The picture shows a simple linear power supply concept I found in an old product. The AC input range is 70 ~ 150 VAC from mains (nominal 115 VAC). R2 is a metal film resistor. C2 is ceramic capacitor (Digi-key part). The transformer is this one 14A-10R-20. The V_UNREG output feeds this switching regulator. I'm trying to simulate this circuit with LTspice coupling 4 inductors to get the transformer. The input AC source waveform is ok, but the voltage at C2 is practically zero. Why could this happen? In the real world this design should have some flaws though. R2 goes open circuit. The transformer also gets warm. Some thoughts I have are the circuit not good handling surges. Like, a surge may damage R2 and C2 easily. I don't see the MOV activates though. Also, what's the real purpose of R2/C2? Values don't look reasonable if the intent was a low pass filter. Any circuit analysis thoughts are welcome.

 

Hi uns,
Welcome to AAC.
If the posted circuit image you posted is the LTS asc file, I don't see the mutual coupling factor?
eg: K1 L1 L2 L3 L4 1.
Where did you get the transformer LTS model?

E

 

Hi uns,
This version of the LTSpice sim runs OK.
E

 

Perhaps you connected the primaries backwards?

They are in parallel now, phasing matters.

 

Hi,
R2 can work as a fuse. C2 damps sparking, when mains is removed. C2 must be a high quality capacitor, rated to 200VAC or higher voltage

 

Hi uns,
This version of the LTSpice sim runs OK.
E
View attachment 301867

The only difference I see between your circuit and mine are the values of the inductors. I have value in micro Henries. Which leads me to think now, how I can choose them right for the simulation in a way that behave like the transformer from the datasheet.

 

hi uns,
This PDF may help.
E

Added:
https://www.allaboutcircuits.com/te...chpt-9/mutual-inductance-and-basic-operation/

 

I believe that R2 should not be a fixed resistor but rather a NTC thermistor if the purpose is to limit the inrush current.
A fixed resistor will cause both a poor load regulation and lowered efficiency.

 

hi uns,
This PDF may help.
E

Added:
https://www.allaboutcircuits.com/te...chpt-9/mutual-inductance-and-basic-operation/

Thank you! It helped. My simulation seems to work as expected from a theory standpoint. Unfortunately I can't blow things with LTspice

 

I believe that R2 should not be a fixed resistor but rather a NTC thermistor if the purpose is to limit the inrush current.
A fixed resistor will cause both a poor load regulation and lowered efficiency.

I think the same. Also, should C2 be a X7R 200V ceramic cap? I think that a mains surge may blow that 200V cap.

 

No serious problems with the circuit as posted, however I would not put a capacitor across the mains winding of a transformer, because there is no benefit in doing it. and if a simulator does not provide a model of a transformer I would not create one with inductors.
Transformer efficiency is always less than 100% and so all transformers get warm, and some of then get quite hot.
The series resistor is an inefficient power waster and after the first second beyond power-on it provides no benefit at all. When resistors are used to reduce turn-on surge current, they are usually bypassed after the first few seconds. If the switch selected for controlling the mains input is adequate for the application there will be no sparking problem when the supply is switched off.
The 4diodes, 1N4007 are rated 1 amp maximum forward current and up to 1000 peak revers volts. That is far beyond what is required for a 0 volt transformer.
And normally the negative of a power supply is not tied to earth ground.

 

The picture shows a simple linear power supply concept

View attachment 301841

BTW, in post #1 you have your Earth and GND symbols backwards.
In most cases where possible I always reference supply DC common to earth GND.

 

BTW, in post #1 you have your Earth and GND symbols backwards.
In most cases where possible I always reference supply DC common to earth GND.

View attachment 301951

Ok, although both earth GND and chassis are tied together since R1 is 0 ohms. And talking about grounds, is it okay to have the AC input source floating?

 

What is AC input floating exactly?
You mean the earth GND unconnected?

 

No serious problems with the circuit as posted, however I would not put a capacitor across the mains winding of a transformer, because there is no benefit in doing it. and if a simulator does not provide a model of a transformer I would not create one with inductors.
Transformer efficiency is always less than 100% and so all transformers get warm, and some of then get quite hot.
The series resistor is an inefficient power waster and after the first second beyond power-on it provides no benefit at all. When resistors are used to reduce turn-on surge current, they are usually bypassed after the first few seconds. If the switch selected for controlling the mains input is adequate for the application there will be no sparking problem when the supply is switched off.
The 4diodes, 1N4007 are rated 1 amp maximum forward current and up to 1000 peak revers volts. That is far beyond what is required for a 0 volt transformer.
And normally the negative of a power supply is not tied to earth ground.

The capacitor serves to reduce voltage spikes at the moment the mains is switched off. This is due to inductive imedance of the transformer. And, in addition, it reduces voltage spikes from the mains. The real life is much more complicated than a picture in a simulator...
As about the resistor-it serves as a fuse and a surge limiter. Power loses for a 10 VA transformer is about 0.05W. Is it significant?

 

Hi uns,
For the floating mains source, this is one method for LTS, use the COM symbol F4.
E

 

One of the major benefits of an input power transformer is isolation. Thus connecting the mains to the output, either directly or indirectly, is a poor choice in most applications.
AND as for a shunt capacitor reducing spikes, the switch off spike does no harm and affects nothing. and consider the source impedance of any line spikes, how much will a capacitor affect them??