Problem understanding and applying different types of transistors.

Other Hardware General Electronics
1

Ok, so I'm struggling to figure out how to get a motor to turn with the use of transistors.
So far my circuit looks like this : (MotorCircuit1.bmp)

The motors that came with my robot chassis unfortunately didnt come with any specifications. However they do work and seem to be not too bad.

The transistors i'm using are a BC546 NPN transistor. >>> BC546 Datasheet pdf - Transistor Da Finalidade Geral - Korea Electronics (KEC)

I'm using an arduino uno and the output im supplying does work, I've tested it across LEDs and looked at it with an oscilloscope. I'm just using the basic brightness/fade code that they have setup. I'm using the variable voltage purely for testing.

The main thing that happens is when i run the code, i hear a small whirring sound from the motor, but thats about it - however when i try to spin the motor by hand, it sometimes requires less torque, indicating that atleast some voltage is getting through i suppose.
I have tried lowering the resistance into the base to increase the current. Once it gets low enough, i do have success and the motor acts as normal - however the transistor gets incredibly hot within a matter of seconds.

My main theory at this stage is that i have the wrong type of transistor. (when i bought them i foolishly assumed all transistors would work the same and with the same currents and voltages.. I assumed i could just apply some current into the base and the wire then turn theoretically into a closed switch.)

I appreciate that all the transistors have different values for many many things and from testing i can only speculate that right now my transistor is working as normal - (an applied current to the base will allow current across C to E) However - once it does allow that current, the current passing through the transistor (from the battery) is so high that it just fries the transistor straight up.
PS - i think i may have fried a few already and have been replacing them for testing, I only say this to show the question of whether it is broken can be ruled out.

So my main questions to you friendly people are:
If im right in assuming i've got the wrong transistor - what would be a more suitable one
AND
if possible, could someone explain what values to look at in the transistor manuals in order to suit my needs (and why).

any help at this stage would be massively appreciated :smiley:

also: some minor questions which would be nice answered if anybody has the time :slight_smile:

  • can the base pin in a transistor have too much current being applied and screw up the whole thing?
  • if the transistor is saturated, will the collector current always be equal to the emitter current (+- minimal differences) or are there circumstances which it can be really weird?
  • is the reason my transistor was getting hot because of the size of the current into the base / the voltage into base / the voltage across C-E / current across C-E or any combination etc.

again, thanks in advance guys! :smiley:

MotorCircuit1.bmp (736 KB)

2

With that 330? you've probably base current enough , approx 10mA.
Are your connections certain, esp. the transistor?
Note the depiction on the top page of the datasheet --

Motorola BC546B datasheet pdf <<<
There needs a diode across the motor, too.

! > ! >! > Oy!
More important though is that transistor has a max collector current of 100mA. That's probably not enough for any motor, practically. What's the market for such a thing?? Call that "a transistor", do they?

3

You really want a Logic Level, Low Rds, N-Channel MOSFET.
Logic Level - will switch full on with 4.5V signal
Low Rds - only 40-50 milliOhm (mOhm) when on, so all your battery voltage is used by the motor and not heating up the transistor
N-channel - for Low Side switching.
Searching at Digikey:
N-channel MOSFET >> Instock >> FETS-Single >> Through Hole >>
Sort by price, browse past the 1000-lot pricing, keep going to the Low Rds parts, will find this part on page 2:

4

Runaway Pancake - ahh, see i didnt realise transistors had a max collector current >.< haha.. As in the datasheet - will it always be refered to as 'Collector Current — Continuous'?
----also, when you say there needs a diode across the motor, any specific type of diode? or is a diode just a diode lol.

CrossRoads - when you say Low Rd's - what exactly does that mean? obviously a resistance, but a resistance of what? and where in a manual/booklet thing will that value be?

Thanks for your help guys - and apologies for the noobyness hah, my transistor knowledge is pretty amateur

5

[quote author=Runaway Pancake link=topic=159846.msg1196459#msg1196459 date=1365684353]
More important though is that transistor has a max collector current of 100mA. ... What's the market for such a thing?? [/quote]
OT: Perhaps they were used to drive MOSFETs from logic level devices, before IGBTs became commerically available.

6

Not "Low Rd's", but "Low Rds". Rds is the drain-source resistance. The power dissipated by the MOSFET will determined by the current flow and its internal resistance.

Power = Current * Voltage
P = IV, or, using Ohm's Law V=IR, and I=V/R, and doing mathematical substitution: P=IIR, or P=VV/R

So if you had a motor that needed 1 amp, and the Rds was 0.033 ohm, then the power dissipated by the MOSFET would be
P=IIR = 11*.033 = 33mWatts.
You could also calculate the voltate drop across the MOSFET, V=IR, 1*.033 = 0.033V.

For an NPN transistor, the voltage drop from Collector-Emitter is typically 0.3 to 0.7V, so with 1 amp of current:
P=IV = 1*0.7 = 700mW. Lot more!
The voltage loss might keep the motor from turning, or the transistor might heat up to the point of failure.

7

mick1r:
Runaway Pancake - ahh, see i didnt realise transistors had a max collector current >.< haha.. As in the datasheet - will it always be refered to as 'Collector Current — Continuous'?

That or **IC**max, Yes.

mick1r:
----also, when you say there needs a diode across the motor, any specific type of diode? or is a diode just a diode

You don't have to lose any sleep there, any 1N4000-type diode ought to be adequate. Some use Schottky diodes, for their speed, others go with a 1N4148.

What sort of motor are you working with?