Looking for a suitable transistor

[SLP]

All the time since I’ve ever touched a soldering iron until now I’ve always been building things where a complete schematic was already available.

And now as I need to do something *myself*, my knowledge of electronics has pretty fast reached its end.

Here’s my problem:

I’ve got a simple circuit with a NE555 (timer/clock IC) and a CMOS4022 IC (8step counter). Everything is connected to a 12V battery. Now I’ve got 8 LED arrays that run with 12V and draw 150mA each. I want to connect each LED array to one output of my 4022.

And that’s the point where I need a transistor.

I know of course what a transistor is and what it does, but it’s not that there is just one type of transistor available. There are thousands of types.

I know that I need a NPN one, but I gave up searching for the right one when I came to those characteristic curves and to several I’s and U’s. Including the I_CEO. (what’s a CEO got to do with transistors? ok - that was a joke)

So,.. Does anybody know how to proceed in such cases?

Or, can anyone suggest me a transistor that works in my case?

thank you very much,

regards,

matthias

[stryd_one]

Heh, I had this same situation tonight! Coincidence…

What I did, was googled around to find a similar circuit to what I was making, and then used a similar part… There’s definitely a large amount of voodoo around this subject, I hope someone can enlighten us both!

[SLP]

coincidence?

I’d call it “soulmates”  ;D

[bugfight]

use the uln2803 8 darlingtons in 1 handy pkg…

[stryd_one]

Thx bugfight..

FWIW I used a BS250 to make a (pulled-down) NOT gate. I had a schem for a NOT gate using a BS170, which is NMOS, but I needed mine to pull down,not up… I found this schem:

and managed to find that the BS250 is the equivalent in PMOS through google bruteforce. Not the most efficient thing ever done

So, SLP and I have our immediate issues solved, but I’m still curious about the general “how do you pick a transistor” subject:

I know of course what a transistor is and what it does, but it’s not that there is just one type of transistor available. There are thousands of types

…

So,.. Does anybody know how to proceed in such cases?

…yaknow… aside from creative plagiarism/googleducation

[SLP]

Thank you very much, jimp bugfight!

Is is correct that I have to connect Pin 10 to V_DD (V_BAT)?

[stryd_one]

who the f*** is jimp?

Yeh, 10 <– Vbat

[bugfight]

oops, i overlooked that you are running at 12v.

you want the uln2804.

or you can add ~8k input resistors to the uln2803, which is designed for 5v inputs.

this would be a pain, but if you have the chip in hand already…

iirc pin 10 is for inductive loads, but it won’t hurt to go ahead and

connect it to vdd

[SLP]

oops, i overlooked that you are running at 12v.

no problem,.. I read tha datasheet, so I already got that point

the ULN2804A has a 10.5kOhm input resistor for 6-15V CMOS

iirc pin 10 is for inductive loads, but it won’t hurt to go ahead and

connect it to vdd

what do you mean by that?

[bugfight]

what do you mean by that?

inductive loads such as relays can kick back a high voltage spike when they stop.

the common pin shunts those spikes back to the supply line…

[SLP]

inductive loads such as relays can kick back a high voltage spike when they stop.

the common pin shunts those spikes back to the supply line…

Alright…I was just wondering where to connect V_IN, and pin 10 was the only one left…

[SLP]

So, does nobody know how to find the right transistor that fits your needs?

The Transistor is after the Resistor and the Capacitor the most basic electronic component.

Isn’t there someone who can explain how to act in such cases?

[stryd_one]

Well, we’re boned!

[_tilted]

Well it all depends on your needs… ;D

Unfortunately with transistors, as opposed to resistors or capacitors, the needs are more numerous and more precise.

Situation: I need a resistor for current limiting for an LED.

Easy.  (V_supply - V_led)/I_led = R_limiting

Where V_supply is obvious, V_led is the forward voltage drop of your LED, I_led is the desired operating current and R_limitingis the resistance needed (pick the closest value).

Then you just double check your power dissipation as P_resistor = I_led²xR_limiting

Simple, right? Well, at least there are only two variables to determine.

Capacitors are slightly more involved, as their applications vary, as do the types and values available.

Transistors are yet more varied, and come not only in a variety of devices and subsets of devices (BJT(NPN and PNP), Darlington BJT pairs (NPN and PNP again), FETs (NMOS and PMOS MOSFETs, IGBTs, JFETs,), UJTs, SCRs, TRIACs…

Then you need to know the current gain (HFE) required, the amount of power which may need to be dissipated…

Alas, there is no simple formula which will spit out a model number for the transistor you need. Truth told, there is no such formula for resistors or capacitors, but when was the last time you ordered one of those by model number?

[stryd_one]

That wasn’t an answer, that was just a more wordy description of the problem

Well, we’re boned!

Is it bad form to quote yourself? ;D

[SLP]

Is it bad form to quote yourself? ;D

Don’t worry, I’ll do that for you:

Well, we’re boned!

;D

[_tilted]

If it’s a straight forward answer you’re looking for, I believe nILS used to have it as his tag line:

Go to University.

;D

[stryd_one]

4 years of learning stuff I don’t need to know, in a manner I don’t agree with, that doesn’t work effectively, fed by people I don’t like, for an exorbitant cost I and most others can’t afford, so I can spend the rest of my life paying off a student loan…

Straightforward? You’ve spent too much time in brainwashing camp school.

[_tilted]

Straightforward? You’ve spent too much time in brainwashing camp school.

Sorry, I got the syntax wrong there. That was more of a straight, forward answer than a straightforward one. 

I guess what I’m getting at is that there is, as has been said, quite a bit of voodoo to be found in this area.

If you know how much current you can have, and how much current you want, and the polarity of the signals involved, it is not too awful.

Hfe =I_collector / I_base

Hfe (also called beta) is the forward current gain of the device. Literally, the current you get, divided by the current you apply to ‘switch on’. This is the number one determining factor in selecting a transistor.

The power dissipation is a little trickier.

You’ll need to know the C-E Voltage drop, which varies from one device to the next. You multiply this by the absolute value of current in/out of the collector pin.

P_junction = V_ce x I_c

This gives you the power dissipation of the junction in watts. Such a rating usually includes a temperature the rating was taken at, or a maximum junction temperature. If you are going to dissipate a lot of power, you need to sink the heat to avoid destroying the device.

You’ll also want to know the maximum reverse bias voltage, particularly for switching applications. This is generally the maximum CE voltage which is tolerated with the device in the ‘off’ state, before breakdown occurs and the device shorts or arcs over. You’ll also get ratings for the voltages CB and EB, to represent how high a voltage can be tolerated before the device sends voltage from your switching side (usually a higher voltage, higher current area of the circuit) back into the control circuit, be it a PIC, some discrete logic, etc… to fill this field, you need to find a trade-off between safe isolation and the ridiculous. Generally speaking, the higher the power dissipation and reverse bias voltage ratings, the bigger the device tends to be physically.

You could think of an analogy where you have a large metallic toggle switch, to turn on a mains powered circuit. You are going to operate this switch by hand, so you want the largest resistance possible between the switching element and your own fingertip. On the other side of the equation is Jones from accounting, who doesn’t see the need to install a 40kV contactor as the 240v power switch on the panel.

There are a handful of other terms here like saturation voltage and maximum collector current, which refer to the transistor at it’s maximum ‘on’ state, or saturation. The saturation voltage is the voltage drop accross the junction while in this state, and the collector current is the maximum allowable current through the collector while in this state.

Am I helping yet? ;D

[stryd_one]

Niiiice antivoodoo Thanks dude!!!