I'm on a witch-hunt about walwarts. I started this witch hunt when I noticed that my son's RC car batteries didn't last very long even though we were being careful about their charging times and such. I kept wondering why his batteries were dying so quickly, so I started checking the voltage outputs from the walwarts that came with them and boy was I in for a shock (sorry for the bad pun!).
What I found was most of them are putting out WAY over their rated voltage.
I know that most of these things are unregulated and unfiltered, but I thought...surely not ALL of them are so bad? Yet that's what I find. I've checked all sorts of walwarts from all sorts of products and they almost always show to be putting out anywhere from 50% - 100% more than their rated voltage.
I've checked them with several different DMMs and even on occasion an old analog multimeter. But the results are almost always the same, so I know it's not my meter.
Does this sound reasonable, or am I missing something?
I have noticed that the more expensive the power supply is, such as for computers and cameras, the closer the rated output voltage matches what I measure. So I expect that it's just the bad design or bad components of the cheap ones that's causing the over-voltage.
Or is there something going on with these multimeters that somehow gives an inaccurate reading from cheap walwarts? Maybe something to do with the semi-sine wave voltage that they put out? I'm hoping that it's something like that, because I'm afraid now to plug in any kind of device that comes with a cheap walwart for fear of burning out the circuitry or batteries inside it, etc.
Has anybody else noticed this phenomenon?
Question about WalWarts
Re: Question about WalWarts
What you are looking at is the unloaded voltage from the walwart (as you said,
most are unregulated). When you put a load (whatever the design specifies) on
the walwart, the voltage will drop down to very near its rating. Try measuring
the voltage again while the supply is under load, ie, charging the batteries, I'm
sure you will find the voltage is much more satisfactory. You might also check it
at different times during the charge process: begining, middle, and (near) end.
That will give you an idea of the battery charge curve vs. PS voltage.
CeaSaR
most are unregulated). When you put a load (whatever the design specifies) on
the walwart, the voltage will drop down to very near its rating. Try measuring
the voltage again while the supply is under load, ie, charging the batteries, I'm
sure you will find the voltage is much more satisfactory. You might also check it
at different times during the charge process: begining, middle, and (near) end.
That will give you an idea of the battery charge curve vs. PS voltage.
CeaSaR
Hey, what do I know?
Re: Question about WalWarts
Hi tedleydo,
As Ceasar has mentioned, a lot of times we measure the unloaded output voltage and that
seem too high. Another problem with the unloaded voltage is that often there is a large
electrolytic capacitor inside the wall wart, and this charges up to the full PEAK voltage of
the wall wart and we end up measuring the peak voltage rather than the average voltage
or RMS voltage which is usually the actual dc output available under load.
Of course this doesnt mean that the wall wart is not actually putting out that higher voltage
at the time of measurement and this has to be kept in mind too as it may blow out some
devices that are made to run on the lower average or rms voltage. Once the wall wart
is loaded down however the voltage drop closer to the actual rated value.
Thus, we have to be a little careful about using wall warts for purposes that they were not
actually designed for.
There is a new problem on the rise now too though, and that is that there is a new type of
wall wart being marketed that actually has a small circuit inside with a small step down
transformer that operates at high frequency. It uses a chopper regulator circuit with isolation
transformer to get the lower output.
The problem is, these type are different in that the unloaded output measurement may be
very very close to the loaded output voltage. The cheaper older type design wall warts
used a transformer too, but only operating at line frequency, and they typically put out
1.4 times the 'rated' or 'stamped' voltage on the wall wart label. The newer ones however,
are regulated so they put out almost exactly the same voltage as they are stamped.
This means an older type wall wart with 10v stamped on it may read 14 or 15v unloaded,
but the newer regulated types would put out 10v even unloaded.
This difference can be a problem if you order a wall wart with a given voltage because you
have to know ahead of time if it is regulated or not, and sometimes it is not shown for some
web sales items on some sites.
For example, one time i ordered a 3vdc wall wart hoping to use it with a 3.3v white LED.
How 3.3v with only 3vdc wall wart? Well, the unloaded voltage of many wall warts is
1.4 times the stamped voltage, so i figured 3 times 1.4 is 4.2v peak and that would mean
at least the LED would get some current and run to some degree. Well, when i got the
wall wart in the mail i was very disappointed to see that it was actually a 3v regulated
wall wart, not a simple transformer/cap older type wall wart. It would never work at all
with a 3.3v LED because it never puts out 3.3v, only 3.0v.
The only hope then is to take it apart and hope the circuit can be modified to put out more
like 3.5v or so. Many of those regulated type can be modified with the change of one
resistor, but i dont know if this one can be yet or not, i'll have to check.
So the long and the short of it is that there are two basic types of wall warts now:
1. Unregulated that put out 1.4 times the stamped voltage (peak).
2. Regulated, that puts out almost exactly what the stamped voltage is.
To determine what type you have, you can try loading it with a current that is near the
max output and see if the voltage drops down to about 70 percent. If it drops significantly,
then it is unregulated.
As Ceasar has mentioned, a lot of times we measure the unloaded output voltage and that
seem too high. Another problem with the unloaded voltage is that often there is a large
electrolytic capacitor inside the wall wart, and this charges up to the full PEAK voltage of
the wall wart and we end up measuring the peak voltage rather than the average voltage
or RMS voltage which is usually the actual dc output available under load.
Of course this doesnt mean that the wall wart is not actually putting out that higher voltage
at the time of measurement and this has to be kept in mind too as it may blow out some
devices that are made to run on the lower average or rms voltage. Once the wall wart
is loaded down however the voltage drop closer to the actual rated value.
Thus, we have to be a little careful about using wall warts for purposes that they were not
actually designed for.
There is a new problem on the rise now too though, and that is that there is a new type of
wall wart being marketed that actually has a small circuit inside with a small step down
transformer that operates at high frequency. It uses a chopper regulator circuit with isolation
transformer to get the lower output.
The problem is, these type are different in that the unloaded output measurement may be
very very close to the loaded output voltage. The cheaper older type design wall warts
used a transformer too, but only operating at line frequency, and they typically put out
1.4 times the 'rated' or 'stamped' voltage on the wall wart label. The newer ones however,
are regulated so they put out almost exactly the same voltage as they are stamped.
This means an older type wall wart with 10v stamped on it may read 14 or 15v unloaded,
but the newer regulated types would put out 10v even unloaded.
This difference can be a problem if you order a wall wart with a given voltage because you
have to know ahead of time if it is regulated or not, and sometimes it is not shown for some
web sales items on some sites.
For example, one time i ordered a 3vdc wall wart hoping to use it with a 3.3v white LED.
How 3.3v with only 3vdc wall wart? Well, the unloaded voltage of many wall warts is
1.4 times the stamped voltage, so i figured 3 times 1.4 is 4.2v peak and that would mean
at least the LED would get some current and run to some degree. Well, when i got the
wall wart in the mail i was very disappointed to see that it was actually a 3v regulated
wall wart, not a simple transformer/cap older type wall wart. It would never work at all
with a 3.3v LED because it never puts out 3.3v, only 3.0v.
The only hope then is to take it apart and hope the circuit can be modified to put out more
like 3.5v or so. Many of those regulated type can be modified with the change of one
resistor, but i dont know if this one can be yet or not, i'll have to check.
So the long and the short of it is that there are two basic types of wall warts now:
1. Unregulated that put out 1.4 times the stamped voltage (peak).
2. Regulated, that puts out almost exactly what the stamped voltage is.
To determine what type you have, you can try loading it with a current that is near the
max output and see if the voltage drops down to about 70 percent. If it drops significantly,
then it is unregulated.
LEDs vs Bulbs, LEDs are winning.
-
Janitor Tzap
- Posts: 1709
- Joined: Sat Aug 12, 2006 5:17 pm
- Contact:
Re: Question about WalWarts
Hi tedleydo,
What MrAl, and CeaSaR are saying is quite true about wall-warts.
I've had a few RC Car's that I would race.
An old "LaTrax's" and a Tamiya Hornet.
These were not cheap RC cars.
They were Kit Cars 1/10 scale, that I had to assemble.
Then add the remote controls, and battery packs.
Plus, a special quick charger.
Basically on just the Hornet, I paid just around $200 for everything.
{Probably a lot more now.}
If your son is running one of the RC cars that you can get from Radio Shack, or Walmart.
The wall-warts that come with them to charge the Nicad is very simple.
{Because it is cheaper than making a regulated charger to include with the car.}
A.k.a: "You get what you pay for."
If your son is running a Kit RC car....
Then you should go to your local RC, or hobby shop.
And get a specialty charger, that is made for the Nicad racing batteries.
Most specialty RC chargers come with instructions on how to properly charge,
and maximize the amount of time you can get out of your racing battery packs.
There are two styles that are available.
One that you connect to a 12V Car Battery.
The other is a 120Vac, but is well regulated.
Both will have timers. But the features will vary from one manufacturer to another.
If you already have a 12V regulated bench supply.
Then just get the 12V Car Battery version one.
That's what I did.
Signed: Janitor Tzap
What MrAl, and CeaSaR are saying is quite true about wall-warts.
I've had a few RC Car's that I would race.
An old "LaTrax's" and a Tamiya Hornet.
These were not cheap RC cars.
They were Kit Cars 1/10 scale, that I had to assemble.
Then add the remote controls, and battery packs.
Plus, a special quick charger.
Basically on just the Hornet, I paid just around $200 for everything.
{Probably a lot more now.}
If your son is running one of the RC cars that you can get from Radio Shack, or Walmart.
The wall-warts that come with them to charge the Nicad is very simple.
{Because it is cheaper than making a regulated charger to include with the car.}
A.k.a: "You get what you pay for."
If your son is running a Kit RC car....
Then you should go to your local RC, or hobby shop.
And get a specialty charger, that is made for the Nicad racing batteries.
Most specialty RC chargers come with instructions on how to properly charge,
and maximize the amount of time you can get out of your racing battery packs.
There are two styles that are available.
One that you connect to a 12V Car Battery.
The other is a 120Vac, but is well regulated.
Both will have timers. But the features will vary from one manufacturer to another.
If you already have a 12V regulated bench supply.
Then just get the 12V Car Battery version one.
That's what I did.
Signed: Janitor Tzap
Re: Question about WalWarts
Yes. The rated voltage must include the rated current.What I found was most of them are putting out WAY over their rated voltage.
[cut & weld]
Does this sound reasonable, or am I missing something?
Details can be found in the above posts.
What you might consider is buying two batteries. (Not two cells, two batteries). Couple those with a constant-current 0.1C
charger. Easier on the cells and less headache for you. Use one battery while charging the other.
IMO, without a very well designed and implemented circuit any sort of quick charger can be a lot more trouble than the benefits
are worth. In fact, I've taken to designing many of my battery power supplies using the small (e.g. 6V 1.3A) sealed lead acid
batteries that can be found easily on the surplus market. Just so I don't have to bother with any elaborate charging system, or
tossing NiCd/NiMH cells before their time.
Re: Question about WalWarts
I will throw in a few cents here. A battery must be charged with a voltage somewhat higher than the "rated" voltage. For instance a car battery will float just under 13 Volts even though it is considered a "12 Volt" battery. The electronics used in cars (think CB, radar detector, ham radio etc.) are rated at 13.8 Volts. When the engine is running the alternator is charging the battery and you will see about 14.2 Volts or even higher when the electrical system is under light load.
Think of it this way, the voltage has to be higher in order for the battery not to be drained by the charger. The higher voltage depends on the number of cells. For something like a 1.2 volt single cell NiCad the "typical" voltage to charge it would be about 1.5 volts.
There are several types of wall warts as well. Cheap versions use a step down transformer, pair of diodes, and a filter cap. These will (as others have already stated) float at a much higher voltage. I had one 12V that would get as high as 18V unloaded. I thought something was wrong with it but if I put a small load (such as a 12 volt automotive light bulb, i.e. a 194 insturment panel bulb) the voltage would settle down to about 12.5 volts. Regulated can be either linear or switch mode units. But in reality a wall wart is not truly a charger.
A battery charger and wall wart are typically not the same, even though wall warts are used as chargers. A real battery charger has current limiters. This is so a battery is not overheated by rapid charging. The so called "fast chargers" have higher current (and likely voltage). A typical NiCad battery arrangement should be charged for 14 or more hours. If you are using a system that is shortening this charge time, even if this is what the manual/instuctions say is okay, it is a fast charger. The PRO to this is a shorter charge time. The CON is shorter battery life. As suggested by sofaspud multiple batteries and a slow charger are better. I use NiCads in my digital cameras. I typically carry three or more sets of charged batteries with me and charge them in a slow charger. Some of my batteries have been in use for three or more years.
NiCads also have something called a "memory". If you do not use a lot of capacity out of a charged battery before recharging it, the battery use time degrades and won't last as long between charges.
Think of it this way, the voltage has to be higher in order for the battery not to be drained by the charger. The higher voltage depends on the number of cells. For something like a 1.2 volt single cell NiCad the "typical" voltage to charge it would be about 1.5 volts.
There are several types of wall warts as well. Cheap versions use a step down transformer, pair of diodes, and a filter cap. These will (as others have already stated) float at a much higher voltage. I had one 12V that would get as high as 18V unloaded. I thought something was wrong with it but if I put a small load (such as a 12 volt automotive light bulb, i.e. a 194 insturment panel bulb) the voltage would settle down to about 12.5 volts. Regulated can be either linear or switch mode units. But in reality a wall wart is not truly a charger.
A battery charger and wall wart are typically not the same, even though wall warts are used as chargers. A real battery charger has current limiters. This is so a battery is not overheated by rapid charging. The so called "fast chargers" have higher current (and likely voltage). A typical NiCad battery arrangement should be charged for 14 or more hours. If you are using a system that is shortening this charge time, even if this is what the manual/instuctions say is okay, it is a fast charger. The PRO to this is a shorter charge time. The CON is shorter battery life. As suggested by sofaspud multiple batteries and a slow charger are better. I use NiCads in my digital cameras. I typically carry three or more sets of charged batteries with me and charge them in a slow charger. Some of my batteries have been in use for three or more years.
NiCads also have something called a "memory". If you do not use a lot of capacity out of a charged battery before recharging it, the battery use time degrades and won't last as long between charges.
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!
Re: Question about WalWarts
Hey Tedleydo! Me Next!
A regular old wallwart has a small power transformer inside.
Every transformer has enough inductance to keep the primary winding current low.
When you make a transformer 1/2 size (in volume), the inductance drops in half, so you have to add 140% more primary windings just to keep the primary inductive current where it was.
Then you might add another 140% to the primary windings to halve the inductive current again to keep the inductive current in scale...smaller like the transformer.
Plus, every time you shrink the core, the space to install windings gets smaller, and you have to use smaller wire to fit the space and the resistance goes up. Same with adding windings to the primary: Guess what? longer AND thinner wire, The series resistance goes up TWICE.
The secondary voltage is determined by the primary:secondary windings ratio. So if you add primary windings, you have to add secondary windings in scale to keep the secondary voltage the same. MORE resistance.
Because of all this added resistance, the voltage will droop when the transformer has a current load. To compensate, the manufacturer adds even more secondary windings in order to bring the voltage back up. He has to use even smaller wire to fit the space too. Is this getting monotonous yet? Winding resistance goes up again and the unloaded voltage can get very high when compared to the rated voltage output at the rated current load.
I find it amazing that they can make small transformers AT ALL.
A regular old wallwart has a small power transformer inside.
Every transformer has enough inductance to keep the primary winding current low.
When you make a transformer 1/2 size (in volume), the inductance drops in half, so you have to add 140% more primary windings just to keep the primary inductive current where it was.
Then you might add another 140% to the primary windings to halve the inductive current again to keep the inductive current in scale...smaller like the transformer.
Plus, every time you shrink the core, the space to install windings gets smaller, and you have to use smaller wire to fit the space and the resistance goes up. Same with adding windings to the primary: Guess what? longer AND thinner wire, The series resistance goes up TWICE.
The secondary voltage is determined by the primary:secondary windings ratio. So if you add primary windings, you have to add secondary windings in scale to keep the secondary voltage the same. MORE resistance.
Because of all this added resistance, the voltage will droop when the transformer has a current load. To compensate, the manufacturer adds even more secondary windings in order to bring the voltage back up. He has to use even smaller wire to fit the space too. Is this getting monotonous yet? Winding resistance goes up again and the unloaded voltage can get very high when compared to the rated voltage output at the rated current load.
I find it amazing that they can make small transformers AT ALL.
-=VA7KOR=- My solar system includes Pluto.
Re: Question about WalWarts
Wow, lots of good info to chew on! It's pretty obvious that I've never dived too deep into power supplies before, but I guess it's time to jump in. 
I'm a whole lot more comfortable with digital stuff and programming, and I always just took the power supply for granted.
I'm brand new here so thanks everyone for taking the time to help me understand this. I really do need to learn more about analog circuit design...looking forward to learning alot on this forum!
I'm a whole lot more comfortable with digital stuff and programming, and I always just took the power supply for granted. I'm brand new here so thanks everyone for taking the time to help me understand this. I really do need to learn more about analog circuit design...looking forward to learning alot on this forum!
Re: Question about WalWarts
An excellent primer or tutor on batteries can be found at www.rcbatteryclinic.com and there is info there on using wall warts for other than their intended purpose. A very good source of lots of pertinent info on the care and use of batteries.
Re: Question about WalWarts
A battery expert with a sense of humor. Thanks for tipping us off to that great site!
These days a person can't have too much battery knowledge.
These days a person can't have too much battery knowledge.
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