The DO-5 package is JEDEC outline DO-203AB: an 11/16 in hermetic hex with a 1/4-28 UNF stud and a flexible pigtail, holding rectifier dies rated 35 A to 60 A and Zener dies rated 50 W. It is not DO-203AA, which several package databases claim. Below are the registered dimensions, the four-way torque table and which entry applies to your heat sink, and what the package still costs.

Key takeaways

• JEDEC designation is DO-203AB. Sources calling DO-5 “also known as DO-203AA” are describing DO-4, a smaller package with a 10-32 stud.
• Stud is 1/4-28 UNF; hex is 17.44 mm across flats maximum; overall height 25.40 mm maximum; mass 14 g to 17 g depending on vendor.
• Torque depends on how the heat sink is built. Vishay allows 3.4 N·m (30 lbf·in) on the nut for a pass-through hole and 4.2 N·m (37 lbf·in) on the hexagon for a tapped hole. Lubricated threads drop both by about a third.
• RθJC runs 0.65 °C/W to 1.1 °C/W across the rectifier families, and case-to-sink is 0.25 °C/W greased. Adding a mica insulator triples that.
• Polarity flips by device type. DO-5 rectifiers are cathode-to-stud by default; DO-5 50 W Zeners are anode-to-case by default.
• Surge capability is the reason to choose it: up to 1050 A peak and 52,500 A²s for fusing on the 60 A part.

[IMAGE 1: Photograph of a DO-5 stud rectifier standing on a machined heat sink next to its 1/4-28 nut, lock washer, mica insulator and shoulder washer, with a torque wrench in frame | alt: “DO-5 package stud-mount diode with 1/4-28 nut, mica insulator and shoulder washer on a heat sink”]

What the DO-5 package is

DO-5 is a JEDEC outline for stud-mounted power rectifiers and Zener diodes, registered as DO-203AB. The body is a welded, hermetically sealed metal-and-glass hex, 11/16 in across flats, with a 1/4-28 UNF threaded stud on one face and a flexible lead on the other. The stud carries the current and the heat.

One error is worth killing before it costs someone a heat sink. At least one component-marketplace package database states that DO-5 is “also known as the DO-203AA” and then describes an axial-leaded part on roughly 10.16 mm pitch, mounted through-hole to a PCB. DO-203AA is DO-4, a smaller stud package with a 7/16 in hex and a 10-32 thread, and neither package is axial-leaded. DO-5 is DO-203AB.

The distinction matters mechanically, not just in a parts database. A DO-4 stud will spin freely in a hole tapped for DO-5, and a DO-5 will not enter a DO-4 hole at all. [INTERNAL LINK: DO-4 (DO-203AA) -> DO-4 stud-mount diode package reference]

DO-5 dimensions, letter by letter

The registered outline below is reproduced in Microsemi document T4-LDS-0138 rev. 2, which covers the JAN-qualified 1N1184 through 1N3768 series. Inches are the controlling units; millimetres are for information.

Ltr	Inches (min–max)	Millimetres (min–max)	Notes
OAH	— to 1.000	— to 25.40	Overall height
HF	0.667 to 0.687	16.95 to 17.44	Hex across flats, nominal 11/16 in
CD	— to 0.667	— to 16.94	Cannot exceed HF
CH	— to 0.450	— to 11.43
SL	0.422 to 0.453	10.72 to 11.50	Stud length
HT	0.115 to 0.200	2.93 to 5.08	Chamfer or undercut optional
HT1	0.060 to —	1.53 to —
B	0.250 to 0.375	6.35 to 9.52
J	0.156 to —	3.97 to —
ΦT	0.140 to 0.175	3.56 to 4.44	Terminal diameter
C	— to 0.080	— to 2.03
M	0.030 to —	0.77 to —

Four notes on that drawing do real work. The maximum pitch diameter of the plated thread is the basic pitch diameter of 0.2268 in (5.76 mm) per FED-STD-H28, so heavy plating can push a stud out of gauge and bind in a correctly tapped hole. The unthreaded shank runs 0.220 in to 0.249 in (5.59 mm to 6.32 mm). Complete threads must reach to within 2.5 threads of the seating plane. And because the chamfer at the hex end is optional, the seating face can be as small as 0.600 in (15.24 mm) in diameter.

Vishay’s outline for the same package agrees and adds a metric option: an M6 × 1 stud in place of 1/4-28 on metric devices. Its terminal diameter reads 3.80 mm to 4.10 mm against Microsemi’s 3.56 mm to 4.44 mm. Package mass is 14 g for the Microsemi rectifier, about 15 g for its 50 W Zener and 17 g for Vishay’s parts.

The stud is the design constraint: torque and which way you tighten

Most stud-diode guidance gives one torque figure. Vishay gives four, and the useful part is the pair of footnotes telling you which one applies to your heat sink.

Tightening method	Thread condition	Maximum torque	Applies to
On the nut	Not lubricated	3.4 N·m (30 lbf·in)	Pass-through holes
On the nut	Lubricated	2.3 N·m (20 lbf·in)	Pass-through holes
On the hexagon	Not lubricated	4.2 N·m (37 lbf·in)	Tapped heat sink holes
On the hexagon	Lubricated	3.2 N·m (28 lbf·in)	Tapped heat sink holes

Read that as a decision, not a range. If the stud passes through a clearance hole and is captured by a nut, torque the nut. If the heat sink is tapped and the stud threads into it, you have no nut to turn, so you torque against the hexagon and the allowance is higher. Vishay quotes these with a tolerance of +0 % and −10 %, and Microsemi’s outline independently states the device must survive 30 in-lb applied to a 0.250-28 UNF-2B nut.

Lubricated threads cut the allowance by roughly a third because more of the applied torque becomes clamping force. Grease on the seating face is required; grease on the threads changes the number you are allowed to use.

The two failure modes are unchanged from every other stud package: over-torque, and cutting threads by driving the stud into an untapped hole. Both warp the hex base and can crack the die, and the part often survives its first test and fails weeks later. Vishay’s mounting note allows only a nut and washer, and requires heat sink flatness under 0.03 mm with surface roughness under 0.02 mm to DIN/ISO 1302, with no particle thicker than 0.05 mm in the joint.

Polarity: rectifiers and Zeners disagree

In DO-5 the default polarity depends on what kind of die is inside, and the suffix means “the opposite of this family’s default” rather than any fixed terminal.

• Microsemi 1N1184 through 1N3768 rectifiers: standard polarity is cathode to stud; an R suffix gives anode to stud.
• Vishay 1N1183, 1N3765, 1N1183A and 1N2128A rectifiers: same convention. The base number is cathode to case, and R gives anode to case, as in 1N1188R or 1N2135RA.
• Microsemi 1N3305 through 1N3350B and 1N4549B through 1N4556B, 50 W Zeners: standard polarity is anode to case, and R gives cathode to case.

So a bare “DO-5” in a bill of materials tells you nothing about which terminal bolts to the chassis. On a shared, grounded heat sink that ambiguity either shorts a rail or forces isolation hardware you did not budget for thermally.

Reading a DO-5 part number

The strings are dense and every field changes what you receive. Microsemi’s nomenclature for the rectifiers is JAN 1N1184 R e3, and the Zener series adds a tolerance letter.

Field	Values	Meaning
Reliability prefix	JAN, JANTX, JANTXV, or blank	Screening level; blank is commercial. Rectifiers qualify to MIL-PRF-19500/297
Type number	1N1184, 1N3768, 1N3319 and so on	JEDEC registered device
Polarity	R, or blank	Reverse of that family’s standard polarity
Tolerance (Zeners)	B, A, C, D, or none	±5 %, ±10 %, ±2 %, ±1 %, and ±20 % when absent
RoHS	e3, or blank	RoHS compliant, commercial grade only

Worked example from Microsemi’s own datasheet: 1N3319RB is a reversed-polarity 20 V Zener with ±5 % tolerance. Note the ordering trap in that scheme, because a plain R with no tolerance letter means reverse polarity at ±20 %, not reverse polarity at the default tolerance.

Terminal finish tracks the grade. Hot solder dip in Sn63/Pb37 is standard on JAN, JANTX and JANTXV parts; RoHS matte tin over nickel is available on commercial grade only. If the assembly is a lead-free line, confirm which you are receiving.

[IMAGE 2: Exploded diagram of a DO-5 mounting stack showing tapped heat sink, thermal compound, hex seating face, mica insulator, fibre shoulder washer, solder terminal, lock washer and 1/4-28 nut | alt: “DO-5 package mounting stack with mica insulator, shoulder washer and 1/4-28 nut”]

How much heat the package will actually move

Junction temperature follows TJ = TA + PD × (RθJC + RθCS + RθSA). The first two terms are published; the third is your problem.

Parameter	Value	Source and conditions
RθJC, 35 A JAN rectifier	0.8 °C/W	Microsemi T4-LDS-0138 rev. 2, DC
RθJC, 35 A to 60 A families	1.1 °C/W down to 0.65 °C/W	Vishay 93492, varying by die size
RθCS, greased	0.25 °C/W	Vishay 93492, smooth flat greased surface
Measured joint, bare metal, greased	0.1 °C/W	Motorola AN1040, DO-203AB at 25 in-lb
Measured joint, bare metal, dry	0.2 °C/W	Motorola AN1040
Measured joint, 5 mil mica, greased	0.6 °C/W	Motorola AN1040
Measured joint, 5 mil mica, dry	0.8 °C/W	Motorola AN1040

Worked example. Take a 1N1190 at 600 V dissipating 40 W, in a 45 °C enclosure, designed to a 150 °C junction against the 175 °C rating. Allowed rise is 105 °C, so the total thermal path must stay at or under 2.625 °C/W.

• Bolted straight to the sink with grease: 0.8 + 0.25 leaves 1.575 °C/W for the heat sink.
• Isolated with greased 5 mil mica: 0.8 + 0.6 leaves 1.225 °C/W.
• Isolated with dry mica: 0.8 + 0.8 leaves 1.025 °C/W.

Isolating the device costs about 35 % of the heat sink budget at this power level. Isolating the whole sink instead, where the mechanical design allows it, is almost always the cheaper answer. [INTERNAL LINK: heat sink sizing -> junction temperature and thermal resistance calculation]

Above 150 °C case temperature the Microsemi part derates linearly at 1.4 A per °C up to its 175 °C limit, so a 35 A device is worth roughly zero amps by 175 °C. Design the case temperature, not the ambient.

Mounting hardware and isolation

The DO-5 stack is not improvised. Keystone’s 4730 kit, stocked by major distributors, is a five-piece assembly covering DO-5 as well as TO-48 and TO-61: a mica insulator, a fibre shoulder washer, a solder terminal, a lock washer and a 1/4-28 nut. Specifying it by part number removes a class of assembly error.

Two mounting rules carry over from Motorola’s application note and are worth restating. Where an isolated stud device uses an internal beryllium oxide layer, hold the hex with a wrench while the nut is torqued so the ceramic never sees shear. And the flexible lead is a glass-to-metal seal that takes neither bending nor axial load, so route it with slack in braided or stranded wire and never let the eyelet support the device.

What ships in DO-5 and what it costs

Part	Type	Key ratings	Sourcing note
1N1184 to 1N3768 (R)	JAN-qualified rectifier	100 V to 1000 V, 35 A at TC 150 °C, IFSM 500 A, RθJC 0.8 °C/W	MIL-PRF-19500/297, JAN through JANTXV
VS-1N1183 series (Vishay)	Standard recovery	50 V to 600 V, 35 A, IFSM 500 A at 60 Hz, I²t 1140 A²s	TC to 190 °C
VS-1N3765 series	Standard recovery	700 V to 1000 V, 35 A, IFSM 400 A	The high-voltage arm of the family
VS-1N1183A series	Standard recovery	50 V to 600 V, 40 A, IFSM 800 A, I²t 2900 A²s	VFM 1.3 V at 126 A
VS-1N2128A series	Standard recovery	50 V to 600 V, 60 A, IFSM 900 A, I²t 3700 A²s	Highest current in the outline
UES806 (Microchip)	Fast recovery, trr 50 ns	400 V, 50 A, VF 1.25 V at 50 A	$71.18 at qty 1, 100 in stock
S34160 (Microchip)	Standard recovery	1600 V, 45 A, VF 1.15 V at 90 A	$33.86 each at 100, 45-week lead time
1N3305–1N3350B, 1N4549B–1N4556B	50 W Zener	3.9 V to 200 V	1N4554A about $46.44 each at 100, 42-week lead time

Two sourcing facts matter more than the prices. Lead times on the non-stocked Microchip parts run 42 to 45 weeks, so a DO-5 device is a buy-ahead item rather than something you find during a build. And distributor parametric data on these old part numbers is unreliable: DigiKey currently lists the 50 W 1N4554B as a 500 mW device. Read the manufacturer datasheet, not the filter field.

Counterfeit exposure follows the same pattern as every hermetic mil-grade package: high unit value, old JEDEC numbers, heavy brokerage, and marking that is easy to reproduce. Buy franchised where the part is JAN-screened. Where you cannot, curve-trace forward voltage and reverse leakage against the datasheet before the part goes into a traction, welding or aerospace assembly. [INTERNAL LINK: counterfeit screening -> detecting counterfeit and remarked semiconductors]

When DO-5 is the right answer

1. Check surge and I²t before average current. The reason to accept a stud package is capability like 1050 A peak and 52,500 A²s of fusing energy on the 60 A part, which small packages cannot reach.
2. If continuous current is under about 25 A and surge is undemanding, DO-4 is smaller, lighter and cheaper. Above 60 A, DO-5 is out of headroom and you want a module.
3. Confirm the polarity variant against the node your heat sink sits at, using the datasheet line rather than the package name.
4. Decide isolation early. It sets the heat sink, and at 40 W it costs roughly a third of the sink budget.
5. Match torque method to heat sink construction: nut for pass-through, hexagon for tapped.
6. For a new design that is not constrained by hermeticity, JAN screening or surge, price an isolated modern package against the 42 to 45 week lead time before committing.

Frequently asked questions

Is DO-5 the same as DO-203AB?

Yes. DO-203AB is the JEDEC registration for the outline originally registered as DO-5, and datasheets print them together as “DO-5 (DO-203AB)”. Some package databases wrongly pair DO-5 with DO-203AA. DO-203AA is DO-4, a smaller stud package with a 7/16 in hex and a 10-32 thread.

What size is the DO-5 stud thread?

1/4-28 UNF, on an 11/16 in hex body measuring 16.95 mm to 17.44 mm across flats. Vishay offers a metric variant with an M6 × 1 stud. Maximum pitch diameter of the plated thread is 0.2268 in (5.76 mm), which is the figure to check when a stud will not turn freely.

How much torque should a DO-5 diode get?

It depends on the mounting. For a pass-through hole with a nut, 3.4 N·m (30 lbf·in) dry or 2.3 N·m (20 lbf·in) lubricated. For a tapped heat sink where you tighten against the hexagon, 4.2 N·m (37 lbf·in) dry or 3.2 N·m (28 lbf·in) lubricated, per Vishay document 93492.

What is the difference between DO-4 and DO-5?

Size and capability. DO-4 has a 7/16 in hex and a 10-32 stud, weighs about 7 g and carries roughly 12 A to 35 A. DO-5 has an 11/16 in hex and a 1/4-28 stud, weighs 14 g to 17 g and carries 35 A to 60 A with a 50 W Zener option. Their mounting holes are not interchangeable.

Is the stud the anode or the cathode on a DO-5 diode?

It depends on the device family. DO-5 rectifiers such as the 1N1184 and 1N1183 series are cathode to stud by default, with an R suffix reversing it. DO-5 50 W Zeners in the 1N3305 series are anode to case by default, with R reversing that. Always read the polarity line.

What mounting hardware does a DO-5 diode need?

At minimum a 1/4-28 nut and a washer, plus thermal compound on the seating face. Where the stud cannot share the heat sink potential, add a mica insulator and an insulating shoulder washer. Keystone’s 4730 kit bundles all five pieces for DO-5, TO-48 and TO-61 cases.

What to do next

If you are replacing a failed part, match the JEDEC number and the polarity suffix exactly, then torque by the method your heat sink demands: the nut for a clearance hole, the hexagon for a tapped one. If you are selecting for a new build, size the heat sink from the insulated interface figure rather than the bare-joint one, because at 40 W that single choice moves the requirement from 1.575 °C/W to about 1.0 °C/W. And order early. A 42 to 45 week lead time turns a late package decision into a schedule problem, not a cost one.