Applied Sensors - Chapter 6 - Pump
MichelB
Posts: 154
Hi, what is the "job" of the 10 ohm resistor between transistor emitter and Vss?
P.S.: if I replace it by a 20 ohm resistor is it dangerous?
Post Edited (MichelB) : 8/26/2009 4:37:14 PM GMT
P.S.: if I replace it by a 20 ohm resistor is it dangerous?
Post Edited (MichelB) : 8/26/2009 4:37:14 PM GMT
Comments
The pump will run slower and with less pumping power. It is a 3 Volt nominal pump, and the combination of the transistor and the resistor reduce the voltage at the pump motor from 5 down to 3 Volts. Try it and see, also with 5 ohms.
-- Tracy
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Tracy Allen
www.emesystems.com
The problem is that the pump that I originally specified for the class (a 3 volt pump from Edmund Scientific) was discontinued by the manufacturer. Despite a search, we were not able to identify a replacement that was both within budget and available. So Parallax left in the chapter, but asked people to find their own pump. And they also asked people to use an external power supply for the pump just in case the pump was more power hungry that the original. The external power supply they suggested was 5 volts capable of at least 300 mA.
Now, the circuit in the text will not be appropriate for many pumps you might find. For example, you might find a 12 volt pump, and that will require a different circuit. The circuit in the text is an emitter follower, and the output will be about 4 volts, and less if a resistor is included. The voltage supply to the collector of the transistor can be 5 or 9 volts, that does not make a difference in the output, which is regulated by the voltage into the base of the transistor, which is 5 V. However if that the supply is say 9V instead of 5V, the transistor has to dissipate more power. Suppose the pump draws 200 mA, then the transistor would dissipate about 1 W (HOT) with a 9V external supply and 0.2 W with a 5 volt external supply.
If you have an external supply of 3 volts to run a 3 volt pump, it is true you will not need the external resistor, but it is also true that the emitter follower circuit is not appropriate. You need to change it to a common emitter circuit. Do you know how to do that? Overall, to support a variety of small DC pumps, the common emitter or common source circuit is more suitable. You should also use a diode in parallel with the pump to catch the switching spikes.
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Tracy Allen
www.emesystems.com