Hi there,
I went to Jaycar the other day and bought what I thought was a 4amp collector current transistor but it turns out the one they gave me was 15amp collector current (the 2N3055, http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF). I am using a 3V Tamiya 70097 Twin-Motor Gearbox kit (Pololu - Tamiya 70097 Twin-Motor Gearbox Kit) with a stall current of 2.1amps for each motor. Just wondering if there will be any issues with using a transistor with such a high max voltage/current. Also will I need a heatsink? And when I'm soldering, can I just solder the wire anywhere on the case (which is the collector)?
Thanks for your help! 
I don't think you solder to the case. You bolt it to something and connect to that.
I might be wrong.
Hi, it depends on the case it is in, usually they are TO-3, however the 15A spec is with a decent heatsink.
The 2N3055 will work but you may have to include another transistor to suppply the base with enough current.
2N3055 is a very old design transistor, older than a LM555 and there are more efficient power transistor around.
Tom...
Nick:
Yeah I think you might be right. In the diagram about half way down the page under Connecting 2N3055 Transistors in Parallel (Understanding 2N3055 Transistor Spec Sheet) they seem to be connecting it right on the edge of the case. So I guess it would be wise just to try and do that?
Tom:
I previously bought some 2N2222A (NPN Transistors - PNP Transistors) transistors but the collector current is only 800mA so I think they're too small to use. However could I use them as you said to supply the base with enough current? Would you be able to give me some tips on how to do that? Also, the case is TO-3 so when you say
5A spec is with a decent heatsink
, you mean its in built and I don't need an external one?
Thanks both of you for the help
The 2N3055 transistor has rather low large signal current gain (the hFE parameter in the data sheet), at 2 amps somewhere above 20. Assuming 20 as the worst case, you have to supply base current = 2 amps/20 = 100 mA to get the motors to start moving. The collector current drawn will be less while the motor is running, but that base current is essentially wasted and drains your batteries that much faster.
100 mA is too much for an Arduino to supply from an output pin, so you need a second transistor like your 2N2222A to supply this, forming a Darlington connection. To build this circuit (twice for two motors) would be a good learning experience, but you would be much better off buying a simple dual motor controller like the following one from Pololu, which will handle the required motor current: Pololu - DRV8833 Dual Motor Driver Carrier
For only US$ 4.95, that is pretty hard to beat. Or you could research how to build the same circuit, but with much more efficient MOSFET transistors.
Ok, so I went to jaycar and asked about the dual motor controller you recommended but they didn't have it (and due to time constraints I can't wait to buy it online). So instead I chose a TIP122 following example 1 from: (How to Use A Transistor as a Switch | Teach Me To Make). Do you think this will do? It seems to match all the specs I require. Quick question though, in the last paragraph of example 1 in that link, it talks about choosing a resistor, do I need one even if I have 3v going into the transistor and 3v coming out (so does that resistor have any other function other than to regulate voltage or is my thinking wrong)? Apart from that I am planning on using a capacitor and diode in the circuit with the transistor to minimise noise and prevent the reverse voltage spike, does this sound like a good plan to you?
A TIP121 will work up to a point, however it has a large voltage drop (up to 2V @ 3A). That application really calls for a logic-level N-channel power mosfet. Unfortunately for you, Jaycar doesn't appear to sell any - none of the power mosfets they list has logic-level gate drive.
If you use the TIP121, you definitely need a resistor between the Arduino pin and the base.
Just to clarify I bought a TIP122 but I don't think that would change much. Well in that case could I just apply say 4.5-5V assuming a 2V drop to still provide my motor with 3V or is that too risky? And the vehicle doesnt have to move fast, it just has to navigate a course and climb a step so how much do you think the voltage drop would affect it?
Sorry, I meant TIP122. Yes you can use a higher supply voltage to allow for the voltage drop in the TIP122. The voltage drop will be less at lower currents, and 2V is the maximum drop at 3A. So you will probably find that 4V is enough. You may even get good enough performance from your motor with a 3V supply, depending on how much torque you need.
If the motor spends much time stalled or under heavy load, you will need to put a heatsink on the TIP122 to prevent it overheating.
Well I think that the maximum stall current for the motors (i have a dual gearbox motor) is 2.1A each and I think that the only time it will be struggling is over the step, so I think voltage drop may be fine if I use 4V supply as you suggested? What are the issues caused with too high supply voltage? Do I just risk burning out my motor? I just read online (and it just a forum post so I don't know how reliable it is) but it said most well built motors can withstand nearly double their stated maximum voltage, so if that is correct I guess it is better to supply slightly too much rather than too little voltage? And I think I will do some trial runs and see how hot the transistor gets to determine if I need a heatsink.
Thanks for all your help by the way!!
Your 3V motor should be fine driven with a couple more volts when it is lightly loaded.