Can a 1k resistor be too low for a transistor's base?

Thread Starter

Rocky_circuits

Joined Nov 1, 2011
57
Hello!

I am creating a Nixie Tube watch that should be really cool and I'm excited to share it (when it works)!
A problem I'm having, is that I designed the watch to use 4 shift registers (3.3v) that output into 1k resistors, and then to a MMDTA42-7-F (datasheet) transistor.
I may have jumped the gun on assuming I could use a 1k resistor for the base of these transistors to simply put it into saturation. Does it stand to reason that I burned out these transistors with an incorrectly chosen base resistor? My nixie tubes are meant to run around 1.5mA to 2.5mA. I think my order of magnitude is wrong and I should have used a base transistor along the lines of 10k-30k. I wasn't planning on using the base resistor to limit the current in the transistor in any way..

Anyway, what are your thoughts on the subject?
 

dendad

Joined Feb 20, 2016
3,612
The 1K base resistor will be ok. It could be higher, like 10K, but 1K will not damage the transistor.
What value resistor do you gave in series with the Nixie tube +HV?
Post a circuit and that will help.
 
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Hymie

Joined Mar 30, 2018
879
The data sheet shows the transistor gain (HFE) to be a minimum of 25, therefore you need to ensure that the transistor base current is at least 1/25 of the collector current in your circuit application.

As a general rule of thumb, by default I assume a transistor gain to be 10, but some high current devices can be less than this.
 

BobTPH

Joined Jun 5, 2013
2,584
If you are trying to use the base resistor to limit the current, that is a very bad design. It depends on a specific gain for the transistor, and gain is not well controlled. You need a resistor in series with the nixie tube to limit the current.

Bob
 

Thread Starter

Rocky_circuits

Joined Nov 1, 2011
57
Sorry guys, I should have specified more. I do have a 20k resistor in series with each nixie tube. The HV power supply is of my own design and delivers 175V (only 1v off of my LTSpice simulation! :))
@BobTPH In this case, I'm not trying to current limit the tubes with the transistors

Here's a few images of the circuit in question

1591448264928.png

1591448218561.png
 

Tonyr1084

Joined Sep 24, 2015
5,220
I am creating a Nixie Tube watch that should be really cool
I should have specified more. • • • The HV power supply is of my own design and delivers 175V
WOW! All that in a watch? 175 volts on my wrist? Wondering how you got the power supply to be that small. Or it's a really really big watch. (wrist watch)

Do you mean a clock? (as opposed to a watch? Even a pocket watch with that kind of power would be scary as heck)
 

dl324

Joined Mar 30, 2015
11,872
I may have jumped the gun on assuming I could use a 1k resistor for the base of these transistors to simply put it into saturation.
The base resistor value depends on the saturation current you want in the transistor. For 2N series, a beta of 10 is used, for many BC5 series, 20 is used.
Does it stand to reason that I burned out these transistors with an incorrectly chosen base resistor?
Not with 1k and a 3.3V drive signal.
 

dl324

Joined Mar 30, 2015
11,872
Here's a few images of the circuit in question
This schematic drawing style is atrocious to read and the drive circuitry is mostly drawn backwards.

Inverted grayscale version with the relevant details:
clipimage.jpg

Here's a better way to draw it:
clipimage.jpg
clipimage.jpg

He uses the wrong beta value for any transistor, but his schematic is easier to read.
 

Thread Starter

Rocky_circuits

Joined Nov 1, 2011
57
@Tonyr1084
That's right! It's definitely still big, but I'm pretty proud of myself for that power supply.
Check it out!
1591582001344.png
(I'm sorry it's out of focus)
While it's not pleasant to shock yourself with the 175V, it's not too bad either. Feels kind of like a hot needle prick. This will eventually be housed in a 3D printed enclosure, protecting me from accidental shocks


@dl324
I appreciate your criticism, but I much prefer a black background in my schematic. I know that is not standard, but it's a pet peeve of mine to use any program with a white background. I do also see that it's better to go from left to right, I definitely did that wrong, but this is just my own project and that's how it developed as I started laying things out. I gained all of my circuit knowledge on my own, and I make a lot of silly mistakes because of that (my degree is in Computer Science). Ultimately, I know a weird amount of advanced things, without knowing some basic things. For example, this nixie watch is using a BQ40Z80 fuel gauge chip for battery management. It has a 300 page technical reference manual that I know very well (I used it on a previous project). Yet I barely know how to properly calculate the resistor for a transistor, but I just knew that a 1k would put it well within saturation for my purposes.
Thank you for that excerpt from that article, it's very enlightening. So it appears a 10-12k resistor would be more appropriate in this instance.

Anyway, I'm sorry to rant a little, but I was driven out of the career of engineering because it feels like everyone is always yelling at each other for not doing it the way they like, including me. It just felt unhealthy I guess.

Also,
I found the issue! The two clock pins were shorted together very slightly.
1591581470429.png

I can't wait to show you guys the finished version of this project :D
 
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