7805 regulator: pinout, the 2 V dropout and the heat sink

The last two digits are the output voltage: a 7805 gives 5 V, a 7812 gives 12 V, a 7815 gives 15 V. The letters in front name the maker (uA and LM for TI, L for ST, MC for onsemi) and the ones after it the package and grade, so a uA7805CKCS and an LM7805CT are the same job in the same TO-220.

The pins and the tab are the same on every member of the family. Only the output and the input range change, so read the number on the body before you swap one for another.

The absolute maximum input is 35 V in both sheets (40 V for the 7824). TI recommends 7 to 25 V for the 7805, and the LM7805 sheet says the input must stay at 7.5 V or more to hold regulation at 1 A.

Two thermal figures matter. The 54 °C/W is the LM340 sheet's figure for a TO-220 standing in free air with no heat sink, and it is in a footnote, not the thermal table. The 19 and 23.9 °C/W in the thermal tables are for TI's standard test board, which a part on wires or perfboard never sees. Design to 54 °C/W without a sink, and to the 4 °C/W junction to case when you add one.

The dropout is 2 V at 1 A in both sheets, and that is a typical figure, not a maximum. So a 7805 wants about 7 V in at the very least, and 7.5 V to be sure at full current. A 6 V battery or a 5 V USB supply is not enough.

The bias current, up to 8.5 mA over temperature, runs from the input to ground whether there is a load or not. It is small beside the load current, but it is heat too, and it is why a 7805 drains a battery even with nothing connected.

The capacitors are small. TI's circuit uses 0.33 µF on the input and 0.1 µF on the output, and the uA78xx sheet says the output capacitor is not needed for stability; it only helps the transient response. The input capacitor is the one that matters when the regulator sits away from the supply's filter capacitor. If you fit a large capacitor on the output, the LM340 sheet asks for a diode from output to input, so the output capacitor cannot discharge back through the chip if the input is shorted.

Every member needs its output plus at least 2 V, and more at the top of the family. The uA78xx sheet gives each one its own input window and output limits at 25 °C:

• 7805: 4.8 to 5.2 V out at 25 °C, 7 to 25 V in.
• 7808: 7.7 to 8.3 V out at 25 °C, 10.5 to 25 V in.
• 7810: 9.6 to 10.4 V out at 25 °C, 12.5 to 28 V in.
• 7812: 11.5 to 12.5 V out at 25 °C, 14.5 to 30 V in.
• 7815: 14.4 to 15.6 V out at 25 °C, 17.5 to 30 V in.
• 7824: 23 to 25 V out at 25 °C, 27 to 38 V in.

A 7812 from a 12 V battery does not work: it needs 14.5 V. The heat sum below works the same way for each of them, with the part's own output in place of 5 V.

Both exist (6 V and 9 V out) in the same TO-220 with the same pins, but neither TI sheet covers them, and the ST and onsemi sheets that do could not be reached for this page. We give no limits for them here rather than borrow the 7805's. Take the input range and dropout from the datasheet of the maker whose part you hold.

• 5 V for logic, sensors and small boards from a 9 V or 12 V supply, at modest current.
• A clean, quiet rail after a transformer, rectifier and filter capacitor.
• A local regulator on a card fed from a higher, noisier bus.

It is a poor choice where the drop is large and the current is high, or where a battery has to last. From 12 V it wastes more than half the input power, so a buck converter such as the LM2596 does that job better. For an odd voltage, the LM317 sets its output with two resistors; for 3.3 V from 5 V, see the AMS1117.

An Arduino board, a relay module and a few LEDs draw about half an amp. Model them as a 10 Ω load on 5 V: 500 mA. The 7805 has 0.33 µF on its input and 0.1 µF on its output, as in TI's circuit, and the air around it is 35 °C.

A heat sink's maker quotes its rating in °C/W; compare that number, plus the pad, with 21.0 °C/W before you buy. The regulator thermal design lesson works the same sums for other packages.

• Work out the heat before anything else: (input − 5 V) × load current. Above about 1.7 W at 35 °C, plan a heat sink.
• Feed it from the lowest supply that stays above 7.5 V at full load and lowest mains. Every extra volt is another half watt at 500 mA.
• The tab is ground, so a grounded chassis makes a free heat sink. Use thermal paste or a pad; a dry metal-to-metal joint adds its own thermal resistance.
• Put the input capacitor right at the pins if the bulk filter capacitor is more than a few centimetres away.
• With a big capacitor on the output, add a diode from output to input (cathode on the input) so a shorted input cannot pull current back through the chip.

Any 7805 from any maker fits the same footprint. The LM7805 and LM340-5 from TI are the tighter-spec versions of the same part. The 7805 vs LM317 vs AMS1117 page compares the three side by side, and LM2596 vs 7805 shows when a buck regulator beats it.

1. Power off, meter on continuity: the tab should read 0 Ω to the middle pin.
2. Feed 12 V from a current-limited supply with no load. The output should read 4.8 to 5.2 V.
3. Add a 10 Ω resistor rated 5 W or more as the load (500 mA, 2.5 W in the resistor), with the 7805 on its heat sink. The output should barely move.
4. Turn the supply down slowly and note where the output starts to fall. The gap between input and output there is the dropout, about 2 V.

The common mistakes: the pins read as output, ground, input (that is the order from the back), no heat sink at half an amp from 12 V, a 7812 run from a 12 V battery, and a large output capacitor with no protection diode.

Use a 7805 from 8 to 9 V where you can, with 0.33 µF in and 0.1 µF out at the pins. From 12 V, fit a heat sink of 21.0 °C/W or better for half an amp, or keep the load under about 220 mA bare. If the current is high or the input is battery power, use a buck converter instead.

The 7805 is easy to find, loose in TO-220 or on small regulator boards with the capacitors and a heat sink fitted. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes. Buy the heat sink at the same time and check its °C/W rating.