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BigBoy1
09-12-2010, 09:52 AM
My neighbor and I have a six-pack riding on the answer to this question.

We talking about the electrical power consumption of devices in the home. The item in question were low-voltage converters for recharging electronic devices like cell phones. The ones which use 120 VAC input and step it down to DC power that charges the battery of low-voltage electronics. He contends that when the device is disconnected from the charger, there is no electricity flowing in the converter because the device has been removed.

I contend there is electricity flowing in the converter device as long it is plugged into the wall, even though the device being charged has been removed. The only way to stop the current flow in the converter is to unplug the converter from the wall. This would then reduce the power consumed by devices in the house.

The analogy I tried to explain is a simple transformer with a single primary and single secondary. If power is applied to the primary, a magnetic field will be produced as long as power is applied to the primary, independent of what is happening to the secondary electrical current produced. I contend the low-power chargers are exactly the same. He says not.

Would appreciate any help in this discussion.

Richard-TX
09-12-2010, 09:58 AM
You are correct. A charger will consumer power when plugged in regardless if it is charging a device or not. Although the current drawn is very very small, there is current being drawn when not actively charging.

Dr Stan
09-12-2010, 10:04 AM
You are correct. A charger will consumer power when plugged in regardless if it is charging a device or not. Although the current drawn is very very small, there is current being drawn when not actively charging.

That's what I've also heard. To completely stop power consumption it must be unplugged. Ever notice how the lights stay on cordless power tool chargers.

J Tiers
09-12-2010, 10:48 AM
Yes, you are correct.

I have designed a few devices of that general type.... in most cases they are still active with the powered device out. In a few cases they are in a "sleep" mode which draws a VERY little current , just enough to power something that determines if there is a load plugged in.

SOMETHING has to determine whether there is a load, and that "something" needs power.

JoeFin
09-12-2010, 11:22 AM
Yes, you are correct.

I have designed a few devices of that general type.... in most cases they are still active with the powered device out. In a few cases they are in a "sleep" mode which draws a VERY little current , just enough to power something that determines if there is a load plugged in.

SOMETHING has to determine whether there is a load, and that "something" needs power.

Soaking the step down transformer at best. The remainder of the circuitry depend on the powered device to complete the circuit and draw current.

But I agree - it will draw current - how ever minuscule it may be

J Tiers
09-12-2010, 11:50 AM
Soaking the step down transformer at best. The remainder of the circuitry depend on the powered device to complete the circuit and draw current.

But I agree - it will draw current - how ever minuscule it may be

Most cellphone chargers are SMPS.

The SMPS will usually go into a "burst" mode, where it turns on to produce real power every so often, but is drawing only the small "startup" power most of the time. Some pass "energy star" requirements, which are under a half watt.

A device with an actual 60 Hz transformer will draw a percentage of max current as "magnetizing current"..... with small cheap transformers, that is often significant.... could be a couple watts of power. Way more than "energy star rating" allows.

It's easy to find out.... see if it gets warm...... if warm with no load, it draws significant power.

kf2qd
09-12-2010, 11:57 AM
That's what I've also heard. To completely stop power consumption it must be unplugged. Ever notice how the lights stay on cordless power tool chargers.


That charger with the lights (LEDs) and such is a different animal than just a transformer. There is other circuitry that is being powered even when a battery is not being charged. A transformer should, idealy, draw no current when nothing is attached to the secondary. BUT, in the real world, the small ones (wall warts...) are designed to be cheap and easy to make so I doubt much care goes into making them as efficient as possible.

Large transformers can have quite an inrush current when first powered, but the primary current with no load is minimal. The problem comes with time and things loosening up and any vibration gets turned into noise and heat. The more noise and heat, the moe losses.

gnm109
09-12-2010, 12:28 PM
One way to find out would be a short test harness with a milliameter plugged inbetween the wall socket and the unit....I think the amount of current would be quite small but there would be some given that we are talking about a transformer which can probably use some current, even in an idle state.

In fact, I've noticed that some of the "wall warts" will stay warm even if left plugged in without a load. Of course, I'm excluding any current drain from an indicator LED....those can draw 20-30 ma.

arcs_n_sparks
09-12-2010, 04:56 PM
The old style, linear type regulator wall warts were pretty thirsty, and soaked up a fair amount of juice even with the load off.

Through a series of regulations driven by the DOE, small wall power supplies are getting quite efficient. Without load, many do not get warm to the touch. As someone mentioned, they are pretty much switched mode power supplies at this point.

Nevertheless, plugged in, they are drawing some current.

Black_Moons
09-12-2010, 05:49 PM
Any circuit draws current, even the best electrical switchs still given a rated 'off' resistance, Infact even mechanical switchs have a insulation resistance.

On the one hand, a charger can sense/detect a battery, or even have a mechanical switch that disconnects the charger from mains without a battery inserted (havent seen one that does this yet but..)

On the other hand, even a off switch still allows a few picoamps of current through. And most supplys will draw a lot more if they are just electricaly sensing the battery.

Basicly: yes everything draws power pluged in, a mechanical off switch is the best at preventing signifigant power usage, while electrical sleep modes have varying success depending on thier design, but will allways have some parasitic power draw.

PS: Did you know for most nicad/nimh chargers, its very bad to leave a fully charged battery in the charger for long periods? Most chargers trickle charge the battery once fully charged, and many will end up overcharging it given enough time.

Paul Alciatore
09-12-2010, 06:38 PM
I have to agree with everybody. They will all draw some small amount whenever they are plugged in. That amount will vary, primarily depending on the design and quality of the transformer. The primary winding of the transformer is essentially a wire connected across the two power leads. Only the self inductance of this coil opposes the current. In a good transformer, it will produce a high percentage of the line Voltage as a back EMF (Voltage) and the current will be quite small, but low quality ones there will be less back EMF and the current drawn will be more.

I like to use a switched power strip to hold these devices. Turn it off when they are not in use. Of course, it will probably take 100 years to make back the cost of these strips.

Arcane
09-12-2010, 09:12 PM
Large transformers can have quite an inrush current when first powered, but the primary current with no load is minimal.

Not true, (unless you consider 2 to 5 % minimal.) From http://elektro.fs.cvut.cz/en/SSem/2141025.old/Transformer.pdf:
"The value of idling current is between 2 and 5 % of rated current in big transformers and up to 15 % in smaller transformers."

J Tiers
09-12-2010, 09:54 PM
Not true, (unless you consider 2 to 5 % minimal.) From http://elektro.fs.cvut.cz/en/SSem/2141025.old/Transformer.pdf:
"The value of idling current is between 2 and 5 % of rated current in big transformers and up to 15 % in smaller transformers."


And it is at a horrible power factor........ which causes the power company trouble.

whitis
09-12-2010, 10:01 PM
You win a six pack.

The control circuitry on the hot side takes some small amount of power to run. It may not produce pulses on the transformer as long as the output capacitor is fully charged. However, the low voltage side circuitry (which also needs a small amount of power) signals to the high voltage side that it doesn't need power through an optoisolator. Driving the LED on the optoisolator will take a few milliamps (at the DC voltage, not AC) and will slowly discharge the capacitor until a pulse is required to top it off. I just measured the current on my cell phone charger (small switching supply) with no load: 1.5mA. Then I measured a USB charger, also switching. 1.44mA. So, about 1/6 of a watt or about 1.5KWhr/year.

lakeside53
09-12-2010, 10:02 PM
And it is at a horrible power factor........ which causes the power company trouble.

Ah.. but that trouble doesn't mean much on the residential watt meter. Crappy power factor is is great :)

lakeside53
09-12-2010, 10:04 PM
You win a six pack.

.... 1.5mA. Then I measured a USB charger, also switching. 1.44mA. So, about 1/6 of a watt or about 1.5KWhr/year.

Did you use a watt meter? If not, what about the power factor? of course at these levels it's splitting hairs :)

EVguru
09-13-2010, 05:48 AM
Switched mode supplies are now cheap enough to be the majority, but there are still some horrendous transformer based supplies around.

I used to have to repair quite a few where the thermal fuse had failed, which usually meant cracking the glued case seal with a hide hammer before I could get in.

Even quite expensive equipment was often supplied with the cheapest transformer it was possible to get away with. The Chinese were best at supplying this 'demmand' and built transformers with too little iron and high impeadance windings to limit the saturation current. It was not uncommon for them to pull more current and run hotter when NOT supplying their equipment and indeed this was the common cause of failure.

Now the failure is usually capacitor failure caused by accumulated hours, or running at high temperature (I see lots blanketed with dust or burried under stuff). My PC setup at work and the one at home both run on a UPS. Once the system is shut down, the UPS is powered down and everything is properly off. So far, I've never had the PSU fail in one of my PCs.

BigBoy1
09-13-2010, 08:54 AM
Thank you guys for the replies. I'll print them off and go over and collect my six-pack!