In addition to being an electronics engineer and programmer, I’m an avid Maker and have been tinkering with electronics for over 40 years.  During that time I’ve build dozens of electronic devices for use in and around my house and have helped friends build all types of projects.   Two questions I’m often asked are; “How do I figure out what size power supply I need to power my device?”, and “How can I turn my motor, 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 you how to determine what the voltage and current requirements for your device are.  Other Instructables will show you how to select a power source, how to select interface components, and how to connect controllers 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 and current required by your device.  If you want to construct an interface for controlling your device, say from a microcontroller or switch box, than you just need to have a general idea of the voltage and current requirements.

Step 1: Is It AC or DC Powered?

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

Alternating current (AC) devices are ones you plug into a wall outlet and include things like household light bulbs, appliances, and fans. Some AC powered devices are powered by a transformer that reduces the 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 panel lights, automotive accessories, portable electronics and most toys. Some DC devices will be powered by 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 what country you live in.  All of North and Central America use 110 to 130 volts, while most of Europe and Asia use 220 to 240 volts.  Most AC powered devices will have a rating label on them – one that shows the voltage and, current or wattage, requirements.  If the label only lists the voltage and wattage, you can get a rough estimate of the current by dividing the wattage by the voltage.  It’s not exact, but it’s close enough for determining what interface components you can use.  If you have a device without a rating label or spec sheet you’ll need to measure the current.  I show you how in later steps.

Step 3: Determining DC Voltage Requirements

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 on the converter for the voltage and current.  If it’s battery powered, multiply the number of batteries used by the battery voltage, which should be marked on the battery.  The common, household alkaline battery is rated at 1.5volts.   Other types, like Nickel Cadmium, Nickel Metal Hydride or lithium-Ion, can have much 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 following steps.

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 your device:
- 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'll connect the meter in series with your device and the power source – the current will run from your power source, through your meter and then into your device.  The following steps will show you how to make these connections.

Step 5: AC Current Setup

WARNING - this procedure involves voltages that can cause severe shocks, burns or even kill you.  If you don’t feel confident working with these voltages have an experienced person perform this step for you. To ensure your safety, the ends of all wires used in this procedure should 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 that to the green or yellow/green wire of your power cord. This is a safety connection to prevent you from getting shocked in the event your device has a short circuit.

2. Neutral Wire - Connect the white wire on your device to the white wire of your power cord.  Use the Blue 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’s brown 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 on some 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 one switch and the current range with another.  Other meters will have one switch that selects both.  Consult your 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

Direct current (DC) devices have a positive lead and a negative lead.  For some device it’s important to connect 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 the negative terminal of your device.  These terminals may be marked with a - sign or may be marked in black.

3. Connect the black test lead to the positive terminal of your device. This terminal may be marked with a + 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 on some 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 one switch and the current range with another.  Other meters will have one switch that selects both.  Consult your 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

You can use the voltage and current values you've obtained for determining what size power supply your device 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.
<p>I am looking for a printer to use in my semi. my DC lugs(power outlets) are 12v10 amp and I also have one 12v20 amp lug. how do I calculate if my tiny 300w(200constant/300surge) dc-ac converter will power ac device requiring 100-240 VAC@50/60hz? or better yet, how to calculate max VAC output of my DC to AC converter? </p>
<p>Hi BrandonF39,</p><p>Depending on the type of printer you buy you might be able to use a DC to DC converter instead of the AC one(if it doesn't suit your needs). Some printers have those external AC to DC power supplies, and some are designed to be small and portable with built in batteries.In that case it would only be a matter of matching the (DC)voltage and amperage required by the printer.</p><p>It all depends on the existing converter you have ( exact specs and power input required should be with the unit or look the model up online) and what printer you need or would like to buy. If you need more help send me a message, I'm not an expert but I'd be happy to try and assist you. It shouldn't be too difficult to sort out. You have plenty of options .</p>
<p>Hi, I have a bluetooth speaker device that takes a DC input of 8V to 24V, How would I wire up a switch to allow me to power it via these 3 different power inputs? </p><p>1. a 12v car battery for when we're travelling with a cig socket on it, </p><p>2. a 12v wall wart for when we're at home, &amp;</p><p> 3. an 18v Makita Lithium Ion battery for when we're camping. </p><p>I'm able to do the wiring up so that each works as a separate plug in, but to have all three options on a switch would be easier than plugging/unplugging internal plugs every time.</p><p>Cheers</p>
<p>EssS1,</p><p>That would be relatively easy depending on your skill/experience level. Since the speaker has a wide voltage input of 8-24v all you need to do is wire a connector that works with all 3 power sources. Most &quot;wall warts&quot; use those male 2.1 x 5.5mm style connectors. So your best bet would be to make a male to female cable suited to those plugs. So the male end plugs in to the speaker, the female end takes the male end from the power sources. Would be any easier option than having 3 inputs as you won't(can't) use them together at the same time.</p>
<p>I'm a little confused. If I have 4 devices that convert 12V to 5V 3A (usb chargers) that would be 60W (4 * 5V * 3A). Would I need:</p><p>a) a 12V 5A PS because 12V * 5A = 60W</p><p>or</p><p>b) a 12V 12A PS because 4 devices * 3A = 12A and 12V * 12A = 144W (144 watts seems like a lot for a handfull of USB chargers)</p>
<p>I was taught </p><p>P ower = W atts</p><p>I nductive Amps = A mps</p><p>E lectromotive force = V olts</p><p>R esistance = O hms</p><p>Watts law Ohms law</p><p>P E</p><p>_ _</p><p>I * E I * R</p>
Thanks for the great info. But I have another question: At the end of this instructable, we are left knowing how to find the total current draw. But what about total voltage? Did I miss something? I looked at step 3, but it just said that we needed the current draw to determine total voltage. But that step to get from current draw to voltage was never explained. I thought about using Ohms law, but I'd have to have total resistance, which in not sure how to go about finding. Please help! And thanks again for what you've already done.
<p>I too am left wondering &quot;how do i know how many volts i need for my circuit&quot;. Have you came across anything else yet? This is another post (link) that I have found. which may be of use to you (wasn't for me).</p><p><a href="http://electronics.stackexchange.com/questions/130710/how-to-determine-how-much-current-and-or-voltage-a-given-circuit-needs/130738#130738" rel="nofollow">http://electronics.stackexchange.com/questions/130...</a></p><p>But doesn't seem to answer &quot;how do i determine how many volts I actually need to send the circuits required current&quot;</p>
is this ON LINE TEST or device pin is unpluged ?
As indicated in step 3 you need to &quot;plug the power cord into the wall outlet&quot;.

About This Instructable




Bio: UConTroll is a Wi-Fi enabled controller that lets, Makers and hobbyist control their devices using their iPad, tablet PC, smartphone or laptop. Visit our Web ... More »
More by UConTroll:A Wi-Fi based Controller for Model Trains, Halloween Props,  Energy Monitoring and More! 9V Battery Snap Connector from Scrap Powering and Controlling Your Devices; Part 1 - Determining Voltage and Current Requirements 
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