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DC voltage drop across choke vs resistor!!!

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Posted on March 17, 2017 at 18:25:10
TomWh
Audiophile

Posts: 630
Location: Tucson Az
Joined: August 7, 2003
Was informed that using the resistance of a choke vs resistor in measuring voltage drop to calculate current is different. So I did the experiment on both channels. This tube has its own power supply. So across the 1st choke got 18ma. Across a resistor I got 16 ma. So the choke must be doing something to the DC which is over my head.

I guessing the resistor is the accurate one seeing that is how everyone seems to measure it but what is the choke doing to the get the different reading?

Thanks Tom

 

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RE: DC voltage drop across choke vs resistor!!!, posted on March 17, 2017 at 20:38:50
Chip647
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Posts: 1681
Location: The South
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If the voltage drop is less across the choke than the resistor (less DC resistance) then the operating point of the amp will change and draw more current.

 

RE: DC voltage drop across choke vs resistor!!!, posted on March 18, 2017 at 06:55:43
TomWh
Audiophile

Posts: 630
Location: Tucson Az
Joined: August 7, 2003
My confusion is that with everything in place with only only one power supply to one tube no other branches the current should be constant. So a resistor in series can be measured for voltage drop then ohms law we have the current in circuit or through the tube.

The question is in regards to why when you measure the voltage drop across a choke with known DC resistance it is different than a resistor with the same resistance.

So in the above example using the voltage drop the choke shows we have a 18 ma circuit and the voltage drop across the resistor shows a 16 ma circuit. This is b+ to a tube circuit.

So I am guessing the DC circuit is seeing the choke as more than just its resistance. My question is why.

Thanks
Tom

 

RE: DC voltage drop across choke vs resistor!!!, posted on March 18, 2017 at 07:29:36
Tre'
Industry Professional

Posts: 12157
Location: So. Cal.
Joined: February 9, 2002
"My confusion is that with everything in place with only only one power supply to one tube no other branches the current should be constant."

The amount of current the tube will draw depends on the bias condition of the tube.

If the tube has more voltage across it (plate to cathode) with the choke in place vs. the resistor in place then it will draw more current.

I'm not sure why you think the current should be constant.

Power supplies, unless they are current regulated supplies, supply whatever current the tube circuit demands.....not the other way around.

"I am guessing the DC circuit is seeing the choke as more than just its resistance. My question is why."

The DC will only see the resistance of the choke. The AC (music signal) will see the resistance of the choke and the reactance of the choke in series.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: DC voltage drop across choke vs resistor!!!, posted on March 18, 2017 at 08:47:53
TomWh
Audiophile

Posts: 630
Location: Tucson Az
Joined: August 7, 2003
Hi Tre

I am not explaining this correctly. The circuit is biased with a diode. This is why I do not check the voltage drop there. So I would normally would check the voltage drop across a choke with known resistance and do the math from there. A buddy told me the choke measurement is not as accurate as a series resistor. So I put a resistor in series and then measured across the resistor and the choke. After the math it was 16 ma for the resistor and 18 ma for the choke.

So everything is constant the diode is setting the bias for the tube the b+ is settled in to what ever the current is. So all I was doing was checking what the measured current is.

So my question is why would the drop across the choke DC resistance measure different than a standard resistor?

Thanks Tom

 

RE: DC voltage drop across choke vs resistor!!!, posted on March 18, 2017 at 09:16:54
JKT
Audiophile

Posts: 469
Location: Midwest
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This is only a guess, but there could be some AC ripple riding on the DC ahead of the choke that the choke is blocking and your meter is responding to. Check the voltages with a 'scope and see what you find.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

DCR is DCR, posted on March 18, 2017 at 10:07:58
Triode_Kingdom
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Posts: 5993
Location: Texas
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"why would the drop across the choke DC resistance measure different than a standard resistor"

It won't. Something else is happening that you're not aware of. As someone else suggested, maybe there's an AC component present.



--------------------------
Buy Chinese. Bury freedom.

 

RE: DC voltage drop across choke vs resistor!!!, posted on March 18, 2017 at 10:23:57
TomWh
Audiophile

Posts: 630
Location: Tucson Az
Joined: August 7, 2003
I did measure it on the 1st choke in the power supply. There are 2 more will measure there for the fun of it. Should get Closer down the line if ripple is being reduced.

Can the caps to ground cause this issue also?

Thanks
Tom

 

Can you post a schematic? (nt), posted on March 18, 2017 at 10:27:42
Tre'
Industry Professional

Posts: 12157
Location: So. Cal.
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.
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: DCR is DCR, posted on March 18, 2017 at 10:27:54
TomWh
Audiophile

Posts: 630
Location: Tucson Az
Joined: August 7, 2003
That is what I was thinking, in regards to dcr. There are caps to ground in this power supply. Would that cause this measurement issue.

Thanks Tom

 

RE: Can you post a schematic? (nt), posted on March 18, 2017 at 11:17:59
TomWh
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Posts: 630
Location: Tucson Az
Joined: August 7, 2003
I do not have schematic software but here is what it is. Full wave damper diodes to a .33 uf cap to 30hy choke to 2uf cap to 10hy choke to a 12uf cap to 5hy choke to a 100uf cap to interstage transformer to plate.

Pretty standard power supply. Caps to ground chokes in series. This is to one tube only so current should be the same from beginning to end.

Thanks Tom

 

RE: Can you post a schematic? (nt), posted on March 18, 2017 at 12:31:09
Blackdog
Manufacturer

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Location: Ontario
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Did you add the resistor in series with the choke or in place of the choke.
Dan Santoni

 

RE: Can you post a schematic? (nt), posted on March 18, 2017 at 13:58:04
TomWh
Audiophile

Posts: 630
Location: Tucson Az
Joined: August 7, 2003
It was put in series after the last cap. Everything else is the same.

Thanks
Tom

 

RE: Can you post a schematic? (nt), posted on March 18, 2017 at 14:03:02
Tre'
Industry Professional

Posts: 12157
Location: So. Cal.
Joined: February 9, 2002
The resistor lowered the voltage to the tube so the current decreased.



Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Can you post a schematic? (nt), posted on March 18, 2017 at 14:31:40
cpotl
Audiophile

Posts: 724
Location: Texas
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"The resistor lowered the voltage to the tube so the current decreased."

I think he's saying he puts the resistor in place, and then measures the voltage drop across the resistor, and also across the choke (first choke, if I understood correctly). He then uses Ohm's law for the resistor, and for the know DCR of the choke, and comes up with the seemingly discrepant results for the current.

Many things could be going on involving the AC component across the choke causing complications, as has been suggested. Especially if it is the "first choke," which I would take to mean the one closest to the rectifiers. There will be a lot of AC across that choke. Who knows how the voltmeter, set to measure DC voltage, will respond when a small DC voltage has a large AC voltage superimposed? It could depend a lot on the specifics of the particular meter that is being used.

Of course another possibility could be a leaky smoothing capacitor. But on balance, I would expect the AC superimposed on DC complications in the measurement to be the more likely explanation.

Chris

 

RE: Can you post a schematic? (nt), posted on March 18, 2017 at 15:21:11
TomWh
Audiophile

Posts: 630
Location: Tucson Az
Joined: August 7, 2003
Chris you got the whole picture. Did both channels close to same results so probality not a cap issue. I think the AC check with the scope is next up. The meter was a fluke 87 but I have others. Might try different dvm's just for the education.

Thanks Tom

 

RE: DC voltage drop across choke vs resistor!!!, posted on March 18, 2017 at 16:44:45
xaudiomanx
Audiophile

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I am wondering that if a choke is used and the DC resistance of it is measured without any voltage going through it the DC resistance is different than with voltage running through it whereas the DC resistance in a resistor is measure the same whether voltage is running through it or not.

 

Copper resistance increases with temperature., posted on March 18, 2017 at 17:13:27
sser2
Audiophile

Posts: 2228
Location: Pittsburgh USA
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Copper has one of the highest positive thermal resistance coefficient (TRC) of all metals. For this reason, cold choke has lower DCR than when it is heated by current passing through and warmer environment of working amplifier. Metal in resistors has much lower TRC than copper, so value doesn't change much with temperature. Carbon resistors have negative TRC (their resistance drops with increasing temperature.

 

RE: DCR is DCR, posted on March 19, 2017 at 08:21:21
Triode_Kingdom
Audiophile

Posts: 5993
Location: Texas
Joined: September 24, 2006
The caps will only affect it if there's AC present, or if you don't allow enough time for the measurement to settle.



--------------------------
Buy Chinese. Bury freedom.

 

RE: Can you post a schematic? (nt), posted on March 20, 2017 at 14:17:49
gusser
Audiophile

Posts: 2403
Location: So. California
Joined: September 6, 2006
A scope may not even tell the whole story. Unless you have a current probe you can only measure voltage.

I have done work on large megawatt class UPS systems where you must use a current probe to check SCR commutation waveforms. A voltage measurment won't wotrk.

 

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