Replacing a 7805 linear regulator with a K78 switching module
Why a linear regulator on a 24 V rail wastes most of its input, what a K78 module changes, and how to read the part number.
Who this is for: Engineers replacing a linear regulator that runs too hot, and buyers sourcing the replacement.
A 7805 on a 24 V rail delivering 1 A dissipates about 19 W as heat while delivering 5 W of useful power. That is not a rounding error — it is a heatsink, a thermal design problem and an efficiency figure around 21%. On a 12 V rail it is better but still poor.
What a K78 module changes
The MORNSUN K78 family is a non-isolated switching step-down module in the same three-pin footprint and pinout as a 78xx linear regulator. Efficiency is up to about 90-92% instead of 21%, so the heat that was being dumped into the board largely disappears. In most designs the swap needs no layout change at all.
What you do need to check:
- The minimum input voltage. It rises with the output voltage — the 12 V part needs considerably more headroom than the 3.3 V one.
- Output ripple. A switcher has switching ripple where a linear had essentially none. For most digital loads this is irrelevant; for a precision analogue front end, add local filtering or keep a small LDO downstream.
- EMC. A switching module radiates where a linear did not. Keep the input and output capacitors close, keep the loop area small and follow the datasheet layout note.
- Height. The module is taller than a TO-220 lying flat.
Reading the part number
K7805-1000R3 = K78 family · 05 5 V output · 1000 1000 mA ·
R3 package and revision. So the 3.3 V version of the same module is K7803-1000R3, and
the 2 A version of the 5 V module is K7805-2000R3.
The parts we publish
| Part | Output | Input | Efficiency (max.) | Size (mm) |
|---|---|---|---|---|
| K7805-500R3 | 5 V / 0.5 A | 6.5-36 VDC | 90% | 11.60 × 7.55 × 10.16 |
| K7805-1000R3 | 5 V / 1 A | 8-36 VDC | 90% | 11.50 × 9.00 × 17.50 |
| K7805-2000R3 | 5 V / 2 A | 8-36 VDC | 92% | 11.50 × 9.00 × 17.50 |
| K7803-1000R3 | 3.3 V / 1 A | 6-36 VDC | 90% | 11.50 × 9.00 × 17.50 |
| K7812-1000R3 | 12 V / 1 A | 16-36 VDC | 90% | 11.50 × 9.00 × 17.50 |
From the current MORNSUN datasheets. Note how the minimum input voltage tracks the output: 6 V for the 3.3 V part, 8 V for the 5 V parts, 16 V for the 12 V part.
Worked example
A 24 V panel feeds a board that needs 5 V at 800 mA.
- With a 7805: 19 V dropped × 0.8 A ≈ 15 W dissipated. That needs a real heatsink and raises the temperature of everything near it.
- With a K7805-1000R3: roughly 4.4 W drawn from the 24 V rail for 4 W delivered. Under a watt of loss, and the 24 V load list drops by about 2.2 W as well.
Isolation
The K78 family is not isolated — input and output share a ground, exactly like the 7805 it replaces. If you need galvanic separation, you need an isolated DC/DC converter instead; ask us and we will quote from MORNSUN's isolated range.
Written by Raitop. Product parameters are taken from MORNSUN official datasheets; see each product page for the source, the datasheet revision and the date it was checked.
