# Choosing a transistor for a low power application

simulate this circuit – Schematic created using CircuitLab

I have looked at a few guides, both on this forum and elsewhere, for choosing a transistor. I know to look for things like HFe and the Collector-emitter voltage, but on the datasheets I am seeing multiple values and don't know which one to look at. In my application I will have a comparator (powerd by 5v) connected to the base of the transistor. The Emitter will be connected to GND and the emitter will be connected to the GND of a 1.5V vibration motor. Based on those voltages, how should I go about looking for a transistor? Also, here is an example of multiple HFE values. For the future, how would I decipher which one to use for calculations?

Edit: I have included a schematic. "U2" is the HC-SR04 Distance Sensor and the Speaker is a 1.5V 50ma vibration motor. That Op-amp and Comparator are the main point of contention. The functionality I am looking for is it to compare the voltage coming from the Distance sensor to some reference voltage i can find later and if it is above that voltage, open up a gate that allows current to flow from the battery to the vibration motor.

• Well, I'm not even sure you will be using the comparator correctly. Please note that some of them already come with an open drain or open collector output. So you might start out by considering one of those. Probably the most important details you left out are the worst case vibration motor current and the minimum allowable operating voltage for the motor. $\beta$ is way down on the list, right now. Do you have a spec sheet for the motor? – jonk Oct 25 '17 at 18:52
• Since you say this is for a low power application, don't use a bipolar transistor, use a MOSFET instead. The latter are voltage driven and do not waste current driving the base. But to answer your question, you always have to design with the worst case value. – Trevor_G Oct 25 '17 at 18:54
• Please put in a schematic so we can double-check stuff. In your written description, you mention emitter twice. I think it is the collector that is connected to the motor. But who knows? Be sure to list the true part number for your comparator. – mkeith Oct 26 '17 at 4:32
• To add a schematic, edit your original question. Then, at the top of the edit area, there is a button that looks like schematic symbol elements. Click on that button to bring up the schematic editor. – mkeith Oct 26 '17 at 4:33
• I just updated my post to include a schematic – Gabe Spound Oct 27 '17 at 13:38

Since you say this is for a low power application, you would be better off ditching the bipolar transistor, and using a MOSFET instead. The latter are voltage driven and do not waste current driving the base.

But to answer your question, you always have to design with worst case values.

Moreover, when presented with minimum and maximum values, it is often prudent to recalculate your equations with both to determine if the circuit will function within your requirements across the full variation of the part.

For example, you need to use the minimum $\beta$ value to calculate the largest base resistor you can use for a desired collector current, but at the same time you may need to use the maximum $\beta$ value to check your collector current is not too large with the resistor you calculated in the first part.

Ultimately, which is important when is dictated by the rest of the circuit, or the application of the part.

As for your design, as Jonk mentioned, most comparators have open-collector outputs anyway, so, with the right comparator, you may not even need a transistor.

• Could you link one that might work for this application? I'm finding ones with 2 reference voltages but none with an open collector. – Gabe Spound Oct 25 '17 at 20:41
• @GabeSpound "Ones with two reference voltages?" Me thinks your idea of what a comparator is may be suspect. Post a link of what you are looking at. (I also cant suggest a part sine you have supplied too little information of what you are doing.. and no schematic.) – Trevor_G Oct 25 '17 at 20:52
• +1 - 100k pull is 300uA, nothing fun ultra low power – Jan Dorniak Oct 26 '17 at 6:57
• I updated my post to include a schematic and an explanation in the "edit" section – Gabe Spound Oct 27 '17 at 13:38