Tutorial: How to design a transistor circuit that controls low-power devices

Transistors are “ok”.
Pwr FETs are better.

I’m a little confused. A FET is a field effect transistor. Most common types of transistors are


Then of course we can talk about materials. For example GaN transistors have superior high voltage performance.

When you say PWR FET do you mean a MOSFET with a LDMOS structure? It’s just a vertical transistor that has better RDSon.

Most likely MOSFET.
Voltage driven switch as opposed to current driven.
N channel - low side switch to gnd.
The term “transistor” is generically applied to many different types

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Sorry I was referring to why you said “transistor” in quotes. A PWR FET is a transistor.

I don’t like that this guy, and other similar online tutorials, show show to engineer the transistor into the active region then just wave their hands at how to keep the transistor in saturation by using a slightly lower resistance. They also don’t speak to power dissipation which is even worse in the active region. Why can’t the guy just use the saturation voltage from the 2n222a datasheet (2v) rather than the PN junction voltage (0.7v)?

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It’s bad engineering for sure. Datasheets aren’t the scary thing most people make them up to me. I’ve worked with app engineers who spent a lot of time making sure the data sheet described the operation of the device well.

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Really bad design. When he placed the cap and diode parallel to the NPN, I was shocked, shocked I tell you…

Please see Boylstad & Nashelsky (any of their 11 editions) for S (stability) in bipolar transistor circuits. They also do chapters in Vacuum Tube and MOSFET design, depending upon the edition and revision.

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wow … over $100 for that book and $150 new … maybe the Electronics Committee might buy it

I had one in college, somehow I lost it. It’s definitive.

I’ll admit in school we mostly focused on MOSFETs so I’m more familiar with MOSFET analysis than bipolar. For most integrated power devices we mostly only use bipolars for the internal bandgap circuit implementation. Otherwise transistor usage is solely MOSFET. I probably should get better with bipolars.

Depending on what you’re trying to control, I use these little boards with escape room props: https://smile.amazon.com/gp/product/B073D399M1/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

while its not strictly required, having it on a little board allows me to add a diode for coil loads and have screw terminals, etc if needed.

If you’re going to move BIG POWER, look into IGBT modules (isolated gate bipolar transitor). Some of these are the big 3 phase hogs we use to drive our 100 hp motors on our wide format presses. I never knew you could move this much power without thyristors…