Mains voltage with non zero DC, how bad is it ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: February 26, 2014, 04:44:52 am
content: 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.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 26, 2014, 05:13:43 am
content: 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.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 26, 2014, 05:50:02 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2014, 06:32:35 am
content: Quote from: Dawgie on February 26, 2014, 05:16:32 amThough 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

author: BravoV
author_rank: Super Contributor
timestamp: February 26, 2014, 07:51:41 am
content: Thanks for all replies, learned something new today, even though its probably a very basic 101 of electricity.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 26, 2014, 01:50:26 pm
content: 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

author: SeanB
author_rank: Super Contributor
timestamp: February 26, 2014, 07:02:07 pm
content: 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.

author: kizzap
author_rank: Unknown
timestamp: February 27, 2014, 01:17:38 am
content: 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.

author: BravoV
author_rank: Super Contributor
timestamp: February 27, 2014, 02:01:09 am
content: Quote from: The Electrician on February 26, 2014, 01:50:26 pmDon't worry, I used Fluke 287 which is safe to use for such purpose, see my reply to kizzap below.Quote from: The Electrician on February 26, 2014, 01:50:26 pmThank 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.Quote from: SeanB on February 26, 2014, 07:02:07 pmThe 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.Quote from: kizzap on February 27, 2014, 01:17:38 amI 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.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 27, 2014, 02:09:24 am
content: Quote from: BravoV on February 27, 2014, 02:01:09 amHair 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.

author: BravoV
author_rank: Super Contributor
timestamp: February 27, 2014, 03:01:42 am
content: Quote from: The Electrician on February 27, 2014, 02:09:24 amMy 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.

author: AlfBaz
author_rank: Super Contributor
timestamp: February 27, 2014, 03:23:40 am
content: 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

author: c4757p
author_rank: Super Contributor
timestamp: February 27, 2014, 03:30:42 am
content: Quote from: AlfBaz on February 27, 2014, 03:23:40 amAnd a spherical cow?

author: BravoV
author_rank: Super Contributor
timestamp: February 27, 2014, 03:38:01 am
content: Quote from: AlfBaz on February 27, 2014, 03:23:40 amYes, 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 ?

author: AlfBaz
author_rank: Super Contributor
timestamp: February 27, 2014, 04:08:36 am
content: Quote from: c4757p on February 27, 2014, 03:30:42 amMy need for an emoticon with shit flying over a head is growing by the day

author: AlfBaz
author_rank: Super Contributor
timestamp: February 27, 2014, 04:11:49 am
content: Quote from: BravoV on February 27, 2014, 03:38:01 amGiven 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 thepeakmin/max readings might show this "inbalance"

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 27, 2014, 05:18:35 am
content: 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.QuoteSounds like a good one.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 27, 2014, 09:54:59 am
content: Quote from: BravoV on February 27, 2014, 03:01:42 amIt 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.

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2014, 11:08:42 pm
content: Quote from: The Electrician on February 27, 2014, 02:09:24 amYes, 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.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 27, 2014, 11:16:15 pm
content: Quote from: Hero999 on February 27, 2014, 11:08:42 pmOnly 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?

author: AlfBaz
author_rank: Super Contributor
timestamp: February 27, 2014, 11:20:59 pm
content: Quote from: The Electrician on February 27, 2014, 11:16:15 pmGirls Dorm on a Friday night?

author: BravoV
author_rank: Super Contributor
timestamp: February 28, 2014, 02:08:09 am
content: Quote from: The Electrician on February 27, 2014, 09:54:59 amThanks, 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.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: February 28, 2014, 02:27:31 pm
content: Do you see a potential between Neutral and Ground? If so, how much?

author: c4757p
author_rank: Super Contributor
timestamp: February 28, 2014, 02:39:52 pm
content: Quote from: AlfBaz on February 27, 2014, 04:11:49 amJust saying - if you want to measure millivolts on top of 170/340 peak, by assuming symmetry, you're going to need averysymmetric sine wave. Like, within 300-600ppm or so. And mains is usually very sloppy.

author: Zero999
author_rank: Super Contributor
timestamp: March 01, 2014, 07:56:18 pm
content: Quote from: The Electrician on February 27, 2014, 11:16:15 pmI 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.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 01, 2014, 08:30:49 pm
content: Quote from: Hero999 on March 01, 2014, 07:56:18 pmWhat I mean by distribution transformer is the pole mounted "pole pig":You're probably thinking of the larger type found in substations:

author: Zero999
author_rank: Super Contributor
timestamp: March 01, 2014, 09:31:22 pm
content: Lol, I've never heard the term pole pig before. Where I live there aren't any transformers on poles. There's a substation at the end of the road which powers half the village.

author: sync
author_rank: Frequent Contributor
timestamp: March 01, 2014, 09:32:18 pm
content: Quote from: The Electrician on March 01, 2014, 08:30:49 pmThese are almost non-existent here in Germany. One of our distribution transformer supply up to a few hundreds domestic homes. It outputs 400/230V tree-phase which is directly distributed to the houses. And we are using underground cables. A typical distribution transformer looks like this.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 01, 2014, 09:49:52 pm
content: This situation cries out for some measurements. You guys could have many hair dryers on at the same time in the morning when people are getting ready for work. Build the low pass filter I described and see how much DC you observe.On my street, no more than 3 houses are served by one pole pig, so the DC from one house where a hair dryer is used on half power isn't communicated to more than 2 other houses. You might have many hair dryers on at the same time, all on the same circuit!

author: sync
author_rank: Frequent Contributor
timestamp: March 01, 2014, 10:20:05 pm
content: Our plugs are non-polarized. So the hairdryers cancel each other out statistically.I made the test. I measured the DC of an outlet directly with a mulltimeter (1000V range, 100 PLC integration time). Without the hairdryer the DC level was <30mV. With the hairdryer set to half power (600W) it was ~1V on the outlet of the dryer and ~0.4V on an outlet connected to a different circuit breaker but on the same phase.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 01, 2014, 11:45:12 pm
content: That's probably enough to cause the saturation effects (in an audio amplifier with a toroidal power transformer) reported by golden ears audiophiles.

author: johansen
author_rank: Super Contributor
timestamp: March 02, 2014, 12:39:57 am
content: meh, tell the folks that can hear the bad mains power to increase the PSRR of their tube amps.. oh, wait, my bad. what's PSRR?anyhow, yes, 1 volt of dc is enough to blow up your mains.I stick weld with a variety of nonstandard voltages and currents.running various rods say, 7018 on ac, when they are meant for dc, sometimes you can hear the transformer saturate because the arc is pulling more dc amps than ac on various polarities. i've noticed this with other rods as well. so sometimes i switch to dc, because that rectifying action reduces the power available at the arc.

author: sync
author_rank: Frequent Contributor
timestamp: March 02, 2014, 01:38:32 am
content: Quote from: The Electrician on March 01, 2014, 11:45:12 pmDamned! How can I protect my amplifier from this evil DC? Does a blocking capacitor works? How big must it for 1.5kW?And how can I make the noisy hairdryer quieter so I can hear these bad saturation effects?Seriously, I wounder how much DC a transformer tolerates before it goes into saturation.

author: johansen
author_rank: Super Contributor
timestamp: March 02, 2014, 04:25:19 am
content: Quote from: sync on March 02, 2014, 01:38:32 amthree diodes can be configured to deliver .3v drop in one direction and .6v drop in the other (use 30 amp Schottkys) for testing this yourself.switch to silicon diodes to get a 1v dc bias, and a D cell battery to get 1.5V, etc.

author: ampdoctor
author_rank: Frequent Contributor
timestamp: March 02, 2014, 05:04:56 am
content: Everybody is talking about all kinds of odd possibilities but I've yet to see any mention of ghost or phantom voltage.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: March 02, 2014, 07:14:33 pm
content: Quote from: ampdoctor on March 02, 2014, 05:04:56 amHow about *just* ghosts or phantoms?

author: BravoV
author_rank: Super Contributor
timestamp: March 03, 2014, 06:35:29 am
content: Decided to take a short video on the test, apologize for the amateur video quality.Few notes on the measurements :- Hairdryer is Philips 1600 watts with 3 selections for the heat & air speed, plugged at cheap watt meter at the front for monitoring the power consumptions.- Fluke 287 was set using DC over AC mode, while 87V was set at DC Voltage mode.- Both dmms range were set manually at 500V for the 287 and 600V for the 87V.So the hair dryer vs DC results :Off -> DC = 0 VoltLow @50 watt -> DC = 1.2 VoltMid @650 watt -> DC = 0.6 VoltHigh @1300 watt -> DC = 0 Volt@The Electrician, I can't find my motor start capacitor, probably buried somewhere in the pile of component junks, and I don't have any other high volt cap available, so no simple RC LPF measurement yet as you've suggested, sorry.Comments are welcome.Attached below photos with the measurements results as above video for better details.

author: BravoV
author_rank: Super Contributor
timestamp: March 04, 2014, 06:04:29 am
content: Quote from: ampdoctor on March 02, 2014, 05:04:56 amIs possible that I might interpreted wrongly on my above test that the DC result is a phantom voltage ?

author: BravoV
author_rank: Super Contributor
timestamp: March 04, 2014, 07:07:00 am
content: Quote from: CaptnYellowShirt on February 28, 2014, 02:27:31 pmSorry, missed this question, Neutral to Ground voltage is about <= 1 Volt AC.

author: johansen
author_rank: Super Contributor
timestamp: March 04, 2014, 07:08:25 am
content: you should be able to measure some dc between neutral and ground during the load test, due to the resistance of the neutral line.the remainder of the dc would be formed by the resistance of the hot line and the incoming supply.

author: BravoV
author_rank: Super Contributor
timestamp: March 05, 2014, 06:00:31 am
content: Quote from: johansen on March 04, 2014, 07:08:25 amThe DC measurements between Live (L) and Neutral (N) vs Ground (G) with negative black probe was attached permanently at Ground line :Off -> L to G = -0.1 V and N to G = 0 voltLow @50 watt -> L to G = 0.6 V and N to G = -0.6 voltMid @650 watt -> L to G = -0.4 V and N to G = -0.2 voltHigh @1300 watt -> L to G = -0.1 V and N to G = 0 voltWhat does this mean ?

author: johansen
author_rank: Super Contributor
timestamp: March 05, 2014, 07:21:03 am
content: Quote from: BravoV on March 05, 2014, 06:00:31 amit means i greatly suspect the .6v reading for the low setting.the medium setting shows that a third of the dc voltage is across the resistance of the neutral line, the other third across the hot line, and the other third appearing at the breaker box.the 1300 watt datapoint of .1 volts dc line to ground is also suspect.

author: BravoV
author_rank: Super Contributor
timestamp: March 05, 2014, 09:57:07 am
content: johansen, thanks for reply, but honestly, I still don't get it.