Mains voltage with non zero DC, how bad is it ?

[BravoV]

How bad is it if your mains has DC volt ? When measured with DMM that can show AC + DC voltages separately.

At which level it will become bad thing for common house hold electronic appliances ? CMIIW, I'm thinking those transformers in power supply must be heating up unnecessarily, right ? Anything else ?

What is causing it if this really happened ? or its very unlikely at residential area ?

Its just my curiosity since my mains shows zero DC all the time.


Update :

Posted a short video on a hair dryer that made a DC offset in AC mains line at post #37 -> Here.

[HackedFridgeMagnet]

Funny thing is that DC stands for Direct Current.
So DC volts obviously a common term, is Direct Current Volts?

I'll take a stab at this.
The local substation can't really supply DC volts as the transformer supplies an emf proportional to the change of flux. So the voltage is naturally is AC at the point of supply.

But with non linear loads and especially half wave rectifiers loading this (however insignificantly) then current at the point of use can have a dc component.
Therefore due to imperfect transmission lines your point of use voltage will not necessarily be purely AC.

I doubt it would be in anyway significant or easily measurable though from a decent supply authority.

[peter.mitchell]

If the DC offset did exist, an SMPS would not suffer at all; potentially it would be even more efficient because it would be easier to achieve a power factor closer to 1.

[T3sl4co1l]

You couldn't get a DC bias on a normal mains supply because it's fed by a sub-station transformer, and transformers don't pass DC.

Anyone/anything trying to force a DC bias onto the normal mains supply by other means would be faced with an extremely low DC impedance to overcome - sub-station transformer along with all the other user transformers on the same supply/phase.

Though uncommon these days, half-wave rectified loads draw DC.  Perhaps a lot of low quality LED lights?

Quite possible one could also see mere residual measurement error in the instrument.  Averaging out 120 or 240V AC to get just a few volts leftover is no small feat.

Tim

[BravoV]

Thanks for all replies  , learned something new today, even though its probably a very basic 101 of electricity. 

[The Electrician]

You can't just connect your voltmeter to mains and then select DC on some low range; the meter will be overloaded by the AC component.

Have a look at this rather long thread.  Toward the end I discuss the possibility of DC on the mains in your house:

http://forum.allaboutcircuits.com/showthread.php?t=37358

[SeanB]

Dc on the mains was a problem years ago when colour TV sets were being introduced. Phillips made the G11, which used a half wave switchmode power supply ( a thyristor connected in series with the mains and controlled by the HT voltage, it would typically draw a 11A pulse every second mains cycle) that caused quite a few distribution transformers to catch fire from the high DC current saturating the core, as it had never been designed to have a 100A or so DC current flow ( as these were all UK sets they were all wired correctly so all the current pulses added up) in addition to the regular load, and the core material ran out of it's linear region and saturated. Later transformers were designed with larger cores and a small air gap distributed around the core to reduce saturation. That is why half wave rectifier power supplies are outlawed in most countries for any load over 10W or so.

[kizzap]

I am curious as to how you were measuring the DC voltage, as in what it was relative to. If it was ground, aka a ground pike plugged into ground, then yes it can be a bad thing. There is a reason that Neutral is tied to Earth in the power distribution box.

[BravoV]

You can't just connect your voltmeter to mains and then select DC on some low range; the meter will be overloaded by the AC component.

Don't worry, I used Fluke 287 which is safe to use for such purpose, see my reply to kizzap below.

Have a look at this rather long thread.  Toward the end I discuss the possibility of DC on the mains in your house:

http://forum.allaboutcircuits.com/showthread.php?t=37358

Thank you, great info  , it may take a while for me to digest it all though. 

But the experiment with hair dryer is interesting, I will try it and report back.

Dc on the mains was a problem years ago when colour TV sets were being introduced. Phillips made the G11, which used a half wave switchmode power supply ( a thyristor connected in series with the mains and controlled by the HT voltage, it would typically draw a 11A pulse every second mains cycle) that caused quite a few distribution transformers to catch fire from the high DC current saturating the core, as it had never been designed to have a 100A or so DC current flow ( as these were all UK sets they were all wired correctly so all the current pulses added up) in addition to the regular load, and the core material ran out of it's linear region and saturated. Later transformers were designed with larger cores and a small air gap distributed around the core to reduce saturation. That is why half wave rectifier power supplies are outlawed in most countries for any load over 10W or so.

The half wave rectifier, is that a direct rectifying the mains using high voltage rectifier without using a transformer ?

Are there other common house hold electronics that you aware of that can caused that DC flow ? Even they're banned now.

I am curious as to how you were measuring the DC voltage, as in what it was relative to. If it was ground, aka a ground pike plugged into ground, then yes it can be a bad thing. There is a reason that Neutral is tied to Earth in the power distribution box.

I used Fluke 287, it can display something like this below, but in my experiences, I never spotted any DC voltage, as they're always zero, while the AC voltage is spot on.

Example from Fluke manual, these are just the variations on how to display AC and DC at the Fluke 287/9 display.

[The Electrician]

The half wave rectifier, is that a direct rectifying the mains using high voltage rectifier without using a transformer ?

Are there other common house hold electronics that you aware of that can caused that DC flow ? Even they're banned now.

Hair dryers use the rectifier in series with the heating element to get the low or medium power setting.  No transformer involved, just a rectifier diode in series with the heater.

I don't know of any other common household devices that use a rectifier like that.

For measuring the DC on the mains you need to use a low pass filter consisting of a resistor and capacitor as I explained in that long thread.  The DC you're trying to measure will be in the millivolts and the 120 VAC will be applied to the DC range of the meter as well as the DC you're trying to measure.  It won't hurt the meter but it will overload the sensitive DC range and give an erroneous reading.

[BravoV]

For measuring the DC on the mains you need to use a low pass filter consisting of a resistor and capacitor as I explained in that long thread.  The DC you're trying to measure will be in the millivolts and the 120 VAC will be applied to the DC range of the meter as well as the DC you're trying to measure.  It won't hurt the meter but it will overload the sensitive DC range and give an erroneous reading.

My Fluke 287 when set to AC and DC measurement, since my mains is 220 V, the voltage range for both AC and DC will use the 500 Volt range, as above example picture that the Auto Range was set at 500 V.

At this range, the DC part has 10 mili Volt resolution, is this enough for this "hair dryer" test ? 

As I mentioned, when measuring in this mode at mains, usually the DC part is flickering at the least significant digit at 0.01 Volt.

[AlfBaz]

Assuming a fairly symmetrical sine wave from mains wouldn't measuring peak min/max values give you some idea of the dc content?
I don't know anything about the fluke 287 but the 87 has peak min/max so good chance the 287 does also

[c4757p]

Assuming a fairly symmetrical sine wave from mains

And a spherical cow?

[BravoV]

Assuming a fairly symmetrical sine wave from mains wouldn't measuring peak min/max values give you some idea of the dc content?
I don't know anything about the fluke 287 but the 87 has peak min/max so good chance the 287 does also

Yes, it has peak min, max and average, apart from the live one that keeps running.

How to read if there is a DC content ? An unbalanced voltages between avg-min vs max-avg ?

[AlfBaz]

And a spherical cow?

My need for an emoticon with shit flying over a head is growing by the day

[AlfBaz]

Yes, it has peak min, max and average, apart from the live one that keeps running.

How to read if there is a DC content ? An unbalanced voltages between avg-min vs max-avg ?

Given c4757p's response it looks like I may be completely wrong but if there is and dc content it's effect will be to shift the ac above or below 0V. Using the peak min/max readings might show this "inbalance"

[HackedFridgeMagnet]

The point is BravoV is trying to force the voltage sine wave into asymmetry with the hair dryer, and then to measure the resultant DC volts, so why should he assume symmetry?

Read the electricians thread on all about circuits, it reveals all.

My need for an emoticon with shit flying over a head is growing by the day

Sounds like a good one.

[The Electrician]

For measuring the DC on the mains you need to use a low pass filter consisting of a resistor and capacitor as I explained in that long thread.  The DC you're trying to measure will be in the millivolts and the 120 VAC will be applied to the DC range of the meter as well as the DC you're trying to measure.  It won't hurt the meter but it will overload the sensitive DC range and give an erroneous reading.

My Fluke 287 when set to AC and DC measurement, since my mains is 220 V, the voltage range for both AC and DC will use the 500 Volt range, as above example picture that the Auto Range was set at 500 V.

At this range, the DC part has 10 mili Volt resolution, is this enough for this "hair dryer" test ? 

As I mentioned, when measuring in this mode at mains, usually the DC part is flickering at the least significant digit at 0.01 Volt.

It would be easy enough to use a 100k resistor and a 1 uF film capacitor to make a simple low pass filter.  Then you can measure the voltage across the capacitor with the meter set to a low DC volts range, and compare with what you get with the 500 volt AC plus DC measurement.

[Zero999]

Hair dryers use the rectifier in series with the heating element to get the low or medium power setting.  No transformer involved, just a rectifier diode in series with the heater.

Yes, I've seen that before, although if the hair driers were made with 50% of them with one polarity and 50% with the other, it will average out.

[The Electrician]

Hair dryers use the rectifier in series with the heating element to get the low or medium power setting.  No transformer involved, just a rectifier diode in series with the heater.

Yes, I've seen that before, although if the hair driers were made with 50% of them with one polarity and 50% with the other, it will average out.

Only if several of each kind are in use in a given residence.  How often is more than one used at a time in a home?

[AlfBaz]

How often is more than one used at a time in a home?

Girls Dorm on a Friday night?

[BravoV]

It would be easy enough to use a 100k resistor and a 1 uF film capacitor to make a simple low pass filter.  Then you can measure the voltage across the capacitor with the meter set to a low DC volts range, and compare with what you get with the 500 volt AC plus DC measurement.

Thanks, will try this when I get home this weekend and share the result here.

Hopefully the SWMBO's hair dryer is the half wave rectifier type.

[CaptnYellowShirt]

Do you see a potential between Neutral  and Ground? If so, how much?

[c4757p]

Yes, it has peak min, max and average, apart from the live one that keeps running.

How to read if there is a DC content ? An unbalanced voltages between avg-min vs max-avg ?

Given c4757p's response it looks like I may be completely wrong but if there is and dc content it's effect will be to shift the ac above or below 0V. Using the peak min/max readings might show this "inbalance"

Just saying - if you want to measure millivolts on top of 170/340 peak, by assuming symmetry, you're going to need a very symmetric sine wave. Like, within 300-600ppm or so. And mains is usually very sloppy.

[Zero999]

Hair dryers use the rectifier in series with the heating element to get the low or medium power setting.  No transformer involved, just a rectifier diode in series with the heater.

Yes, I've seen that before, although if the hair driers were made with 50% of them with one polarity and 50% with the other, it will average out.

Only if several of each kind are in use in a given residence.  How often is more than one used at a time in a home?

I don't know about where you live but where I am, a distribution transformer will power a whole street or housing estate so it's pretty likely more than one hair dryer will be used simultaneously.