Question about the transistor TIP122

[Jeff27]

Hi Guys,

Can I just ask you some questions about the TIP122 ST Trans Darlington Transistor please. I just learned how the transistor works, basically after receiving a HIGH signal, Arduino sends a signal to the Base Pin of the transistor through a resistor, and then the transistor will connect its Collector Pin and Emitter Pin to finish the circuit.

Got a simple question here though, why the current from solenoid or from the connected Emitter Pin of transistor (e.g. at 120mA) will not flow to the GND on Arduino which will then damage Arduino, but will flow to the cathode of the power source, please refer to my attached picture. I guess I am really asking how does the negative common ground or buses on the breadboard work. The digital Pin on Arduino also requires a complete circuit to work right ? For example, it goes like from the digital Pin to a resistor then to the Base Pin of transistor and exits from the Emitter pin and back to the GND on Arduino ?

I am using the TIP122 transistor and have listed its specifications as below. I think it's switched by current, so I have to supply the correct current to its Base Pin, but I can not find what is this correct current from its information below. And can you tell me if I understand those information correctly ?

DARLINGTON TRANSISTOR

Transistor Polarity:NPN ---------- not sure what NPN differs from PNP
Collector Emitter Voltage V(br)ceo:100V -------------- the maximum voltage that can flow through the transistor ?
Power Dissipation Pd:2W
DC Collector Current:5A -------------- the maximum current that can flow through the transistor ?
DC Current Gain hFE:1000 -------------- really no idea
Transistor Case Style:TO-220
No. of Pins:3
SVHC:No SVHC (15-Dec-2010)
Av Current Ic:5A -------------- average current 5A ? no idea
Collector Emitter Voltage Vces:2V ----------- how does this differ from the 100V ?
Continuous Collector Current Ic Max:5A --------- what does the term 'Ic' mean....
Current Ic Continuous a Max:5A ---------- no idea
Current Ic hFE:3A -------------- no idea
Device Marking:TIP122
Full Power Rating Temperature:25°C
Hfe Min:1000
No. of Transistors:1
Package / Case:TO-220
Power Dissipation Pd:2W
Power Dissipation Ptot Max:65W
Termination Type:Through Hole
Transistor Type:Darlington
Voltage Vcbo:100V ransistor

So how do I know what is the correct current I should send to its Base Pin from Arduino ? And it seems I need to connect a resistor between the Base Pin and Arduino digital Pin, how do I work out the resistor ?

Thank you very much guys for helping a learning noob here

Regards, Jeff

[system]

With such a high gain from a darlingtom pair, your 2k resistor is fine.

[alnath]

Hi Jeff27,

I (or others here) could answer your questions one by one ( Ic = collector current.... Hfe = current gain etc... ) , but I think you'd better have a look here :

Bipolar Transistor Tutorial, The BJT Transistor

and then here (more about darlington transistors)

Darlington Transistor and the Sziklai Darlington Pair

you'll get answers to all your questions, and more

[MarkT]

Jeff27:
Hi Guys,

Can I just ask you some questions about the TIP122 ST Trans Darlington Transistor please. I just learned how the transistor works, basically after receiving a HIGH signal, Arduino sends a signal to the Base Pin of the transistor through a resistor, and then the transistor will connect its Collector Pin and Emitter Pin to finish the circuit.

The current through the collector/emitter circuit depends on the current through the
base/emitter circuit, the ratio being called the "gain".

Got a simple question here though, why the current from solenoid or from the connected Emitter Pin of transistor (e.g. at 120mA) will not flow to the GND on Arduino which will then damage Arduino, but will flow to the cathode of the power source

Because the collector/emitter and base/emitter circuits are different circuits. Circuit
means a loop, current flows round a loop. The fact the emitter is common to
both circuits cannot affect the current, only bring the voltage to a common value
at the emitter. The high current is though the solenoid so it has to flow round the
collector/emitter circuit since the Arduino isn't allowing large currents (due to the
base resistor).

I am using the TIP122 transistor and have listed its specifications as below. I think it's switched by current, so I have to supply the correct current to its Base Pin, but I can not find what is this correct current from its information below. And can you tell me if I understand those information correctly ?

DARLINGTON TRANSISTOR

Transistor Polarity:NPN ---------- not sure what NPN differs from PNP

All the charge carriers are reverse polarity in a PNP compared to NPN,
its complementary.

Collector Emitter Voltage V(br)ceo:100V -------------- the maximum voltage that can flow through the transistor ?

Voltage is developed across a component, it doesn't flow

Power Dissipation Pd:2W
DC Collector Current:5A -------------- the maximum current that can flow through the transistor ?

The maximum current through the collector without risking damage

DC Current Gain hFE:1000 -------------- really no idea

Ratio of collector current to base current.

Transistor Case Style:TO-220
No. of Pins:3
SVHC:No SVHC (15-Dec-2010)
Av Current Ic:5A -------------- average current 5A ? no idea

Yes, this is the same parameter as above

Collector Emitter Voltage Vces:2V ----------- how does this differ from the 100V ?

The is Vsat, the saturation voltage when the transistor is hard on at 5A. Ideally it
would be 0V

Continuous Collector Current Ic Max:5A --------- what does the term 'Ic' mean....

I is always current (no idea why), Ic = collector current

Current Ic Continuous a Max:5A ---------- no idea
Current Ic hFE:3A -------------- no idea
Device Marking:TIP122
Full Power Rating Temperature:25°C
Hfe Min:1000
No. of Transistors:1
Package / Case:TO-220
Power Dissipation Pd:2W
Power Dissipation Ptot Max:65W
Termination Type:Through Hole
Transistor Type:Darlington
Voltage Vcbo:100V ransistor

So how do I know what is the correct current I should send to its Base Pin from Arduino ? And it seems I need to connect a resistor between the Base Pin and Arduino digital Pin, how do I work out the resistor ?

You want the output transistor of the darlington to carry about 0.1 times the Ic value,
and the first transistor has a gain of about 100 probably, so Ib of 1/1000
Ic or more. If the load is 2A you want at least 2mA into the base, so a base resistor
of (5 - 1.5)/0.002 ~= 1800. 1k is a good default value likely to work with most loads.

Thank you very much guys for helping a learning noob here

Regards, Jeff

[Jeff27]

You want the output transistor of the darlington to carry about 0.1 times the Ic value,
and the first transistor has a gain of about 100 probably, so Ib of 1/1000
Ic or more. If the load is 2A you want at least 2mA into the base, so a base resistor
of (5 - 1.5)/0.002 ~= 1800. 1k is a good default value likely to work with most loads.

Thank you very much for the detailed explanation. So if my solenoid valve is 12V 120mA which means the working current or load is 120mA, then I want at least 0.12mA into the base, so a base resistor of (5-1.5)*1000/0.12=29,166 Ohm or about 29K Ohm (btw, 5V is the Arduino voltage, and what is the 1.5V for ?) is required ? This is much higher than 1K. Did I miscalculate something here ?

Thank you very much.

[MarkT]

Everyone always uses 1k into a Darlington, laziness - it will work.

For a 120mA load a 2N2222 will be sufficient BTW, with a 470R base
resistor.

Don't forget the freewheel dode.