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Power supplies, wires and connectors

Wires

There are three things you need to specify for a power supply. You need the correct voltage and how much it can be varied. Then you must ensure that there is sufficient current. Even small stepper motors use quite high currents of up to 2A and a multimotor controller probably uses about 2A. If you have four motors you need at least 10A. It is a very good idea to overspecify the unit, so I would choose one that can produce at least 15A. This will run cooler and protect against surges. Modern power supplies use switching technology so are very efficient at about 80%. Even so the unit will use about 300 W.

Gauge and current rating

A wire has a copper centre with one or more round conductors, surrounded by an one or more insulating sheaths. These are made from PVC which melts quite easily or PTFE, which doesn't. This is called a core.

A core is specified by the number and size of the conductors or by the cross-sectional area of all the conductors together. For example 7/0.15 means seven conductors each 0.15 mm in diameter. This has an area of 0.124 sq mm and is rated able to carry 4 amps with PTFE insulation. In the US a different sizing system is used called American Wire Gauge (AWG). The above wire is 26 AWG.

The more conductors there are, and the thicker they are, the more current the wire can safely carry. There are lots of tables on the web for the current ratings of different wires. None is the single correct answer. How much current is safe depends on whether the current is steady or intermittent, whether the cores are in free air or trapped together in a sheath or conduit, the temperature at which the insulation melts and so on

Most mains power supplies are 400W or less, so 0.5 sq mm three core flex will be fine on 230 volt mains. This flex can carry 3A continuously. For very high power supplies you might move to 0.75 sq mm. 7/0.2 should be fine for motor currents of up to 2A. For control signals the wire can be very thin, as only a few tens of milliamps are needed. However to save money there is no harm in using the thicker wires you have for motors, provided they can be made to fit into the plug and socket headers you want to make.

Connectors

Jumper wires like these are essential for making quick circuits using Arduino boards and peripherals. This is a male jumper that plugs into the tiny sockets. They hold in surprisingly well.

 

This is a female jumper that fits over a post sticking out of the board.

 

Jumpers are best bought, but when setting up the completed circuits it is best to make leads out of these crimp terminals. They are snapped off the band and then crimped onto the wires using needle nose pliers. You can buy a tool for nearly £50 but don't need it. Insert the completed wires into the housings below.

 

These are the female crimp terminals.

 

You buy these housings in strips, often called PC header housing. You clip off the amount you need, trim the rough edges and finally push in the connectors until they click. These housings are not polarised, meaning that they can be used the wrong way round. You can buy polarised housings, like Molex, but this is unnecessary if you colour code the wires.

 

Here are home-made leads using the crimps and housings.

 

A cheaper way to make up multiway plugs

The crimps shown above work well, but at about 12p per crimp the cost mounts up if you are making lots. I have devised a much cheaper way of doing it.

Instead of crimps, use these sets of pins called header strips. Snap off the number you need.

This time I needed a strip of six pins.

 

Pull the pins out with some needle-nosed pliers. If you don't, you will melt the plastic part of the strip when you solder, making the whole thing useless.

Tin one end of the pin.

Tin the bared end of the wire. You need it to be about 3 mm long.

Solder the wire to the pin.

Push the pin back into the plastic strip.

All six wires have been soldered into place.

 

Slip on lengths of heat shrink sleeve. They must be about 10 mm long and 2.4 mm bore. Shrink the sleeve using a heat gun. Take great care to use the minimum heat possible to avoid melting the plastic strip.

Power leads

 

I think power leads are best soldered and then strengthened and insulated with heat shrink sleeve. Crimps are usually reliable but a power lead coming loose can destroy circuits and components as well as being potentially lethal.

In this case the connectors are crimp fork terminals adapted for soldering.

 

 

How I do it

This is the crimp fork as bought.

 

Remove the insulation.

 

Slip on the heat shrink and solder the wire into the terminal.

 

Use a heat gun to shrink the sleeve into place.

 

Job done

 

 

How to get the voltages you need

It is common to find that you need different voltages. The Arduino board needs 9V at the main power socket or 5V if you feed it in at the Vin socket. Stepper motor controllers and drivers need voltages somewhere between 5V and 36V. Generally it is best to buy a power supply for the highest voltage you need (usually the motor controller) then step the voltage down using a ready-made device or one of the 7800 series of chips that step a dc voltage down. They only produce about 1 amp but that is more than enough for Arduino boards. You use a 7805 for 5V or a 7809 for 9V and other voltages are available. The 1000uF capacitors are electrolytics of at least 36V and the 100nF is a ceramic or polyester. Total cost including components, board, heat sink and terminals is about £2.50 if you buy sensibly on eBay. Obviously if you need a current of more than about an amp at the lower voltage, you will have to use a separate power supply.

This is simple to turn into a small device.

Or you could buy one of these tiny adjustable voltage changers for about £4.00.

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(C) Peter Scott 2012

Last edit 23 December 2015