I’m pretty new to the whole electronics thing, I’m trying to figure out how to decide between a BJT and a mosfet for my output. I’m turning on a relay and have found circuits for both, but can’t quite figure out why I’d choose one or the other.
I’m curious to hear if anyone has a good answer for this too, I had the same question working on my sprinkler system and did a tonne of research and still don’t have a good answer. I think it would be helpful to know what the specs of the relay are and what type of load you’ll have on the relay.
So both obviously work, since you’re seeing circuits for both, Sam.
The thing that I usually tell people is that the current requirements are going to be the key piece:
- BJT - If you’re driving from another part of a circuit that has sufficient current, then a BJT will work fine. If not, you won’t be able to drive the element “hard enough” to turn it on.
- MOSFET - Allows for a lower current control, so this is what I normally suggest. There are some downsides as well, but the low current requirements for turning on a MOSFET as a switch is not one of them.
In general, I tell beginners to start with MOSFETs. This is because most beginners are starting with a microcontroller of some sort (ie. an ATMEGA on an Arduino) which has a current limit on the pins of about 20 mA. This is more than sufficient to drive a MOSFET, but likely will not be able to drive a BJT hard enough to turn on a relay (without using 2 to set up a darlington type circuit).
MOSFETs are super cheap and come in a variety of packages. I recommend keeping something like 2n7002’s around your bench for when you need to turn something on (datasheet)
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Wait, if the current doesn’t matter like you said, why does it state maximum drain currents for that part?
As far as I know about mosfets, the current Chris was talking about going into the gate. The current shown in that datasheet is about current flowing through the part (drain to source). I’m still learning about fets though.
I used to use BJTs everywhere. But Chris’ “Current Sink or Swim” project in CE v2.0 introduced me to mosfets and I’ve never looked back! (Also available as part of the hew Contextual Electronics V3.0.) I now think of BJTs sort of as current amplifiers - in that they need current at their input to do their thing. MOSFETs are controlled by the input voltage with almost no current being drained from whatever is controlling the MOSFET. Good for switching powerful things on and off (especially with an arduino or teensy), and a lot of other things.
This is by far the best way to think about them, but I think most people have trouble viewing circuits from the role of current at first. When you can make the switch, it really helps! We will work on that in future courses.
I dont’ understand how they are current amplifiers? do you mean that they actually increase power if you put current through them?
Hi tnichols,
Consider the most commonly used BJT, an NPN transistor:
When you make current flow into the base (b), it comes out of the emitter (e) and, as a result of semiconductor physics, the transistor will try to pull current from the collector (c). The current pulled into the collector will also come out the emitter.
The collector current is X times the base current, where X depends on the internal construction of the transistor and other factors, such as temperature. This effect is what is meant by “a current amplifier”. It is the power supply that will actually deliver the collector current, not the BJT itself – it’s not a battery 
The datasheet of the transistor will give an indication of how big X is. You can expect values from around 10 to 800 upwards.
Regards,
Niels.
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