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Topic: Transistors in parallel (Read 14124 times) previous topic - next topic

BillO


humm.. 10v for switching. Will this work?


took from other topic: http://arduino.cc/forum/index.php?topic=75747.0

(need to learn about mosfets...i think it will took a year).



Holy saturation Batman! 4K7 on the base and 22K on the collector?  You could get away with 200K on the base quite nicely.

tomasi


Grumpy_Mike

He is just suggesting that you could use a bigger base resistor because the transistor has more gain than you are using. It won't make much difference to what you are doing though it just makes the transistor turn off a slight touch faster.

dc42

#18
Oct 29, 2011, 11:00 am Last Edit: Oct 29, 2011, 11:08 am by dc42 Reason: 1

I found this FET.
http://www.datasheetcatalog.org/datasheet/irf/irlz44n.pdf

It´s a logic level FET, so I think a "pre transistor" to achieve RDs(on) won´t be necessary. Am I right?


That's almost right for the job, however you want to switch 25A and the Rds(on) for that mosfet is quoted as 25 milliohms at Vgs = 5v and 25A. The power dissipation at 25A will be 15W and it will need a good heatsink. I'd look for a logic level mosfet with a lower Rds(on) than that quoted at a current at least as high as the 25A that you want to switch. Alternatively, you could use 2 of those mosfets in parallel (unlike bipolar transistors, mosfets can be paralleled). This will reduce the power dissipation to around 7.5W total, shared fairly well between the 2 mosfets. They'll still need a heatsink or heatsinks, but not such a large one.

You can drive the gate of a logic level power mosfet from an Arduino pin through a 100 or 120 ohm resistor.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

tomasi

Thanks  BillO , Grumpy Mike and DC42. My plan is to isolate these fets with an opto. I´ve bought some   STP80NE06-10 and  HUF75652G3 http://www.fairchildsemi.com/ds/HU/HUF75652G3.pdf. Ill study a bit more to conclude what suits my needs. I´ve learn a lot on this thread.And still have a lot to learn.

dc42


Thanks  BillO , Grumpy Mike and DC42. My plan is to isolate these fets with an opto. I´ve bought some   STP80NE06-10 and  HUF75652G3 http://www.fairchildsemi.com/ds/HU/HUF75652G3.pdf. Ill study a bit more to conclude what suits my needs. I´ve learn a lot on this thread.And still have a lot to learn.


Both of those have a nice low Rds(on), however they are not logic level, so you need around 10v gate voltage to drive them reliably.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

tomasi

Hi again.I hope this is not a dead topic. :)
Found this FET and I have some questions about it.



Whats the real current this FET  supports?
VGS of 10V is the maximum voltage gap between gate voltage and source voltage? As this FET has a RDS(on) of 5V , can I use on  28V? VGS is not clear to me yet. :(
   

  Cheers  from Brazil

GoForSmoke

You might want to look up "Darlington Array". You can get them packaged in chips with back-EMP protection for not a whole lot of money.

1) http://gammon.com.au/blink  <-- tasking Arduino 1-2-3
2) http://gammon.com.au/serial <-- techniques howto
3) http://gammon.com.au/interrupts
Your sketch can sense ongoing process events in time.
Your sketch can make events to control it over time.

tomasi

Darlingtons are measured by it´s Hfe , right?

dc42

IMO Darlingtons are rarely a good idea in Arduino projects. The problem is that they have high saturation voltages. If you're switching more than about 0.5A then this means they get hot and you need to use a heatsink, whereas a properly chosen mosfet will drop a much smaller voltage and run cool. If you're switching less than 0.5A, then a single transistor such as BC337 will do the job with a lower voltage drop. The voltage drop is important if you are running LEDs or 7-segment displays from the 5v supply, because the LED itself will often drop around 3.2v, giving you only 1.8v to play with.

The only time I would use darlingtons is if I was switching a voltage higher than 5v at quite a low current (say 200mA or less) and I needed several such switches. Then I would use a ULN2803A - not because its elements are darlingtons, but because it combines 8 transistors + base resistors + kickback diodes (if I need them) in a convenient package.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dc42

#25
Nov 26, 2011, 09:56 am Last Edit: Nov 26, 2011, 09:58 am by dc42 Reason: 1

Whats the real current this FET  supports?
VGS of 10V is the maximum voltage gap between gate voltage and source voltage? As this FET has a RDS(on) of 5V , can I use on  28V? VGS is not clear to me yet. :(


I found the full datasheet at http://www.vishay.com/docs/91328/91328.pdf. Here is an excerpt:

ABSOLUTE MAXIMUM RATINGS (TC = 25 °C, unless otherwise noted)
Drain-Source Voltage VDS 60V
Gate-Source Voltage VGS ± 10V

Drain-Source On-State Resistance RDS(on)
VGS = 5.0 V ID = 31 A  0.028 Ω
VGS = 4.0 V ID = 25 A  0.039 Ω

Vgs is the voltage applied between gate and source. The absolute maximum rating for Vgs of this device is +/- 10v. If you are driving it from an Arduino pin, you will be feeding the gate 5v to turn it on and 0v to turn it off.

The rating that determines how many volts you can switch is Vds. The abs max rating for Vds of this device is 60v. So you can switch a 28v supply with it.

You should keep the current below the value at which Rds(on) is quoted, otherwise you may be operating the device outside its fully-on region and the voltage drop across it may be higher than the Rds(on) value suggests. So if you can guarantee that your Arduino 5v supply really will be at 5v, then 31A is the maximum. If you want to play safe, use the 4v figure instead, so 25A is the maximum.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

tomasi

Thanks for helping  again, dc42. Very illustrative and easy to understand for noobs like me.Now this soup of letters are easier to read. :)

As far as I understand, RDS(on) is the best value for switching.Volts above this value, gets the FET unresponsive to swittching? 9V on gate could be achieved with a set of NPN PNP and with a voltage divider for supplying desired  volts on gate. Maybe a LM324, although I never  played with it.
I can tell you my new hobby is becoming a passion.And this is the most supportive forum I´ve ever seen of my  ~15 years of internet use.

btw, Im using the CS-100a and its running like a charm. 25-30 amps readings without even heating  a bit.
 


  cheers.

tomasi

#27
Nov 26, 2011, 11:16 pm Last Edit: Nov 26, 2011, 11:18 pm by tomasi Reason: 1
Ok,


here is what I came up to:
The FET is a IRFP264.Added datasheet infos on picture.



Cross fingers or will this work?

Grumpy_Mike

Quote
Cross fingers or will this work?

No it won't work.
There are no resistors in the transistor bases, and there is nothing to pull up the base of the PNP transistor to 28V to allow it to turn off. So melted transistors all round.

dc42

If you use a logic-level mosfet instead of the IRFP264 then you won't need the extra transistors, just a resistor from the gate of the mosfet to the Arduino pin.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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