Hello all
I have a 5v Nano. I want to switch various 'things' and I usually use N-Channel transistors.
Namely, TIP121 and TIP122.
These seem to work pretty well and are cheap as chips.
But, I keep reading that I should be using Mosfets. I always have issues here. Never yet found one that seems to switch as well (and isn't like £6). I know they need to be logic level.
Is it that tragic that I stick with the transistors?
Please post the schematic of anything you ever did with a logic level MOSFET that you think performed worse than the schematic you post if the same thing done with an NPN transistor.
Specify the criteria by which one switches "better" than the other.
a7
The downside if those TIP transistors, when used as a switch, is that they drop about 1volt.
That's almost 10% is you switch a 12volt load, and a loss of 20% for a 5volt load.
Current is also a problem. 1 Watt of heat is generated inside the transistor for every Amp.
Mosfets, when used properly, can reduce those losses significantly.
Leo..
No -
but, when you're ever here about something "not working", make the TIPxxx usage clear from the beginning.
Thing is that these are darlington device,s sometimes called a super alpha pair, which are two transistors connected together.
While the gain - input current to output - current is high the Vbe - voltage between the base and emitter when on is double what a single transistor would normally give.
Here is the data sheet to see for yourself.
TIP120.pdf (534.6 KB)
The cheaper FETs are surface mounting and you can solder them between tracks on a strip board:-
A four amp, N channel SOT23 MOSFET costs 4 cents or less each.
It’s an easy decision.
However, if you have the TIPxxx already, use them.
The secret to MOSFETs switching properly is the Vgs (Voltage gate source). The data sheet will tell you what voltage it starts enhancing (turning on) and when it is fully enhanced. Realize the MOSFET knows nothing about your design and references its source. It will conduct via the substrate diode if connected backwards. Also when checking the Vgs of a P channel it is negative in reference to the source where as a N channel will be positive. When you get a chance have some fun with them.
I personally come across only very rare instances where an NPN is more appropriate than a MOSFET. These instances revolve generally around issues related to capacitance and gate charge, or available switching voltage. The latter is a doubtful case for NPN's given the availability of MOSFETs with very low full-on Vgs, but sometimes a common 2N2222 does outperform an equally common 2N7002 in certain applications. The capacitance/charge issue is more relevant and can be an issue especially in HF applications, or situations where e.g. release of gate charge messes up something else. To illustrate the latter: I was working on an integrating transimpedance amplifier once that really did perform a lot better if I used a small HF BJT instead of a FET or CMOS switch, since the charge injection of those created problems in relation to the very weak signals that were being measured.
Long story short, for most common jobs, MOSFETs are superior. Of course, BJT's still work. Then again, so do vacuum tubes....
No, why would it be.
I see you had a related topic on choosing the right MOSFET.
Did you buy them and test them as you said you would?
BTW: A MOSFET is a transistor.
In certain applications with low voltages (say 3v3), you know you can safely use more or less any small signal BJT device because these all switch at 0.6v. but would have to exercise more care in choosing a suitable mosfet. For example, the low side of a high side switch where a 2n3904 will always work, however, care is required in the selection of a suitable mosfet for the same task because these often require a higher voltage to switch and may have a relatively low voltage tolerance (Vds) and possibly require an additional resistor (gate pull down).
Thanks.
I will have a look later for some options.
I know that a TIP121with a 1k on the base, pulled low with a 10k works fine off a 5v processor.
I need to find an equivalent then in the Mosfet world.
I have a pile of IRLML2502TRPBF mosfets here, but I think they might be a bit small for my needs (4.2A?).
Datasheet time
Didn't you learn anything from your other post
Hard to tell without knowing what those needs are.
Max continuous drain current as you say is 4.2A. TIP121 is 5A. That's not much of a difference. The main difference is that the MOSFET is a tiny SOT23-3 package and will dissipate next to nothing when passing a few amps, while the TIP121 comes in a TO220 and will require a heatsink under the same conditions.
Anyway, easy enough to find some MOSFETs that will pass a mass of current with only a small signal applied to the gate without dissipating lots.
You can't actually run a TIP121 at 5A. Yes it is in the absolute maximum section of the data sheet, but there is no way it can work at that rate in the real world.
This is because there are other ratings in the absolute maximum table that are holding you back. Like those in Pc, and the Tj section of the data sheet.
Look at Figure 5 for the power derating against case temperature.
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