Questions about RCC switch mode power supply – Circuit by Andy

Thread Starter

samy555

Joined May 24, 2010
116
Hi
From: http://mmcircuit.com/understand-rcc-smps/

I read all the page and have some questions and hope someone (like Jony130) respond

Q1) Cansomeonegivevalues for thecircuit below as it is,,, I know it is a basic circuit but I want to build it and see some waveforms on my new oscilloscope.




Q2) If Rs = 100Kohm and VCC = 12Vdc then IB = about 113uA

This current will maintain the Q1 always ON. How Q1 be OFF in other period of time?

TomorrowI will putmore questions

Thank you
 
Last edited:

Kermit2

Joined Feb 5, 2010
4,162
The base is not connected to VCC. It is part of another winding on the transformer.
What happens in this winding when you take VCC from nothing to 12 volts?
After looking more closely at this schematic, I do not think it can work as currently configured.
 

benta

Joined Dec 7, 2015
101
This is a really nasty circuit, but was often used in the '60s and '70s. It is one of the noisiest DC/DC converters ever designed.
The transistor is on, first driven by RS, then by the secondary winding through RB, until at some point the transformer saturates and the magnetic field collapses.
This will turn off the transistor. releasing the stored magnetic energy into the secondary winding while reversing the voltage.
I am missing a diode in the primary, though (as well as lots of snubber components).

Not nice.

Benta
 

MrAl

Joined Jun 17, 2014
7,512
Hi,

Yes, looks like a "Jewel Thief" with a secondary winding.
I built one of these a long time ago after realizing that you can simply wind another winding on the Jewel Thief like circuit and get an isolated power source. I needed it to power a multimeter intended for use in an automobile on a regular basis because the meter needed an isolated supply.
I found out later that the meter was not very good for use in the car because it picked up too much noise from the ignition, so i never used the circuit for longer than about 2 days. I think i still have it around somewhere though. It was so simple i didnt use a circuit board, just a piece of fish paper to mount the parts on :)
BTW the core was very small, maybe about 1/2 inch diameter so it was very compact.
 

Thread Starter

samy555

Joined May 24, 2010
116
The base is not connected to VCC. It is part of another winding on the transformer.
What happens in this winding when you take VCC from nothing to 12 volts?
After looking more closely at this schematic, I do not think it can work as currently configured.
No
The base is connected to VCC through Rs
 

Thread Starter

samy555

Joined May 24, 2010
116

Thread Starter

samy555

Joined May 24, 2010
116
The transistor is on, first driven by RS, then by the secondary winding through RB, until at some point the transformer saturates and the magnetic field collapses.
This will turn off the transistor.
Benta
Thank you Benta for reply
I can not imagine that the transistor is OFF while there is a continuous DC base current flowing to its base!!!!
 

Jony130

Joined Feb 17, 2009
5,157
I can not imagine that the transistor is OFF while there is a continuous DC base current flowing to its base!!!!
Transistor will be OFF because of the "transformer action" and the negative voltage induced in AUX winding (7-8).

Today I quickly build Joule thief and capture some waveforms.

10b.PNG

Notice the voltage polarity across transformer.

And o-scope waveforms
10.PNG

In BLUE you have voltage at Q1 base.
RED trace is a voltage at Q1 collector.
And notice that at some point the voltage at the base is negative (-8V). And this is why Q1 is OFF.
 

benta

Joined Dec 7, 2015
101
I can not imagine that the transistor is OFF while there is a continuous DC base current flowing to its base!!!!
The transformer base winding supplies significantly more current into the base than your competing 100 kohm resistor and will dominate operation.

Jony130 was very helpful in quickly building an equivalent circuit and providing the waveforms.

It is basically a self-oscillating DC/DC converter, where frequency is highly dependent on the magnetic properties of the transformer core.

Not a nice circuit today, when so many good ICs are available. And like I said, it is extremely noisy (radiation-wise).

Benta.
 

Thread Starter

samy555

Joined May 24, 2010
116
Transistor will be OFF because of the "transformer action" and the negative voltage induced in AUX winding (7-8).
You did not explain it in the Q1 figures. There is no AUX winding (7-8)
I don't know why in the left transformer figure the current direction in the collector coil is upward where I expect to be downward when Vcc is first applied.

Today I quickly build Joule thief and capture some waveforms.

View attachment 96986

Notice the voltage polarity across transformer.

And o-scope waveforms
View attachment 96987

In BLUE you have voltage at Q1 base.
RED trace is a voltage at Q1 collector.
And notice that at some point the voltage at the base is negative (-8V). And this is why Q1 is OFF.
Yes I noticed. So wonderful
I carried out the following circuit

I am committed to the values given but experiments conducted around the ferrite cure shape
Toroid , E-I shape (from computer power supply) and finally simple ferrite rod (From AM radio antenna). All of which gave me almost the same results and the 3.5V White LED shined from a bad and weak 1.5 volt battery.

The next step I'm going to do is some experiments on the number of turns for each coil of the transformer and i'll record them againest the frequency of oscillations and the output peak voltage.

thank you Jony130 very much

Sorry
I forgot to tell you that your circuit has a capacitor While mine do not.
What is the role of this capacitor?
thank you very much
 
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Jony130

Joined Feb 17, 2009
5,157
You did not explain it in the Q1 figures. There is no AUX winding (7-8)
Look at this diagram, and notice the voltage polarity across transformer winding.
RCC_01.png

RCC_02.png

https://en.wikipedia.org/wiki/Polarity_(mutual_inductance)#Terminal_layout_conventions

I don't know why in the left transformer figure the current direction in the collector coil is upward where I expect to be downward when Vcc is first applied.
This blue arrows show the voltage polarity, not the current.
 
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