I need help selecting a transistor

So I’ve used MOSFET’s before but it looks like basic BJT transistors are a lot cheaper. So I would like to figure out how to select one of them. My basic project is a 4 digit 7 segment display, common anode. So each of the 4 digits get’s it’s own supply and all the individual segments get grounded, through a 220ohm resistor, to turn on. I checked with my DMM and each of the single digits can pull close to 60mA. Since I’ll be multiplexing they’ll need to turn on and off at least 50 times a second. So where do I go from here in selecting a transistor? What other information do I need to do this? I’m thinking I’ll be using a 9V battery to power this project with a 5V regulator. So the transistor will probably switch the same amount of current it will be feeding,+5V.

Look at ULN2803 to see if it fits your needs.

Any npn will do… I can’t think of a transistor which can’t handle less than 20ma…

The table halfway down this page might help.

Sounds to me as if you want a MAX7219.

It does have an internal 7 segment decoder for common cathode displays, but there is no need to use it even with common cathode displays, indeed you cannot if you want digits other than 0 to 9 and H,E,L,P.

It certainly makes things easier than all the wiring you would be using otherwise.

Should you nevertheless choose to use transistors and given that they are red LED displays, you can use NPN BC547-like transistors in emitter follower mode with no base resistors and their collectors connected to the 9V supply bypassing the regulator.

LarryD:
Look if ULN2803 to see if it fits your needs.

No good with common anode display. red913 needs a high-side switch for each digit, if I understand/guess correctly.

Normally a high-side switch (meaning between the positive supply and the component being switched) is done with a pnp transistor (for example bc327). A base resistor would be needed eg. 4K7.

Paul__B's suggestion of using an npn as an emitter follower also works and has the advantage of not needing a base resistor (the base current is naturally limited in this configuration).

The disadvantage of npn as emitter follower is that it drops quite a lot of the supply voltage (0.7V or more). As Paul__B says, with red or green displays, which have a low forward voltage (around 2~2.5V), this won't be a problem.

With blue or white displays (rare) the forward voltage is higher (up to 3.5V) and they may not work with the emitter followers, as there might not be quite enough remaining voltage from 5V to drive them, even with very low series resistors (must not leave those out!). The pnp would have an advantage here because they will only drop around 0.3V.

If you connect the npn to 9V as Paul__B suggested, you won't have any more voltage than if you connected to 5V, so still can't run blue/white displays. (For that you would have to use PNPs as high-side switches, with NPNs to switch the PNPs on.) But at least you could run your displays directly from the 9V supply, avoiding that load on the Arduino's regulator.

Previous paragraph heavily edited following advice and corrections from Paul__B!

Paul

PaulRB:
If you connect the NPN to 9V as Paul__B suggested, you will still have plenty of volts left, so no problem even with blue/white displays. But don't forget to recalculate your series resistors for 9V rather than 5V!

May I suggest you re-think your advice here?

red913:
So I’ve used MOSFET’s before but it looks like basic BJT transistors are a lot cheaper. So I would like to figure out how to select one of them. My basic project is a 4 digit 7 segment display, common anode. So each of the 4 digits get’s it’s own supply and all the individual segments get grounded, through a 220ohm resistor, to turn on. I checked with my DMM and each of the single digits can pull close to 60mA. Since I’ll be multiplexing they’ll need to turn on and off at least 50 times a second. So where do I go from here in selecting a transistor? What other information do I need to do this? I’m thinking I’ll be using a 9V battery to power this project with a 5V regulator. So the transistor will probably switch the same amount of current it will be feeding,+5V.

No problem. Pretty much any BJT will do all of that (and much more).

Paul__B:

PaulRB:
If you connect the NPN to 9V as Paul__B suggested, you will still have plenty of volts left, so no problem even with blue/white displays. But don't forget to recalculate your series resistors for 9V rather than 5V!

May I suggest you re-think your advice here?

Unreserved apologies for any confusion I have caused! I will edit my post and fix before any more confusion happens. You will have to give me a little clue. I have re-read your message any my own but I'm not seeing it. (If you hear some strange noises after, that will probably be me kicking myself)

An emitter follower - using a bipolar transistor, in this case NPN - pulls the emitter up to Vbe (about 0.7V) less than the base. The collector can if necessary, be pulled down to Vcesat less than 0.7V to effect this or more practically, you could pull the emitter up very close to the Vcc by pulling the base up above Vcc. Of course, you are rarely going to do that (except on the high side of a Totem Pole power converter switch as you must do using FETs).

The point is that on a 5V supply, presuming the chip output will pull up to nearly 5V, the emitter can be pulled up to nearly 4.3V - less actually as the chip output - when driving current - can not pull up to nearly 5V. Since it is in fact, an emitter follower, it makes almost no difference whether the collector is supplied from 5V or 9V.

So you do not re-adjust the current control resistors - which would be on the cathode side of the LEDs - for a 9V collector supply.

The reason why it is such a good idea to power the collectors from 9V is that you use the individual transistors to buffer the current rather than the 5V regulator.

Schematics ALLWAYS says more than 1000 words

image.jpg

Paul__B:
The point is that on a 5V supply, presuming the chip output will pull up to nearly 5V, the emitter can be pulled up to nearly 4.3V - less actually as the chip output - when driving current - can not pull up to nearly 5V. Since it is in fact, an emitter follower, it makes almost no difference whether the collector is supplied from 5V or 9V.

I actually did no know that, although it made sense as soon as I read it. Many thanks Paul__B. I have amended the offending paragraph - please review it again for me?

Pelleplutt:
Schematics ALLWAYS says more than 1000 words

Only if you can read the schematic...

And.... sadly, the schematic is sort of wrong. You do not put the current limiting resistors on the common display pins, you place the resistors on the individual display 'segment" pins. If you don't do this, when you have only 2 segments lit to display a "1", each led gets gets 1/2 the current from R and if you display an "8", all segments... each only gets an 8th of the current. So, with 330 ohms, and displaying a "1" each led gets ~5mA.. With an '8' on the LED each gets ~1mA.

And then you will be coming here and saying... why are my LED's so dim?

A UDN2981 might be suitable as a high-side driver?

It's like a ULN2803, only it drives the high side.

PaulRB:
Many thanks Paul__B. I have amended the offending paragraph - please review it again for me?

Looks fine now!

Now, Pelleplutt has got it ass-backwards! Did he draw that bad diagram, or did he get it from somewhere?

It looks like an attempt to have the resistors dissipate some of the power rather than the transistors, however it would badly fail to distribute the current to the segments properly and if the transistors saturate, it will pull down the driving pins on the chip and tend to overload it.

Bad, bad, bad. :astonished:

Yes, I got it "somewhere", Google is your friend.
Do you know what driver it is on the low side, perhaps with current limit?
I can not identify the part number.

On the highside what voltage as input?

I know it was an bad schematic, but I did not have time to do a better.

Pelle

This is not anything from the net.
This need an software sign generator so you can make your own signs.

Pelle

Sju segment.jpg

I hope the OP has gone away happy with his answer, because this thread it getting a bit silly now! 2 ICs and four exotic surface-mount-only FETs just to drive a cheap red 4 digit display…

He may well have had 12 Arduino lines available and just wanted 4 bog-standard NPNs.

BTW, did anyone else notice that the OP’s question was 10 sentences long, of which 5 began with the word “so”? (am I turning into Sheldon Cooper?)

PaulRB:
I hope the OP has gone away happy with his answer, because this thread it getting a bit silly now! 2 ICs and four exotic surface-mount-only FETs just to drive a cheap red 4 digit display...

Did I not introduce the only sensible fork of this quite promptly when I pointed out "way back then" that a MAX7219 will perform the job perfectly? Least components and board space, least code/ multiplexing dealt with, least pins used. All other versions are merely exercises in head-banging "for the academic exercise".

PaulRB:
He may well have had 12 Arduino lines available and just wanted 4 bog-standard NPNs.

But that is limited in the available drive current anyway.

PaulRB:
BTW, did anyone else notice that the OP's question was 10 sentences long, of which 5 began with the word "so"? (am I turning into Sheldon Cooper?)

You mean you are not Sheldon Cooper already? :astonished:

{Like most of us.}