Can I safely use the TIP120 transistor at 12v, 5a? The specs say it can, but is it safe and how much heat would it generate? Also would the output be less, so if 12v, 5a goes in, would a lower voltage/amperage, go out? I am new to higher powered electronics, so I need some help. Thanks.
Let’s make an assumption.
Say the transistor Vce(sat) was 1.2V @ 5 amps. (could be as much as 4v)
That would be 6 watts, heat sinking is needed.
A MOSFET would be a better choice.
https://aws1.discourse-cdn.com/arduino/original/3X/7/2/7253db9f14a32e3568824488ca7d3ba8cae7aea6.pdf
A IRF3708 has a .01 ohm Rds(on).
At 5A, Vds will be .05V, about 1/4 watt.
Let's look at the saturation graph of the datasheet.
It says in the top/right that this is with a base/collector current ratio of 1/250 .
So we need 5000mA / 250 = 20mA base current for 5Amp, to reach these values.
At that base current we have a 2.3volt BE drop, so with a 5volt-logic processor there will be 5 - 2.3 = 2.7volt across the base resistor, resulting in a value of 2.7volt / 0.02A = 135ohm.
Use 120ohm because of some loss in the Arduino pin.
Looking at the CE saturation line, we see a collector/emitter dropout voltage of 1.3volt at 5Amp.
That will generate 1.3volt * 5Amp = 6.5watt of heat in the transistor.
If you don't want the heatsink to rise above 65C (burning fingers) at 25C room temp,
then we need a heatsink with a thermal resistance of (65C - 25C) / 6.5watt = 5C per watt, which is the size of a small fist.
Yes, much better to use a modern mosfet instead of a 50-year old darlington transistor.
Leo..
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All depends if OP wants to not use it as a switch but as an emitter follower.
OP did not specify any goal...
"it depends".
Can you provide a schematic and more detasils of what and how it will be switched? It DOES matter because if you are turning "something" (undisclosed) on and off frequently the power dissipation will be less than if the "something" is turned on for considerable periods.
However generally a FET would be a better choice.
Sorry for not clarifying. I want to use the TIP120 as a switch. It will be used to turn on and off a 12v 5a peltier TEC for longer periods of time. The circuit diagram I am using is found in the link below. I am switching out the battery for a 12v 5a dc power supply and the motor is being replaced with the TEC.
Hello, I am new to the TIP120 and am using it to switch a peltier TEC. I was wondering what power resistor to use to connect it to an Arduino. The power source is a 12V 5A dc power supply and the peltier draws 12V and 5A. I don't know how to calculate the resistance I need, so some help would be appreciated.
Hi,
if your peltier consumes 5A, the source that will supply current to it must support at least 20% more, that is 7A, otherwise you run the risk of either the peltier not working properly or of burning the source.
And the TIP120 supports a maximum of 5A, so it is not the recommended transistor for your project.
TIP120 datasheet : https://www.onsemi.com/pdf/datasheet/tip120-d.pdf
I suggest using a MOSFET to switch the peltier.
I second that suggestion: but be sure to use a logic level MOSFET. One of many examples
Why are you starting a new thread ?
From the data sheet, Ib = 20mA and Ic = 5A
A 5V Arduino output will need a base resistor:
(5v - 1.4v)/.02A = 180 Ω
But Vce(sat) can be 4V 
I took the time to fully explain in your previous thread.
Rather rude to ignore that previously given advice,and open a new thread.
I even calculated the resistor value for you there.
Leo..
Not surprising, though, given the OP's chosen username.
I have not ignored your advice. I find it amusing that you were offended by me creating a new thread. Anyone with common sense would know that multiple opinions on a subject are better than one. I simply created this thread to ask a more specific question so I could get some more opinions from people who knew what they were doing. It is sad to see that people are so egoistic and get offended by such trivial things. I understand that you put time and effort into helping me, and I appreciate that, but as stated before, I prefer to get multiple opinions on a subject, especially when on the internet.
Thanks for the advice. I created a new thread for a couple of reasons.
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I am new to this platform (arduino forum) and didn't know if replying on my previous thread would make it appear for everyone (on the main list that is visible when forum is opened) or only the people involved in my thread.
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I wanted to ask a more specific question and get some more opinions on it.
As I have explained to Wawa, I wanted to get several opinions on a more specific question. I do not see what is so problematic with this. In addition, what would my username have to do with this? I do not find anything stupid about asking multiple experts about a topic instead of just one. It is also quite hypocritical of you to reply to Wawa when you are unaware of what is actually happening, and then call others unintelligent.
Welcome to my ignore list!
It’s best to keep the same project questions in one thread.
This gives some framework that your responders to review and build on.
When it comes to transistors, it is always urged that you measure the (BJT) Vce(sat) and (MOSFET) Vds(on) to confirm the transistor is saturated and whether the circuit will work properly as a switch.
My list has been updated with your user name too.
Thank you for the advice and helpful information!
Here yo go.
Four opinions of frequent posters with a combined experience of I guess 150-200 years.
Note that this is not the average internet forum.
People correct each other here if they talk nonsense.
Leo..
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