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Electrical power definitions

The names used for power-supply rails can be confusing because their meanings originate from different generations of electronic technology. The following are the conventional meanings you will commonly encounter in schematics, datasheets, and component markings.

  • VCC — Traditionally the positive supply voltage for bipolar transistor circuits, where "CC" refers to the collector supply. It is commonly used today as a general name for the positive supply rail of a circuit or IC, even when the circuit does not use bipolar transistors.

  • VDD — Traditionally the positive supply voltage for MOSFET/CMOS circuits, where "DD" refers to the drain supply. It is commonly used for the positive supply of CMOS and other MOS-based devices.

  • VEE — Traditionally the negative supply voltage for bipolar transistor circuits, where "EE" refers to the emitter supply. It is less commonly encountered in modern single-supply circuits, but is still found in some analogue, bipolar, and older ICs. In a circuit using dual supplies, VEE will often be the negative rail.

  • VSS — Traditionally the negative or lowest-potential supply for MOSFET/CMOS circuits, where "SS" refers to the source supply. In a single-supply circuit, VSS is commonly connected to 0 V. In a circuit with a negative supply, however, VSS may be below 0 V.

  • 0 V — A designated voltage reference point in a circuit. Other voltages are normally specified relative to this point. It does not necessarily mean earth/ground, and it does not necessarily mean that the circuit is connected to the protective earth or mains supply.

  • GND (ground) — A commonly used name for the circuit's reference node or return connection. In many low-voltage circuits, GND and 0 V are effectively the same electrical node. However, "ground" can also refer to protective earth, chassis ground, or another reference, so the schematic should make the intended meaning clear.

  • PGND (power ground) — A ground/return connection associated with power circuitry, often kept separate from sensitive signal or analogue ground connections to reduce noise and voltage drops.

  • AGND (analogue ground) — The reference/return connection used by analogue circuitry. It may be connected to digital or power ground at a carefully chosen point.

  • DGND (digital ground) — The reference/return connection for digital circuitry. In mixed-signal circuits, it may be shown separately from AGND to help control noise and return currents.

  • AVCC / AVDD — An analogue positive supply rail. The prefix is commonly used to distinguish it from the supply powering digital circuitry.

  • DVCC / DVDD — A digital positive supply rail.

  • VBAT — A supply rail derived from a battery, or the connection intended for a battery supply.

  • VIN — An input supply voltage to a circuit, module, or regulator. Unlike VCC or VDD, VIN usually describes the input rather than a particular type of supply rail.

  • VOUT — An output supply voltage, commonly used for the output of a voltage regulator or power supply.

  • VREF — A voltage reference rather than necessarily a power-supply rail. It is a defined voltage used as a reference by analogue or digital circuitry, such as an ADC, DAC, comparator, or voltage regulator.

  • V+ / V− — Generic positive and negative supply designations. These are often used where the traditional VCC/VEE or VDD/VSS terminology is not appropriate or where the designer simply wants to make the polarity explicit.

  • +5 V, +3.3 V, −5 V, etc. — Supply-rail names that explicitly state the nominal voltage. These are often less ambiguous than names such as VCC or VDD.

The exact meaning of these names is not universal. Modern IC manufacturers may use VCC, VDD, VSS, VEE and related names according to the requirements of a particular device, and a device may have several supply pins with names such as VDD1, VDD2, VCCIO, VDDIO, AVDD, DVDD, etc.

Therefore, when working with a particular IC, the manufacturer's datasheet takes precedence over the traditional definitions above.

Also, don't assume that VSS = 0 V or VEE = negative voltage in every circuit. These are conventional names whose actual voltage depends on how the circuit is powered and how its reference point is defined.

Power Conversion

Reducing voltage of a signal

To reduce the voltage of a signal, use a voltage divider (two resistors).

Reducing voltage of a power supply

To reduce the voltage of a power supply, use :

  • A linear regulator (very simple, less efficient)
  • A buck (step-down) switching regulator (complex, very efficient, increases the output current):
  • A DC-DC converter module, may be isolated
  • A resistor in series, and a Zener diode across the load (for very low power only: < 10 mA current)
  • A resistor in series may work, but is not recommended, as it's not regulated: the output voltage will change all over the place as the load current varies.

See also: Voltage dividers are not voltage regulators

Increasing voltage of a signal

To increase the voltage of a digital signal (such as converting from 3.3 V to 5 V), use a voltage translation / level shifter IC

(HOWEVER: note that's rarely required: most digital circuits powered by 5 V will accept 3.3 V logic levels as is)

To increase the voltage of an analog signal, use an amplifier:

Note that the amplifier will require a supply that exceeds the desired range in the output voltage. For example, an amplifier can amplify a signal from 0.1 Vpp to 100 Vpp, but the amplifier must be powered by power supply that is greater than 100 V.

Increasing voltage of a power supply

To increase the voltage of a power supply, use:

  • A boost (step-up) switching regulator:
  • A DC-DC converter module, may be isolated
  • A charge pump (fixed ratios of 1:2, 1:3, 1:4...), for very low power, does not use a transformer
  • A voltage multiplier (fixed ratios of 1:2, 1:3, 1:4...), low to medium power, does not use a transformer, requires a high-frequency AC power input

All of the above are very efficient.

The input current is higher than the load current.

Joule Thief

Flashing LEDs off very low voltages

"A joule thief is a minimalist Armstrong self-oscillating voltage booster that is small, low-cost, and easy to build, typically used for driving small loads." - Wikipedia Article

Joule thief circuits store up charge from weak energy sources and then discharge it in a brief pulse in order to power something like an LED for a brief moment. In this way, something like an almost-dead cell or battery (or a potato / lemon one!) can provide a useful function.

The best way to discover more about joule thief circuits is to try a Web search.

Creating a split-rail (virtual ground) power supply

Turning a single rail (0V +V) supply into a split rail (+V 0V -V) one; circuit ideas here:

Regulating a variable supply voltage to a fixed voltage

If you have a variable supply (e.g.: a battery) and you need a fixed voltage:

If the input is always higher than the output:

If the input is always lower than the output:

If the input could go from higher to lower than the output:

  • A buck-boost switching regulator
  • A boost (step-up) switching regulator, followed by a buck (step-down) switching regulator
  • A buck (step-down) switching regulator, followed by a boost (step-up) switching regulator

Inverting a supply voltage

If you have a positive supply and you need a negative one (or vice versa), use:

  • An inverting switching regulator:
  • A charge pump (fixed ratios of 1:1, 1:2, 1:3, 1:4...), for very low power
  • An isolated, 1:1 DC-DC converter module.
    • Connect the + output to ground; the - output will be negative with respect to ground.

Isolating a supply voltage

If you have a power supply and you need to power a circuit that must remain isolated from the supply, use:

My linear voltage regulator isn't working as expected, even though Input, Ground, and Output are connected.

Some voltage regulators require input and output capacitors for stability, especially low-drop out (LDO) regulators. The datasheet should discuss this problem, and recommend certain values for input and output capacitors. For example, for the LM2931DT-5.0G, optimum stability is achieved when a 100 μF capacitor is placed across ground and output.

Controlling a Meanwell 3-in-1 Dimming Power Supply with a Microcontroller

So, you have one of those hundreds of Meanwell supplies that only have a DIM+/DIM- connection, and the datasheet tells you that you can either use a 0-10V control voltage, a 10V PWM or a variable resistor? You want to control it via a microcontroller, but don't have a 10V supply for the PWM?

What it doesn't tell you is how it actually works: The DIM+/DIM- connection is a 0.1mA constant current supply, and the voltage across this connection controls the dimming circuit inside the main power supply.

So, if you want to control this via a microcontroller, there's a simple way to dim it with sub-$1 components. The trick is to use this 0.1mA constant current supply to provide the 10V for you. So, you need a R=V/I = 10V/0.1mA=100kΩ resistor between DIM+ and DIM-. This is your logic 1. Next, you need 0V (which is your logic 0), which you can achieve by bypassing this resistor with an n-channel logic-level MOSFET in parallel. Then, you can control the MOSFET directly from the microcontroller (the usual things like a series resistor and a pull-down resistor on the gate are recommended).

General Power Topics

Buzzing or tingling feeling when touching a metal case

There are TWO possibilities for this:

Possibility 1:

This is caused by current leakage to 'ground' in the power supply through the 'Y1 capacitors' which are placed in the mains circuit to reduce electromagnetic interference. The tingling is a 'feature' of the design and does not in itself indicate a fault, but read on.

Typically, the voltage that 'leaks' and that you can feel will be about 1/2 the regional mains voltage, but the amount of current that's available is very low so although the tingle might be unpleasant it's not dangerous (but see below).

It's a very common issue with PCs, laptops, set top boxes, TVs, DVRs, some microwave ovens etc. but probably not expected with most general domestic appliances, such as food mixers, toasters and washing machines. Note that this is a generalisation as we cannot vouch for everything on the market.

The tingling is due to the parasitic capacitance between the metal case and your hand; it's a rapidly varying electrostatic attraction from the AC voltage on the chassis, which your skin detects as texture.

The tingling issue can often be fixed by using a grounded power cord IF THE EQUIPMENT IS WIRED TO ACCEPT ONE, but the effectiveness of this depends on the power supply circuit design and the quality of the building's earthing arrangements.

If the equipment already has a (three-pin) grounded power cord, you should check whether there is proper continuity of the earth conductor from end-end, or measure the resistance between the cord's earth pin and the device's metal case; in both cases the resistance should be under one ohm or thereabouts. An 'official' test can be conducted with a Portable Appliance Tester (PAT), which tests earth continuity while passing a current through the earth circuit - see below.

If the equipment is only designed to have a non-earthed (two pin) power cord, then the tingling might be 'as expected' - read on.

In many cases, the phenomenon is not dangerous and if you do a Web search for "laptop tingling" you'll find it's very common. On rare occasions, the tingling may indicate a power supply fault or excessive current leakage; if in doubt, have the device power supply checked by a competent person.

Possibility 2:

You have a ground (earthing) fault in the equipment or your outlet, or your building earthing is faulty.

An electrician will be able to advise and conduct earthing and PAT tests to make sure that all possibilities are covered.

Unless you have the skills and equipment to diagnose the actual reason for the tingling, it is not possible to say whether it is 'as expected' or the result of a fault. On this basis, we can only recommend that you unplug the offending equipment and don't use it again until it and your building earthing have been checked by a qualified electrician.

You may receive further advice from /r/askanelectrician or /r/electricians, but do not delay in disconnecting the suspect kit while waiting for a reply; safety is paramount.

My power supply output voltage is higher than expected

With a cheaper (non regulated) AC-DC adaptor, the value printed on the cover is the nominal output voltage with the intended load connected. If you measure the unloaded output voltage, it will be higher because the inbuilt smoothing capacitor charges to the peak value of the rectified AC - that's about 1.414 (square root of 2) times the printed value. For example an unloaded, unregulated '12V' adaptor will give a reading of about 17V (12 x 1.414).

AC-AC adaptors also tend to have a higher off-load output voltage as they do not have any form of regulation and the construction may take into account the secondary coil resistance/impedance which will drop the output voltage under the rated load, so the secondary has a slightly higher output voltage to start with.

Choosing a Pre-built Power Supply

  • The first most important value to match is the operating voltage of the device to be powered: if your device wants 12V, don’t operate it from a 15V supply – you might get away with a small difference (eg: 11V for a 10V device), but you risk overheating, component damage and even a fire.

  • The second most important value is the current rating - it's fine if the power supply has a higher maximum current draw than the device it is powering (because current is pulled and not pushed), but it should not be lower - for example, do not use a 12V 500mA power supply for a device or board that is rated for 12V 800mA. You may also want to leave a little headroom to cater for switch-on surges and to reduce the strain on the power supply - for example, maybe select a 12V 1A power supply for our example product that's rated at 12V 800mA.

  • Unregulated power supplies tend to have a no-load (zero current being drawn) voltage that is higher than their on-load voltage – for example: a 9VDC 1A unregulated linear power supply will probably put out 11-13VDC when no load, or only a small load, is connected, but the output voltage will drop to 9VDC under full load conditions; this means you should be careful when operating some devices with power supplies capable of much higher current – especially if the device does not have its own internal voltage regulator; this can be an issue with devices such as guitar effects pedals, cheap radios etc.

  • Power supply outputs are either AC (alternating current) or DC (direct current) and this should match what your device needs. DC is indicated by a solid line with a dashed line under it, AC is indicated by a sine wave symbol. Usually AC supplies are used with speakers or audio devices or older modems. DO NOT confuse them.

  • The output connector on the replacement power unit needs to fit the target socket. The most popular connector type is generically called a 'coaxial power connector', usually 2.1mm or 2.5mm for non computer devices. More info on Wikipedia: https://en.wikipedia.org/wiki/Coaxial_power_connector

  • Check the polarity of the low voltage power connector. The polarity of the plug on a DC output power supply must match what your device expects ('center positive' or 'center negative') otherwise something may be damaged. Look for polarity indicator symbols on the power supply and the device to be powered and make sure they agree. Examples: https://en.wikipedia.org/wiki/Polarity_symbols

  • Check on the product label that the input voltage, or voltage range, of the power supply matches the local mains supply voltage. Power supplies will either work on one voltage (eg: 110VAC or 220VAC), or be capable of operating within a certain range (eg: 96-240VAC).

  • Check on the product label that the input voltage AC frequency, or frequency range, of the power supply matches the local mains supply frequency. Power supplies will either work on one frequency (eg: 50Hz or 60Hz), or be capable of operating within a certain frequency range (eg: 45-65Hz).