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Old 05-20-2017, 10:45 AM   #61
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Default Re: First circuit design needs to be checked please?

It's never as simple as one thinks at first...
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Old 05-20-2017, 07:52 PM   #62
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Default Re: First circuit design needs to be checked please?

Here is the completed circuit up close and personal ... with a question at the end...

I made a video!
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Old 05-20-2017, 07:58 PM   #63
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Default Re: First circuit design needs to be checked please?

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It's never as simple as one thinks at first...
Boy ainít that the truth ... and no more truer with any discipline other than electronics.
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Old 05-20-2017, 08:19 PM   #64
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Default Re: First circuit design needs to be checked please?

I think it was already stated before: Silicon and other semiconductor materials have a negative temperature coefficient - it gets more conductive as it gets hotter (at a certain point). As it gets more conductive, resistance goes down, and the voltage drop across the transistor goes down. Since power is getting dissipated in the solenoid as it has the highest impedance (1A 12V drop = 12W and R=~12 ohms) luckily this does not cause the destruction of the transistor. If the lowest resistance element of the circuit is actually the transistor, the power would be dissipated in the transistor and potentially cause runaway heating and thus destruction of the transistor.

Oh, and also wanted to add:

Copper has a positive resistance coefficient, so as the solenoid heats up, it becomes more resistive and thus the voltage drop across the solenoid will increase. So you have both the semiconductor and the solenoid working together to show you that strange effect. I would think the solenoid heat up actually is generating more of the effect than the semiconductor, actually.

Last edited by eccerr0r; 05-20-2017 at 08:25 PM..
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Old 05-20-2017, 08:24 PM   #65
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Default Re: First circuit design needs to be checked please?

Yeah, it seems in this case, the recipe is just right...
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Old 05-20-2017, 08:28 PM   #66
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Default Re: First circuit design needs to be checked please?

Quote:
Originally Posted by eccerr0r View Post
I think it was already stated before: Silicon and other semiconductor materials have a negative temperature coefficient - it gets more conductive as it gets hotter (at a certain point). As it gets more conductive, resistance goes down, and the voltage drop across the transistor goes down. Since power is getting dissipated in the solenoid as it has the highest impedance (1A 12V drop = 12W and R=~12 ohms) luckily this does not cause the destruction of the transistor. If the lowest resistance element of the circuit is actually the transistor, the power would be dissipated in the transistor and potentially cause runaway heating and thus destruction of the transistor.

Oh, and also wanted to add:

Copper has a positive resistance coefficient, so as the solenoid heats up, it becomes more resistive and thus the voltage drop across the solenoid will increase. So you have both the semiconductor and the solenoid working together to show you that strange effect. I would think the solenoid heat up actually is generating more of the effect than the semiconductor, actually.
That sounds logical .. and thank you for the explanation ... so as the solenoid gets warmer, its resistive properties increase, and as a result, it takes on more voltage over time ... is that an accurate description of whats going on there?

Also, is there any possibility that such a phenomena could shut down the transistor ... under difference circumstances perhaps?
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Old 05-20-2017, 09:07 PM   #67
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Default Re: First circuit design needs to be checked please?

As the voltage across the transistor goes down, it's going deeper and deeper into saturation, this is fine. I'd be more worried about any thermal meltdown of the solenoid as it's dissipating 12W worst case. I don't know what the maximum duty cycle of the solenoid is rated for...
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Old 05-20-2017, 09:54 PM   #68
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Default Re: First circuit design needs to be checked please?

The solenoid is going to be on just long enough to open the hidden compartment.
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Old 05-20-2017, 10:21 PM   #69
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Default Re: First circuit design needs to be checked please?

Unless it's typical for the solenoid to heat up. Not sure if he expect to use it until it starts heating up frequently, but yes, ideally one would trigger, open, release within a few seconds. There should be plenty of thermal mass to not detect any heat...
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Old 05-21-2017, 07:04 AM   #70
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Default Re: First circuit design needs to be checked please?

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Unless it's typical for the solenoid to heat up. Not sure if he expect to use it until it starts heating up frequently, but yes, ideally one would trigger, open, release within a few seconds. There should be plenty of thermal mass to not detect any heat...
Under normal use, the solenoid will be actuated for less than two seconds ... long enough to open the door ... then when Iím done doing whatever, long enough to close the door. And that door might be accessed two to three times a week at best?

I think this thing should last quite a while.
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