12V Charger Delivers 11V?

[spetch]

I have a 12V wall charger for charging a 12V Power Wheels ride on car. It says it delivers 12V out at 150 mA. When I plug it into the wall and put a volt meter on it, it says 11V. Is it possible that a wall charger/transformer would do this with no load, but then would deliver 12V when a battery is connected? I have no way to check the voltage under load, because of the way the connector is.

Fisher Price was no help. Their lady just said "Oh...11V is acceptable for our product." She had no idea what no load versus a load meant.

 

[Willbe]

11/12 = 0.92
0.92 x 120v = 110v.
Is your house very far from the pole transformer?
In MD they're allowed to go between 114v and 126v.

 

[spetch]

Hmmmm...never thought of that. The transformer box (ground unit) that services several houses around me is right in my side yard. I guess I should try to put my AC voltmeter in the wall outlet and see what I have.

 

[Roff]

It is possible that your charger output is not DC, but is instead pulsating, similar to full-wave rectified AC. A meter would give you the average DC voltage, but a battery would charge to the peak value.

 

[Diver300]

It is possible that your charger output is not DC, but is instead pulsating, similar to full-wave rectified AC. A meter would give you the average DC voltage, but a battery would charge to the peak value.

I have found this with a 12 V car battery charger. On a DC voltmeter, it shows less voltage with no load than when charging a battery. There is no smoothing capacitor in the charger because it isn't needed and it would have to be huge to actually reduce the ripple voltage.

 

[MrAl]

Hi,

I have to agree that the reason you may be measuring 11v instead of 12v is
because it is a full wave rectified sine wave with no filter capacitor.
With a load of 10 megohms and no cap the output could measure between
10 and 11 volts, roughly, depending on the exact transformer turns ratio
(sometimes set a little higher than 12vac) and the drop of the diodes.

To make sure this is really the case, add a small capacitor, even 0.1uf,
across the output and measure again with the same meter. You could
see a dc voltage much higher than 12v, like 16vdc. The cap should be
rated for 20v or higher.
If the result of this test shows 16v (or near that) then the device is
most likely working just fine. If the result shows 12v or less, then the
device is not working properly. If the result shows a voltage between
12v and 14v then the device may or may not charge the battery in
a reasonable amount of time, you'll have to try that.

 

[spetch]

Thanks, guys. I have this big 1.0uf 100V capacitor laying around, so I hooked it up to the charger leads. When I measured the voltage under this load it was 16V, just like MrAl said. I guess this thing is working fine. I learned something today!

 

[Willbe]

Thanks, guys. I have this big 1.0uf 100V capacitor laying around, so I hooked it up to the charger leads. When I measured the voltage under this load it was 16V, just like MrAl said.

Full wave rectified giving 16 peak volts x (2/pi)= 10.2 vdc. I guess the battery gets charged on only a small portion of the cycle. I would have called this a 10v xformer. My 12vdc xformers, unloaded, probably read 14vdc.
Strange. . .

 

[Leftyretro]

Full wave rectified giving 16 peak volts x (2/pi)= 10.2 vdc. I guess the battery gets charged on only a small portion of the cycle. I would have called this a 10v xformer. My 12vdc xformers, unloaded, probably read 14vdc.
Strange. . .

A transformers voltage is normally rated in RMS AC input and output voltages and assumes a pure sine wave at a specified frequency. In the OP's case the device is an assembly consisting of a transformer and a rectifier, so it's output voltage is not a sine wave but rather a pulsating DC voltage. These are quite different waveforms of voltage and require some thought and analysis when trying to measure.

A simple DMV can be misleading as the OP found. If he had measured the chargers output AC voltage component ( using the AC voltage function) with his DVM he would have found that is was not zero and perhaps that might have got him thinking about it depending on his electronics training and experience. A true RMS DVM would have read even a different value and of course a O-scope would allow one to see what was actually going on. It's not always obvious, with consumer power packs/charges, where the various functions of rectification, filtering, possible regulation and charge rate control resides, either in the pack or in the device being charged.

I learned a long time ago to not put full faith when using DVM meters. They are wonderful instruments, but they will always display a number. However that value is not always correct as it is based on a few assumptions (voltage waveform, meter input impedance, etc). Education and experience is a necessary part of circuit measurement and analysis.
Lefty

 

[Willbe]

It's not always obvious, with consumer power packs/charges, where the various functions of rectification, filtering, possible regulation and charge rate control resides, either in the pack on in the device being charged.

Lefty

I assume filtering is downstream of the xformer since capacitors are bulky. Maybe not. . .

 

[MrAl]

Thanks, guys. I have this big 1.0uf 100V capacitor laying around, so I hooked it up to the charger leads. When I measured the voltage under this load it was 16V, just like MrAl said. I guess this thing is working fine. I learned something today!

Hi again,

What we are doing when we connect the cap is sort of catching the
peaks of the rectified sine wave, so the result is called the 'peak'
voltage.

If we wanted to measure the 'average' voltage, we could connect
a 1k resistor across the wall wart output and connect a 100k
resistor in series with the 1uf cap, connect that also across the
wall wart output, then measure the voltage across the cap. The result
should be near 9.6 volts, unless the manufacturer compensated for
the diode drops, which then would make it around 10.8 volts.