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Offline Robotboy86Topic starter

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Question about transistors.
« on: July 15, 2007, 08:04:31 PM »
I am building a basic H-bridge and was reading around a few places and saw(I believe on this site even) that transitors are more effient if I turn them on/off with higher voltage.  I.E At 5 volts it might turn on, but it would become much hotter then if I did it at say 40 volts?

If that is true, is there a voltage amplifying IC? 



Offline KambeiX

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Re: Question about transistors.
« Reply #1 on: July 15, 2007, 09:40:37 PM »
Hmm you are getting it wrong...

Normal transistors are current amplifiers, so a few mA in the base will give you a max of base current X gain (hFE)

However, all transistors have a saturation point and a cutoff point, so under certain current the transistor will act as an off switch and over certain current the transistor will act as an "on" switch.

BUT, normal transistors can only tolerate up to certain voltage at the base, so if you put too much current they will fail and won't give you any benefit, so you MUST use a current limiter resistor in the base depending on the voltage, gain, etc (you need to calculate).

On the other hand, there are FET transistors (MOSFET, JFET, etc) that are voltage to current amplifiers. In that case, higher voltage will give you a higher gain and 5V won't be enough for driving high power MOSFETS so you will need a driver circuit.


So 5V will be more than enough for some applications and too little for other applications.

 

Offline Robotboy86Topic starter

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Re: Question about transistors.
« Reply #2 on: July 16, 2007, 02:50:07 AM »
I don't understand..  these mosfet voltage to current amp's work at any voltage up?  or what?

My understanding is that *all* transisitors require a certain base amount of voltage to turn on...    so?

Offline Robotboy86Topic starter

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Re: Question about transistors.
« Reply #3 on: July 16, 2007, 03:03:37 AM »
"Another note on MOSFETS, they are more efficient with higher Gate voltages. So you probably do not want to just apply a simple binary 5V. Instead you should amplify it with say an operational amplifier or even better, a MOSFET Driver IC. All are quite cheap and easy to implement. I highly recommend amplifying the Gate voltage because it would save on battery power, and significantly reduce overheating. Which reminds me of another point . . . "

That is from this site..  it would appear he is saying it is better to hit it on with higher voltages??


Sorry if I am annoying but this is just an important point and I wanna make sure I understand it

Offline Tsukubadaisei

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Re: Question about transistors.
« Reply #4 on: July 16, 2007, 04:46:55 AM »
There are only 2 things you must understand. You must know that mainly 2 types of transistors exist,i.e., PN channel transistors and MOS-MET. And you must understand their basic principles.
Depending on the task and conditions the kind of transistors you must use is different. Not only being a MOSFET or PN but their sub-categories(like PNP or NPN or nMOS or pMOS or dep or enh or JMOS transistors) and specifications. For that you have to look in their datasheet. All the data you want is in the datasheet. You have to read the datasheet. There are MOS-FET are operational at 1.2V but there are some that would take 40V. There are lots of kinds of circuits including motor-drivers. Depending on your objective NPN transistors may be better than MOS-FET and so on. IF you are just learning the principles then focus on the principles. All those details will just drive you crazy. If you REALLY want to learn the principles then do like me and study some basic qantum physics by yourself, then you can trust me, transistors(and diodes) will never be a problem anymore.
A.I.(yes those are my initials)

Offline Robotboy86Topic starter

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Re: Question about transistors.
« Reply #5 on: July 16, 2007, 11:00:53 AM »
So I should always use the rated voltage on the transisitors, and to have more effient transisitors a higher voltage works better right?


Offline Robotboy86Topic starter

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Re: Question about transistors.
« Reply #6 on: July 16, 2007, 11:27:10 AM »
I guess it might be easier to understand if we work from a real transisitor..

http://www.vishay.com/docs/71146/71146.pdf

I just found this under google, if I am understanding this correctly when it says "Gate Source Voltage +_20V" it means the gate will tolerate UP to 20v's to turn it on/off..   As such, running 15v might turn it on.. but it is best if I run as close to 20v as possible right?

but wait :(

IT says: Gate Threshold Voltage VGS(th) VDS = VGS, ID = 250 μA, with 1 volt as minium, and 3 as maxium.  So which do I use then? lol

So, if I were to design something to use that mosfet I should get a 20v voltage regulator, and run something like 24v through the regulator, and take its output to run the mosfet?  Also, I always see resisitors on these transisitor schematics and it usually goes from the source to drain..  Are those pull-down resisitors meant to keep the transisitor line as empty as possible when I am not sending a voltage??

Thanks..

P.S. I promise I do learn things, and I am googling like mad.. but I thought I had a fair understanding until I posted that and Now I am feeling as if I don't
« Last Edit: July 16, 2007, 11:29:36 AM by Robotboy86 »

Offline KambeiX

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Re: Question about transistors.
« Reply #7 on: July 16, 2007, 03:05:05 PM »
Look page 2 at the bottom, there are 2 graphics.

How many amps will your motor need? Match that with VGS (gate voltage). As you will see on the 2nd graph, 5V is more than enough for that mosfet.

And as you will see on the 1st graph (left), 3v won't work, 4V will work for up to 10 amps and over that (5 through 10V) there won't be much difference.

So, for that MOSFET in particular, 5V is OK as gate voltage, but 4V isn't and higher voltage won't do much difference.


Offline Robotboy86Topic starter

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Re: Question about transistors.
« Reply #8 on: July 16, 2007, 03:15:28 PM »
Night and day!

Thanks man!  One of those "uh.. duh" moments there. :)  Just need about 8 of those then, 8 diode's, a few resistors..  op-amp's and such :)

Thanks guys :)  I didn't want to buy an h-bridge if I could make one myself and all..

now time to figure out the pwm stuff lol

yay

Offline Robotboy86Topic starter

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Re: Question about transistors.
« Reply #9 on: July 16, 2007, 05:03:05 PM »
and the last stupid question for me in this thread lol..

I am looking to provide ABOUT 5-10amps via h-bridge for motor driving..

I came to these:

http://www.fairchildsemi.com/ds/FQ/FQAF19N20.pdf
http://www.fairchildsemi.com/pf/FQ/FQAF19N20.html

I figured I could just use an op amp and push the 5v to 10v and be done with it.

I noticed that there thermal disappation is kind of ridicilious..  40c to every W to air..  so that'd let me have about 3w's of power to air before melting..  so I'd defintelly need to heat sink these, correct?

Are all transisitors so easily overheated?  I mean 3a's is about 500ma to a motor at 6v..  Thats far from powerful.


Thanks again :)

P.S.  I assumed I would get 8 of those, and 8 of P types...
« Last Edit: July 16, 2007, 05:15:18 PM by Robotboy86 »

Offline dunk

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Re: Question about transistors.
« Reply #10 on: July 16, 2007, 05:48:21 PM »
hey Robotboy86,
so i've been sitting out of this discussion because i've never taken the time to read up on how transistors work.
as a result i can't tell you anything on why or how they work.
also the language i use may not be correct.

where i can possibly help is how you can identify a MOSFET that will work for you and how to use it.

if you want to use a transistor as a simple on/off switch MOSFETs work well.

MOSFETs expect a certain voltage level to switch them on.
different MOSFETs will require different voltage levels to switch them on. this will be displayed as a graph in the datasheet, typically called "SATURATION CHARICTERISTICS".
if you don't trigger them with enough voltage then they still appear to work but are more wastefull of power, generating more heat and reducing the life of the component.

MOSFETs will always generate some heat. if you are switching a big load then you will need to attach a heatsink.

forget using an op amp to boost the voltage from your micro to the MOSFET.
use a MOSFET driver IC. you will get far better results.
or (better solution) pick a MOSFET that can be driven from TTL voltages. (they are out there, you just have to search.)

that's about it.
o, one more thing,
this might save you a bit of searching,
here's a MOSFET that can handle 30A @ 60V and be driven straight from a microcontroller's 5V logic:
http://www.fairchildsemi.com/ds/RF/RFP30N06LE.pdf
take a look at Figure 7. on page 4. of the datasheet.
it shows you what your Gate voltage (Vgs) needs to be for different loads.
you can clearly see that the 5V logic from your microcontroller could power a load of 60A at 4.5V or over.

someone please correct me if i'm wildly off the mark with any of this.
it's only what i have picked up from datasheets.

dunk.

Offline Robotboy86Topic starter

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Re: Question about transistors.
« Reply #11 on: July 16, 2007, 06:20:36 PM »
Thanks Dunk :) You guys in this thread all really cleared a TON up for me :)

Offline Admin

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Re: Question about transistors.
« Reply #12 on: July 22, 2007, 11:16:07 AM »
yeap dunk you are 100% correct.

Robotboy86, keep your eye out for something called 'PWM Optimized MOSFET'
They are fairly common now.

Also, when you first power up your circuit, start with smaller motors and test if anything overheats. This will tell you if you need heatsinks or not (if you arent confident of your calculations). My multimeter has an electronic thermometer on it - really useful!

 


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