78H24 Pinout Explained: What It Really Is (and What to Use Instead)

We dug through datasheet archives, distributor databases, and manufacturer documentation to find out what "78H24" actually refers to. Here's the honest answer, along with what to use if you actually need a 24V regulator that can supply 5A.

What "78H24" Would Mean, If It Existed

The naming logic itself is sound, which is probably why this part number keeps showing up in searches. Breaking down the "78H24" designation using standard 78xx-family conventions:

• 78 — indicates a positive linear voltage regulator, part of the broad 78xx family that also includes well-known members like the 7805 and 7812.
• H — in this family, the H suffix denotes the high-current variant. Standard 78xx parts in TO-220 packages typically deliver 1A to 1.5A; the H-series steps that up to 5A, housed in a metal TO-3 can to handle the extra heat.
• 24 — would indicate a fixed +24V output, following the same voltage-suffix pattern used across the 78xx family (05 = 5V, 12 = 12V, and so on).

So on paper, "78H24" reads as "a 5A, 24V positive linear regulator." The problem is that we could not verify this specific combination was ever manufactured.

The 78H Series: A Real 5A High-Current Family

To be clear, the 78H family itself is genuine and not something we're guessing at. It dates back to Fairchild Semiconductor and National Semiconductor's original 78xx lineup, and several members are documented across multiple datasheet archives and distributor catalogs:

• 78H05 / 78H05A — 5V output, 5A rated current, TO-3 package. Listed by Fairchild Semiconductor and cross-referenced by multiple distributors as a "5-Volt 5-Amp Voltage Regulator."
• 78H12A — 12V output, 5A rated current, TO-3 package.
• 78H18 — 18V output, three-terminal regulator, also documented by Inchange Semiconductor.
• 78HGA / 78HGKC — the adjustable version of the same 5A, TO-3 platform, described as a "Positive Adjustable 5-Amp Voltage Regulator."

Notice what's missing from that list: a 24V version. That gap is the whole story behind this article.

Why We Can't Find a 78H24 Datasheet

Before publishing this, we checked Alldatasheet, Datasheet Archive, datasheetspdf, and datasheet4u — the same databases that reliably surface 78H05, 78H12A, and 78H18. None of them return a manufacturer-issued datasheet for "78H24." Searching Alldatasheet for the exact term "78H24" doesn't even return a voltage regulator; it returns unrelated Grayhill DIP switch parts (78H01T, 78H02T) that happen to share the "78H" prefix.

That's a strong signal — not proof of nonexistence, but proof that if a 78H24 was ever produced, it left essentially no trace in the documentation ecosystem that every other 78H part shows up in. It's possible it existed as a limited-run or regional part decades ago, or that the "78H24" search term is simply a mix-up between the 78H prefix and the unrelated 7824 (standard 1A version). Either way, we're not going to publish a spec table for a part we can't verify — if you find a genuine 78H24 datasheet from a named manufacturer, we'd genuinely like to see it.

Practical takeaway: if you're specifying a part for a real design or BOM, don't write "78H24" without a manufacturer datasheet in hand. Use one of the verified alternatives below instead.

78H Series vs. LM7824: Don't Confuse These Two

Most of what currently contents for "78H24" is actually written about the LM7824, which is a completely different, much more common part. Here's how they compare:

If your actual goal is "I need +24V out of a three-terminal regulator," the LM7824 is almost certainly the part you're looking for — you just won't get more than about 1A to 1.5A out of it without help.

TO-3 Pinout for the 78H Family

For the verified members of the 78H family (78H05, 78H12A, 78H18), the TO-3 metal-can pinout follows the same layout used across virtually all three-terminal TO-3 linear regulators:

• Pin 1 (Input): unregulated DC input voltage.
• Pin 2 (Output): regulated DC output voltage.
• Case / metal body (Ground): the entire metal can serves as the third terminal and the ground connection, typically bonded to the PCB or heatsink through the mounting screw.

For comparison, the standard TO-220 package used by the common LM7824 follows a different three-pin order (front-facing, text side toward you, pins down):

1. Pin 1: Input
2. Pin 2: Ground
3. Pin 3: Output

Need 24V at 5A? Here Are Real Options That Work

If your project genuinely needs a regulated 24V rail at up to 5A, here are two well-documented paths that don't rely on an unverified part number:

Option 1: LM7824 + External Pass Transistor

The classic approach for boosting any 78xx regulator's current beyond its rated maximum is to add an external power transistor that carries the bulk of the load current, with the 7824 itself only supplying the base drive and voltage reference. This is a long-standing application-note technique for the 78xx family, though the exact resistor and transistor values depend on your specific load and thermal budget.

Getting this circuit right starts with correctly identifying the transistor's base, collector, and emitter pins before you wire it in — mixing these up is one of the most common reasons a pass-transistor boost circuit fails to regulate or overheats immediately. If you're not confident reading a transistor's pinout from its datasheet, our transistor pinout guide walks through how to identify each pin on common NPN and PNP packages before you commit to a layout.

Option 2: LM338 Adjustable 5A Regulator

The LM338 is a genuinely documented, currently-produced three-terminal adjustable regulator rated for over 5A continuous output across a 1.2V to 32V range — comfortably covering 24V. It's available in both TO-3 (K-suffix) and TO-220 (T-suffix) packages, and only needs two resistors to set the output voltage. Because it's still in active production and thoroughly documented by Texas Instruments and STMicroelectronics, it's a far safer choice for a new design than chasing down a part with no confirmed datasheet.

Option 3: A Switching (Buck) Regulator Module

For most modern designs, a 24V/5A requirement is better served by a switching buck regulator than any linear part. Linear regulators like the 78H series or LM338 dissipate the voltage difference as heat, which gets expensive fast at 5A. A buck converter module handles the same job at a fraction of the power loss and heatsink size.

Thermal Dissipation: Why High-Current Linear Regulators Run Hot

Whichever linear regulator you choose, the heat math is unavoidable. Power dissipated as heat in a linear regulator is calculated as:

P(watts) = (Input Voltage − Output Voltage) × Output Current

For example, regulating a 30V input down to 24V while delivering 5A means dissipating (30V − 24V) × 5A = 30W as heat. That's a substantial amount of power to manage — it requires thermal paste between the regulator and heatsink, a heatsink sized for the junction-to-ambient thermal resistance of your package, and careful attention to the regulator's thermal shutdown threshold. This is also the core reason Option 3 (a switching regulator) is often the more practical choice at these current levels.

FAQ

Does the 78H24 voltage regulator actually exist?

We could not find a datasheet published by any traceable manufacturer for a part called "78H24." The 78H family itself is real and well-documented for 5V, 12V, and 18V versions (78H05, 78H12A, 78H18), but a 24V member of that family doesn't appear in any of the major datasheet archives we checked.

What's the difference between 78H24 and LM7824?

They're not the same part, even though they're often confused. The LM7824 is a well-documented, widely available 24V regulator rated for 1A to 1.5A in a TO-220 package. The 78H series is a separate, higher-current (5A) family in a TO-3 package — but a 24V version of that higher-current family isn't confirmed to exist.

What should I use if I need 24V output at 5A?

The LM338 adjustable regulator is a solid, currently-produced option that covers 24V within its 1.2V–32V range at over 5A. Alternatively, boosting an LM7824 with an external pass transistor, or switching to a buck converter module, are both proven approaches.

What is the TO-3 pinout for 78H-series regulators?

Pin 1 is the input, Pin 2 is the output, and the metal case itself serves as the third pin and ground connection — this is standard across virtually all TO-3-packaged three-terminal linear regulators.