Wall Wart Voltage.
February 10, 2013 1:24 PM Subscribe
Are wall wart output voltages load dependent? I am testing out a salvage piece of electronics (Sanyo micro cassette transcriber). The 9 VDC power supply is missing. I have a 9VDC, 1200 MA, class 2 wall wart in my salvage pile. When polarity tested after soldering on a new connector, I got an unloaded voltage reading of 12.34 VDC. Should this voltage drop down after it sees a load?
Unregulated adapters are load dependent. At the VA rating, the output voltage specified is as labeled.
A regulated supply will be stable up to at least the rated output current. A random box of used adapters usually contains some of each.
What you have is an unregulated supply.
posted by FauxScot at 1:41 PM on February 10, 2013
A regulated supply will be stable up to at least the rated output current. A random box of used adapters usually contains some of each.
What you have is an unregulated supply.
posted by FauxScot at 1:41 PM on February 10, 2013
Yeah, pick up something like this 16 ohm, 50 watt chassis-mount resistor if you want to test the power supply at 50% load without risking your transcriber.
posted by scose at 2:34 PM on February 10, 2013
posted by scose at 2:34 PM on February 10, 2013
Limor from Adafruit has a good tutorial on DC power supplies that shows how the load affects voltage in the cheaper, unregulated converters. Her conclusion for that style of wall-wart:
Now we can at least understand the thinking behind saying "9V 200mA" on the label. As long as we are drawing less than 200mA the voltage will be higher than 9V.posted by autopilot at 4:32 PM on February 10, 2013
You can tell by picking them up - if they're heavy, it's because they have a lot of metal in them - a big transformer. Those are highly load dependent and the only way to get a steady voltage similar to what the label says is to have a steady current draw similar to what the label says.
If they're light (and/or small) and feel like the heaviest part of them might be their plastic case, those are switch-mode voltage adaptors, and they are voltage-regulated (and generally just better), and you can trust them to do what's on the label.
posted by anonymisc at 5:17 PM on February 10, 2013
If they're light (and/or small) and feel like the heaviest part of them might be their plastic case, those are switch-mode voltage adaptors, and they are voltage-regulated (and generally just better), and you can trust them to do what's on the label.
posted by anonymisc at 5:17 PM on February 10, 2013
Anonymisc has half of it: The heavy transformer-based supplies may or may not have a regulator after the rectifier. The only way to be sure is to measure with and without load.
The light switch-mode type are all regulated, and more efficient too.
posted by Myself at 5:28 PM on February 10, 2013
The light switch-mode type are all regulated, and more efficient too.
posted by Myself at 5:28 PM on February 10, 2013
As a very informative extension, have a look-see at this blog post which is a lab test of a dozen different USB AC adapters. Particularly relevant to the discussion at hand are the charts about halfway down the page which show the load characteristic; that is the relationship between the current draw and the voltage of the supply.
posted by metroidhunter at 5:56 PM on February 10, 2013
posted by metroidhunter at 5:56 PM on February 10, 2013
The heavy transformer-based supplies may or may not have a regulator after the rectifier.
For what it's worth, I've taken apart quite a lot of these over the years, and I've never seen one that does have a regulator. Some of them don't even have smoothing capacitors.
posted by flabdablet at 6:10 PM on February 10, 2013
For what it's worth, I've taken apart quite a lot of these over the years, and I've never seen one that does have a regulator. Some of them don't even have smoothing capacitors.
posted by flabdablet at 6:10 PM on February 10, 2013
The heavy transformer-based supplies may or may not have a regulator after the rectifier.
For what it's worth, I've taken apart quite a lot of these over the years, and I've never seen one that does have a regulator.
Actually, the small, light-weight converters, called off-line converters are a relatively recent innovation. Traditionally, a heavier AC line voltage transformer was used for galvanic (safety) isolation followed by either a linear or switching regulator to provide a regulated output voltages. Off-line converters provide galvanic isolation using the switching transformer itself at the switching frequency. The higher frequency allows a smaller transformer. But until recently, both regulated and unregulated converters used heavy AC transformers, so you really can't tell without seeing the specs or testing.
posted by JackFlash at 7:57 PM on February 10, 2013
For what it's worth, I've taken apart quite a lot of these over the years, and I've never seen one that does have a regulator.
Actually, the small, light-weight converters, called off-line converters are a relatively recent innovation. Traditionally, a heavier AC line voltage transformer was used for galvanic (safety) isolation followed by either a linear or switching regulator to provide a regulated output voltages. Off-line converters provide galvanic isolation using the switching transformer itself at the switching frequency. The higher frequency allows a smaller transformer. But until recently, both regulated and unregulated converters used heavy AC transformers, so you really can't tell without seeing the specs or testing.
posted by JackFlash at 7:57 PM on February 10, 2013
Quite so. And to clarify the original question: if you have a wall wart nominally rated 9VDC that tests at 12.34VDC open circuit, that's pretty normal for a simple filtered but unregulated type.
The usual circuit for these things consists of a mains frequency transformer (operating at 50 or 60Hz necessitates a relatively heavy core and windings, which is why this kind of wall wart is bigger and heavier than the newer off-line switching type) followed by a full-wave rectifier and (if you're lucky) a filter capacitor.
The output of an unfiltered full-wave rectifier is a train of half-sine-wave pulses at twice the input frequency. If you were to feed that into a purely resistive load and measure the average power dissipated by that load, you'd be able to work out what voltage you'd need from a perfect direct current supply to get you the same power dissipation. That value is the wall wart's RMS output voltage, and is a reasonable thing to specify as "the output voltage" for an unregulated, unfiltered supply.
If you add a filter capacitor to the output of such a rectifier and don't load it, then on each pulse the capacitor will charge to the pulse's peak output voltage and stay there. The peak of a half-sine pulse is √2 times its RMS value, so a half-sine pulse train at 9.00V RMS will have a peak voltage of 12.7V and this is about what you'd see if you used a typical voltmeter to measure a filtered (but unregulated) 9VDC wall wart unloaded and plugged into the exact mains voltage it was designed for.
The voltmeter reading you'd get from an unregulated, unfiltered, unloaded 9V RMS wall wart depends on the design of your voltmeter. Some show RMS voltage, some show peak voltage, and some show something in between.
posted by flabdablet at 10:45 PM on February 10, 2013 [1 favorite]
The usual circuit for these things consists of a mains frequency transformer (operating at 50 or 60Hz necessitates a relatively heavy core and windings, which is why this kind of wall wart is bigger and heavier than the newer off-line switching type) followed by a full-wave rectifier and (if you're lucky) a filter capacitor.
The output of an unfiltered full-wave rectifier is a train of half-sine-wave pulses at twice the input frequency. If you were to feed that into a purely resistive load and measure the average power dissipated by that load, you'd be able to work out what voltage you'd need from a perfect direct current supply to get you the same power dissipation. That value is the wall wart's RMS output voltage, and is a reasonable thing to specify as "the output voltage" for an unregulated, unfiltered supply.
If you add a filter capacitor to the output of such a rectifier and don't load it, then on each pulse the capacitor will charge to the pulse's peak output voltage and stay there. The peak of a half-sine pulse is √2 times its RMS value, so a half-sine pulse train at 9.00V RMS will have a peak voltage of 12.7V and this is about what you'd see if you used a typical voltmeter to measure a filtered (but unregulated) 9VDC wall wart unloaded and plugged into the exact mains voltage it was designed for.
The voltmeter reading you'd get from an unregulated, unfiltered, unloaded 9V RMS wall wart depends on the design of your voltmeter. Some show RMS voltage, some show peak voltage, and some show something in between.
posted by flabdablet at 10:45 PM on February 10, 2013 [1 favorite]
But until recently, both regulated and unregulated converters used heavy AC transformers, so you really can't tell without seeing the specs or testing.
I don't know what value of "recently" you're using, but if someone were tasked with a day to find such an ancient beast in the wild, even going through the dusty box of old and orphaned supplies collecting in the corner at the thrift stores, I'd put my money on coming up empty.
I'm old enough to remember the days before switch-mode was consumer-level, and I wouldn't call it "recent". I don't think I even still use any appliances from that era.
posted by anonymisc at 11:11 AM on February 11, 2013
I don't know what value of "recently" you're using, but if someone were tasked with a day to find such an ancient beast in the wild, even going through the dusty box of old and orphaned supplies collecting in the corner at the thrift stores, I'd put my money on coming up empty.
I'm old enough to remember the days before switch-mode was consumer-level, and I wouldn't call it "recent". I don't think I even still use any appliances from that era.
posted by anonymisc at 11:11 AM on February 11, 2013
I'd put my money on coming up empty.
You'd lose. About half the wall warts in my house are the 50Hz transformer type.
Speaking as an IT technician, it's only been in the last two or three years that I've stopped being pleasantly surprised to find that this router or that network switch or the other portable DVD player comes with something light and cool rated at 100V-250V input that isn't slightly wider than the socket spacing on the typical Australian power strip, and started expecting this as normal.
posted by flabdablet at 4:26 PM on February 11, 2013
You'd lose. About half the wall warts in my house are the 50Hz transformer type.
Speaking as an IT technician, it's only been in the last two or three years that I've stopped being pleasantly surprised to find that this router or that network switch or the other portable DVD player comes with something light and cool rated at 100V-250V input that isn't slightly wider than the socket spacing on the typical Australian power strip, and started expecting this as normal.
posted by flabdablet at 4:26 PM on February 11, 2013
You'd lose. About half the wall warts in my house are the 50Hz transformer type.
You're not following. Half of them are transformers, and those transformers will not be regulated transformers. It is a safe assumption in this day and age that if a wall wart is light, it's regulated, if it's heavy, it isn't.
posted by anonymisc at 3:07 PM on February 12, 2013
You're not following. Half of them are transformers, and those transformers will not be regulated transformers. It is a safe assumption in this day and age that if a wall wart is light, it's regulated, if it's heavy, it isn't.
posted by anonymisc at 3:07 PM on February 12, 2013
if someone were tasked with a day to find such an ancient beast in the wild
It was not clear to me that the "ancient beasts" you were betting against were mains-frequency transformer based wall warts with regulators, as opposed to mains-frequency transformer based wall warts in general.
As I believe I said earlier, I've never actually encountered a mains-frequency transformer based wall wart with regulation.
Are we in heated agreement yet?
posted by flabdablet at 7:13 PM on February 12, 2013
It was not clear to me that the "ancient beasts" you were betting against were mains-frequency transformer based wall warts with regulators, as opposed to mains-frequency transformer based wall warts in general.
As I believe I said earlier, I've never actually encountered a mains-frequency transformer based wall wart with regulation.
Are we in heated agreement yet?
posted by flabdablet at 7:13 PM on February 12, 2013
This thread is closed to new comments.
How much voltage drops under load depends on the supply internal resistance and the amount of current draw.
posted by Diplodocus at 1:36 PM on February 10, 2013