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Author Topic: Current limiting problem.  (Read 2375 times)
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When you used battery capable of sourcing more current, the solenoid will draw more current.

So this meant that the solenoid was pulling more current than the tip 122 could handle because it had access to more current @ 24V with the marine batts? 
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Portland, OR
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That is my first thought but that doesn't make complete sense to me if you were only powering one solenoid at that draws 2.5 amps.

I'm a little unclear as to the details of you actual project, were you only ever testing this with one servo or did you have all seven hooked up?
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just one solenoid per TIP 122 circuit.
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Well then, I can't say I know what exactly happened as a battery that can provide 7 amps should have done the same damage. Maybe something happened with your code that kept the solenoid powered longer during the tests with the second set of batteries.

Regardless, mosfet's are the way to go when switching this much current. The tip122 you used is actually two transistors in one package to get you greater current amplification, which leads to double the voltage drop of one transistor and double the power dissipated as heat. The need to do this was largely eliminated with the invention of mosfet's which can be used in similar applications with more efficiency and less wasted heat.
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Cool thanks.  I will check my cables and things and report back.  I will deifinatley be switching to MOSFET. 
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While a MOSFET is the way to go the TIP122 dose not double the ammount of heat compared to a normal transistor.
The two transistors have their emmiters and collectors in parallel not seriese. The only thing doubled is the base emitter voltage which has no bearing on power dissipation.
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Thanks Mike, obviously I didn't know that and don't want to spread bad info.
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I was thinking,

could it be that I am powering the Arduino with my computer through a wall socket while using the batts to power the solenoids on the same circuit?  Would that lead to the transistor blowing up?
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Would that lead to the transistor blowing up?

No, transistors blow when one or more of their specification limits are exceeded, too much current, too much voltage, too much heat disspation, etc.


Lefty
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But if the arduino is on a different ground than the transistor would that overload the transistor?
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But if the arduino is on a different ground than the transistor would that overload the transistor?

If the transistor is a mosfet then yes the floating gate condition from not having a common ground reference could force the transistor into partial conduction with a high Ron resistance from source to drain and exceed the heat dissapation for the mosfet. If it's a BJT transistor (npn,pnp) then it should not cause a burnout (just no conduction), but still not function of course.

Lefty
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But if the arduino is on a different ground than the transistor
As Lefty says it will not work.
See:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html
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