DC Offset Blocker/Killer - where to buy in the USA

If Al is still following this thread I am sure he will have a better technical answer.

Not necessarily, Jim :-)

Your finding that having the hairdryer plugged into the outlet where the measurement was taken resulted in a DC offset of around 730 mv, while measuring at the same outlet but with the hairdryer plugged into various other circuit branches on the same leg resulted in around 115 mv, is indeed a bit of a headscratcher.

But not being familiar with the design of hairdryers I did a little research and found this interesting paper:

http://www.idc-online.com/technical_references/pdfs/electrical_engineering/MEASURING_ACOUSTIC_NOISE_EMITTED_BY_POWER.pdf

As stated on pages 4 and 5 and as depicted in Figure 13, at least when hairdryers are operated "at lower power" they place a half-wave rectifier diode in series with the load they place on the AC, or at least a substantial part of the load. So DC offset results from the difference between the amount of current that is drawn during the positive half-cycle of the AC waveform and the amount of current that is drawn during the negative half-cycle, and the differing voltage drops that occur in the resistance of the AC wiring between the two half-cycles as a result of that current difference.

In tests 2, 3, and 4 essentially zero current was being drawn through the dedicated wiring between the service panel and the outlet where the measurement was taken, so no voltage drop would have been occurring in that wiring. And the DC offset that was measured would have resulted essentially from the voltage drop differential between half-cycles that was occurring in the panel and in the outside wiring, since the additional voltage drop in the wiring between the panel and the outlets where the hairdryer was plugged in would not have been in the path to the outlet where the measurement was being taken. And presumably the outside wiring is considerably heavier gauge than the 10 gauge Romex used for the dedicated line, and therefore it would present a lower source resistance (per unit length, at least) for DC offset to develop across as a result of the asymmetrical current draw.

Whereas in test 1 that asymmetrical current was being drawn through an additional resistance in the path to where the measurement was taken, corresponding to the sum of the resistances of the dedicated line’s two 75 foot 10 gauge conductors, which amounts to about 0.15 ohms.

Looking at it quantitatively, if my theory is correct the difference in the amount of current drawn by the hairdryer between the positive and negative half-cycles would be:

(730 mv - 115 mv)/0.15 ohms = 4.1 amps.

In the context of the large current draw of an 1875 watt hairdryer, and one that is placing a diode in series with much of the load it presents, I suppose that is consistent with your findings.

Best,
-- Al

almarg8,581 posts01-15-2019 6:12pm

If Al is still following this thread I am sure he will have a better technical answer.

Not necessarily, Jim :-)

Your finding that having the hairdryer plugged into the outlet where the measurement was taken resulted in a DC offset of around 730 mv, while measuring at the same outlet but with the hairdryer plugged into various other circuit branches on the same leg resulted in around 115 mv, is indeed a bit of a headscratcher.

Al,

Ah, that’s nothing....

In another room there are also several 120V convenience wall outlets that are fed with a 20 amp separate circuit. Wire is #12-2 with ground Romex. Home run feed to first outlet, closet to electrical panel. Branch circuit Romex is run around the perimeter of the room to outlet boxes in an in and out configuration. Up and down, and all around. The length of the Romex from the first outlet to the elecrical panel is approx 45ft.

The length of the Romex from the first outlet to the farthest, last, outlet in the room is approx 50ft. Approx 90ft total from the last outlet to the electrical panel.

The test.
I first plugged the hair dryer into the farthest duplex receptacle outlet. (High heat, low blower speed).
Mains voltage unloaded, 120.8Vac.
Hair dryer on, loaded, 115.8Vac.
5Vac VD.

Plugged in the low pass filter in the same duplex outlet.
Fluke measured 1.577Vdc . . ??? Higher than my previous post test of 0.734Vdc. What are the differences between the two branch circuits? Length of the branch circuit wire and the AWG wire gauge size. (0.734Vdc 75ft, 10-2. Mains loaded VD approx 2.5Vac) (1.577Vdc 90ft 12-2. Mains loaded VD 5Vac)

I then plugged the Low pass filter into the first outlet. (Outlet closest to electrical panel. First outlet on home run feed.)
Hair dryer left in farthest outlet.
Fluke measured 0.578Vdc....... (Note DC offset is decreasing)
I then plugged the low pass filter in the outlet just below the electrical panel. (Basically connecting the low pass filter to the electrical panel itself. The branch circuit source)
Turned on the hair dryer. (High heat, low blower speed).
Fluke measured 0.106Vdc (106.3mVdc) meter then set to mV.

Note the harmonics, DC offset, decays the closer its gets to the source the electrical panel. Or does the source have anything to do with it?

Final test.

I plugged the hair dryer into the outlet directly below the electrical panel. Plugged the low pass filter into the first outlet closest to the electrical panel.
Fluke measured 0.115Vdc, it then settled down to 0.114Vdc

I then plugged the filter into the farthest outlet on the branch circuit.
The Fluke measured 0.115Vdc. Same thing here it settled down to 0.114Vdc

Both measurements were taken without a connected load on the branch circuit.
For a load I plugged in a quartz construction work light in the farthest outlet. I think it’s a 500 watt but can’t remember for sure. I’d have to check it.
I then measured the DC offset again. This time it measured 0.112Vdc.
Jim

.

That all seems perfectly consistent with the explanation I proposed in my previous post, Jim. The hairdryer draws considerably different amounts of current in the positive half-cycle than in the negative half-cycle. The resulting DC offset corresponds to that difference in current x the resistance of the wiring that current is drawn through, **for wiring that is in the path between the outdoor AC wiring and the measurement location.**

For example:

I then plugged the Low pass filter into the first outlet. (Outlet closest to electrical panel. First outlet on home run feed.)
Hair dryer left in farthest outlet.
Fluke measured 0.578Vdc....... (Note DC offset is decreasing)

At that measurement location the DC offset resulting from the current differential between the two half-cycles will not reflect the resistance of the wiring between the first outlet and the last outlet of that branch.

Regarding the 1.577 vs. 0.734 difference, note that 12-2 has about 60% more resistance per unit length than 10-2, and the length of the 12-2 was about 20% greater than the length of the 10-2.

0.734 VDC x 1.6 x 1.2 = 1.41 VDC.

The additional difference of 1.577 - 1.41 = 0.167 VDC is probably accounted for by a combination of the resistances of the connections to the several intervening outlets in the room with the 1.577, and imprecision in the estimates of the run lengths.

Best,
-- Al

Thanks Al, (almarg) for the response.

The additional difference of 1.577 - 1.41 = 0.167 VDC is probably accounted for by a combination of the resistances of the connections to the several intervening outlets in the room with the 1.577, and imprecision in the estimates of the run lengths.

I don’t think so. All in and out wire connections along with the pigtail extended for the receptacle connection are twisted together and then made mechanically tight using Electrical Spring Connectors.

The live wire steel spring inside the connector is designed to expand and contract with the copper conductors. Varying loads placed on the circuit conductors can cause the copper conductor to expand from heat and contract from cooling as load falls off. The connector’s wire spring always keeps the connection tight.



Jim

After reading this thread, I am second guessing a similar issue I’m having with an old Arcam P85 where I’ve noticed it buzzing, and the corresponding A85 integrated which has been switching off intermittently with a message that reads “DC Offset - Check Connections”.  The connections are all fine.

Figured the P85 was just old and was toast, but now I will try taking it somewhere else and plugging it in.

I’m concerned as I am considering an amplifier upgrade with a McIntosh MC462 as a candidate.  Also, this problem only started after we moved into a house we fully renovated, including a full replacement of the electrical.

Of note, there is no buzzing from the A85.