dc filter

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
If you are experiencing mechanical hum from your
transformer, it is often caused by the presence of
DC on the line. Usually this comes from some appliance
using current asymmetrically, such as a lamp dimmer.

The hum comes usually from toroidal transformers, which
saturate easily with DC, and when they recover, they
draw an extra pulse of current, causing the noise.

You can put a pair of back-to-back electrolytics in series
with the AC power line to block this, and it works fine.
Makes sure the current rating of the electrolytics is
high enough, and the they are joined at a like polarity,
such as + to +.
 

leroy

Member
2001-10-31 10:52 pm
I'm wondering how the back-to-back electrolytic capacitors will behave during a fault condition such as a short circuit. What I'm trying to say is that the capacitors could limit the current and thus prevent the fuse to trip which may lead to dangerous situations.
 
Note also that transformers can hum if they get heavily loaded.
I've got a fairly large EI transformer that I picked up cheap once upon a time. It hums. Turned out that one of the laminations had a tab sticking off to one side that wasn't held firmly against the previous lamination (right in the middle of one side, away from the screws that hold the thing together). The flux made it vibrate against the rest of the transformer. I slid a piece of paper between the laminations and the hum stopped completely.
One of these days I'll get out some shellac and do a proper job of fastening that little fella down...

Grey
 
thanks for the replies!

In my case I think it is DC causing the hum. It´s not always the same but about a month or two ago it has become louder as before and can be heard when all is silent. (and gets very very loud when somebody uses a hairdryer)

Any suggestions for a specific type of capacitor? I assume they need a 400V rating (230v mains) but I´m not sure what capacity will be sufficient.

william
 
William,

In Australia, we have three phase powerlines. Every third house is connected to the same phase to preserve a balanced power drain. You could request your power supply company or utility to change the phase to your house.

I use electrostatic speakers and valve amps at home. I had similar problems and I suspected DC on the powerlines. The power utility company changed the phase to my house and the hum disppeared.

I would imagine that the density of appliances connected to each phase would be a great deal higher in Germany than in Australia and so perhaps this suggestion may not be applicable. Nelson's suggestion is a good one.

Mohan
 
It seems like they put the diode across the capacitor so that it can only charge in one direction. That would be the direction from which the dc is trying to get through. So, if it doesn't solve the problem hooked up one way, my guess is that you would have to reverse it.

The 25 volt rating assumes that the impedance of the load, say a transformer is much less at 60hz than the load. Xsubc = 1 / 2*pi*f*c. Xsubc, the impedance of the capacitor comes out to be .12ohm. I guess this process is what is going on.
 
I had that problem with a large toroid at one time (intermittent humming)... you might want to try reversing the polarity of the primary winding -- this seemed to fix my problem completely. Also, unbalanced load on the secondary can cause hum on some toroids. This could occur if your protection relays or other circuits are powered only from the + or - rail and not both rails evenly. Hope this helps!!
 

mlloyd1

Member
Paid Member
2001-02-25 7:10 pm
Northern Iliinois
one version of DC blocking circuit ....

I found the circuit I mentioned in an earlier post in this thread. It is very similar to what Nelson described. I have not built it, but I will....

.....eventually :)

Enjoy and if someone builds it before me, please report to the group. It includes some surge protection and HF line noise supression, which could be removed if you desired.

Michael
 

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Re: one version of DC blocking circuit ....

mlloyd1 said:
I found the circuit I mentioned in an earlier post in this thread. It is very similar to what Nelson described. I have not built it, but I will....

.....eventually :)

Enjoy and if someone builds it before me, please report to the group. It includes some surge protection and HF line noise supression, which could be removed if you desired.

Michael
Hi,
I' m afraid this circuit has a DC path ACROSS the capacitors via the diodes and will block no DC at all... The surge & HF supression part will probably be the only cause of improvement you can expect from it !
 

mlloyd1

Member
Paid Member
2001-02-25 7:10 pm
Northern Iliinois
I' m afraid this circuit has a DC path ACROSS the capacitors via the diodes and will block no DC at all... The surge & HF supression part will probably be the only cause of improvement you can expect from it !

I susepct the diodes are in the circuit to protect the caps from excessive reverse voltage. Under normal operating circumstances, the diodes do not conduct at all - the caps wouldn't allow enough forward voltage to bias the diodes.

I initially thought as you did; then I though some more. :) I also tried an experiment using much lower voltages than power line levels. The diodes did not turn on.

Michael
 
Filter

On the filter schematic by Mloyd1 if D1 and D3 are removed DC will be blocked and the capacitors still protected. The caps see very low DC because of the diodes. Make sure to use high ripple current capacitors. Digikey has some good for about 4 - 5 amps and you could use more of them. The P6421 is 33000 at 16 Volts and good for 5.2 amps at 85 degrees C. The diodes need 400 PIV rating.
 
It's not just reverse voltage protection, its forward protection too (notice how two diodes are used). Care must be taken in choosing capacitors with enough current capacity, especially with larger amps. I would stay away from polarized electro's in this configuration... they can make a really big mess when reversed with enough current (even at low voltages). :cool:
 
I´m just building a part of the circuit. Only the capacitor and the two diodes (this is all that´s in the LC-Audio filter). They us a 22000microF/25v cap. I´ll use two 6800/35V paralleled.
I´ll see if there´s any dc-blocking done by this.

Unfortunately (or typically):rolleyes: my power amp is very silent today so I probably can only measure the filter today.

william
 
mlloyd1 said:


I susepct the diodes are in the circuit to protect the caps from excessive reverse voltage. Under normal operating circumstances, the diodes do not conduct at all - the caps wouldn't allow enough forward voltage to bias the diodes.

I initially thought as you did; then I though some more. :) I also tried an experiment using much lower voltages than power line levels. The diodes did not turn on.

Michael
:confused:
Let me think twice :
First, let's us assume DC only :
- as soon as the DC voltage on the upper mains terminal is greater than two diode drops (say 2 V with big rectifiers), D1 and D4 will conduct and bypass the capacitors...
- If the DC is reverse, i.e. negative on the upper terminal, D2 and D4 will be direct polarized and then bypass again the capacitors...
Then AC only : If I consider the bottom terminal as a reference, the voltage at the top terminal will change from +325V to -325V peak, assuming a 230V mains (european practice...). The diodes will be polarized as soon as the voltage across them is greater than two diode drops, but now they are shunted by the impedance of the capacitor. If the capacitor is big enough, this low impedance will keep the diodes from conducting...
Now let's us consider DC + AC :
-The AC component will pass through the capacitors, if they are large enough, but the DC component will be blocked by them, as they have a very high impedance and will take the less impedent path at zero frequency, through the diodes, except the first 2 V...
Maybe I missed a point and I would be happy that someone shows me where... A spice simulation showed the same blindness as me...
I fully agree with you that the diodes can be non-conducting under normal circumstances, i.e. no DC, but DC seems not to be blocked !