Some electronics questions

[ahmedragia21]

Hi, i've some questions in my mind while reading my text books
1. Why we use a series resistor with zener diodes ? to limit the current but i dont understand really what if i didnt put it , what would occur ?
2. Using transformers for power supply as an isolater , how it can isolate the main supply and why we need to isolate it ?
3.In a series regulator circuit using zener there's a load current and a variable resistor , doesnt we put a constant value of the load resistor ? why the book talks about variable load and hence variable load current IL ?

thanks

 

[Dean Huster]

Well, that series resistor isn't there only to limit zener current. It's to provide a voltage drop for the difference between the supply voltage and the zener voltage. Remember that the series resistor, zener and supply are a series circuit. The zener voltage plus the voltage across the resistor equals the supply voltage. If the zener were connected directly across the supply, it would try to "zener" at (for instance) 5.1 volts while the supply wanted 12 volts at its output; guess what would win out on that one?

Transformers isolate for safety reasons. You don't want the "hot" side of the mains to be connected to any metallic part of the equipment that can be touched by the user. IF the user was grounded and touched a hot chassis, he/she could be shocked or electrocuted. Even modern switchers include a power transformer, albeit because of the switching frequency, is a lot smaller and lighter -- and still there for the isolation. In addition, power transformers can provide multiple secondaries to supply various voltages that otherwise might require a very inefficient voltage divider or zener regulator to derive.

Your text is likely using a variable resistor to simulate a load current that's changing to illustrate how the zener operates with the series resistor to stabilize the output voltage. They just don't usually completely open the load resistor (zero output current) or take it all the way to a dead short (maximum output current) as the first could cause overdissipation of the zener and the second would cause the series resistor to go up in smoke.

Dean

 

[ahmedragia21]

Thanks alot,
Suppose your repairing the TV set how a short circuits happens to let the fuse be burnt ? also like you mentioned above a short circuit (maximum output current ) , i need to understand this a bit more ?

Also for the zener's series resistor what i understood is the series resistor is there to make a voltage drop to fullfills kirchoof's KVL 12V= 7Vresistor + 5.0
Zener correct ?

Also how an open circuit causes overdissipation of the zener ?

Many thanks

 

[Dean Huster]

Suppose your repairing the TV set how a short circuits happens to let the fuse be burnt ? also like you mentioned above a short circuit (maximum output current ) , i need to understand this a bit more ?

Not sure about your question here. But a short circuit where the power supply is allowed to accidentally connect to ground (supply return) causes the supply to put out as much current as it can. A fuse, rated for much less current than the supply is capable of, will "blow" or open to protect the supply and other circuits from damage.

Also for the zener's series resistor what i understood is the series resistor is there to make a voltage drop to fullfills kirchoof's KVL 12V= 7Vresistor + 5.0 Zener correct ?

You understand that part correctly.

Also how an open circuit causes overdissipation of the zener ?

When the load draws more current, more voltage will be dropped across the series resistor. This allows less voltage across the zener and the current through it will lessen. This then lessens the current through the series resistor and the voltage drop remains close to the original drop. All of this happens so fast that you cannot actually "see" it happening.

If the load draws less current, less voltage will be dropped across the resistor, more will be applied to the zener, it will pass more current and this current will then flow through the resistor raising the voltage drop back to nearly what it was. Again, happening extremely fast.

If the load is disconnected, then zener current is limited by the value of the series resistor. The voltage drop will be a lot less than with the load connected and the zener will be passing as much current as the resistor will allow. The current will actually be the difference between the supply voltage and the zener voltage divided by the value of the series resistor. This current could be large enough on some designs to over-dissipate the zener and cause it to (typically) short out. This usually happens when the series resistance is made small so that the regulator can work with larger load currents. But the danger to this is when the load opens. That's why zener regulators these days are usually limited to simply supplying very-low-current voltage references and we use electronic regulators such as the 7812 or LM317 to handle larger load currents with much better results.

Dean

 

[Hero999]

The neutral is connected to the earth before the distribution board. This means that you can get a shock if you touch the live wire.

An isolation transformer isolates the neutral from the earth. This means that you have to touch both the live and neutral to get a shock.

 

[mneary]

A fuse, rated for much less current than the supply is capable of, will "blow" or open to protect the supply and other circuits from damage.

Sometimes true, but: Most fuses can't protect semiconductors from damage; semiconductors are too fast. Fuses are more often used to reduce the risk of fire.

 

[Hero999]

You can get special semiconductor fuses but they're very expensive and only worth it for very big and expensive pieces of equipment.

 

[ahmedragia21]

Q: a transistor is a device which called current controller current source, and its called only current amplifier not voltage ampilifier , but how could be that correct and there's a voltage gain in most BJTs circuits ?

 

[dknguyen]

It's due to the current running through a "current-to-voltage converter".

Resistor = current to voltage converter!

It's like taking a voltage and turning it into a current, amplifying the current, then change it back into a voltage.

 

[cdstahl]

as for transistors, you can define things like transconductance for them as well. but they usually are called current controlled devices as there is ideally a constant ratio between base current and collector current. still you know that the base emitter junction is like a diode, and thus can find a base current for a given base-emitter voltage, and from this find the collector current. -- this would give current per volt.

of course this would not be a constant -- higher base-emitter voltages would mean MUCH higher base currents and thus EXTREME increases in colltector current. for this reason, BJTs normally have a higher small-signal transconductance then FETs. moving from 0.5V to 0.51V on the base of a BJT can cause current to change from microamps to milliamps (depending on device, temp, ect).