The Power

Every circuit we have built so far has run on a 9 V battery. Plenty of projects run on a wall adapter instead, the black box on a cord that plugs into the wall. The one on the bench is labelled 9 V DC, 2 A. DC, short for direct current, means the current flows one way only, as it does from a battery. The adapter keeps its plug at a steady 9 V, and 2 A is the most current it can give. Do you think it pushes 2 A into each of the four devices on the bench?

It does not. A device takes the current it needs, and the adapter gives it, up to what it can. The 2 A on its label is called the current rating, the most it is made to give before its voltage starts to drop. That is why the blower was too much for it. A real adapter pushed past its rating gets hot, so never leave one working like that.

A lot of small projects run on AA cells instead of a 9 V battery. The holder on the bench below takes a row of them and feeds a small motor. One tray has ordinary cells, called alkaline, and the other has cells you can recharge. There is also a flat coin cell, and a 9 V battery that opens to show what is inside. In The Voltage we saw that cells joined + to − add their voltages. Which row do you think turns the motor faster, four ordinary cells or four rechargeable ones?

The ordinary cells turned it faster. The rechargeable ones are NiMH cells, short for nickel metal hydride, and each gives a little less voltage than an ordinary cell. What a cell is made of sets its voltage.

With any of these cells, never join the + straight to the − with a wire. That is a short circuit, and even one AA cell soon makes the wire too hot to touch. Bigger batteries are worse. A shorted car battery can burst, and a shorted lithium battery can catch fire. Never open a real battery either.

A computer’s USB socket gives 5 V, and a device may take only so much current from it. On the bench below, a small USB meter sits in the socket and shows the voltage and the current, with the socket’s limit beside it. How do you think the socket treats a device that needs more than it allows?

It switched the cup warmer off. Any device may take up to 100 mA as soon as it is plugged in. To take more, it has to ask first, through two small pins in the socket that carry messages, and the phone did. The cup warmer took more than it was allowed, so the socket’s protection cut its power. Newer USB-C sockets can agree with a device on more current, and even on a higher voltage.

The plug at the end of an adapter’s cord is a round barrel plug, with a metal sleeve outside and a contact in the centre. Either one can be the + side. Most adapters put + in the centre, but not all, and on an old adapter the mark that says so is often worn away. The two adapters below both say 9 V DC, 1 A, and neither shows which side is +. How would you find out with a multimeter?

They read opposite signs. With the red probe on the centre and the black on the sleeve, a + reading means the centre is +. A − reading means the sleeve is. Which contact is + and which is − is called the plug’s polarity. So before an adapter with no mark goes into anything, we check it with the meter.

The radio below needs 9 V DC and up to 500 mA, and its socket is marked + in the centre. Four adapters hang on a rack, each with a clear label. Which one would you plug in?

The radio played only on the 9 V, 1 A adapter with + in the centre. Its current rating is higher than the radio needs, and that does no harm. Before plugging an adapter into a real device, read its label beside the device’s. The voltage and the polarity have to match, and the adapter’s current rating has to be at least what the device needs.

At a real bench, we usually power circuits from a bench supply, which plugs into the wall. One knob sets its voltage, and the other sets a current limit, the most current it will let flow. It shows the voltage and the current it is giving, and it has two small lights marked CV and CC.

Here the supply powers a 470 Ω resistor and a red LED. It is set to 9 V, with the limit at 20 mA. A stray jumper wire lies beside the board. Will the LED survive if the jumper lands across the resistor?

It did. With the jumper across the resistor, nothing but the LED was left to hold the current back. The supply kept the current at its limit and let its voltage drop instead. The CC light, for constant current, came on to show it. Until then the CV light, for constant voltage, was on, because the supply was holding the 9 V it was set to. With the limit at 2 A, the same mistake burnt the LED out, as it would a real one. So set the limit just above what the circuit should take, before you switch the supply on.

Current is what hurts people, too. A shock is current flowing through the body, and Ohm’s law decides how much, with the body as the resistance.

Our skin keeps the current through the body small. Dry skin has a high resistance, so a small voltage can push only a tiny current through it. Wet skin has much less resistance. The bench below uses a model of the body. It works out the current from three batteries, an AA cell, a 9 V battery and a 12 V car battery. How do the three currents change as the skin gets wetter?

All three rose together. With soaked skin, the 12 V car battery pushed past the 10 mA line the table marks as dangerous. Through dry hands, the same battery would not be felt at all. Dry your hands before you touch a circuit, even a low-voltage one. The voltage at a wall socket is at least ten times higher, and there skin cannot be trusted at all. Everything we build in the lessons ahead runs on low voltages like these, with the high voltage shut away inside the adapter’s case.

Five questions, if you want to check yourself.

Sometimes one part of a circuit needs less voltage than the source gives, such as 3 V from a 9 V battery. How can we get it? In The Divider we will make a smaller voltage from a bigger one, using two resistors.