Current limiting problem.


I recently changed the power supply for this solenoid circuit to two 12V marine lead acid batts in series supplying 24V @ 70 amps. Unfortunately the transistor completely blew up and Im trying to figure out why.

The circuit had originally worked with a power supply of 2 lead acid batts in series supplying 24V @ 7 amps so I think its a current limiting issue. Can anyone verify this for me and suggest a proper current limiting circuit?

The solenoid circuit I built uses a TIP 122 rated for 10 amps.

Please help. Thanks.

What is the current draw of your solenoid? It may have been a bit starved under the old source.

By that I mean the other batteries are ratted at a lower amperage meaning they are only promising to provide up to 7 amps.

This 7 amps is within the rating of your transistors, but when you switch to the new supply the limiting factor is now the solenoid.

I would think about switching out that darlington for a properly rated power mosfet, heat should be less of an issue.

Anyway this is speculation without knowing what solenoid you have.

The "rating" of your transistors is not as useful a number as their power dissipation. Manufacturers like to state the largest possible number they can to impress people, while it is not really a practically achievable current without aggressive heat sinking and control.

Consider your TIP122 "rated for 10A" (the Fairchild data sheet only specifies 5A absolute maximum...weird). At only 5A the Vce(sat) voltage (voltage across the transistor) is 4V, meaning the transistor will dissipate 4V*5A = 20W...for a couple of seconds before it blows up.

At these current levels a nice low-resistance MOSFET (e.g., NDP6060L) is a better way to start.

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there are two applications. One uses a 24V 1 amp solenoid and the other uses 24V 2.5 Amp solenoid. Could I just replace the TIP 122 with a Mosfet in the circuit? Is that what your saying?

I fried the transistor using the 1 amp solenoid. I should also mention that the solenoid on the fried circuit did not engage at all.

Yes, in the circuit you posted you could replace the TIP122 with a logic-level N-channel MOSFET. I suggested the NDP6060L because it's one of my go-to devices but there are slightly-cheaper options that would work equally well for only 24V 2.5A operation.

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and this would be able to handle a 70 amp power source even if im only pulling 2.5 amps max? Sorry if its a stupid question im a noob.

The power source simply is a capability, it does not "force" 70A of current. A "70A source" simply means that it could supply 70A if so demanded. What is actually determining the current is your solenoid. If you have a "24V 1A" solenoid then it will draw 1A from the 24V power source (approximately).

Really, your 70A battery configuration is major overkill for a 2.5A solenoid. You should be able to quite safely go back to your 7A setup if you use a low-resistance logic-level MOSFET instead of a TIP122.

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The reason why I need to use the 70 amp source is because Im actually driving 7 solenoids and some LEDs off the same power source for three days straight. I need the capacity.

Will the MOSFET work with this power setup or only with the 7 amp setup as you suggested?

Well, don't confuse current capability with battery capacity. A "70A" rating is a current capability: it says the battery can safely supply 70A of current. It doesn't say anything about how long that current will last, i.e., capacity. Now, I'm betting your 70A marine batteries are pretty high up there in capacity too, just wanted to make the distinction.

Are you sure you don't have a "70Ah" battery? Now that would be a measure of capacity (70 Amp-hours).

And yes, the MOSFET will work fine. Again, it doesn't care about your power supply, it only cares about the voltage across it (24V) and the current through it (2.5A). An NDP6060L (or similar) will handle those conditions just fine.

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ok cool. Thanks.

I understand that the solenoid pulls power across the transistor @ 1amp or 2.5 amp but if the transistor is rated for max 5 amp why would the 1amp solenoid blow up the transistor?

We're back to heat. The fact that the transistor is "rated for 5 A" is pretty much irrelevant. Of course, the manufacturer doesn't want you to think that, they just want to impress you with their product so you'll buy it.

The problem is your transistor dissipates heat. Too much heat and the transistor "blows up". The power dissipated by your transistor is current through it (1A) multiplied by voltage from collector to emitter. The TIP122 datasheet says this is 2V at 3A, or 4V at 5A. Let's say it's 1V at 1A, then it would be dissipating 1W of power and its temperature will be about 65 degrees higher than ambient (or 90C if room temperature is 25C).

That's not hot enough to blow it up, so something else must have been going on, most likely higher-than-expected current. Your 2.5A solenoid, for example, would dissipate about 2.5A * 2V = 5W and that will definitely blow it up since its temperature would reach 25 + 5*65 = 350C and that will definitely kill it.

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Yes i completely understand now about the heat dissipation.

But hears the thing I was able to use the 2.5 amp solenoid with the 7 amp batt successfully with the tip 122. The solenoid is only powered for 80 milliseconds.

so does this mean that it is a problem somewhere else in the circuit or could the current from the battery need to be limited? Is there a circuit or resitor I could use to do this without pulling the Voltage down too far?

Well, in only 80 milliseconds there isn't a lot of time for the transistor to heat up to the point of failing so that could explain why you're getting away with it.

Sorry, not following your last question. I think you're heading off in the wrong direction with this whole limiting-current-from-the-battery thing. I'm going to leave it at my MOSFET suggestion.

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ok thanks for the help. much much appreciation. ;)

Ill try the MOSFET thingy.

I think you are still confusing the max current a battery can provide with what it will provide to a given load (if able).

Most power sources, including batteries act as voltage sources, not current sources. That is they stay at a certain voltage and as load is added the current flowing increases to keep the voltage where it was, up to a point. This point is the current rating you are talking about with 7 and 70 amps.

A current source would try to push the same current all the time and therefore a greater load would require a greater voltage between the poles of the source. Given a voltage source, like you have, a current limiter doesn't make a ton of sense, as you presumably want you solenoid to recieve the max amount of current it can draw so it operates as designed.

The solenoid will try to draw a certain amount of current at a given voltage, the battery may or may not be up to providing that current. If the battery is not up to providing the current the voltage will droop, you may have seen this if you have ever shorted a circuit while measuring what should be a non-zero voltage, the voltage drops and something will start getting hot.

When you used battery capable of sourcing more current, the solenoid will draw more current. There is a difference between the total amount of power a battery will over a charge cycle, measured in Amp hours, and the amount that it can output at any given time.

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?

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?

just one solenoid per TIP 122 circuit.

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.

Cool thanks. I will check my cables and things and report back. I will deifinatley be switching to MOSFET.