Thyristor Package Description TO-94 Bolt Mount Style
 TO-94 Stud-Mount Package | TO-94 Terminal Identification: The threaded bolt is the Anode and is used to support the device to what ever the component is mounted to, a metal plate for example. The thick metal cable, attaches to the Cathode within the device. The large diameter hole at the end of the cable is used to attach that lead of the semiconductor to the circuit. The small diameter wire attaches to the Gate of the semiconductor, and also has a small attachment for a bolt. A secondary wire terminal acts as an auxiliary Cathode lead. However if this extra terminal is not required it may be cut off before installation, or anytime after installation [and left unused]. The redundant terminal wire is used for shielding stray signals which might trigger the device. Note the wires may be color coded. |
The physical dimensions are listed in the table below. The call-outs in the graphic above match those in the table. The outline graphic to the left is only provided to show another example, but there are no dimensions for that pictorial. |
 TO-94 Component Dimensions |
In both examples the threaded bolt represents the Anode of the Thyristor. The thicker cable is the Cathode lead, and the single wire is the Gate of the Thyristor. Although the auxiliary Cathode lead is a separate wire, the terminal is actually attached to the base of the thicker Cathode lead. So this is a three terminal SCR Thyristor, which happens to have two individual Cathode leads, although both are attached to the Cathode of the semiconductor.
The Anode or bolt portion of the component body should be attached to a Printed Circuit Board [PCB] or metal heat sink while the opposite terminals of the component [Cathode and Gate] use lead wires to attach to the circuit.
This example of the TO-94 package is used by a Thyristor, but any two or three terminal semiconductor could use the package. The drawing calls out specific wire insulation colors for the Gate and Cathode line, however those colors used in the drawing may not be standard and could change depending on the manufacturer. In addition, there is no guarantee that another device does not reverse the Anode and Cathode connections on the component. However if a lead reversal did occur the component would either have a completely different part number, or have a 'R' appended to its original part number to indicate that the functions had been reversed.
A number of Thyristors use this package style including; 2N1910, 2N1911, 2N1913, 2N1915, 2N1916, 2N18095, and 2N1806 devices. Although these part number are just examples, many other semiconductors use this package type.
Note that one of the drawings uses both a designation of TO-94 and a TO-209 designation. This usually indicates that TO-94 is the older package designation, and the TO-209 is the newer designation. The AC suffix indicates that this package [dimension] is a part of a larger family which includes other dimensions and variations included under the TO-209 drawing.
Part Substitution
Other component packages may be physically similar to the TO-94, but different enough to cause concern. Many of these bolt mount pages don't have wire leads, they have fixed metal studs [see the list to the left]. In this case, a TO-94 uses cables which are integral to the body of the part, which then attach to the circuit. A similar package having rigid metal terminals would require lead wires to connect the device terminals to the circuit. The lead wires would be a new line item in the parts list. In addition a new drawing might be required showing the placement of the lead wires running between the semiconductor and the circuit.
The point of course is to point out that some times finding a replacement part, that also happens to be smaller, may still cause additional work. In this case the additional work might be adding a new call-out for stripped-wire. Changing the original part number in the parts list is a requirement, or at least having to add the new part as a substitute.
The only reason to mention part substitution at all is because most of the 3-terminal stud-mount package listed on this site are fixed-stud. Same basic package, but one style uses flexible wire and the other style uses a metal lug.
