List of Components for Interfacing

TerryFricker

New Member
Having trawled many articles about interfacing micros with the outside world, it appears that half a dozen devices would handle the majority of switching needs. The old darlington pairs and traditional transistors figure less and less. Many of these 'newer' devices have characteristics approaching the 'Ideal Switch', and need few, if any, peripheral components. Do the electronics buffs agree with this, and if so, could they help with drawing a part code list compatible with the PIC? This would be a useful resource for schools and the non-electronics Picaxe hobbyist.

eg
a) For low frequency switching (<1kHz)of <1A loads use: (perhaps logic level MOSFET?)
b) For low frequency switching (<1kHz)of 1A to 10A loads use: (perhaps logic level MOSFET driver plus power MOSFET)
c) For high frequency switching ie PWM projects use ..........etc etc
.
.
x) For low power and battery projects use a low drop-out regulator eg .... and 'high side' MOSFET eg .... to isolate the supply from the rest of the circuit.

 

Chris DeHut

Senior Member
HI TSF,

I can't add to your list but I can confirm that I like the idea of the list. I am learning electronics and PICs at the same time. I have programming experience so programming the PICAXEs is the easy part for me, the hard part is the electronics.

Chris
 

Adiman

Member
I cant give you a list but I can confirm a few things.

For low frequency switching, less than 1khz - a mosfet is ideal. When only switched at 1khz you can drive most normal mosfets gate straight off the output pin of the picaxe even the high power 150 amp + types without problems.

When you need to go above 1khz lower amperage mosfets will still work fine up to around 40khz or so, but at that point you really should be running a mosfet gate driver in between the picaxe and the mosfet. Microchip.com make a great range of gate drivers, for instance a TC1413N is great for driving a high amp N channel Mosfet at high frequencies off of a picaxe
 

hippy

Technical Support
Staff member
I read a word of caution somewhere that Mosfets are highly sensitive to being damaged by static while being handled ... is that true ?

I have no experience with Mosfets so some pointers to interfacing examples for PICAXE would be ideal.
 

premelec

Senior Member
You might add the IGBT which is sort of a hybrid FET and bipolar. Insulated Gate Bipolar Transistor...

MOSFETS have fairly large capacitance in the gate for higher current types this can cause a delay if driven from a high impedance.

MOSFETs are static sensitive but ordinary precautions seem to work ok... don't leave the gate floating [that gate capacitance also acts as a 'memory' of any voltage and gate impedance is very high so it takes a long time to drain the charge on gate capacitance]. Anyhow the MOSFETs are cheap enough and durable enough to jump in and use 'em! I have seen some cases where the MOSFET got 'leaky' from static damage perhaps from poor handling.
 

Rickharris

Senior Member
MOSFETS like many FET n=based chips are suppposed to be static sensitive, i have yet to see one damaged in this way.

Most normal environments are a leaky electrically as the oft quoted sieve as any one who has tried to generate static electricity knows.

normal care is all that is required. As said they are fairy cheap anyway.

L293 and derivitives are good for motor driving. High current outputs can be made by using the L293 as a driver for MOSFETS.

I quite like the darlington chip as a source of several drivers in a compact package.
 

Adiman

Member
I've run a IRF1405 N channel mosfet directly off the output pin of a picaxe 18x to run a high frequency buck dc converter (around 40khz) but i found the mosfet was getting warmer than such a powerful mosfet should, and it was for that reason mentioned in a earlier post, the gate capacitence. The fet wasn't being switched fully on, and fully off quick enough by the picaxe.

It gets worse as the frequency goes up, and better as it goes down. lower amperage rated mosfets(<30A - IRF540N for example) normally have much lower capacitence on there gates, so frequencies up to around 100khz directly connected to the picaxe is possible without problems.

Also mosfets work better with a voltage of around 10 volts on their gate - a normal picaxe circuit is 5 volts - that's where the gate driver chips come into use - gate driver chips actually have 2 little fets internally inside them, one to turn the mosfet gate on, one to pull it back down to ground - sort of like using a very high speed relay to switch a bigger circuit.

There are special fets about that are specially designed to run from a pic directly but since most normal types run fine it's normally unnecessary.

I've played with a lot of fets connected to the picaxe, i've never had one fail and I take no static precautions. The only thing you have to worry about as always is back EMF from motors and such, normally a free wheeling high speed diode on the DRAIN (middle pin) of the N channel fet to the positive line is all that's needed to prevent this.
 

evanh

Senior Member
For simple DC switches the MOSFET is pretty ideal alright. Though, sometimes it is easier to make use of the diode drop of BJTs, Ie: The FET doesn't work well as a voltage follower.

A logic level FET is the one to use if driving directly with a picaxe or anything else that can't manage atleast 10 volts.

Consider the IGBT as the replacement for the old power transistor and is best used in high voltage applications where the power FET struggles.

Both the MOSFET and the IGBT are highly sensitive to static, don't do your assembly work on synthetic carpet and always transport them inside antistatic bag/tube or tinfoil.

when wiring them into the circuit add a series resistor between the driver and gate plus a zenor/transorb across the gate-source pins. This mostly serves to absorb capacitively coupled spikes coming back through the FET but I'm confident it also rugardises the FET. The resistor is spec'd to match the driver current capacity.

Thinking about the fact that any decent positive spike on the FET will turn it on and thereby snubb it there is not really a need for the zenor as such. A Schottky diode will be superior reducing the need for the series resistor - not that I'm saying remove it.

As for switching speed, it's quite a wonderful sight to see it on the scope, you get this capacitor charging curve up to about 4 volts then it flatlines while the drain comes racing down then it carries on up at a new charging rate.

My one and only test so far was lightly loaded so the flatline stage would be more sloped under full load I figure. That said, the duration of the flatline is not a function of the load but rather the gate charge vs the driver strength. I think gate charge is a function of it's capacitance and open circuit drain-source voltage.


Evan

Edited by - evanh on 9/10/2005 6:12:32 PM
 
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