# MOSFET shopping, recomended devices for the experimenter?

#### Hamlet

Joined Jun 10, 2015
398
I need a selection of MOSFETS for my kit. I think I understand
the difference between P & N channel, volts, amps, logic level,
but what I don't get is when the device is rated for ohms...

http://www.taydaelectronics.com/t-transistors/fets-mosfets.html

There seems to be a wide range in ohm ratings, RDSON? From
high, say 8ohms, to .8 ohms, & very low 9Meg? I don't know enough

I'd appreciate any & all recommendations for individual
devices that I should include in my kit. I'm experimenting,
and learn best with an iron & breadboard. I expect two or
three each of perhaps 12 different popular mosfet devices should
cover 90% of the projects I anticipate building, but that's
just a guess.

#### dl324

Joined Mar 30, 2015
12,462
There seems to be a wide range in ohm ratings, RDSON? From
high, say 8ohms, to .8 ohms, & very low 9Meg? I don't know enough
It depends on your application. For power switching applications, low on resistance is desirable. For most other applications you want resistance because that's how you control current.
I expect two or
three each of perhaps 12 different popular mosfet devices should
cover 90% of the projects I anticipate building, but that's
just a guess.
What you stock depends on what you need. Because of the relative cost between MOSFETs and BJTs, I have 1 NMOSFET and 1 PMOSFET on hand, and a dozen or two different BJTs.

#### bertus

Joined Apr 5, 2008
21,156
Hello,

Have a look at the

Bertus

#### dl324

Joined Mar 30, 2015
12,462
I got tired of making $15 orders to digikey every time I need a$1 part, so I picked up one of these:
That kit has 1 N channel MOSFET (qty 6 2N7000) and no P channel.

#### MrSoftware

Joined Oct 29, 2013
1,974
That kit has 1 N channel MOSFET (qty 6 2N7000) and no P channel.
Great point, I didn't realize that. It is heavy on the BJT's isn't it...

Is there a reason that BJT's might be used more often than MOSFETS in lower power electronics?

#### bertus

Joined Apr 5, 2008
21,156
Hello,

Here I have a zip file with 2 excel sheets.
One for N mosfets and one for P mosfets, all from IRF.

Bertus

#### Attachments

• 63.8 KB Views: 3

#### dl324

Joined Mar 30, 2015
12,462
Is there a reason that BJT's might be used more often than MOSFETS in lower power electronics?
Cost comes to mind. Many GP BJTs are a nickel apiece. The only MOSFET that comes close is 2N7000. I haven't found any P channel MOSFETs at that price point.

I'm careful when I handle MOSFETs, but I've damaged several 2N7000 in the last few weeks. They worked fine and then started exhibiting largish leakage currents. Can't remember the last time I had a damaged BJT...

The main advantage for MOSFETs is negligible bias current, but the largish and not tightly controlled threshold voltages are often problematic; particularly for low voltage circuits. Just my opinion...

#### crutschow

Joined Mar 14, 2008
26,388
The ON resistance of MOSFETs is important in switching applications where, ideally you want zero resistance when ON (some MOSFETs have ON resistance in the low milliohm region).
Not only does that deliver more voltage to the load but it minimizes dissipation in the MOSFET which might otherwise require a heatsink at high currents.

One MOSFET variable to note is the minimum gate-source voltage to fully turn the device ON.
Standard MOSFETs generally require a Vgs of 10V to be fully ON.
Logic-level type MOSFETs will fully turn on at logic level signals (3 to 5V Vgs depending upon the particular MOSFET).
Note that this is NOT the Vgs(th) (threshold voltage) which is typically a few volts, it is the Vgs used where the Rds(on) value is specified in the data sheet.

MOSFETs are generally preferred in power switching applications since they take no steady gate current to be ON (but they do have a significant gate capacitance that must be charged/discharged) and they can have a much lower ON resistance than a BJT.

The rule of thumb is that BJTs need a base current about 1/10th of the collector current to guarantee them to be fully turn on (saturate) which is independent of the transistor Beta gain (as shown in their data sheets), and that can waste a fair amount of power and require a large base driver at high collector currents.

BJTs are useful in many lower power switching and amplification applications since they have a low input ON voltage of only about 0.7V and are relatively cheap.

Just by looking at the application requirements you can usually see whether a MOSFET or a BJT is likely the better device to use.
Often you will see a mixture of devices in a circuit, where the BJTs handle the low level signals and MOSFETs handle the higher level/output signals.

#### hp1729

Joined Nov 23, 2015
2,304
I need a selection of MOSFETS for my kit. I think I understand
the difference between P & N channel, volts, amps, logic level,
but what I don't get is when the device is rated for ohms...

http://www.taydaelectronics.com/t-transistors/fets-mosfets.html

There seems to be a wide range in ohm ratings, RDSON? From
high, say 8ohms, to .8 ohms, & very low 9Meg? I don't know enough

I'd appreciate any & all recommendations for individual
devices that I should include in my kit. I'm experimenting,
and learn best with an iron & breadboard. I expect two or
three each of perhaps 12 different popular mosfet devices should
cover 90% of the projects I anticipate building, but that's
just a guess.
TO-92 case? 2N7000 or BS170 for N-MOSFET. VP0106 or BS250 for P-MOSFET. Stock maybe 10 P-MOSFETs and 40 or 50 N-MOSFETs. N- run maybe $0.10, P- may run$0.70. I use a lot more N-.

TO-220? Buy as needed. Too pricey to stock stuff that may go obsolete before you can put it to use. It seems like newer and better devices come out every year.

Re: RDSON
Source to Drain resistance. Lower resistance means less voltage lost across the transistor. You will find higher current and higher wattage devices have lower resistance.

#### Hamlet

Joined Jun 10, 2015
398
I just dropped $40 on parts, and there wasn't a mosfet amongst them. I've been yanking mosfets from old boards, and building a datasheet library for them. I have a small collection, so I can try this one, or that one, and compare. I wish there was an all around go to mosfet, like the TIP series of bjt. I hate having an idea, but waiting on parts, or paying$10 in shipping
for \$2 in parts. I've been goofing with power supplies, so I know I need pots,
0.1ohm power resistors, and TIP35c, and so forth. China is killing the small
parts business, and say what you will, it's the best way for me to get into the hobby.

I loved this, thanks for the tip:
Components Selection Guide