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The main advantage is that a FET gate is basically insulated from the other pins (source and drain)
So it is easier to isolate the input part of the circuit from the output part.
Ie, you can use a relatively low powered drive circuit to turn it on and off.

A BJT transistor on the other hand requires a much larger current source to switch high current loads. Due to the way a BJT works you actually need some maths to make sure it is being switched on properly. The advantage of a BJT is that they generally have higher current capacity for the same size part. But this is rarely an issue.

There are other advantages of FETs when you use several in parallel.

In the end it doesn't really matter, there are many circuits out there that use both, I generally pick a FET first because it is basically easier to use from a technical point of view.

If you have FETs use them, if you have BJTs use them !




Yep great way to do the PWM, probably easy to find designs on the web.

555 PWM circuit

I haven't used these, just got simple ones off the interweb.
N type MOSFET

This one needs a few mods to make it reliable, like some resistors on the FET gate etc.

PNP transistor

Do we want a circuit on a reversible motor ? with speed control?
What would you do with it?

As I already mentioned on another thread, haven’t been to Electronics_101 forum since Picaxe appeared in au.
Surprised to find its offshoot on BYAP.

There’s lot of online El. Calculators but a nice downloadable is Electronics Assistant V4.2 for those interested.

There is a great android up that does tons of electronics calculations, ill dig out the name if anyone is interested. Not that I have used it yet, it still seems easier to websearch.
But it's "cool"

the picaxe version of the PWM circuit would be ...



and then from manual 2 ...

Syntax:
PWM pin,duty,cycles
- Pin is a variable/constant which specifies the i/o pin to use.
- Duty is a variable/constant (0-255) which specifies analog level.
- Cycles is a variable/constant (0-255) which specifies number of cycles. Each
cycle takes about 5ms at 4MHz clock frequency

so the code might be...

How about some more basic education.

Like using a multi meter.

eg I'd love to know how to test a transistor to see if I fried it or not.

But multi-meter basics might be a nice thread.

And speaking of reversing motors a post on switches

And soldering

And fault finding tips

How many do you want

Just keep it coming (everyone), this stuff is great

To test a diode, put multimeter on diode mode, the meter should get a reading of around .6v one way and 0v the other.
The number it shows is the voltage drop.
A germanium diode will measure a lower voltage drop, around .3v
A transistor is basically two diodes stuck together, so depending whether npn or pnp with one lead on one leg it should read .6 on both of the other legs. Just keep testing till you work it out. Or google the part number and look up the data sheet to get the pinout.
Some transistors will have a diode across base and emittor legs, so sometimes you may get a funny reading, but that is mainly pretty beefy power diodes, not the kind of stuff you are likely to be working on.

But the general rule is if it doesnt measure short across all legs, the transistor is fine. 99.9% of the time a transistor fails by going short, in fact so do diodes and FETs. Resistors fail by going open circuit 99.9% of the time.

A new capacitor will be faulty if bulged or the top has popped open, that is why they have a cross on the top, its a pressure release.
Old capacitors may have electrolytic paste leak out of them, if it happens generally the copper on the board turns black from the acid etching it.
Or they just dont work as well with no visible sign. Capacitors cause over 50% of faults in electronic goods in my experience.
Unless you wired the circuit up yourself, then it is almost guaranteed to be your fault!

I was always taught to remove a transistor or diode before testing it, but that is a load of crap. If it tests well in circuit, it will be ok, if it tests short, remove then retest.
A resistor measured open in circuit will measure open out of circuit. No need to retest.

Always short out larger caps before testing around them, if you dont have an expensive multimeter and try to measure with a stored voltage in a cap you may be replaceing your multimeter before finding your fault.
I have been zapped by smoothing blocks with 340v 2 weeks after they were unplugged.
Really funny to see, hurts if its you.

I could write a book on fault finding, start with the above.
But if a circuit you just made doesnt work, all you can do is check your work, if you are really unlucky you will damage something with your faulty circuit, then you may fix your work, test and have it still not work.
However resistors are forgiving, a cap under voltage or reverse wired will smoke or explode, so obvious to fault find. Always check the semiconductors if you wired something up wrong, they will be the first to fail.
If you did reverse wire a capacitor, throw it out and replace it. They cost 20c, wear it!

Make sense?

Andrew

Forgot to add
The multimeter measures the diode/transistor by putting a low voltage out of the leads and measuring the voltage drop.
Semiconductors need some voltage to turn on, a multimeter on resistance setting has no voltage going out, so cant turn on the semi conductor, so they always measure open circuit.
Generally your meter will have a continuous beep if you hold the two leads together on diode mode. Or it will have a sound mode.
If you have that beep it is brilliant, you can test semiconductors without looking at the meter.
If you hold the leads on the component and there is no noise, it is open circuit, constant beep is short circuit, short beep means there is a voltage drop. A diode should get a short beep one way, not the other, if you get a constant beep both ways it is either faulty, or there is a resistor across it somewhere else in the circuit, thats why you always retest when removing from the circuit if it tested faulty in circuit.
Transistor short beep with one lead on one leg and the other lead on each of the other legs.

As per your drawing, a transistor with arrow pointing away from base leg is npn.
Arrow pointing towards base is pnp.
There is a few ways to remember which way means what, but the only one I can remember isnt suitable for a public site!
To measure, put positive lead on the base and negative lead on collector, you should have a short beep and ~0.6 on the meter. Then leave the positive lead on base and put the negative on emitter leg, you should get the same reading.
PNP transistor, just put negative lead on base and move the positive.

NPN needs a +ve on base to turn on, PNP needs -ve.

Soldering, there are heaps of tutorials on youtube.
But its pretty easy once you get the hang of it.
1) Let the iron get hot
2) clean tip by rubbing on a wet sponge, it should go silver after a clean, not stay black. It will turn black pretty quick so dont clean and leave for 30 mins. Always clean before starting a new join at first. It also removes excess solder off the tip.
3) Once clean, add a little solder to the tip of the iron.
4) Gently lean iron tip against both surfaces you are soldering.
5) Add solder to the iron on one side of the leg you are soldering, then drag it across the leg to the iron on the other side.
6) Pull away solder, then pull away iron.

A good join will have a concave shape, not a blob and not convex!

If the solder doesnt melt on contact, the iron isnt hot enough. Stop and wait for it to heat up.
The hotter the better.
Add more solder than you think you need at first, smoke coming off the join is good, that is the flux burning doing its job.
You should be able to solder a joint in less than a second.
Any longer and you may damage the component if it is a semi conductor.
Practice on resistors and capacitors. They arent heat sensitive... to a point!
Always solder on clean joints, corroded /oxidised copper or tin wont stick. Use steel wool or a fibreglass pen to clean a copper board, cut and restrip wire if the copper isnt shiny, other wise the solder just doesnt stick.

Practice makes perfect.
I used to solder surface mount 200+ pin ICs with a normal soldering iron.

Bad solder joints make up for alot of faults in appliances, they come second to capacitors. Parts that get hot or vibrate (coils/transformers) can detach from the solder on the board. The term is a dry joint.

Andrew

Thats a ship load of facts.

Thanks.

Can you PM me the mnemonic for transistors. I find them fascinating. I can almost see the tweed wearing professor in the 40's telling the new students the nice racist rhyme for remembering the resistor colour bands.

I wish a few more of them were directed at white middle class middle aged unfit men, just to balance things out a little, but I still get offended enough by them to have them stick in my head

Resistor remember rhyme
Bad
Beer
Rots
Our
Young
Guts
But
Vodka
Goes
Well

PNP Pointing iN Properly - arrow points in
NPN Not Pointing iN , arrow points out

PNP "equivalent" to P FET
NPN .... N FET

If you want to make soldering ONE BILLION times easier get a flux pen. Link previous post. Another great tool is a solder sucker, spring powered vacuum sucks up hot solder. INVALUABLE for replacing parts. Best way to use sucker is to great solder from one side of board, and while iron still there suck from other side. If part still stuck add more fresh solder repeat.

LoL Wow what a flash back, I still use it today