Product Overview
National Semiconductor LM340T-12: a 12V Rail in a TO-220 Case
Condition
Listed as used surplus and sold untested. Stock is deep here, so the photographed piece is representative of what is on hand rather than the individual part you will receive. On that piece the plastic body is clean and the printing reads clearly: the National script logo followed by -29FJ on the top line, LM340T12 on the second, and 7812 P+ on the third. The three leads are full length and untrimmed with no solder on them in these photos. The bare metal tab carries fine surface marks and light scuffing around the mounting hole, most visible in the close-up and in the reverse view. We do not power these up, so output voltage, regulation and dropout are not confirmed here - verify against the LM340 datasheet and your own bench check before use.
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Product Overview
The LM340T-12 is National Semiconductor's three-terminal positive fixed voltage regulator set at 12V with a 1A output rating, housed in the familiar TO-220 package with a drilled tab for chassis or heatsink mounting. It is the National house designation for the part the rest of the industry catalogs as a 7812, and the photographed piece carries both markings on the case along with the P+ code National used for its Product Enhancement tested parts. Like the rest of the LM340 family it needs no external components to hold its output beyond the input and output bypass capacitors the datasheet recommends, and it carries internal thermal overload shutdown, short-circuit current limiting and safe area protection for the output transistor. On the standard 78xx TO-220 pinout the metal tab is electrically common with the center ground pin, so check it with a meter before you bolt it to a grounded chassis. Output tolerance grade, maximum input voltage and thermal derating are not confirmed here - verify against the LM340 datasheet before use. Sold each from a stock of many; the photographs are the spec for the photographed piece only.
Key Features
- Fixed 12V positive output, 1A output rating (LM340T-12, industry 7812 class)
- TO-220 three-lead package with drilled mounting tab for heatsink or chassis
- Photographed piece marked with the National script logo, LM340T12, 7812 P+ and a lot line reading -29FJ; lot lines vary across stock
- Internal thermal overload shutdown, short-circuit current limit and output transistor safe area protection
- P+ Product Enhancement tested marking present on the photographed piece
- Leads full length and untrimmed on the photographed piece, ready for through-hole work
- Tab is common with the ground pin on the standard 78xx TO-220 pinout - confirm with a meter on your piece
- Output tolerance grade and input voltage limits not confirmed - verify against the datasheet before use
Applications
- A vintage audio restorer rebuilding a preamp supply who wants the original style linear 12V rail instead of a switcher
- A maintenance tech repairing a legacy control board where the nameplate calls out an LM340T-12 or a 7812 and the board must match
- An amateur radio builder dropping a transformer and rectifier supply down to a clean, quiet 12V for relays, logic and small accessories
- A bench builder stocking a handful of TO-220 regulators in the parts drawer so a 12V rail never stalls a Saturday project
Frequently Asked Questions
What exactly is printed on the part?
Three lines on the photographed piece: the National Semiconductor script logo with -29FJ beside it, then LM340T12, then 7812 P+. The -29FJ line is a factory lot or trace marking and we do not interpret it as a date. The P+ is National's Product Enhancement tested designation.
Is the LM340T-12 the same as a 7812?
They are the same class of device: a three-terminal positive regulator fixed at 12V in a TO-220 package, and this case is marked with both designations. Pin function and package match the common 78xx footprint. Exact tolerance grade and specification limits vary by manufacturer and suffix, so check the LM340 datasheet against your design before you commit.
Has this regulator been tested?
No. It is sold untested as used surplus. We describe what the camera shows and what the case says, and we leave the electrical verification to your bench.
Does it need a heatsink?
That depends on your input voltage and load current, since a linear regulator turns the difference into heat. At meaningful current with an input well above 12V, plan on heatsinking the tab. Work the dissipation out from your own numbers and the thermal figures in the datasheet.
It is sold used and untested - what am I actually taking on?
You are taking on the electrical test. What you are ordering is the LM340T-12 in the TO-220 package; the markings described above are the ones legible on the photographed piece, and lot lines vary across the stock on hand. What you are absorbing is the verification step we did not perform: put it in a circuit or on a curve tracer and confirm the rail before it goes into anything that matters. For a 12V regulator that is a two minute check, which is why these move at surplus terms rather than distributor terms. Shipping is a flat $8, free once your order passes $28, and orders leave our Idaho warehouse the same or next business day.
Two names live on one small case here: National's own LM340T12 and the 7812 designation every parts catalog on earth understood. Prove it out on your bench, then let it go back to doing the quiet work of holding a 12V line steady.
