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Trafo primary resistance too low?
Trafo primary resistance too low?
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Old 11th October 2021, 05:29 AM   #1
donovas is offline donovas  South Korea
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Default Trafo primary resistance too low?

Ive noticed the shunt resistor on my line filter heats up quite a bit so by troubleshooting i found out that the cheap trafos ive been using measures 150ohm across its primary winding. The shunt resistor is 100k ohm. Parallell that with the 150ohm and its been acting like a 149ohm resistor!

My question is about the trafo. Why is it wound this way and what benefit/harm is there to have the primary resistance so low?

The trafo itself works fine in the circuit, tho with a noticable self hum.
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Old 11th October 2021, 05:59 AM   #2
mirlo is offline mirlo  United States
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Trafo primary resistance too low?
The 100 kOhm resistor will dissipate about 0.48 W = 220/100,000 across the 220 V line.
Depending on its dimensions it may have thermal resistance as high as 70 or even 100 K/W. What is the power rating of the resistor?
So a temperature rise of 35 C above ambient would not be unexpected.
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Old 11th October 2021, 06:30 AM   #3
Mooly is offline Mooly  United Kingdom
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Trafo primary resistance too low?
Transformer primaries do read low resistance. You have to remember that resistance is a DC quantity, when fed with 50 or 60Hz line voltage the transformer winding then behaves as an inductance and has reactance rather than resistance. That value will be much higher.

A 100k is always a 100k. The value is unaffected by anything placed across it. The total resistance of the two in parallel will change but not the individual resistances.

Also remember that resistors have voltage ratings and it is essential this is specified when using resistors in high voltage situations. To not do so means they will not only fail but could also pose a safety/fire risk.

So always use resistors specified for the voltage in question. Remember that 220 volts AC has a peak value of over 300 volts.
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Old 11th October 2021, 07:07 AM   #4
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by donovas View Post
the cheap trafos ive been using measures 150ohm across its primary winding.
WHICH trafo?
WHAT primary rating?
WHAT secondary rating? .... alternatively: WHAT secondary voltage /current or WHAT VA?
No data no answer.
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
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Old 11th October 2021, 07:28 PM   #5
martin clark is offline martin clark  Europe
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More than that - what is the 150ohm resistor actually doing..?
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Old 11th October 2021, 07:37 PM   #6
Mooly is offline Mooly  United Kingdom
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Trafo primary resistance too low?
Isn't the 150 ohm the DC resistance of the primary? That is how I read it.
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Old 11th October 2021, 07:44 PM   #7
russc is offline russc  England
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In my ignorance, I would guess it's only there to bleed caps safely.
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Old 11th October 2021, 07:47 PM   #8
adason is offline adason  United States
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Quote:
Originally Posted by donovas View Post
Ive noticed the shunt resistor on my line filter heats up quite a bit so by troubleshooting i found out that the cheap trafos ive been using measures 150ohm across its primary winding. The shunt resistor is 100k ohm. Parallell that with the 150ohm and its been acting like a 149ohm resistor!

My question is about the trafo. Why is it wound this way and what benefit/harm is there to have the primary resistance so low?

The trafo itself works fine in the circuit, tho with a noticable self hum.
How did you measure the resistance of the trafo? It does not work using normal multimeter in ohm setting. You need to use Kelvin sensing. Read this:
https://www.debassociates.com/blog/i...r-measurement/
how to measure dc resistance of inductor Kelvin sensing - Google Search
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Old 11th October 2021, 07:49 PM   #9
martin clark is offline martin clark  Europe
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Doh, of course I'd misread. Apols.

100K bleeder on a nom 220VAC mains (from your S Korea flag) is dissipating about 0.5w so yes - it will be hot. Esp. if mounted close to the pcb / not elevated to allow a bit of convection. I'd suggest that it needs to be a part rated for 2-3w and 500V ideally.
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Old 13th October 2021, 03:07 PM   #10
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by donovas View Post
My question is about the trafo. Why is it wound this way and what benefit/harm is there to have the primary resistance so low?
That's actually quite high. You want the primary resistance to be as low as possible so its not wasting power as heat. However copper and iron cost money and weight, so you have to pick a compromize.

Say I had a 100W transformer running at 240V. That's 0.42A in primary. If the primary were 100 ohms I'd be wasting 17.6W as heat in the primary (and a similar amount in the secondaries too probably). That's not great. If the primary were 30 ohms the loss would be 5W, and say the secondaries were the same, thats 10W loss for 100W load, or about 90% efficiency - much better.

The actual mains current that flows is everything to do with the inductance of the windings and the load on the secondary, and almost nothing to do with the winding resistances (in an efficient transformer). Assuming no DC component in your mains supply (that's a whole issue itself).
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