The picture above is my circuit schematic. I have been trying to power a water pump and use a BJ transistor as a switch instead of a relais. The max values for the pump are Imax=200mA and Umax = 5V. When I put the circuit together with my breadboard, my laboratory power supply says that im only drawing 1 mA, which is too little to drive the water pump. It does get enough current when I increase the load supply voltage. It is a fairly simple circuit so I am kinda embarrassed to ask this: Why does my pump not get enough current without increasing the load supply voltage? Should i get a different transistor with a bigger gain?
From what to what?
from 5V to ~20-25V
The BC546A is totally inadequate for motor currents -- the absolute maximum current is 100 mA.
Use something like a 2N2222 (800 mA) or a logic level MOSFET instead, like this one: https://www.sparkfun.com/products/10213
A 2N2222 or BC337 with 220 ohm base resistor might just do the trick.
Don't forget that kickback diode across the motor if you want your transistor to survive.
Leo..
Welcome, you start with a great schematic which explained what your doing nicely and a good question.
A simple rule I use for a common emitter circuit as you have drawn has a hard gain of 35 and a soft gain of 50 if appropriately driven. Worse case for the pump is 200mA / 50 = 4 mA current into the base of the transistor. The Vbe will be about 0.7V so assume 1V. Using Ohm's law to get 4 ma from the 5V source you need to subtract the Vbe from 5 rounding that leaves you 4 volts. This states your 1K is close enough. This varies with temperature and transistor chosen but it will get you in the ballpark.
My suggestion is to use a avalanche rated MOSFET, in the 10 amp range or so, no problem oversizing and the cost difference will be a few pennies at most. Then you hook it up the same as your transistor but you need to add a 10K resistor from the arduino port pin, not the MOSFET to ground. This guarantees it will remain off until you gain control in your setup routine. You will not need a flyback diode if you make a good choice on the MOSFET, you will always need one with a transistor connected to an inductive load.
Thank you very much for every reply. I will probably get some 2N2222 for my future projects. What are some good MOSFETs to use?
See post #4.
That's the worst module you can get. It has an IRF530, which works poorly with 5volt-logic processors and not at all with 3.3volt processors. Other modules with SMD mosfets are much better.
Leo..
Had no issues with 5 volt logic here.
3 volt I can understand
Did you try with a 5Amp load?
The poor Rds(on) at 5Vgs could quickly overheat the fet.
I wouldn't recommend this fet for more than 1Amp with 5volt logic.
Leo..
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Show us good images of the actual wiring.
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What is Vce when the transistor is ON ?
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As mentioned, reduce Rb to 220R.
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Motors need a kickback diode.