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Using Arduino => General Electronics => Topic started by: stoopkid on Mar 20, 2011, 01:58 am

Title: How to pick PNP transistor for nixies
Post by: stoopkid on Mar 20, 2011, 01:58 am
So ive given up on my old power transformer and im ordering parts to build a new one. While im at it I want to get some PNP transistors because I plan on multiplexing my nixie tubes. I really don't understand these things a whole lot yet, how do I pick one that can switch 200ish volts and not fry my arduino?

DC collector current, power dissipation, transition frequency, dc current gain max, emitter voltage..... what the heck do these correlate with?
Title: Re: How to pick PNP transistor for nixies
Post by: CrossRoads on Mar 20, 2011, 04:24 am
Go to digikey.com, search for PNP transistor, select Transistor (BJT) Single.
Filter on "Voltage - Collector Emitter Breakdown (Max)" and pick 200V,  then look at the ones that meet your current requirements.

I think you will need another transistor in front of it to drive it, something like this, so the base voltage can swing high & low enough to control it.

(http://www.crossroadsfencing.com/NPN-PNP_control.jpg)
Title: Re: How to pick PNP transistor for nixies
Post by: stoopkid on Mar 20, 2011, 02:18 pm
okay thanks... quick question though, im using a different site to get my other stuff, and on it they have some things listed as a negative voltage. is -200 the same as 200?
Title: Re: How to pick PNP transistor for nixies
Post by: CrossRoads on Mar 20, 2011, 03:05 pm
In this case, yes.  The + or - is all based on the reference point.
Title: Re: How to pick PNP transistor for nixies
Post by: davekw7x on Mar 22, 2011, 04:27 am
We always used MPSA42 for NPNs and MPSA92 for PNPs for HV circuits like this.  Cheap and In Stock at Digi-Key (among others) .


Regards,

Dave
Title: Re: How to pick PNP transistor for nixies
Post by: MarkT on Mar 22, 2011, 01:41 pm
When you see specs like 'Vbe' and 'Vce' these mean max voltage of base with respect to emitter, max voltage of collector with respect to emitter.  For PNP transistors these will be negative because the emitter is the most positive voltage present.  In the two transistor circuits above both transistors need a high voltage rating, and you'll need resistance to limit the collector current of the NPN transistor or it'll be dissipating way too much power.  Remember power = voltage x current so that at high voltages even a few milliamps can cause overheating.

Another possibility is to use opto-isolators to control the high-side multiplexing - then you just have to ensure the isolator's output transistor can handle the high voltage.  There's also much less risk to the Arduino this way.
Title: Re: How to pick PNP transistor for nixies
Post by: davekw7x on Mar 22, 2011, 10:15 pm

... Remember power = voltage x current so that at high voltages even a few milliamps can cause overheating....


That is, of course, true, but the idea is to drive the transistors so that they are either "Off" or "Saturated."

If a transistor in a circuit like this is "Off" then it has 200 Volts across it, but the current is less than a microamp.

If the transistor is "Saturated" it may have the full two milliamps (or whatever) of Nixie current through it but the voltage across it will only be about a half of a volt.



Now, I have never actually multiplexed Nixie tubes, so I don't know how effective it might be.  I mean the eye tends to be a peak detector, so if you are multiplexing, say, 10 digits, you really don't have to drive each of them with ten times its normal current.  I'm not sure how much you do have to drive it.

Nixie tubes that I worked with, years and years (and years) ago typically "looked good" at about 130 volts and 2 mA, but there was considerable variation depending on size, exact color, etc...

Anyhow...

If anyone is interested, I have attached what I think is a practical driver circuit that is "typical" for various high-voltage driver applications.  I mean, I gave the title "Nixie Tube Anode Driver," but...

Bottom line: In this circuit the transistors are driven with something like 1/2 mA and 3 mA, respectively, which is enough to ensure saturation when they are conducting.

Do I have to say it?  "IWFMYMMV"  (It Works For Me; Your Mileage May Vary.)



Regards,

Dave
Title: Re: How to pick PNP transistor for nixies
Post by: MarkT on Mar 23, 2011, 06:01 pm
Look at the circuit again - the NPN transistor's collector is always at high voltage whether its on or off - it can't saturate.
Title: Re: How to pick PNP transistor for nixies
Post by: davekw7x on Mar 23, 2011, 06:52 pm

Look at the circuit again - the NPN transistor's collector is always at high voltage whether its on or off - it can't saturate.

The proposed circuit is based on the following assumptions:

The collector of Q2 is feeding the anode of a Nixie tube.  One of the Nixie's cathodes goes to approximately zero volts to light up a particular digit on that tube.

For purposes of analysis, I assume that the Nixie tube is a Zener diode with a voltage of 135 volts.  (I know, I  know, that's not what a real Nixie tube looks like, but the tube I am recalling for this example had a voltage of about 135 volts when it was illuminated, and at 135 volts the current was about 2 mA and it "looked good.")  A real Nixie has a "soft knee" in its Voltage/Current characteristics at the point it starts to illuminate and it has varying dynamic resistance (and capacitance and maybe some other stuff) that I didn't try to model since I am only looking at DC operating points for the two logic levels out of the Arduino.

Here are the back-of-the napkin calculations that led me to the values that I showed:

With a logic 1 from the Arduino, the base of Q1 gets something close to half a milliamp of current. With the values shown for R2 (and R3), for Hfe values greater than 10 or so, Q1 will be saturated, and the collector of Q1 goes to Vcesat, a few tenths of a volt.  Note that the voltage across R3 will not be more than a few tenths of a volt with this circuit, since it is across the forward-biased base-emitter junction of Q2.

Anyhow...

With Q1 saturated, the base of Q2 is driven by the voltage divider consisting of R2 and R3 going between 200 Volts and (approximately) ground.  With the values shown, this gives a base current in Q2 of something between 4 and 5 mA, so Q2 will be saturated, and almost all of the 200 volts is applied to the Nixie.  The Nixie fires somewhere around 135 volts, and R4 limits the current to something a little less than 2 mA, which was the target.  These calculations may be performed more precisely for any given Nixie, but if you are going to multiplex them you will more than likely have to determine the value of R4 experimentally to obtain the desired visual results anyhow.  (Unless I had extremely detailed device characteristics and had done extremely precise calculations, I would usually start with circuit values that give something less than nominal current to the device and work my way up.)

With logic 0 from the arduino, Q1 does not conduct.  There is no current (other than the very small leakage current---less than 0.1 microamps) through Q1, so there's essentially no current through R2.  That means that the base of Q2 is pulled up to 200 Volts by R3 (0.1 uA through R3 is not enough to bias Q2 into any kind of meaningful conduction).  Since the base of Q2 is at essentially the same potential as its emitter, Q2 does not conduct, and there is no current through the Nixie.

I hate to repeat myself, but I haven't actually used this to do Nixie multiplexing.  (I have used circuits like this to switch higher voltages in other applications.)  My hope was (and is) that the stuff that I posted may be useful as a starting point using cheap and readily available semiconductors in what I think is a practical circuit.  There are other ways...

[/begin Important edit]
Resistor R2 in the diagram will dissipate more than a Watt when the Arduino applies logic 1.  I won't go back and change it more appropriate values, but it can definitely be improved.  And it really should be improved --- see my quick take on a (possibly) better starting point in my post a couple of replies down from this.

I regret the "little lapse" in my analysis.

Anyhow...

Thanks to MarkT for pointing this out!
[/end Important edit]



Regards,

Dave

Footnote:
The transistors dissipate very little power when they are cut off or when they are saturated.  Now, a Nixie tube fires and extinguishes rather slowly (compared to transistor switching time), and the transistors may dissipate a little during transitions.  For multiplexing a reasonable number of things that people are going to look at (each device visually "refreshed" a few tens of times a second), the multiplexing frequency will be low enough that I wouldn't expect any anode's Q2 would spend enough time in the transition region to cause concern.  However (do I have to say it again?):
YMMV
Title: Re: How to pick PNP transistor for nixies
Post by: MarkT on Mar 23, 2011, 07:06 pm
Ah, if there's a resistor between the two transistors that'll allow Q1 to saturate - just check the dissipation in that resistor (and possibly its voltage specs - resistors do have a maximum voltage specification!).

Sounds fine to me.  I often regret that there is a very limited choice of transistor available in array packaging BTW - so much easier soldering 1 DIL package than 8 individual TO92s!
Title: Re: How to pick PNP transistor for nixies
Post by: davekw7x on Mar 23, 2011, 07:28 pm

...just check the dissipation in that resistor...

Oops: The dissipation in R2 in the circuit that I showed is over a Watt when the Nixie is turned on.  We might increase the value of R3  somewhat (will probably have to increase R3 also) to make the dissipation less.  If this is going to be used to multiplex, say, 10 digits, we might get away with something less than a 2 Watt resistor, but remember that software is driving this stuff, and a software bug (or hardware glitch just might let some of the smoke out.  It is definitely not acceptable (to me) to dissipate a whole Watt of wasted heat in anything like this.  See Footnote.  On the other hand, when dealing with 200 Volt power supplies and components that need them, sometimes our way of thinking about system design, not to mention debugging,  has to change also.

Maybe someone else will try a more detailed analysis with different values...

I won't go back through the numbers, but if I were doing it for a project of mine, I might start with 330K for R2 and 3K3 for R3.  Something like that will reduce the dissipation in R2 and still allow Q2 to saturate, I'm thinking.

Anyhow, thanks for pointing it out.


Regards,

Dave
Footnote:

For people who prefer simulation to back-of-napkin analysis, note that Spice (or SWCad or whatever) will sometimes give you nice looking output values for results obtained from trial-and-error twiddling and fiddling (or as some folks call it "heuristic design"), but will not necessarily point out impractical component values.  You still have to remember to check power dissipation and maximum voltage and current values before actually hooking stuff up.  Otherwise you may let all of the smoke out of something important.
Title: Re: How to pick PNP transistor for nixies
Post by: Chrisprols on Mar 24, 2011, 02:10 pm

We always used MPSA42 for NPNs and MPSA92 for PNPs for HV circuits like this.  Cheap and In Stock at Digi-Key (among others) .


Hi.
I've seen quite a few schematics to use those transistor: MPSA42 and MPSA92
Among those, here is a power supply which is using them.

http://davbucci.chez-alice.fr/index.php?argument=elettronica/orolnixie21/orolnixie21.inc
I'm trying to build from this other guy's design ... I might have messed it up: I only get 90V, but I'll try again some time soon :)
Title: Re: How to pick PNP transistor for nixies
Post by: davekw7x on Mar 24, 2011, 11:41 pm

...
Among those, here is a power supply which is using them.
... I might have messed it up: I only get 90V, but I'll try again some time soon :)


First of all, under what conditions are you measuring 90 Volts?  
I didn't try to go through the narrative on the web site that you referenced (dumb, lazy mono-lingual American that I am), but here's how it looks to me:

I think that when processor outputs are high, if the power supply is giving 170 Volts at the point labeled "B+" on the schematic, the anode drivers (Q6-Q9) are supposed to apply something in the neighborhood of 100 Volts to the Nixies.   (The voltage dividers in the bases of the anode drivers cause them to operate as emitter followers with an output about 60% of B+.)

Then, when one of the processor outputs goes low, that anode's driver goes into saturation and it applies the full B+ voltage (minus Vcesat) to the Nixie.  (Al cathodes driven by the decoded active-low output of the 74141 will be at ground potential, but only one of the anodes will be high enough to illuminate a Nixie tube.)

Applying a voltage that is a little lower than the strike voltage of the Nixie tube (a keep-alive voltage) makes it faster to illuminate when the full voltage is applied.  With just the right setting of R20 (and with proper software driving Q1-Q4), one and only one of the Nixies will be illuminated at any given time and the others will have a voltage just below their ionization value.  It seems like a good plan to me, and I don't see any glaring errors on the schematic.  (But I might have overlooked something.)

Anyhow...

How are you testing?  What voltage are you supplying to the '555?  If the load is disconnected from the power supply (disconnect one of the leads to R14) what voltage are you measuring at the cathode end of diode D1?  Adjust the 1K potentiometer (R20) from one end to the other.  By my reckoning, the voltage should go from something between 90 and 100 Volts when the wiper contact of the pot is at the R19 end and should be something over 160 Volts when it is at the R21 end.

If none of this helps you, then I have some other questions for you:

Did you use component values indicated on the schematic or did you substitute?

What kind of 1 millihenry inductor did you install at L1?

What kind of test equipment do you have?  In addition to a voltmeter, a simple oscilloscope that can look at 100 Volt pulses at a frequency of something like 20-30 kHz would be cool.


Regards,

Dave
Title: Re: How to pick PNP transistor for nixies
Post by: Chrisprols on Mar 25, 2011, 10:57 am
wow ! I didn't expect such an expert answer. you actually got me completely lost. ahahah. (I'm nearly a beginner !).
I'm planning to re-do the circuit from scratch. Maybe I did something wrong.
If after this, it goes wrong again, I'll go through your complete troubleshooting process.
fyi, I've got a multimeter (fluke 189) but no oscilloscope (if you have advise on cheap/diy oscilo/DSO, I'll hear them ;)).
... I'll post the conclusions here (don't hold your breath, I really have little time for personnal hobbies those days).
Anyway thanks tons for your answer.
Title: Re: How to pick PNP transistor for nixies
Post by: stoopkid on Apr 07, 2011, 11:41 am
I'm having trouble with Dave's schematic. My power supply is set to light the tube well, but when in that circuit it goes from a very low incomplete glow to a slightly more complete low glow, but still too low. I've already fried an ATmega, so i think I should ask which resistors to play with to change me on/off currents.

Quote
I might start with 330K for R2 and 3K3 for R3.


Is this a good place to start?

Thanks
Title: Re: How to pick PNP transistor for nixies
Post by: davekw7x on Apr 07, 2011, 04:58 pm

I'm having trouble with Dave's schematic.
That was intended to be a starting point for a design.  It assumed that there was a 200 Volt source and that one of the Nixie digits would be fully illuminated at 2 milliamperes with 135 Volts applied to the Nixie anode and ground supplied to a cathode.  That may or may not be true for any given type of Nixie.  Do you have a data sheet?

Even if you have a data sheet, you might find that it is necessary to do some subjective testing to see if the appearance meets your expectations.  (And to see whether the approximations that I assumed for the design are valid for the components that you are actually using.)

Anyhow...

Yes, the starting point is now with R2=330K and R3=3K3


To see if the supply is doing what  I think it's supposed to:

1.  Disconnect everything from the Arduino.  Disconnect the Nixie tube's anode from the power supply.

2.  Connect a 68K Ohm resistor to the point where you would normally connect the Nixie anode (Bottom of R4 on the schematic).  Connect one of the Nixie cathodes to ground.  Measure the voltage across the 68K Ohm resistor.  Should be about zero.

3.  Connect +5 Volts where you would normally connect the Arduino anode output signal (Left-hand end of R1)
Measure the voltage across the 68K Ohm resistor.   Should be somewhere around 125-130 Volts.  (At this voltage the resistor will be dissipating close to 1/4 Watt, so it will feel pretty warm.  Two 33K 1/4 Watt resistors in series would probably make a better load than a single 68K 1/4 Watt resistor.  The exact value isn't terribly important for this test.)

4.  Now, test the Nixie: Disconnect the anode driver circuit from the Nixie.
Apply 200 Volts to one end of a 39K 1/2 Watt resistor.  The other end of the goes to the anode of the Nixie.  Connect one of the Nixie cathodes to ground.  What does it look like?  Vary the voltage, if possible, to get the brightness that you like.  (If the 200 Volt supply is not variable, you can play around with the resistor value.  I would expect it to be in the neighborhood of 39K)

Bottom line: Current through Nixie is (High voltage supply value - voltage across Nixie) / Resistor value.

5.  Given the required Nixie voltage and current, we (you, actually) should be able to fine-tune the driver resistor values R3 and R4 to get good results.

6.  I have been assuming that in the actual circuit the cathode voltage is zero when a given digit is enabled.  The final step is to activate whatever cathode driver you are using instead of directly connecting a Nixie cathode to ground.  The cathode should go really close to ground potential.

Anyhow, when you have verified power supply component values, then you might be ready to attach to an Arduino.


It's kind of hard to troubleshoot by remote control and, since I don't have the same setup that you are working with, I might not be able to get to the bottom of everything, but the idea is to thoroughly test the driver stuff before you connect to an Arduino.

The MPSA42 and MPSA92 are 300 Volt devices and will not (should not) break down with a 200 Volt supply.  However...before connecting anything to an Arduino I would test the power supply and control circuitry separately and make sure there are no sneak paths (due to design errors or mistakes in connections or anything else) that would apply improper voltages (or allow excessive currents) to Arduino pins.

With power supplies, sometimes "smoke test" is, unfortunately, more than a figure of speech.



Regards,

Dave