How to pick the right transistor? - Page 1
Forum section: electronics / beginners

author: Fixed_Until_Broken
author_rank: Regular Contributor
timestamp: March 24, 2020, 12:33:53 am
content: Ok so application I am using a PNP transistor as a switch. I've read a little about it today but some of it still inst making exact sense to me. Lets us an example so lets say I have 14 volts DC. my load 1 amp what do I need to look at on the datasheet to decide if a certain transistor is appropriate or not.Edit: I am open to guides links to guides on this I just keep reading them and either it just explains how it works or it goes above my head.2nd edit: after playing with some transistors from a kit I think I understand what I am looking for in a transistor but I am not sure how to pick the correct Base/gate resistor.

author: bob91343
author_rank: Super Contributor
timestamp: March 24, 2020, 04:11:37 am
content: Rather than attempt a detailed analysis, I recommend you drive the transistor with 1/10 of the collector current. So if your load is 1A then drive the transistor with 100 mA.If the drive source is the same 14V, then use 140 Ohms or so from the base. Add about 1k emitter to base to ensure complete cutoff.If the load is reactive, other considerations apply. Likewise if you need extremely fast switching.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 24, 2020, 09:02:07 am
content: Quote from: Fixed_Until_Broken on March 24, 2020, 12:33:53 amPNP (BJT) as a switch? forget it. use PMOSFET (or TrenchFET etc) instead (i suspect you want to build high side switch like i usually did). nowadays BJT only in the domain of linear (not saturated) region. the day of BJT as a switch is long gone, but from time to time people still got into the trap, including me. ymmv.

author: Jwillis
author_rank: Super Contributor
timestamp: March 24, 2020, 12:11:38 pm
content: Depends. Are you using it like a light switch or are you switching the transistor with a frequency to control current. If it's like a light switch then your running it in the linear (DC) region . If your modulating a pulse to control current then things change.Look at a common PNP like the BD140.https://www.mouser.com/datasheet/2/149/BD140-888626.pdfThe maximum collector current (DC) is -1.5A and -3.0A with a pulse.But theirs a catch . If you look at the Typical Performance Characteristics and at Figure 4. Safe Operating Area(SOA) you will see that at around 14V it begins to fall outside the SOA of the DC range at 1A. If you pulse it 1ms then it's fine but 100us is better.So for 1A operation the BD140 would not be suitable as a linear Switch at 14V.So lets look at something that can handle a little more power like the BD434 .https://www.onsemi.com/pub/Collateral/BD438-D.pdfMaximum Collector Current (DC) is - 4A And if you look at the Typical Characteristics Figure 5. Safe Operating Area You can see the at 14V , 1A of current falls inside the SOA .So look for a transistor that has a maximum voltage rating that is higher than your application and compare the amount of current you need to the SOA at the operating voltage. Just because it's advertised with a certain current rating does not mean it will have that rating at the voltage you require.You need not use the specific transistors I described here but I hope this helped to find your specific needs.One more thing. BJTs are prone to thermal run away regardless if they are NPN or PNP. So suitable resistor biasing at the collector or emitter may be required with adequate heat sinking ..