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Pulsating current

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umer007

New Member
First of all can sumone describe pulsating current to me is it like AC? and second of all wat converts DC to a pulsating current. I have a circuit drawing in which there r fixed capacitors (electrolytic and monolythic), PNP transistor. There is a transformer in it too but my teacher says the pulsating current goes tohrough the transformer so i need to no what changes the DC into pulsating current. Thx
 

Phasor

Member
so i need to no what changes the DC into pulsating current.

1) It is always easier to understand people's requests if they post a schematic. Can you upload a drawing of your circuit?

2) I assume that you are studying an inverter. The pulsating current is caused by the on-off-on-off switching of the transistor. The rate of switching is controlled by the value of the capacitor and the inductance of the transformer.
 

umer007

New Member
I m trying to attach my schematic which i made on paint but when i clik add attachment it says the extension bmp is not allowed. CAn sumone plz tell me how to attach my schematic and i drew it on paint. And the person who said when a transistor is worked like a switch, when it is turned on and off there is a pulsating current. Cud u plz go into depth into that explanation my teacher asks abt every litle thing. And can u also explain more abt wat a pulsating current is how is it different teh AC and DC.

Thx
 

umer007

New Member
Alrite, ive attached teh schematic. CAn u guyz plz describe every component, like transformer, transistor, and plz tell me wt they do. And also how is the pulsating current made. IF it is made by switchin current on and off, wat things r seperated to make it pulsating from DC. Plz explain in detail. Thanx
 

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Russlk

New Member
The circuit is a "blocking oscillator". When the key switch is closed, the PNP transistor is turned on. The collector goes positive, which causes the other side of the transformer to go negative, turning the PNP on more. At some point the current stops increasing due to resistance in the transformer, or the transformer saturates. At that point, the feedback from the transformer goes to zero, but the 10uF capacitor has charged up which turns the PNP off. The 10uF charges slowly thru the base resistors and the cycle repeats.
 

umer007

New Member
The name that i wuz given for this schematic was Machine gun pulse oscillator. I wuz wondering when the key is closed and transistor turnd on, why duz the collector go positive and how duz this make the other end of the transformer go negative? And wt do u mean this makes the transistor turn on more....like it stays on longer or?? And if the capacitor is fully charged up, is that why the transistor turns off. And can u also tell me how isthe pulsating current made?

Thx
 

Russlk

New Member
Current flows in the direction of the arrow on the PNP emitter. It is positive current (not electron flow) so the end of the transformer that the current enters is positive relative to the center-tap which is connected to the battery negative terminal. Because of mutual induction, the current in the other half of the transformer flows out, making that end negative. The pulsating current is caused by the PNP switching on and off. The current thru the base bias resistors is not enuf to saturate the PNP; if it was, the oscillator would stop. The feedback thru the 10uF is enuf to saturate the PNP.
 

umer007

New Member
Cud u explain mutual induction to me plz and wt do u mean by when u say saturates, like the transformer gets flooded wid current and stops working?? ANd also i read on a reply here, it says that the rate of switching of the transistor is controlled by the value of the capacitor and the inductance of the transformer but u said its controlled through the on off switching of key?? Wt duz the value of the capacitor have to do wid it, and wt duz inductance of the transformer mean?

Excuse this noob for all the questions :wink:
 

Russlk

New Member
When current flows, a magnetic field exists around the wire. When the wire is wound into a coil, the fields add up and the field of one turn cuts across all other turns. The field in air is rather weak, the presence of iron in the coil magnifies the field and makes it stronger, but the ability of iron to increase the magnetic field is limited, at some point the increase of current does not cause an increase in magnetic field and the core is said to be saturated. An air core coil cannot saturate.

The magnetic field expands and contracts as the current increases and decreases, but the changing field induces a voltage in the coil that opposes the change in current. This is in accordance with Newton's third law: For every action, there is an equal and opposite reaction. This is the essence of induction. If a constant voltage is applied to a coil, the current will increase in a linear manner depending on the resistance and inductance, due to the back EMF (EMF=ElectroMotive Force = voltage)

At one point, I mentioned saturation of the PNP transistor: that is when the collector-emitter voltage reaches it's lowest value and cannot go lower.
The PNP switch is controlled by the inductance and capacitance. The key switch is just an on-off switch, I think this is a code practice oscillator. The inductance causes the current to increase slowly (linearly) and the capacitor keeps the PNP off while it is discharged by the bias resistors.

Inductance is a measure of the energy that can be stored in the magnetic field : E = L*I^2 where E is the energy in Joules, I is the current in amps. Inductive reactance is a measure of the impedance the coil presents to sine wave current. Note that reactance is only valid for sine waves. The symbol for reactance is X, the symbol for inductive reactance is Xl. There is a voltage drop across an inductor , like Ohm's law: V = i*Xl, but there is no power lost in the inductive reactance . A coil may get hot due to losses in the iron core or resistance, but not from the reactance.
Thats the lesson for today!
 
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