nixie help appreciated

[secretsather]

Hi all,

I'm new to this group, and I would appreciate if somebody could help me or point me in the right direction.

I'm a newbie when it comes to electronics, but I'm building my first Nixie clock, and experiencing some problems with it.

it's before work right now, so I will post a schematic if needed when I get home, but I will describe my set up.

I'm using a 555 Nixie driver to obtain 185 volts. Basically, I have an ATMega 328 hooked up to 595 shift registers, and those are hooked up to transistor Darlington arrays (SN75468N Texas Instruments)

I'm able to successfully program the Atmega328, and get it to work with the shift registers. I can even cycle the numbers on the nixie tube. But here's my problem:

when the 555 Nixie driver comes up to voltage the entire circuit is only drawing 5 Milliamps. When I hooked the first Nixie tube up (the first hour tube which will only display a 0,1,or 2) the whole circuit was drawing a half of an amp, and the LM 7805, which regulates the voltage for the low voltage part of the circuit was getting hot as a result.

i then put a base resistor between the shift registers and the SM 75468 N of 680 ohms, just to try. when I plugged the circuit in after that, it was only drawing 100 mA.

so I guess my question is this: what the heck is going on? I thought the transistor array, according to the data sheet, has a built in 2.2 K resistor, so why would putting an additional 680 ohms on the base of the Darlington array make that much of a difference?

just to clarify, the SN75468n is similar to the uln2003. Also, my common pin on the transistor array is floating, I didn't hook it to anything.

How is it possible that the base of these transistor arrays are sinking so much current? Or do you think I may have a problem elsewhere in my circuit? If so, why would putting an additional resistor between the shift registers and the array make such a difference?

[secretsather]

Here's the schematic: http://secretsather.com/delme/schematic.png

And the pic: http://secretsather.com/delme/IMGP0986.JPG

Thanks again to anyone who's willing to help.

[David Forbes]

Those are SN75468 chips, right?

The want all of their pin 9 connected to a 80-100V DC source. This will keep the
internal commutating diodes from conducting and causing trouble.

--
David Forbes, Tucson, AZ

[Michel]

If you say the whole circuit draws 500mA, you mean the total current
of your HV circuit plus the LV circuit, right? You should measure how
much of this 500mA is drawn by the HV circuit and how much by the LV
circuit. If you're pumping 30mA through your nixie, it is not strange
the HV circuit draws say 400mA, the question is then why does your LV
circuit draw 100mA. Each input of your 75468 draws about 1.5mA when
on, normally you have 6 of them on, one for each tube, say 10mA. If
the current drawn when all is blanked is 5mA as you say, the LV
circuit shouldn't draw much more than 15mA when all 6 tubes are on.

Michel

[Michel]

I just had a look at your picture, what is the function of that
enormous capacitor you have there? It looks like this is C5 in your
schematic to buffer the high voltage. I would say it is way too big
and might even cause the strange problem you describing (dependent on
the type of 12V power supply you are using). It should be much less in
value than C1, say somewhere between 1uF and 10uF, but I think you got
a couple of 100's of uF in your circuit.

Michel



On Nov 22, 8:13 am, secretsather <secretsat...@gmail.com> wrote:

[Michael Gregg]

What resistors do you have around of the transistors? If you do not
have an resistors in front of the base of the transistor, it will draw
as much current as you can supply.

I'd be happy to send you the eagle files that I use for my nixie tubes
for you to compare against.

Also, did you built the HV supply yourself?

Michael-

[secretsather]

@nixiebunny - I will give this a shot. I wonder if this could be the cause? 

@Michel - Yes, it's drawing 500mA total current (both lv and hv), so I'll isolate the LV and check it with a meter to see how much current it is drawing next time I get a chance (hard to do things around the holidays) Also, you're right about the capacitor, I looked at thge schematic I used for the 555 nixie driver, and noticed it's only supposed to be a .47uF cap. I'm using a 220uF, and what was I thinking? This will be the first thing I try (is replace the cap). I'm using a lab power supply if that makes a difference.

@mikegregg - I don't have any resistors leading to the base of the transistors. The datasheet for the transistor array I'm using says they have a 2.2k on the base. Quoting from the datasheet, "The SN75468 has a 2700-Ω series base resistor for each Darlington pair for operation directly with TTL or 5-V

CMOS."

[Michel]

I think you'll find that replacing the 220uF cap with a 0.47uF cap
will solve your problem. Although I realize it may sound strange.... I
think what happens is that when the HV switch opens, C1 cannot supply
the current to charge this huge capacitor and the current will flow in
negative direction through the 7805 which causes it to heat up. You
may want to put a series diode in forward direction to the input of
the 7805, that will also solve the problem I think.

Michel

[secretsather]

Replacing the cap helped. The whole circuit HV + LV together (after the cap was replaced) is only drawing 70mA, which is around what I expected.

I added a second shift register to the one pictured, as well as a second tube. The circuit then was drawing 80mA, which is fantastic. I now have every component soldered to the board, so I'll be firing it up later today (after I do some programming and testing), but I believe this is going to work for me. I attached a pic if you're interested to see it. 

I appreciate you taking the time to help me out. It's one of those weird things that I never would have thought would be the problem. 

[Michel]

That certainly is good news. What a difference a capacitor can
make :-)

Once this works, the rest becomes and easy task.

Michel

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