Voltage regulator

Mokxi teamParts

The models: the 78xx family (7805, 7806, 7809, 7812, 7833), the negative 7905 and 7912, the AMS1117 modules (3.3, 5.0 and 1.8 V), the LD1117-3.3, and the low dropout MCP1700-3.3, HT7333 and AP2112K-3.3. Aliases: LM7805, LM7812, LM7833, 78xx, 79xx, LDO, 3.3 V regulator. For the adjustable LM317, see its own article.

A linear regulator is a transistor between IN and OUT that an amplifier opens just far enough to hold OUT at its voltage. Two things follow. It needs some voltage across itself, the dropout, so below OUT plus the dropout on IN the output follows the input down. And whatever the load draws comes through from IN: a 7805 turning 12 V into 5 V at half an amp burns 3.5 W as heat.

Pins

Pin	What it does
`IN`	The unregulated input.
`GND`	Ground, and the bias current's way out.
`OUT`	The regulated output.

The legs are drawn where the real part has them. The 7805 reads IN GND OUT from the left, printing toward you, with the tab on ground. An MCP1700 in TO-92, flat face toward you and legs down, reads GND IN OUT, and is drawn that way. The AMS1117-3.3 is drawn as the little blue module, whose header reads VIN GND VOUT; the bare AMS1117 in SOT-223 soldered on it is GND OUT IN with the tab on OUT, and not every module puts its header in the same order, so follow the silkscreen on the one in your hand.

The 7806, 7809, 7812 and 7833 read IN GND OUT like the 7805. The negative 7905 and 7912 read GND IN OUT, with the tab on IN. The LD1117-3.3 in TO-220 reads GND OUT IN, tab on OUT. The HT7333 in TO-92 reads GND IN OUT, like the MCP1700. The AMS1117-5.0 and -1.8 and the AP2112K-3.3 are drawn as modules with a VIN GND VOUT header; the bare AP2112K is a five-pin SOT-25 with an enable pin, which the module ties to VIN.

Properties

model is 7805, 7806, 7809, 7812, 7833, 7905, 7912, AMS1117-3.3, AMS1117-5.0, AMS1117-1.8, LD1117-3.3, MCP1700-3.3, HT7333 or AP2112K-3.3.

legs is package (the model's real order, above, which every new part gets) or schematic, which draws IN GND OUT from the left whatever the model. Older projects open with schematic, so their breadboards keep every leg in the hole it was in.

`model`	output	dropout	bias current	current limit	input
`7805`	5 V	2 V at 1 A	4.2 mA	2.2 A peak	7 V to 35 V
`AMS1117-3.3`	3.3 V	1.1 V at 0.8 A	5 mA	1.1 A	4.4 V to 15 V
`MCP1700-3.3`	3.3 V	178 mV at 250 mA	1.6 uA	408 mA	3.5 V to 6 V
`7806`	6 V	2 V at 1 A	4.3 mA	2.2 A peak	8 V to 35 V
`7809`	9 V	2 V at 1 A	4.3 mA	2.2 A peak	11 V to 35 V
`7812`	12 V	2 V at 1 A	4.4 mA	2.2 A peak	14 V to 35 V
`7833`	3.3 V	2 V at 1 A	4.2 mA	2.2 A peak	5.3 V to 35 V
`7905`	-5 V	1.1 V at 1 A	1 mA	2.2 A peak	-7 V to -25 V
`7912`	-12 V	1.1 V at 1 A	1.5 mA	2.2 A peak	-14 V to -35 V
`AMS1117-5.0`	5 V	1.1 V at 0.8 A	5 mA	1.1 A	6.5 V to 15 V
`AMS1117-1.8`	1.8 V	1.1 V at 0.8 A	5 mA	1.1 A	3.3 V to 15 V
`LD1117-3.3`	3.3 V	1.1 V at 0.8 A	5 mA	950 mA	4.4 V to 15 V
`HT7333`	3.3 V	90 mV at 40 mA	3.5 uA	none given	up to 12 V
`AP2112K-3.3`	3.3 V	250 mV at 600 mA	55 uA	about 0.8 A	up to 6 V

The figures are the typical ones from TI's uA7805C, LM340 and LM79xx sheets, ST's L78 and LD1117 sheets, AMS's AMS1117, Microchip's MCP1700, Holtek's HT73xx and Diodes Inc's AP2112 sheets. Neither the HT7333's nor the AP2112K's sheet gives a current limit in its table: the HT7333's is an assumption at 0.3 A, past its 250 mA rating, and the AP2112K's is read off its sheet's graph. The 7833 (sold as the LM7833) is not in ST's or TI's 78xx sheets; its figures are Taitron's LM78xx sheet, which gives only a maximum bias current (8 mA), so the model uses the 7805's 4.2 mA and 2.2 A limit.

Negative regulators. A 7905 or 7912 holds OUT its voltage below GND. Its IN goes to the negative end of a supply, and the readout is negative.

While it runs

The readout is OUT above GND.

What the model gets right

Dropout. The pass transistor is a threshold and a resistance, fitted to meet each sheet's dropout at its test current: the 78xx as 1.7 V plus 0.3 ohm, the 79xx as 0.8 V plus 0.3 ohm, the AMS1117 as 1.0 V plus 0.125 ohm, the LD1117 as 0.99 V plus 0.14 ohm, the MCP1700 as 0.71 ohm, the HT7333 as 2.25 ohm and the AP2112K as 0.42 ohm. So a 7805 on four AA cells (6.4 V fresh) gives about 4.7 V, and an AMS1117-3.3 on one full 18650 (4.2 V) gives about 3.2 V. The MCP1700 holds 3.3 V from the same cell, which is why battery projects use it.

The input current is the output current, plus the bias current, which goes out of GND. A 9 V battery feeding a 7805 and 50 mA sags by (50 + 4.2) mA times its own resistance.

Load regulation. The output droops a few millivolts from no load to full load, fitted to each sheet: 15 mV for the 7805, 7 mV for the AMS1117, 1% for the MCP1700, 45 mV for the HT7333 (from 1 to 80 mA).

One way only. A regulator cannot sink current, and with its input gone it passes nothing back.

What it does not model

The 78xx's foldback into a dead short, the AP2112K's foldback to 50 mA, heat and thermal shutdown, line regulation and ripple rejection, the AMS1117's 5 mA minimum load, and the output capacitors every sheet asks for: a real regulator can oscillate without them, and this one cannot. The body diode that lets a real CMOS regulator conduct backwards is not modeled either. The fits to the dropout are assumptions that meet the sheets at their test currents.

Common mistakes

Feeding a 7805 from less than 7 V. It drops out and gives less than 5 V.

Running an AMS1117-3.3 or an LD1117-3.3 from one Li-ion cell. They need about 4.4 V to make 3.3 V. The HT7333, MCP1700 and AP2112K are the ones for a single cell.

Feeding a 7812 from a 12 V battery. It needs 14 V or more; from 12.8 V it gives about 11 V.

Wiring a 79xx like a 78xx. Its middle leg is IN, and IN goes to the negative supply.

Forgetting the heat. A 7805 dropping 7 V at half an amp needs a heatsink.

See it in action

Solar sensor node makes its 3.3 V rail with an MCP1700 from an 18650. Open it at /templates.

The 7805 voltage regulator circuit holds 5 V from a 9 V battery down to about 6.7 V in, then drops out.