Powering and Controlling Your Devices; Part 1 - Determining Voltage and Current Requirements

In addition to being an electronics engineer and programmer, I’m an avid Maker and have been tinkeringwith electronics for over 40 years. During that time I’ve build dozens of electronic devices for use inand around my house and have helped friends build all types of projects. Two questions I’m often askedare; “How do I figure out what size power supply I need to power my device?”, and “How can I turn mymotor, solenoid, appliance or string of lights on and off remotely?”

This is the first in a series of Instructables on Powering and Controlling devices. This one shows youhow to determine what the voltage and current requirements for your device are. Other Instructables willshow you how to select a power source, how to select interface components, and how to connectcontrollers to your your device.

If you want to buy or build a power supply for your device you’ll need to know the exact voltage andcurrent required by your device. If you want to construct an interface for controlling your device, sayfrom a microcontroller or switch box, than you just need to have a general idea of the voltage andcurrent requirements.

Step 1: Is it AC or DC Powered?

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Posted:Aug 10, 2012

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1 of 8 9/25/2013 5:33 AM

There are three things you need to know about your device before you can select a power source orinterface components; Is the device AC or DC powered, what voltage does it require to operate, andhow much current does it draw.

Alternating current (AC) devices are ones you plug into a wall outlet and include things like householdlight bulbs, appliances, and fans. Some AC powered devices are powered by a transformer that reducesthe voltage from the wall outlet to a lower voltage.

Direct current (DC) devices are things that typically run off batteries; small motors, LED’s and panellights, automotive accessories, portable electronics and most toys. Some DC devices will be poweredby an AC-to-DC converter that you plug into the wall - often referred to as Wall Warts.

Step 2: Determining AC Requirements

If the device is AC powered it will typically require around 120 volts or 220 volts, depending on whatcountry you live in. All of North and Central America use 110 to 130 volts, while most of Europe andAsia use 220 to 240 volts. Most AC powered devices will have a rating label on them – one that showsthe voltage and, current or wattage, requirements. If the label only lists the voltage and wattage, youcan get a rough estimate of the current by dividing the wattage by the voltage. It’s not exact, but it’sclose enough for determining what interface components you can use. If you have a device without arating label or spec sheet you’ll need to measure the current. I show you how in later steps.

Step 3: Determining DC Voltage Requirements

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The voltage required by DC powered devices can range anywhere from 1-1/2 volts to 24 volts or more. Most devices you’ll encounter will operate off 3, 6 or 12 volts. Some will operate off a “wall wart”AC-to-DC converter while others will use batteries. If it uses an AC-to-DC converter, look at the label onthe converter for the voltage and current. If it’s battery powered, multiply the number of batteries usedby the battery voltage, which should be marked on the battery. The common, household alkaline batteryis rated at 1.5volts. Other types, like Nickel Cadmium, Nickel Metal Hydride or lithium-Ion, can havemuch higher voltages.

Your device may have markings on it indicating the current requirements, but that's not too common. Most likely, you'll have to measure the current draw of your device - I show you how in the followingsteps.

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Step 4: What You Need to Measure Current

In addition to your device, you’ll need the following items to determine the current requirements of yourdevice:- Ammeter or Digital Multi-meter (DMM)- Test Leads- Power Source; wall socket for AC, power supply or batteries for DC- For AC powered devices you’ll also need a 3-wire power cord- Wire Nuts (or Electrical Tape)

To measure current you need an Ammeter or a Digital Multi-Meter that can measure current. You'llconnect the meter in series with your device and the power source – the current will run from yourpower source, through your meter and then into your device. The following steps will show you how tomake these connections.

Step 5: AC Current Setup

WARNING - this procedure involves voltages that can cause severe shocks, burns or even killyou. If you don’t feel confident working with these voltages have an experienced personperform this step for you. To ensure your safety, the ends of all wires used in this procedureshould be covered by wire nuts or electrical tape.

Make the following connections, using wire nuts or electrical tape to secure the connections:1. Earth/Ground Wire - If your device has a metal case, or a green or yellow/green wire connect thatto the green or yellow/green wire of your power cord. This is a safety connection to prevent you fromgetting shocked in the event your device has a short circuit.

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2. Neutral Wire - Connect the white wire on your device to the white wire of your power cord. Use theBlue wires for 220-240 volt devices.

3. Hot Wire - Connect the red test lead to the black wire of your power cord. Use the power cord’sbrown wire for 220-240 volt devices.

4. Connect the black test lead to the black wire on your device.

Step 6: Measuring AC Current Draw

1. Connect your test leads to your meter. Plug the red test lead into the socket labeled 10A (20A onsome meters). Plug the black test lead into the socket labeled COM.

2. Set your meter to measure AC on the 10A scale. Some meters require you to select AC/DC with oneswitch and the current range with another. Other meters will have one switch that selects both. Consultyour meter’s manual if you’re not sure.

3. Turn your meter on and plug the power cord into the wall outlet.

4. The meter’s display will show the current draw in amperes - this is the value you need.

Step 7: DC Current Setup

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Direct current (DC) devices have a positive lead and a negative lead. For some device it’s important toconnect these correctly or the device may be destroyed. For others, it doesn’t matter. Light Bulbs – doesn’t matter LED’s – matters (won’t light if leads are reversed) Toys – usually matters (device may be destroyed if leads are reversed) Electronic Devices - matters (device may be destroyed if leads are reversed) Motors- matters (the polarity of the leads determine the direction the motor turns)

Make the following connections:

1. Positive Wire – Connect the red test lead to the positive terminal of your battery or power source. This terminal may be marked with a + sign or marked in red.

2. Negative Wire – Connect a wire from the negative terminal of your battery or power source to thenegative terminal of your device. These terminals may be marked with a - sign or may be marked inblack.

3. Connect the black test lead to the positive terminal of your device. This terminal may be marked witha + sign or may be marked in red.

Step 8: Measuring DC Current Draw

1. Connect your test leads to your meter. Plug the red test lead into the socket labeled 10A (20A onsome meters). Plug the black test lead into the socket labeled COM.

2. Set your meter to measure DC on the 10A scale. Some meters require you to select AC/DC with oneswitch and the current range with another. Other meters will have one switch that selects both. Consultyour meter’s manual if you’re not sure.

3. Turn your meter on. If your power source has a power switch, turn it on.

4. The meter’s display will show the current draw in amperes - this is the value you need.

Step 9: Summary

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Jul 3, 2013. 11:50 AM Reply (CFW0WEAHINO3M2J)

3(/member/UConTroll/)

Jul 3, 2013. 12:23 PM Reply (CNPZLQQHINOEDOA)

You can use the voltage and current values you've obtained for determining what size power supply yourdevice requires, or to determine the components needed to interface your device to a microcontroller(or other control device). Other parts in this series of Instructables will show you on how to do this.

shekharmirajkar (/member/shekharmirajkar/) says:

is this ON LINE TEST or device pin isunpluged ?

UConTroll (/member/UConTroll/) (author) in reply to shekharmirajkar

As indicated in step 3 you need to"plug the power cord into the walloutlet".

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Powering and Controlling Your Devices; Part 1 - Determining Voltage and Current Requirements