How do you represent the electrically isolated transistor case in a schematic symbol?

A recent thread involved a transistor with a TO72 case. This case has four leads: the three usual SGD or BCE plus a fourth lead to the electrically isolated metal case.

My question:
How would you represent this fourth lead on a Kicad schematic symbol?

I would personally do something like this:

I don’t know of a “standard” for it.

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The circle around the transistor is supposed to represent the case of the transistor, so as craftyjon suggested, a pin connecting anywhere to that circle is just fine. Just make sure it can not be confused with transistors having two collector connections such as this one:

So do not draw it too close to any of the other pins.

If you do a picture search for transistors, you see variants both with and without the circle:

I prefer without the circle, because it removes clutter from the schematic. “officially” the symbol without the circle are for on-chip transistors, and maybe for bare dies such as on thick or thin film modules.

[Edit] Oopsy: Emitter -> Collector.


Yes, that was why I asked.

It has been 40 years since I have used one of these. They were always RF and the case lead went to earth. I don’t recall ever seeing that lead on a schematic but it exists and the Kicad footprint has four pads.

In KiCad it is also possible to set the pin to “invisible” and connect it automatically to the GND net. This is used to be common in 74xx TTL logic for example.

But it’s old school. In those times there were hundreds of TTL chips connecting all to the same power supply. and computers were slow, so drawing hundreds of power connections was a chore.

These day you have a few complex digital chips and they connect to multiple different power supplies. (Such as FPGA’s with core, I/O and other voltage rails)

So do I, so how do “we” represent that fourth lead if there is no circle?

I suppose it really is ancient history, but it would have been comforting to know if there was something official.

By the way, does anyone know of any current (modern)(SMT) transistors (not microprocessors) with electrically isolated metal parts.

My preferred solution would be to draw it without circle for normal transistors, and draw it with both a circle and a pin connected to the circle for the shielded versions.

My best guess is that the added capacitance is more detrimental then the added shielding helps for HF stuff, and the practice has ceased completely.
If shielding is needed, then also the tracks etch needs to be shielded, and a metal can further away from the active currents is a better solution.

Have a look at a teardown of a high quality spectrum analyzer on Youtube. Shielded by thick and custom milled Aluminimum.
Transceivers for Telephone poles are another example. Extreme high quality shielding.

Home high power transistors may have isolated metal parts.
Maybe IGBT modules.

I haven’t seen that recently, it is more convenient to connect the thermal pads to one of the active nets rather than isolate it as you can then use a large copper area both for heatsinking and carrying current.

Thankyou @paulvdh and @craftyjon

I was looking for confirmation.

My experiences also showed that isolated metal tags and cases now reside with the team obsolete.

There are other things in this direction.
“Silion on Insulator” for example, but that’s a high speed digital thing.

And for high power & high voltage applications inherent isolation between the dies and heatsink is a good thing.

But the semiconductor industry is not standing still, and getting an overview of modern manufacturing would need some research.

I would identify it as a passive in the symbol.
In the schematic I would simply put a no connect marker. This would be the X on the right tool list.

In EN 60617, symbol 5-5-2 has a junction to the collector to indicate the connection to the envelope.

I believe a junction can also apply in your symbol for a more accurate representation…

I don’t own that standard so I’m not sure exactly what you mean, but for a transistor where the case is not connected to the collector I would not imply that it is with the symbol…

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IEEE Std 315 (PNP) and IEEE Std 315A 8.6.2A (NPN) show similar example symbols like IEC 60617 S00664 (05-05-02). I would guess clearest is probably with an additional connection point (junction dot) where pin 4 touches the envelope in the image above (analog to how a connection to a tube/valve shield is drawn). Some libraries provide access to IEEE documents.

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I think that those 4th leads were called S as shield. I also think that they were shown as short tab close to transistor body, at least in one old HAM radio book.

But transistors with 2 collectors or emitters are much more current problem.

This would follow other conventions on other parts/modules for that time period.

I still have copies of old D.A.T.A. books dated 1981 for diodes and transistors. The fourth lead on the TO72 case was called “case”.

Today I went digging through a box of old TV circuit diagrams from the 1970s. Found some for old Philips TVs, both B&W (with variable inductance click around channel selectors) and CTVs with varicap diode tuning. Philips had pretty good circuit diagram representations.
Both types used transistors with TO72 cases (BF200, BF183, & BF182) but none of the circuits had the fourth lead represented. Google has also been of little use. :slightly_frowning_face:

Ah well, I suppose this will have to be called an “obsolete mystery” ???

I think I remember BF200, in those days transistors had names like Ferrari or MB, who would care about some modern Bgfg65565.

I had somewhere here an old British Ham radio book, but I could not find it now, has Elektor magasine used those kinds of transistors.

But pictures would be nice here.

A picture search:

… mostly shows up nothing special.
The most complicated contraption I found is:


My first post.

I found three examples of a transistor case with four leads on this page: