Transistor and car battery - 4.5V instead of 12V when on

Hello everyone,

This is my first post in this forum and I am happy to be here. Also, sorry for the noob question

I have a clock project that is going to run off of a car battery. I wanted to test the power supply on-off circuit with the hope of multiplexing the 7-segment displays in pairs of two. I am using an Arduino Mega because this is going to expand quite a bit if it works. I have connected the base, through a 1kΩ resistor, to pin 8 and control the transistor from there with a really simple code.

uint8_t TestPin = 8;

void setup() {
  // put your setup code here, to run once:
pinMode(TestPin,OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:

digitalWrite(TestPin,HIGH);
delay(3000);
digitalWrite(TestPin,LOW);
delay(3000);
}

But when the transistor turns on (3sec on 3sec off, to reliably measure with a multimeter) I can only measure 4.5V at the emitter instead of the 12.7V that the battery currently has. Which seems to be correct because when I connect the display (4'' - 12V) it is very faint.

Tried with a BC337 and a BD135. Same result. Maximum load per transistor will be around 80mA.

Any ideas why this is happening? Am I doing something wrong?

There are several ways to wire or mis-wire a transistor. Can we see a schematic? And, make sure you've correctly identified the collector, emitter, and base so you're sure your wiring matches the schematic.

BTW - The emitter is usually grounded so I'd expect to always read zero...

I'll get a schematic ready tomorrow. BTW, several other transistor in the same project all running perfectly well, even in pwm, but from a step down converter to give 5V.

Anyway, it's a bit late over here, will upload a schematic of this test setup tomorrow. Thanks for your time guys

Off Topic but I see that you used code tags on your very first post. Karma++

ExMachina:
But when the transistor turns on (3sec on 3sec off, to reliably measure with a multimeter) I can only measure 4.5V at the emitter instead of the 12.7V that the battery currently has. Which seems to be correct because when I connect the display (4'' - 12V) it is very faint.

Tried with a BC337 and a BD135. Same result. Maximum load per transistor will be around 80mA.

Any ideas why this is happening? Am I doing something wrong?

Yes, you've used the emitter-follower circuit configuration, which is not used for switching as it
has no gain and cannot saturate.

The common-emitter circuit is the one always used for switching a load with a BJT or darlington.

You may be wanting to switch on the high-side - if so you have to use a PNP device and a level
shifter.

Its usually much easier to switch low-side, when an NPN device can be used without level shifting.

This is a common mistake BTW, familiarize yourself with common-emitter circuit - you may never
need to use any other configuration, certainly not if just driving loads from digital or PWM signals.

Hi again, this is a (very) rough draft of the schematics. Again, please bear in mind that I'm still in a learning process here.

P.S. I'll try and link the link to my Google Drive with the pic
Link to download

MarkT:
This is a common mistake BTW, familiarize yourself with common-emitter circuit - you may never
need to use any other configuration, certainly not if just driving loads from digital or PWM signals.

Yup... Felt like a really common mistake!!!

I see what you mean, I'll do some reading up and try to change my circuit to suit. BTW, in my previous post, I uploaded a link to a rough schematic. I think I've done what you suggest.

So, what I would need to do is drive the emitter straight to ground and then intersect with a different line between the battery and collector, right?

Thanks so much for this! It seems I needed a push (shove) in the right direction...

I can’t see any schematic just a link to a blank thing.

Steve

Here. :slight_smile:
7SD Display.jpg

Yes , wired wrong - google using a “transistor as a switch “

Trying to power a 7-segment display directly from 12volt?

Small ones are usually powered from 5volt.
Are these very big segments, like 3 or 4", with a Vf of >=8volt?

If so, then a TPIC6B595 could be a better solution.
If small displays, why the 74LS47.
Mega pins and code can do what that chip does.
Leo..

Wawa:
Trying to power a 7-segment display directly from 12volt?

Small ones are usually powered from 5volt.
Are these very big segments, like 3 or 4", with a Vf of >=8volt?

If so, then a TPIC6B595 could be a better solution.
If small displays, why the 74LS47.
Mega pins and code can do what that chip does.
Leo…

They are six 4’’ ones and two 6’’ ones. So, the 7447 save me 3 pins per display. I am using the mega because there are going to be a lot of stuff added on as time goes by and this project matures.

P.S. Outsider, thanks for the pic repost

Eight TPIC6B595 chips (ebay) can drive all your displays (and more), with only three Arduino pins.
See this page (with code example).
Any other solution will be harder.
Leo..

That is true, but we have a lot of time in our hands and almost next to no money… Besides, learning and troubleshooting is half the fun, right?

Doesn't make sense.
The TPIC6B595 is $0.50 on ebay,
you can use a much smaller Arduino,
and you can learn how shift registers work.
Leo..