A voltage regulator simulator: a 7805 on a 9 V battery
Mokxi is a free online voltage regulator simulator that runs in your browser, with nothing to install and no account needed. A 9 V battery feeds a 7805 regulator, and 100 ohms of load hangs on its 5 V output. One meter reads the current out of the battery, the other the output, and the regulator’s own label shows OUT while it runs.
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- 43boards running now
- 1.00xreal time, on every board
What you can do here
In the circuit above, or in the editor it opens.
- Read the output, the load current and the regulator’s own bias current
- Run the input down and watch the output drop out
- Switch between a 7805, an AMS1117-3.3 and an MCP1700-3.3
- Work out the heat a linear regulator turns the difference into
- Power a board or a sensor from the regulated rail
- Compare a regulator with a Zener diode and a resistor
What the running circuit shows
The load gets 5 V; the battery pays for 9.
The 7805 holds OUT at 5 V above its GND pin, and in the engine the meter reads 5.00 V. The 100 ohm load draws 5 V / 100 ohm = 50 mA, and the battery gives about 54.2 mA: the extra 4.2 mA is the regulator’s own bias current, out of its GND pin. The battery sags from 9.6 V to about 9.51 V under that load.
A linear regulator does not convert the extra voltage; it burns it. About 4.45 V across the 7805 at 50 mA, plus the bias current, is about a quarter of a watt of heat, and less than half of what the battery gives reaches the load. Make the load 10 ohm and the output still reads about 4.99 V at half an amp, but the regulator is now burning over 1.5 W, more than a real 7805 without a heat sink can shed for long.
Swap the battery for four AA cells, 6.4 V, and the output falls to about 4.61 V: a 7805 needs about 2 V more than its output, and four AAs do not have it. Set the model to AMS1117-3.3 and the output is 3.30 V on the 9 V battery; on one 18650 cell at 4.2 V it gives only about 3.18 V, where the low dropout MCP1700-3.3 still gives about 3.29 V.
How the regulator is modeled
A pass transistor, an error amplifier and a current limit.
The regulator is built the way the chip is: a pass transistor between IN and OUT that an error amplifier opens just far enough to hold OUT at its voltage. Its dropout, bias current and load regulation are fitted to the datasheets: the 7805 drops 2 V at 1 A and draws 4.2 mA, the AMS1117 drops 1.1 V at 0.8 A and draws 5 mA, and the MCP1700 drops 178 mV at 250 mA and draws under 2 µA. The current limits are 2.2 A, 1.1 A and 408 mA.
What it leaves out: heat and thermal shutdown, so a regulator here never gets hot whatever it burns; line regulation and ripple rejection; and the input and output capacitors the datasheets ask for, without which a real regulator can oscillate. For a voltage the fixed models do not give, the LM317 adjustable regulator sets its output with two resistors, and the Zener regulator page builds one from a diode.
Questions
Is it free?
Yes. The circuit on this page, every part in it and the editor it opens in are on the free plan, with no card.
Do I need an account?
No. The circuit runs the moment the page does, and the editor opens without one. A free account is only for saving a project to the cloud and sharing it by link.
What is dropout voltage?
The least voltage a regulator needs between its input and its output to keep regulating. A 7805 needs about 2 V, so below about 7 V in its output follows the input down; an AMS1117 needs about 1.1 V, and an MCP1700 under 0.2 V at light load.
Why does my 7805 get hot?
Because everything between the input and 5 V is turned into heat: the voltage difference times the current. 12 V in at half an amp is 3.5 W, more than a bare TO-220 can shed. A switching regulator wastes far less.
Can I power an Arduino or a sensor from the regulator?
Yes. Wire OUT to the board’s 5V or 3V3 pin, or to a breadboard rail, and the board runs from it. The battery then pays for the board’s current too, and the meter shows it.
More simulators
Each one opens on a circuit that is already running.
Load a regulator
Open the circuit, change the battery and watch the output hold, then drop out. Nothing to install, and no account needed.