Why is there a resistor between the primary and the secondary winding?

Thread Starter

Milan Bozic

Joined Dec 2, 2017
5
Is is PSU of Samsung HVAC.

Are resistors R106 and R107 violate isolation DC side from AC main current source?

Is it dangerous to touch GND on DC side with finger?

What is purpose of those resistors?

Moderator Edit: Same question was asked here.
 

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MisterBill2

Joined Jan 23, 2018
18,426
Keep in mind the resistance of that resistor R106. It is 12 MEGOHMS= 12,000,000 ohms. Now calculate the current that 120 volts can force through that resistor. Not a hazardous amount. Also note that the resistors are NOT connected "across the transformer," but see that chassis ground symbol. Just a lazy drafter gives the wrong impression.
 

MrChips

Joined Oct 2, 2009
30,788
"Does it meter?" vs "Does it matter?"

These are both valid questions with different meanings.

"Does it meter?" - This would mean, "Can you measure the resistance?"
"Does it matter?" - This is the same as asking "Is it important?" or "What are the consequences?"
 

MisterBill2

Joined Jan 23, 2018
18,426
Given that we also see a voltage specified at the junction of the two resistors R106 and R107, it is some sort of test point, whose actual value is to samesong service people, unless others have discovered the secret.
And that high a resistance is not going to create a shock hazard. It might even be present as a static drain. OR it might be part of a check process to verify that the neutral supply lead is actually neutral and not live.
 

Thread Starter

Milan Bozic

Joined Dec 2, 2017
5
I am afraid to connect my oscilloscope if DC part is not isolated! And DC ground can be connected by those resistors to neutral or live line.
 

MisterBill2

Joined Jan 23, 2018
18,426
The end of the resistor string is connected to a point also connected to some sort of "ground" symbol.
SO, exactly as I stated previously, it is not mostly "across the transformer", but rather between the primary and the "frame ground."
Really, just lazy drafting.
So before considering a scope connection, connect a voltmeter and observe the voltage between that point and your safety (green wire) ground connection. Check for both AC and DC voltage.

My concern is that if the TS does not read a circuit drawing very well, why are they probing inside a device connected to mains power??
 

Pyrex

Joined Feb 16, 2022
262
Nothing unusual, two resistors in series, 12 Megohm each. Statics drain, that technique is used often enough. Two resistors 1/2W each in series for safety.
And, if someone afraid to connect an oscilloscope to the secondary side of the transformer, keep in mind that an infinite isolation do not exist in reality:) And an isolation of 24 Megohms is safe in any case
 

Thread Starter

Milan Bozic

Joined Dec 2, 2017
5
Nothing unusual, two resistors in series, 12 Megohm each. Statics drain, that technique is used often enough. Two resistors 1/2W each in series for safety.
And, if someone afraid to connect an oscilloscope to the secondary side of the transformer, keep in mind that an infinite isolation do not exist in reality:) And an isolation of 24 Megohms is safe in any case
So, those resistors drain static current and because the max current could be 230V/24E6ohm = 9.6uA, and nothing is perfect, I am concluding that:
1. it is not dangerous to touch DC parts and
2. it is not dangerous to connect oscilloscope to DC parts

Thanks Pyrex, and all other guys :)
 
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