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I noticed that many sites targeted at hobbyists recommend bipolar transistors (mostly NPN) to switch things on and off. Bipolar transistors definitely have their purposes especially in analog circuits, but if you want to switch a relay for example, a MOSFET is a lot easier to work with a lot less downsides.

They can easily switch many amps of current without requiring current themselves. They (almost) don't drop voltage across source and drain which means they won't get hot even at high currents and they are as cheap as bipolar transistors.

Are there any pros to using bipolar transistors in switching applications I am overseeing?

UPDATE:

  • The sites I was talking about where Adafruit and similar sites.
  • I never really looked into THT parts since I only work with SMD parts at work. I just checked Digi-Key and although they have very good and cheap SMD FETs their selection of THT FETs is very limited much more expensive and with higher drive voltages... I suppose THT FETs are commercially only used to drive really high currents.
  • Summarized you could say that since hobbyists are normally limited to THT, suitable BJTs are more easily accessible and a little less sensitive to ESD while tinkering.
  • Basically I just wanted to know if I was missing something and should use BJTs over FETs at work in some cases.
Karsten
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    How old are these "many sites". If they are old enough, logic level MOSFETs may not have been common and so the only choice if you wanted to use an MCU to drive a transistor was a BJT which requires only 0.7V. BJTs are also (or at least were cheaper) I am told. Before my time. – DKNguyen Aug 01 '20 at 17:35
  • Please link to one of those sites. – Andy aka Aug 01 '20 at 17:52
  • According to this article comparing transistor types https://www.allaboutcircuits.com/technical-articles/fet-vs-bjt-vs-igbt-whats-the-right-choice-for-your-power-stage-design/ the BJT is suitable for higher voltage applications compared to the FET. – SystemTheory Aug 01 '20 at 18:25
  • How many amps do you want to put through the relay coil? When driving a relay, total power dissipated (by the relay coil and switcher) is nearly same whether one uses a mosfet or a bjt. At 50 mA, a bjt dissipates about 35 mW which is not a big deal. A cheap bjt easily operates from below 3V to above 30V. This is often also an advantage when driving power mosfets. – Indraneel Aug 01 '20 at 18:44
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    Karsten, There are some reasons, such as availability and cost. For example, I am just a hobbyist, not a professional, and I buy PN2222A's at occasional rock bottom prices of 0.34 *cents* each in 2000 qtys. (About $7.) MOSFETs never are within an order of magnitute. And BJTs are appropriate at certain currents and certain voltages. – jonk Aug 01 '20 at 18:47
  • Karsten, But for an example where MOSFETs would be very nearly impossible to use, I've written about one such discrete BJT switching application [here](https://electronics.stackexchange.com/a/514299/38098) just recently. (These two BJTs are used as switches, not analog amplifiers.) I'd very much love to see how you'd achieve this with discrete MOSFETs. Give it a whirl. Tell me if you think there's any chance at all achieving this with MOSFETs without insanely complex surrounding circuitry. (MOSFETs also have unique situations that BJTs can't do. Low leakage RC timing, for example.) – jonk Aug 01 '20 at 18:54
  • @jonk Yeah, that's true. You can drive BJTs from almost ground-referenced voltage source as long as you pick the right resistor. – DKNguyen Aug 01 '20 at 19:03
  • @DKNguyen In that case, they are providing a well-protected voltage rail for an externally connected low-powered adapter. It's self-limiting and can be shorted accidentally with a screwdriver with little harm or risk of harm. It also provides a similar well-protected internal voltage rail that is nearly identical in value with the external one but for the board itself, where the limit is allowably higher. It does all this with just two BJTs and one resistor. Even considering this with discrete MOSFETs would be a nightmare. – jonk Aug 01 '20 at 19:09
  • Because they're bipolar -- it's just in their nature to switch back and forth. – Hot Licks Aug 02 '20 at 15:33

4 Answers4

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1: Bipolars are cheaper So you can do larger experiments with the same budget.

2: Bipolars don't die an instant death from ESD So there's less risk of the user giving up on electronics because "nothing works as described".

3: bipolars are good enough for many tasks. V_ce(sat) < 0.1V is typical

4: most "logic" MOSFETs need inconvenient drive voltages getting 5V drive from a raspberry pi is going to take an extra MOSFET.

5: The MOSFETS that do exist for 3.3V operation are terrible compared to BJTs they cost ten times as much and aren't half as good.

Jasen Слава Україні
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In general, FETs require much higher drive voltages than BJTs. Let's say you have a circuit running on 5 volts, and you want to switch a relay. If you get a 5-volt relay, you can use a BJT and everything will work off of 5 volts on the control side. The base voltage of a BJT is only about 1 volt, so you have about 4 volts to work with in developing the base drive. Granted, you may have to provide a lot of base current (since BJTs typically want to be switched at a base/collector current ratio of about 1:10).

If you're not using logic-level FETs, you need to provide something like 10 to 15 volts on the gate to guarantee the FET is switched fully on. Doing this with a 5 volt supply (or even a 9-volt battery) is not something that's going to work out well.

So, beginner circuits will typically operate from a single, relatively low supply voltage - and that gives an enormous advantage to using bipolars.

WhatRoughBeast
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  • +1, but "you may have to provide a lot of base current" is unlikely to matter in most cases. If you're switching a relay in the first place, that is definitely going to dominate the current consumption of that part of the circuit. And driving that from a MCU or something only takes a trivial Darlington pair. – TooTea Aug 02 '20 at 14:32
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IMO the biggest issue here is that manufacturers keep making the same throughole components they have always made but they tend not to bother packaging up their cool new components in through hole.

In particular as best I can tell you simply can't get a decent 3.3V switching mosfet in a small through-hole package or at least if you can it's not easy to find it with supplier parametrics.

I went to mouser and set up a search for mosfets with less than half an ohm of RDSon. Unfortunately the parametrics don't tell you what voltage RDSon was measured at. So as a proxy to rule out parts designed for higher voltage I selected athreshold voltage of 1.5 volts or less. I then further restricted it to SOT23 packages. I got about a hundred results. Unfortunately the parametrics don't tell you want voltage the RDSon was measured at but perusing a few datasheets found several parts priced at £0.277 each with an acceptablly low RDSon at 3.3V

In comparison when I selected to-92 I got a grand total of 2 results. Neither of them specified a RDSon value for voltages less than 4.5V.

(unfortunately stackexchange seems to be mangling the links above, they work in the editing preview but not in the posted post).

Peter Green
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  • Links work fine here (using Firefox 52.9.0) – Bruce Abbott Dec 07 '21 at 05:29
  • It seems to be being weirdly unreliable, sometimes the link works correctly, sometimes it drops some of the parametrics I selected. – Peter Green Dec 07 '21 at 05:41
  • Perhaps your browser is truncating the links. Your first link has 504 characters in it, and the second one has 465 characters. – Bruce Abbott Dec 07 '21 at 05:54
  • It's weird, the links work reliablly if I hit the edit button and follow the links in the preview, but not otherwise. And yes the links are very long, it seems that mouser translates a "less than or equal to" parametric search to a list of values in the URL,I tried to use a URL shorterner but stackexchange blocked me from doing so. – Peter Green Dec 07 '21 at 05:59
1

Bipolar transistors definitely have their purposes especially in analog circuits, but if you want to switch a relay for example, a MOSFET is a lot easier to work with a lot less downsides.

I'm not sure that's really true. The current is generally low enough that a generic bipolar transistor will work fine. You need a Base resistor, but a FET may also need a resistor to pull the Gate to ground when the input is floating (eg. when an MCU is starting up or a signal cable is disconnected). Bipolars tend to be more robust in the presence of high voltage spikes (like when you forgot to put a diode across the relay coil), and the required drive voltage and maximum output current can be (roughly) set via the Base resistor. Bipolars work on a wide range of voltages starting at less than 1 V and going up to 40 V or more, whereas low turn-on voltage FETs are usually limited to 20 V or less and still need 2.5 V Gate drive for reliable operation.

Another reason a hobbyist might prefer bipolars is that they are more versatile. Most MOSFETs can only be used as switches, whereas bipolars can also be used as amplifiers etc. This applies to commercial products as well. If you have a design that needs some bipolars then it may be cheaper and/or more convenient to use them for everything - due to lower unit price in larger quantities, lower handling and storage costs etc., and a greater number of substitutes which makes supply less of a concern (an issue that is getting a lot more attention these days...).

~45 years ago I bought a few cheap packs of 100 bipolar transistors. Some have been recycled several times, and they are still just as useful today as they were in 1976.

Bruce Abbott
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  • I agree that bipolars are more versatile. Maybe it's just that I never need any of that versatility and only need simple switches (that actually really switches to 0V and not 0.7V or above). I've come to the conclusion that it's probably because THT MOSFETs are much rarer than I thought. I didn't realize this, because I only use SMD parts in my designs anyway. – Karsten Dec 08 '21 at 17:12