Transistors in parallel

Hi. I have a high current load to control with Arduino (25 amps approx.).
My idea is to join cheap and easy to find NPN transistors in parallel to control the current (pwm current control) capable of 40 amps or more for safety reasons.
Here is my idea:

Will this work?

Thanks in advance.

I haven't actually tried it so don't take this as 100%, but I believe due to the fact that no matter what each transistor setup will have slightly different resistances and flow which will overload some more than others, which would cause a cascading pop of your well spent money as they fail : /

hmm... should i go for a single power mosfet then?

Thanks in advance.

FETs will shair current when put in parallel. Transistors will not.

Yeah I believe you can get a decent mosfet that will work and be cheap, that's what most people do

Yes try MOSFET.. i'm sure it will work for you..

Hi,
Back in the Dark Ages when I first touched power transistors there were no power FETs.

Many high-power power supply designs DID parallel junction transistors. They typically added small resistors in series with the collector of every paralleled transistor, a resistance that would drop about .1 volt at normal operating current. So like .1 ohms for 10 amps, etc.

This equalized the current across multiple transistors.

Also the transistors should be same type, same manufacturer, best if same date code...

Thank you all for the infos.I googled a little and this is what i found:

STP80NE06-10

http://www.datasheetcatalog.org/datasheet/stmicroelectronics/5632.pdf

VDS Drain-source Voltage (VGS=0)60V
VDGR Drain-gate Voltage (RGS=20k?)60V
VGS Gate-source Voltage ±20V
ID Drain Current (continuous) at Tc=25C 80A
ID Drain Current (continuous) at Tc=100C 57A

Im not familiar with MOSFETs.One thing that concerns me regarding this MOSFET is VGS value of 20V.Does it means that i must have 20V on gate for switching?
Sorry for all these questions.Thank you all.

Many high-power power supply designs DID parallel junction transistors.

Yes an in those days many many designs failed. I was on a team that tried that in my first job, I was only an apprentice then but none of the grown ups could make it work. I still remember the pings as they went one after another. In those days just one of these 5A power transistors cost the same as my weeks wages.

One thing that concerns me regarding this MOSFET is VGS value of 20V.Does it means that i must have 20V on gate for switching?

No that is the maximum voltage it will take before you damage it. However you do need 10V to turn this FET on fully. Look at the Rds (on) conditions.

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

took from other topic: Arduino Forum

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

terryking228:
... in series with the collector ...

A small resistance in series with the emitter would be far more effective and would balance the load on each transistor naturally through adjusting the base current.

In a previous life I was part of a team that designed DC motor drives. The biggest I worked on was a 10KW drive that used banks of over a hundred 2N3055s with .1 ohm resistors in series with the emitter. I believe some of them are still in service today.

A small resistance in series with the emitter would be far more effective

Oh. Yeah.. they were in the Emitter for sure..

Hey it was only 45 years ago, how could I forget?

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

Yes that would be fine.

Thanks Grumpy Mike.
Im still studying about FETS. Can I sugest to you more experienced ppl a complete tutorial on playground? As Arduinos are related to robotics and LED arrays, it might be a good start point.
I found this FET.

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

cheers from Brazil

Work but:
You must have a small reistence in emissor and a resistence in base too.
Semicondutor never have the same polarization voltage.

Problem:
The current is not equal in each transistor and you must project with more than 20A.
Edson

tomasi:
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.

Ok, now im confused. :slight_smile:

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.

tomasi:
I found this FET.
International Rectifier irlz44n datasheet 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.

Thanks BillO , Grumpy Mike and DC42. My plan is to isolate these fets with an opto. I´ve bought some STP80NE06-10 and HUF75652G3 Intelligent Power and Sensing Technologies | onsemi. 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.