The ultimate goal is to PWM the bar graph from one of the digital pins.

On another note, the circuit requires 150V but the nixie power supply delivers 200V. Can anyone tell me how I can reduce the voltage? I've been playing with voltage dividers but have fried several 1/2 watt resistors.

Any help is greatly appreciated!

[x88x]

Problem controlling the nixies:
I would have the arduino control a transistor that would switch on and off a raw 5V to the nixie control. Also, put a diode in between the arduino and the transistor control pin, to protect against backflow (just in case).

Problem with powering the nixies:
What value resistor have you been using on the voltage divider? If you're frying 0.5W resistors, you probably just need heftier resistors. To find out what you need, first find the voltage drop across the resistor (in this case, 50V, assuming the resistor to ground is the one we're talking about), then:
I=V/R
That gives you the current through the resistor. Then, you take:
P=IV
So, P = (V^2)/R
To find the power going through the resistor. That gives you the minimum power rating you want on the resistor.

For example, if you were using a 5 Ohm resistor,
I = (50V)/(5 Ohm) = 10A
P = 50V * 10A = 500W

50V is a pretty massive drop for a voltage divider.

Alternately, to work the other way, and get the power rating to something reasonable:
P = (V^2)/R
R = (V^2)/P
So, if you want to use a 0.5W resistor, it would need to be:
R = (50V^2)/0.5W = 5 kOhm

Or, picking a more reasonable value for a power resistor:
R = (50V^2)/20W = 125 Ohm

Then, once you have the resistance you want for that resistor, using the voltage divider formula:
V_out = ((R_2)/(R_1 + R_2))*V_in
And this diagram:

With R_2 being the one we just found a resistance value for.
Therefore:
R_1 = ((V_in*R_2)/V_out) - R_2
So, with the 125 Ohm 20W R_2:
R_1 = (200V * 125 Ohm)/150V - 125 Ohm = 41.667 Ohm
Now, the power requirement for R_1 would be a lot less, but the voltage drop across it would be dependent on the rest of the circuit, so I can't really say what it would be atm.

Disclaimer:
It's been a little while, so my math and circuits might be rusty. If I made an error and someone sees it, please point it out.

[Mach]

Thanks very much x88x for the easy to follow and timely response. Timely because I was reading it at Fry's as I was trying to figure out what to get. You're explanation made things much clearer. I wound up with a set of 1W resistors (3.3K & 10K) to use in my divider.

I used the a site that I found to help calculate the right values and based on the math that you showed.

I = (50V)/(10000 Ohm) = 0.005A
P = 50V * 0.005A = 0.25W

or 10K/(10K+3.3K)*200 = 150.3V

I also found this circuit when searching around more:

From here: http://sites.google.com/site/joscien...coachella-lamp

The capacitor/resistor combo act as a low pass filter. I'm going to give it a go next weekend as its back to work now. Thanks again for the help!

[Oneslowz28]

So is there a reason not to use a 74141 Nixie Driver IC? I like the simplicity of the circuit though.

[Mach]

Not sure, every circuit that I can find uses the mje 340. I believe the 74141 is overkill for the IN-9s as it has only one cathode as opposed to 10.

[Oneslowz28]

Yea after posting this I went back and looked at the tube you were using and came to the same conclusion. I think I may do a Nixie clock sometime this summer. Cant wait to see what you are cooking up Mach!

[Mach]

Thanks I've got a few more projects to finish at work and then its mod vacation time.

[Oneslowz28]

If you need some press with it, just shoot me a PM!

[Mach]

Thanks man, I'll keep you posted.

[nevermind1534]

I was just starting to order parts for a nixie tube clcok.