Incandescent bulbs not lighting via Teensy Arduino - Transistor Bias Problem?

Thread Starter

WShawn

Joined Apr 9, 2017
16
This is my first post on this forum; please let me know if I'm in the wrong place.

I'm attempting to drive a series of four 2.5 V incandescent flashlight bulbs using a chaser sketch running on a Teensy 3.1 Arduino. This is for a Proton Pack I built in 1984. The pack currently uses a very old analog chaser circuit from one of the Forrest Mims books, but it's getting a little flakey. I upgraded the blue lights on the pack using the Teensy to drive a column of LEDs, so now I want to drive the red lights to streamline things.

The chaser in the Teensy is sending pins 16-19 HIGH in sequence. I have those pins going to NPN 2N 3904 transistors to drive the lamps via 1.5K ohm resistors. The lamps won't light, though. Hooking up my old Radio Shack multimeter indicates that the transistors are sending almost the full 3 volts to the lamps. If I replace the lamps with LEDs the LEDs light correctly. This leads me to believe my transistors aren't switching on completely, which I'm speculating is a transistor bias issue. My electronics knowledge is rudimentary, so the formulae I'm seeing online to calculate the bias are over my head.

Here's a rough schematic:




This video (12 seconds in) shows the nature of the red "cyclotron" lights as they currently operate (the oscillating blue lights shown in this video have since been upgraded to a column of LEDs via the Teensy.)


I could just switch over to LEDs, but the incandescent bulbs have a ramping up and down of the brightness that's authentic to the movie prop, and I'd like to retain that. I'm sure there's a way to program such behavior into the sketch, but that's beyond me.

Any thoughts would be appreciated.

Thanks.

Shawn Marshall
Portland, OR
 

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Try increasing the current through the bulb by lowering the 1500 ohm resistor value. You may have to go pretty low on the value, 150 ohms, to get them completely turned on. Try some values...

You may have to verify if the processor can handle all of that current. Otherwise you might try switching to a LOGIC LEVEL MOSFET instead.
 

bertus

Joined Apr 5, 2008
21,940
Hello,

As @drc_567 said, the 2N3904 can only handle 200 mA max.
If you realy want to switch 300 mA, use some 2N2222 as transistors with 150 - 220 Ohms as base resistors.

Bertus
 

dl324

Joined Mar 30, 2015
14,672
You'll have an easier time if you use logic level N channel MOSFETs. Then you don't need to worry about how much current your Teensie can source. If it will sink more current than it can source, you could use P MOSFETs and drive the lamps high side.
 

djsfantasi

Joined Apr 11, 2010
8,392
The maximum current a Teensy pin can supply is 25mA. Better to use a much lower rating, like 15mA.

To drive the transistor to turn on your incandescent lamps, they will have to draw <150mA. If the lamps need more current, you'll have to use a different driver for them.
 

ian field

Joined Oct 27, 2012
6,536
You'll have an easier time if you use logic level N channel MOSFETs. Then you don't need to worry about how much current your Teensie can source. If it will sink more current than it can source, you could use P MOSFETs and drive the lamps high side.
That's the way to go - bulbs have a low cold resistance, so the starting current is much higher than the calculated normal rating. You calculate the required base current for a BJT - but it never gets off the starting block. You need to calculate the base resistor for something like 12x the expected current, which is easier with a Darlington transistor.

In this application; A MOSFET might just as well be infinite current gain - as long as you top VGSthr, the device will conduct its rated current.
 

Thread Starter

WShawn

Joined Apr 9, 2017
16
Thanks to everyone for taking time to reply; I appreciate it.

Some of the responses are above my knowledge level. The first thing I'd like to try is swapping the 3904 transistors for PN 2222A transistors (I forgot the current limitations of the 3904s). My 1986 analog chaser is using 2222A transistors with 1K resistors, and it would simplify things to reuse those parts. I think I might just hook up the outputs of the Teensy to that existing setup to see if my bulbs light. If they do then I'll look to streamline the parts onto a single board.

Before I do that, though, I'm a bit fuzzy on the relationship between how much current the Teensy can safely provide and the resistor/transistor I use. The whole point of the transistor is to switch on when it gets the small (i.e. 10 mA) signal from the Teensy, allowing 300 mA (or whatever) to flow through my light bulb on the other side. How would I know whether my setup is trying to draw too much current from the Teensy output pins?

Best.

Shawn
 

dl324

Joined Mar 30, 2015
14,672
Before I do that, though, I'm a bit fuzzy on the relationship between how much current the Teensy can safely provide and the resistor/transistor I use. The whole point of the transistor is to switch on when it gets the small (i.e. 10 mA) signal from the Teensy, allowing 300 mA (or whatever) to flow through my light bulb on the other side. How would I know whether my setup is trying to draw too much current from the Teensy output pins?
The Rule of Thumb that I and many others use is that base current should be a tenth of the desired collector current. If the bulb requires 300mA, you need to provide 30mA of base current to guarantee saturation.

You could use a Darlington configuration to get current gain, but saturation voltage will be a diode drop higher.

If you use a MOSFET, you don't need any appreciable current from the Teensy. MOSFETs are voltage controlled vs BJT's being current controlled.

With 1.5k base resistors, you're only drawing a couple mA from the Teensy, but that current isn't sufficient to saturate the transistor.

The relevant equation is I = V/R = (Teensy HIGH voltage - Vbe)/Rb
 
Last edited:

ian field

Joined Oct 27, 2012
6,536
Thanks to everyone for taking time to reply; I appreciate it.

Some of the responses are above my knowledge level. The first thing I'd like to try is swapping the 3904 transistors for PN 2222A transistors (I forgot the current limitations of the 3904s). My 1986 analog chaser is using 2222A transistors with 1K resistors, and it would simplify things to reuse those parts. I think I might just hook up the outputs of the Teensy to that existing setup to see if my bulbs light. If they do then I'll look to streamline the parts onto a single board.

Before I do that, though, I'm a bit fuzzy on the relationship between how much current the Teensy can safely provide and the resistor/transistor I use.
Your device might be able to supply enough base current to run the bulb once the filament is hot, but it probably won't turn the transistor on hard enough to light a cold filament.

You need *AT LEAST* a Darlington transistor - a logic level MOSFET is much easier.
 

Thread Starter

WShawn

Joined Apr 9, 2017
16
The Rule of Thumb that I and many others use is that base current should be a tenth of the desired collector current. If the bulb requires 300mA, you need to provide 30mA of base current to guarantee saturation.

You could use a Darlington configuration to get current gain, but saturation voltage will be a diode drop higher.

If you use a MOSFET, you don't need any appreciable current from the Teensy. MOSFETs are voltage controlled vs BJT's being current controlled.

With 1.5k base resistors, you're only drawing a couple mA from the Teensy, but that current isn't sufficient to saturate the transistor.
Thanks for the reply. This rule-of-thumb makes sense.

Can you suggest a MOSFET transistor that would be suitable for the purpose I've described? It would be nice if it came in the same TO-92 package.

Best.

Shawn
 

ian field

Joined Oct 27, 2012
6,536
Thanks for the reply. This rule-of-thumb makes sense.

Can you suggest a MOSFET transistor that would be suitable for the purpose I've described? It would be nice if it came in the same TO-92 package.

Best.

Shawn
0.3A is a bit steep, try looking for some 60mA bulbs. The BS170 MOSFET might just barely do it. There is a relatively new package style that was primarily SMD, but can be had in a through hole format, and it just fits in the same space as TO92. Can't remember the package designation - but its the midget version of the TO220 size SMD package found on motherboards. In fact the midget ones occasionally turn up on motherboards. Plenty of suppliers carry the leaded version, you only need make sure its the right polarity and logic level - you're unlikely to exceed the specifications of the parts in that package style.
 

AnalogKid

Joined Aug 1, 2013
10,098
Here is one way to determine how to bias a bipolar transistor driver. The Teensy output goes to 3.3 V, but not at any significant load current. For a 10 mA load current, let's assume a 3.0 V output. If the Teensy datasheet has an output current plot, great; use whatever it says for the output voltage at 10 mA sourcing.

Next, the transistor. A 2N2222A or 2N4401 can sink at least 500 mA, more than enough for your loads. Yes, there is a higher current when the bulbs are cold, but that lasts milliseconds and does not affect the steady-state power dissipation. To be conservative, let's use 0.7 V for Vbe, the base-emitter diode forward voltage.

We now have enough to calculate a base current limiting resistor value:
R = E / I (a permutation of Ohm's Law)
E = 3.0 - 0.7 = 2.3 V
I = 10 mA
Therefore, R = 230 ohms. The closest 5% value without going over is 220 ohms, a very common part.

While 30 mA falls out of the old-school rule of thumb, that rule is, well, old. My experience with signal transistors developed after the 1950's is that a 30-to-1 ratio of collector current to base current is more than adequate for firm saturation.

While a darlington transistor does require significantly less current from the Teensy, it also will decrease the voltage across the light bulbs by almost 25%.

ak
 

Thread Starter

WShawn

Joined Apr 9, 2017
16
Here is one way to determine how to bias a bipolar transistor driver. The Teensy output goes to 3.3 V, but not at any significant load current. For a 10 mA load current, let's assume a 3.0 V output. If the Teensy datasheet has an output current plot, great; use whatever it says for the output voltage at 10 mA sourcing.

Next, the transistor. A 2N2222A or 2N4401 can sink at least 500 mA, more than enough for your loads. Yes, there is a higher current when the bulbs are cold, but that lasts milliseconds and does not affect the steady-state power dissipation. To be conservative, let's use 0.7 V for Vbe, the base-emitter diode forward voltage.

We now have enough to calculate a base current limiting resistor value:
R = E / I (a permutation of Ohm's Law)
E = 3.0 - 0.7 = 2.3 V
I = 10 mA
Therefore, R = 230 ohms. The closest 5% value without going over is 220 ohms, a very common part.

While 30 mA falls out of the old-school rule of thumb, that rule is, well, old. My experience with signal transistors developed after the 1950's is that a 30-to-1 ratio of collector current to base current is more than adequate for firm saturation.

While a darlington transistor does require significantly less current from the Teensy, it also will decrease the voltage across the light bulbs by almost 25%.

ak
This makes, sense; thanks for explaining it clearly.

Thanks again for all the replies; I really appreciate everyone's help.

I think I'll try using the 2n2222A transistors I have with 220 ohm resistors and see if it all works. If it doesn't I might just switch over to some white LEDs I have kicking around, using 2 or 3 for each bulb. Though LEDs wouldn't ramp up the same way as the bulbs, my batteries should last a lot longer.

Cheers.
 
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