Wall Warts

Introduction To Wall Warts

The most common type of power converison is from 110 VAC (or whatever the line voltage is in your country) to low voltage AC or DC.

For example, "I have spooky eyes that run on a 9-volt battery, how can I reduce battery usage by plugging the spooky eyes into the wall socket?" - A classic requirement for low voltage DC. And "I found some surplus solenoid valves that run off of 12VAC, how do I get that voltage?" - A call for low voltage AC.

In both cases, the simplest answer is a "wall wart".

"Wall wart" is electronic slang for a small transformer with a plug attached, that hangs off of your electrical outlet. [As a humerous note, whenever this term is mentioned on one of the Halloween e-mail lists, one of the list members misreads it as "Wal-Mart", and asks what department carries power supplies.]

[photo] A "wall wart" in its natural habitat.

Wall warts are available in a range of AC and DC output voltages. They are also available for low and high power.

Selecting A Wall Wart

Selection characteristics of wall warts include:

output current
output voltage
output type:
- AC (Alternating Current)
- DC (Direct Current)
  - unfiltered (worst) probably full-wave rectified
  - filtered
  - regulated (best)
mode of operation
- linear - most common
- switching - tends to deliver more current in smaller packages

This page from the All Electronics catalog (Winter 2003 #103) shows some available products. This is just intended to illustrate the variety available.

You must shop for a wall wart that is compatible with the thing that requires the power! Here's what to look for:

Is AC or DC output required?
What voltage?
Is regulation and filtering required?
Mechanical gadgets, like motors and solenoids aren't too picky about how close you get to the desired voltage. So, it might specify "6VDC", but run just fine on 5 or 7 volts.
Electronic gadgets usually like to get exactly what they ask for. They want the power regulated to be exactly "6VDC" or whatever.
Sometimes the maker of the gadget doesn't trust you; they do filtering and regulation inside the unit. One tip off is a dual-power unit that runs off of two penlight cells (total 3VDC), or a 6VDC source of external power. Chances are that he's taking that 6VDC from outside and turning it into the same 3VDC that the batteries would otherwise provide.
Try to find out exactly what is necessary, because regulated and filtered power supplies cost more.
What current (amps or milliamps)?
It's usually OK to use a power supply that provides more current than the device consumes. But you are throwing away money, because beefier supplies cost more. Also, you might not be getting quite the voltage that you think you are getting, because output of cheap wall warts is usually specified while loaded.
You must never use a power supply with a current rating that is less than the gadget specifies.
What connector does it use?
This is the easiest thing to deal with, because some wall warts come with multiple plugs, others come with a set of adapters. If all else fails, you can cut the connector off the end of the wall wart and solder on a connector that matches your gadget that needs the power.
If the gadget runs on DC, make sure you get the polarity right!

Where To Get Wall Warts

Here are some vendors of surplus equipment who often have "wall wart" power supplies in stock:

You can also find them at your neighborhood Radio Shack. If you ask for a "wall wart" and the salesman looks at you like you have grown a spare head, try the keywords "battery eliminator" and "power adapter".

Wall Warts Put Out Wrong Voltage

WARNING: Wall warts are designed to drive a certain output current "load" and perform best when the gadget that they power is near that specified load. When a wall wart is lightly loaded (e.g. it can put out 800 mA and your gadget only draws 20 mA), the output voltage can be much higher than specified! Feeding the wrong voltage into your project can damage it.

To illustrate this point, we put a simple volt meter on a number of DC wall warts from around the home and office. The results were then arranged as tables.

Here's what we found for unregulated linear DC wall warts (probably the most common kind available):

output input description

nominal measured rating Volts AC Amps Watts brand use

3.7 VDC 8.7 V 340 mA 120 VAC 4.8 W Nokia cell phone charger

4.5 VDC 7.4 V 200 mA

7.5 VDC 10.4 V 1.5 A 120 VAC 250 mA Kodak digital photo frame

7.5 VDC 11.8 V 1.0 A 120 VAC 15 W Airlink 16-port ethernet switch

9 VDC 13.2 V 100 mA Thomson video switch box

9 VDC 11.1 V 150 mA Unitech telephone answering machine

9 VDC 14.1 V 600 mA 120 VAC .15 A Belkin 2-port KVM switch

9 VDC 13.7 V 1000 mA 120 VAC .2 A Silicore

12 VDC 17.2 V 1000 mA 120 VAC 21 W 3COM 8-port network switch

12 VDC 16.0 V 1200 mA 120 VAC 30 W Netgear 16-port hub

output	input	description
nominal	measured	rating	Volts AC	Amps	Watts	brand	use
3.7 VDC	8.7 V	340 mA	120 VAC		4.8 W	Nokia	cell phone charger
4.5 VDC	7.4 V	200 mA
7.5 VDC	10.4 V	1.5 A	120 VAC	250 mA		Kodak	digital photo frame
7.5 VDC	11.8 V	1.0 A	120 VAC		15 W	Airlink	16-port ethernet switch
9 VDC	13.2 V	100 mA				Thomson	video switch box
9 VDC	11.1 V	150 mA				Unitech	telephone answering machine
9 VDC	14.1 V	600 mA	120 VAC	.15 A		Belkin	2-port KVM switch
9 VDC	13.7 V	1000 mA	120 VAC	.2 A		Silicore
12 VDC	17.2 V	1000 mA	120 VAC		21 W	3COM	8-port network switch
12 VDC	16.0 V	1200 mA	120 VAC		30 W	Netgear	16-port hub

Here's what we found for small wall wart "switching" power supplies:

output input description

nominal measured rating Volts AC Amps Watts brand use

5.0 VDC 5.23 V 2.6 A 100-240 VAC .45 A IOGear USB hub

5.7 VDC 6.0 V 800 mA 100-240 VAC 180 mA Nokia cell phone charger

output	input	description
nominal	measured	rating	Volts AC	Amps	Watts	brand	use
5.0 VDC	5.23 V	2.6 A	100-240 VAC	.45 A		IOGear	USB hub
5.7 VDC	6.0 V	800 mA	100-240 VAC	180 mA		Nokia	cell phone charger

Here's what we found for small "brick" "switching" power supplies:

output input description

nominal measured rating Volts AC Amps Watts brand use

12 VDC 12.0 V 4.0 A 100-240 VAC 125-175 W HJC

12 VDC 12.1 V 3.3 A 100-240 VAC 1.5 A AComData external disk

12 VDC 12.1 V 2 A 100-240 VAC .55 A Venus external disk

19 VDC 19.3 V 6.3 A 100-240 VAC 1.8 A Toshiba laptop computer

24 VDC 24.2 V 2.3 A 100-240 VAC .75-1.5 A Delta Electronics

output	input	description
nominal	measured	rating	Volts AC	Amps	Watts	brand	use
12 VDC	12.0 V	4.0 A	100-240 VAC		125-175 W	HJC
12 VDC	12.1 V	3.3 A	100-240 VAC	1.5 A		AComData	external disk
12 VDC	12.1 V	2 A	100-240 VAC	.55 A		Venus	external disk
19 VDC	19.3 V	6.3 A	100-240 VAC	1.8 A		Toshiba	laptop computer
24 VDC	24.2 V	2.3 A	100-240 VAC	.75-1.5 A		Delta Electronics

Remember that all of these readings were taken while operating the power supply with no load other than the volt meter.

Get the point?