I am substituting a 2n7000 TO-92 with a 2n2222a TO-18 NPN. According to the diagram I have for the 2n2222a the emitter is closest to the metal tab, the base is the center lead and the collector is opposite the emitter. I need to be sure I am installing correctly. The reason for my confusion is that although it appears when comparing the leads the collector=drain, the base=gate, and the emitter=source. Why do I get continuity from the metal case to the collector? I would think the emitter would be internally connected to the case.

 

Hello,

They can not be substituted.
The 2N2222 is a BJT and the 2N7000 is a MOSFET.

The upper one is for the 2N2222 , the lower one for the 2N7000.

Greetings,
Bertus

 

Depending on the configuration of the circuit you are dealing with, the 2N7000 may not work if all you do is drop it into the circuit in place of the BJT.

hgmjr

 

I have added the board layout so the usage will make sense.


View attachment brd1.pdf

 

It would be easier to tell what is what if you could post the schematic.

hgmjr

 

Does this help?

 

Hello,

The FET or transistor is used to trigger the NE555.
Here is a part of you schematic with the letters put in.

Greetings,
Bertus

 

I don't see any major problem with using the 2N7000 as you are proposing to do.

hgmjr

 

Hello, and thanks for the quick response. If the emitter goes to 0V why do I get continuity from the collector to the case?

 

Hello, and thanks for the quick response. If the emitter goes to 0V why do I get continuity from the collector to the case?

What device package are you using?

hgmjr

 

Hello,

It can be that the collector is mechanicaly connected to the case.
Just take a look underneath the transistor.
On the isolated pins you will see a (most times) a ring.
On the not isolated pin this ring is missing.

Here I have taken a picture of the bottom side of a 2N1711.

Greetings,
Bertus

 

The 2n2222a is a Motorola TO-18 the 2n7000 is a TO-92.

 

What is causing my concern is since I started using the 2n2222a I now get instant motor reversal when I trigger the limit switches and my trim pot has no effect on the time delay that should occur before the motor direction reverses. I don't even know if the NPN is related to the problem. I have received a lot of assistance from L. Chung (a senior member) and was hoping to get a layman's troubleshooting guide so I will know what to test when I experience incorrect operation of the controller.

 

Hello,

Is the housing of the 2N2222 touching some other electriv part?

Greetings,
Bertus

 

No, everything is carefully placed and I clean all of the solder side surface prior to powering up.

 

Hello,

Did you already have a look in the troubleshooting eblc1388 gave you?

Greetings,
Bertus

 

Hello,
Yes, I just printed the information and will begin using his instructions later today. Thanks for your help.
Strpdbas