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Rb for an NPN

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Hello all,

I'm new to electronics & building circuits, I'm finding it interesting.

For my first project Im making a switch (12v actuator) that opens when pressure exceeds a certain value. Im using a PIC 16f690 & everything is going good but I'm having issues with the Rb for the transistor (not switching on).

Im using a TO92-3, 12v, 3A NPN transistor ZTX688B with a hfe ranging from 500 to 100. my solenoid is 12v 500ma 24ohm. I'm currently using a 200ohm 1/2 watt as Rb but my transistor is not switching. Also, I have a 1N4007 in parallel with the solenoid.

When the 16F690 output is "on" I'm getting ~ 4v to the input of Rb & an output of ~
763mv which should be enough to switch the NPN on as the datasheet states Vbe on is max 1V.

Is there something that I maybe misundertanding? Are my values incorrect? Any help with this would be greatly appreciated. Remember I'm new to this so plain english will be needed :)

Thanks
 
Last edited:

MikeMl

Well-Known Member
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To saturate the transistor, the base current (also the current through Rb) should be about ~1/20 of the collector current. That spec is usually on the data sheet. With 1/2A in the collector, that would require an Ib of about 25mA .

Rb = Vrb/Ib = (4-0.7)/0.025 = 132Ω, which is close to what you are using.

I'm guessing something else is wrong, like you have something miswired?

Post a schematic.

added: I just looked here, and you have probably wiped out the transistor. It is only rated at a Vce of 12V, which is what your supply voltage is. Also, do you have a snubber diode around the coil?
 
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Ok

Well looks like I was on the right track atleast, maybe a bad transistor.... Heres the schematic nothing fancy, Im using parts that are well over what I need but I figure it would be good to get familiar with them.

Thanks
 

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Just read ur update, yes i forgot to mention that in the first post, I added it later. im using a 1N007..

Correction: 1N4007
 
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MikeMl

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Shouldn't the solenoid be powered from the unregulated 12V input?
 

audioguru

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Does your 12V solenoid work when it is fed only 4.8V?
Its current will be only 200mA, not anywhere near 500mA.
 
Last edited:

ke5frf

New Member
I thought it was good to always regulate voltage to any part, I read somewhere it's safer to always use a regulator.

No. Not for inductive loads. It is especially bad to share a regulator between logic devices and inductive loads.

If you must regulate the solenoid voltage for some reason (none that I can think of), get a regulator that can handle its power consumption and dedicate this regulator to it. This will save both your ICs and your regulators.

The solenoid shouldn't need to be regulated unless this is some kind of super-precision do-dah something-or-another do-hickey. regulators rob power.

Also, generally speaking you shouldn't try to regulate that close to your supply voltage. You typically need some headroom for regulators to work properly. Some are better than others, but this is generally true.
 
I thought thats what the transistor was for to sink the supply 12v to the solenoid through the transistor?

Maybe Im misunderstanding, as long as the npn base has the required sink voltage shouldnt source from the supply voltage travel through the load to the collector & out NPN to ground?
 

audioguru

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Most Helpful Member
You connected you solenoid to +5V, not to +12V. The transistor turns on and connects the other wire of the solenoid to +0.2V so the solenoid gets only 4.8V.
 
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