First circuit design needs to be checked please?

[EasyGoing1]

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...

Under normal use, the solenoid will be actuated for less than two seconds ... long enough to open the door ... then when I'm 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.

[eccerr0r]

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...

[budm]

Re: First circuit design needs to be checked please?

The solenoid is going to be on just long enough to open the hidden compartment.

[eccerr0r]

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...

[EasyGoing1]

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.

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?

[EasyGoing1]

Re: First circuit design needs to be checked please?

Yeah, it seems in this case, the recipe is just right...

[eccerr0r]

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.

[EasyGoing1]

Re: First circuit design needs to be checked please?

It's never as simple as one thinks at first...

Boy ain't that the truth ... and no more truer with any discipline other than electronics.

[EasyGoing1]

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!

[eccerr0r]

Re: First circuit design needs to be checked please?

It's never as simple as one thinks at first...

[EasyGoing1]

Re: First circuit design needs to be checked please?

You guys are funny! lol

[redwire]

Re: First circuit design needs to be checked please?

The design I ended up with is a design that BUDM and I worked out together. I completely replaced the two resistors I originally speced in exchange for the two I have now which is the 2.2k base to ground resistor then the 220 ohm base to Vcc resistor ...

Whats wrong with the design as it is now? Here is it again for your convenience.

It works well... the transistor no longer heats up and it no longer drains the battery when its not in use.

That looks good part of the fun in electronics is learning by theory and changing parts is easy.
budm, the E-B resistor being 2.2k I was mixing up with the original 2.2k base-drive resistor. Lost 0.6% of base-drive to a good cause.

hFE is higher at lower collector currents, so a few uA of radio I can see amplified with a scope. You can prove it by putting a loudspeaker in place of the solenoid and touch the base or add wire to it. I have picked up the weirdest interference with these kinds of switch circuits so I add parts to stop that. But most people never notice and OP is ok since the reed switch is close, instead of a long cable.

Transistors partially on due to their own leakage currents, I encounter in builds, usually not much but a hot transistor can have high enough leakage current to keep itself on and make heat and stay on.... yuck.

Changing to a Darlington such as TIP122 (or... make one using BOTH MJE172 and 2N6487...), sure it has much more gain but I think higher losses. VCE(sat) is high at 2V which means 10V for the solenoid, and (2*1.7A) 3.4W of losses so it will heat up fast. I don't see a payoff to using a Darlington.

[eccerr0r]

Re: First circuit design needs to be checked please?

Right, we just don't want to dissipate 1.2W on the base for no reason.

[budm]

Re: First circuit design needs to be checked please?

https://cdn.badcaps-static.com/pdfs/...0959f24a0b.pdf
Per page 2 of the spec you can have Ic of 3A with Base current of 600mA with Vb-e sat of 2V, so we are not even close to the limit of the Transistor.

[eccerr0r]

Re: First circuit design needs to be checked please?

Base drive also contributes to total power dissipation too, so there's a sweet spot. Granted since more current flows through the collector, any overdrive in the base will reduce the chance of not saturating, and not saturating will hurt more.

Fortunately we don't have to worry about speed so deep saturation is fine as long as you don't fry the base doping by exceeding max base voltage/current.

And no, omitting the 2K2/R2 resistor will not turn on the transistor with broadcast noise because you're not going to get enough current through the base to make significant difference in collector current due to its low gain. And I sure hope modern silicon transistors will not have C-B leakage at such low voltages, else our quick go/no-go multimeter check from C-E will become invalidated.

[budm]

Re: First circuit design needs to be checked please?

lets complicate things and use a darlington part

Already suggested in post 31.

[budm]

Re: First circuit design needs to be checked please?

Let says right now Vbe is 1V (nice even number), then the Voltage on R1 = 12V - 1V = 11V for VR1, so the current flowing R1 is 11V/220 Ohms = 50mA.
Since VR2 is 1V due to Vbe of 1V) that means the current through R2 is 1V/2200 Ohms = 0.45mA which is really small!
The Base current is then 50mA - 0.45mA = 49.55mA, you can drive it even more to ensure saturation.

[stj]

Re: First circuit design needs to be checked please?

lets complicate things and use a darlington part

[budm]

Re: First circuit design needs to be checked please?

Why are you attached to using the 2.2k base-drive resistor, we're all saying it's 10x too big lol.

If you learn Kirchoff's voltage law KVL and analyze the circuit as two loops, it may be much easier.
Loop 1: battery, R1, R2, E-B of transistor
Loop 2: battery, solenoid, C-E of transistor

Now you are using a smaller transistor so current/heat/temperature have a greater side-effect than the previous big guy 2N6487.

If 2k2 is 10x too big then why do you have 10K in the drawing? Or how about not having no R2 at all then how many time too big will that be?

[EasyGoing1]

Re: First circuit design needs to be checked please?

Why are you attached to using the 2.2k base-drive resistor, we're all saying it's 10x too big lol.

The design I ended up with is a design that BUDM and I worked out together. I completely replaced the two resistors I originally speced in exchange for the two I have now which is the 2.2k base to ground resistor then the 220 ohm base to Vcc resistor ...

Whats wrong with the design as it is now? Here is it again for your convenience.

It works well... the transistor no longer heats up and it no longer drains the battery when its not in use.