Maker Pro
Maker Pro

Design question on microcontroller and TIP122

dashy1981

Jun 19, 2013
83
Joined
Jun 19, 2013
Messages
83
Hi Guyz,

I need design help on a switching circuit. Its going to switch 4V, 250 mA load.
I am considering NPN TIP122 as switch.TIP122 will be controlled through 89c2051.

I have a doubt regarding feasibility of sourcing TIP from 8051 pin. According to my understanding of datasheet it can sink 20mA of current, sourcing should be much lesser I guess. But in this application I need to source current to base of TIP.
So I think I can not directly connect base to port pin... or I am wrong?

I work more on SW, so if this seems a stupid question please ignore.:)

Cheers
dashy
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
If I were you, I would be using the 8051 to turn on and off a mosfet.

This page describes how it's done.
 

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
A TIP122 is way overkill for a 250mA. You could use a 2N2222, it can switch up to 800mA. Assuming a gain of 100, you would need only 2.5mA to the base. I don't know what an 8051 can source, but it seems likely it could do that much.

(or a MOSFET as Steve suggested, while I was typing in my reply)

Bob
 

dashy1981

Jun 19, 2013
83
Joined
Jun 19, 2013
Messages
83
If I were you, I would be using the 8051 to turn on and off a mosfet.

This page describes how it's done.

I'm not an expert on this but I'm very clear on my requirement. Its should be cheapest.
So going through price list this one was what I found cheaper than mosfets.
 

dashy1981

Jun 19, 2013
83
Joined
Jun 19, 2013
Messages
83
:) Agree. I thought that need not mention, since my V/I reqs I had mentioned. but yes that not explicitly mentioned.
 

dashy1981

Jun 19, 2013
83
Joined
Jun 19, 2013
Messages
83
I checked 2N2222, that looks prettier and cheaper than TIP. But I am going to use it to cut off/On a power supply to a load(4V, 250mA). So do I need to consider power dissipation by the transistor. i mean will 2N222 be able to sustain heat ?

BTW how do I calculate this myself ?
 

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
At mouser.com:

tip122 $.050
pn2222 $0.06
NTD4963N (mosfet 44A, 13.9mOhm) $0.19 (this is a great bargain)

So, no, the tip122 is not the cheapest solution.

And if you want to go surface mount, there is:

NX3008NBK,215 (mofset, SMD, 400mA, 1.4Ohm) 0.05

Bob
 
Last edited:

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
A TO92 transistor like the PN2222 can dissapate 1/2W. At 250mA and say 0.2V when saturated, it should be dissapating only 1/20th of a watt. No problem at all.

Bob
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Remember to add the price of the series resistor Bob!
 

dashy1981

Jun 19, 2013
83
Joined
Jun 19, 2013
Messages
83
As I mentioned that application will swith on or off the power supply to the load(4v,250mA). Since the transistor will be connected in between do I need to increase the voltage of power supply to counter fo the drop due to transistor? If yes how do i find out by how much to apply. Question is imp because the my load is costly n i sensitive one.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
A TIP122 will drop about a volt, because it's a Darlington transistor. Therefore you can't use it in your application.

A general purpose NPN transistor such as 2N2222 will drop something like 0.3V at 250 mA load current.

If you use a MOSFET with a low Rds(on) you do not need to worry about voltage drop across the switching element. You do have to worry about the gate drive voltage though. Standard MOSFETs need a few volts to turn them ON properly. You didn't say what voltage your micro is running at.

If you need low gate drive voltage, here's a good option for you: http://www.digikey.com/product-detail/en/SSM3K123TU(T5L,T)/SSM3K123TU(T5LT)CT-ND/2257798 USD 0.49 @1x

Rds(on) is 0.032R@Vgs=2.5V and 0.028R@Vgs=4V.

So even if you only have 2.5V to drive the gate, its resistance of 0.032 ohms will only drop 0.008V. That's 0.2% of your total voltage.

This is the reason why everyone has been telling you to use MOSFETs in this application.
 

dashy1981

Jun 19, 2013
83
Joined
Jun 19, 2013
Messages
83
Hi Kris, I am using an 8051 based MC at 5V. Even though the TIP will drop around 1V it should be fine I guess. Reason being TIP will be switching on/off a seperate power supply which is connected to my load. So if my load needs 4V,250mA then I can always increase power to 5V(its a high quality agilent variable power supply(0-30V,5A)).

During off period my load will be protected anyway. And when TIP is on then 1V drop by TIP + 4V by load => I can make my power supply set to 5V.

Will it be ok ... or I'll have any side effects ?
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
It depends on how the two supplies are connected together. Are they completely independent?

A schematic would explain a great deal. Show the MCU's power supply, the MCU, the other power supply, the load, and the position where you want to insert the semiconductor to control the load.

I don't think it's good reasoning to say that you can tolerate a 1V drop because you can increase the other power supply voltage if necessary. That 1V represents wasted power, which is converted into heat. Admittedly, 0.25W is not a huge amount of power, but it is still wasted. Also, the transistor requires a continuous base current to keep it turned ON, and this is also wasted power.

I do not see ANY reason to prefer a TIP122 over a small MOSFET.

Consider the relative advantages and disadvantages of the TIP122 and a nice MOSFET.

TIP122
NPN Darlington
Collector-emitter voltage drop ~1V at 250 mA
Continuous base current ~1 mA required to ensure saturation
USD 0.60 approx (Digikey, 1-off price)
TO-220 package
Power dissipation ~0.3W

ON Semiconductor NTD4906N
http://www.digikey.com/product-detail/en/NTD4906N-35G/NTD4906N-35GOS-ND/2194521
Drain-source voltage drop 0.002V at 250 mA (Rds(on) 0.008R at Vgs=4.5V)
No continuous gate required to ensure saturation
USD 0.57 (Digikey, 1-off price)
IPAK package, significantly smaller than TO-220
Power dissipation <0.001W

The only disadvantage I see with the MOSFET is that anti-static handling procedures are required.
 

dashy1981

Jun 19, 2013
83
Joined
Jun 19, 2013
Messages
83
Here is the basic schematic.
V1: My power supply which needs to be switched on/off.
R2: is my load.

Pls ignore name of Xistor. It will either be TIP or any other part as suggested by experts here.
One of 8051 pin will be driving it.
 

Attachments

  • Schematic.JPG
    Schematic.JPG
    28.2 KB · Views: 337

dashy1981

Jun 19, 2013
83
Joined
Jun 19, 2013
Messages
83
Updated with some more details... Sorry am not an expert... pls tolerate
 

dashy1981

Jun 19, 2013
83
Joined
Jun 19, 2013
Messages
83
fgt to add schematic..:eek:
 

Attachments

  • Schematic_moredetails.JPG
    Schematic_moredetails.JPG
    50.1 KB · Views: 401

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
That is not the way to use an NPN transistor as a switch. The load should be between the + voltage and collector of the transitor, and the emitter should be connected to ground.

Bob
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
And why are you so resistant to using the correct device for the job?
 
Top