A Newbie question on Zener Diodes

[rational]

Hi,

I am a newbie to electronics and I am trying to build a power supply with output of 12 Volts and 200 Watts. I want to use a zener diode as a voltage regulator in the circuit. However the highest wattage rating zener diode that I can find is of 50 watts. I would like to know if using 4 of these can add up to 200 watts. Could anyone plz give me a circuit block diagram of such a power supply ? I am confused about the zener diodes : should they be used in parallel or in series ?

I would very much appreciate a reply.

 

[Nigel Goodwin]

You don't use a high power zener diode for a 200W PSU - that's completely the wrong approach, and EXTREMELY inefficient.

 

[rational]

Could you plz guide me ? I want a PSU which supports variable load between 80-200 watts.

Thanks.

 

[arhi]

I want a PSU which supports variable load between 80-200 watts.

The PSU that can support load of 200W can support 80W as well ...

So if your output voltage is 12V to get 200W you need 17A hence you need PSU that can output 12V 17A.

Check out high power voltage regulators:
**broken link removed**
Switching power supply
High Current Voltage Regulation - Electric Circuit

 

[rational]

I have one more question. When the reverse breakdown occurs in a zener diode, it allows a large current to flow through it. Assuming that the input voltage is 50v and the zener voltage is 12v, is the difference (50v-12v) completely dissipated as heat. Or does a zener allow this difference to be converted into a large current flow (which occurs when the reverse breakdown takes place) ?

Thanks

 

[Wond3rboy]

It is not that simple you need to see the power rating of the Zener other wise it will burn.Yes the current will increase rapidly after Zener Voltage is reached.It is converted in to heat.

 

[Hero999]

All the energy is converted to heat.

A zener regulator uses exactly the same amount of power when a load is connected as when it's unloaded. This is why zeners are only used for very low power voltage references.

 

[crutschow]

If you have a 50V source and a 12V zener, then 38V of the voltage is dropped in the resistor between the source and zener (you need a resistor to limit the current through the zener) and 12V of the voltage is dropped across the zener. The no-load power dissipated in each is simply the current times their respective voltages.

 

[colin55]

A zener regulator uses exactly the same amount of power when a load is connected as when it's unloaded. This is why zeners are only used for very low power voltage references.

This is incorrect as I have explained in a previous post. It may be correct on a rare occasion but the purpose of a zener diode is to "share" the load with the device being powered and that's why the statement is considered to be incorrect.

 

[crutschow]

This is incorrect as I have explained in a previous post. It may be correct on a rare occasion but the purpose of a zener diode is to "share" the load with the device being powered and that's why the statement is considered to be incorrect.

Depends upon how you interpret "uses exactly the same amount of power". It is true the the power the zener regulator draws from the source is the same whether it's loaded or unloaded. The power that the zener dissipates does change with load but the total power doesn't.

 

[Hayato]

Depends upon how you interpret "uses exactly the same amount of power". It is true the the power the zener regulator draws from the source is the same whether it's loaded or unloaded. The power that the zener dissipates does change with load but the total power doesn't.

It depends, if it is in parallel, it wont change. If it is in series, it will change.

 

[crutschow]

It depends, if it is in parallel, it wont change. If it is in series, it will change.

How do you make a zener regulator with the zener in series? It will drop voltage in series but it won't regulate.

 

[Sceadwian]

I've never seen a zener in series used for a regulator circuit, how would it function?

 

[Hayato]

Well, I was out of the regulator context, sorry.

The zener forward current x forward voltage curve is independent from the load. You can parallel a zener with a 1 ohm resistor, or a 1 Mohm resistor, the curve will be the same, so Hero's statement is very true.

A zener in series, indeed, is not to be used as a regulator, but it can be used as a voltage reference (Vsupply - Vz).

The same thing happens with a bulky design with a zener in parallel with the load, I believe, now a days, nobody uses it anymore. Why not just use them to regulate a transistorized output?

 

[colin55]

A zener regulator uses exactly the same amount of power when a load is connected as when it's unloaded

This is technically not true.
It's a bit like saying a 3-terminal regulator has the same losses, whether the load is applied or not.
A zener regulator, and the load it supplies, are not a single item. They are two separate items.
In the quoted statement, the writer admits the load is a separate item and thus the zener regulator does not consume the same power when loaded as when not loaded.

 

[Hayato]

I understand the quoted statement differently.
I understand, by the quotation, that the zener does consume the same ammount of power, independently if it's loaded or not.

 

[colin55]

Here's an easy explanation:
The zener diode itself, in a shunt regulator, will get hot when the load is not applied and stay cool when the load is applied.

 

[Hayato]

I'm not trying to be a stubborn.

But a zener that is submitted to a reverse voltage higher than the Zener Voltage, represents a very low impedance path to the current, lower than the load itself.
So it will get hot anyways. Unless the load resistence is low enough to cause a higher voltage drop enough to "cut" the zener diode.

Is this the circuit we are talking about?

-----R----+-----+
DZ LOAD
+--+--+
GND

 

[Mikebits]

I'm not trying to be a stubborn.

But a zener that is submitted to a reverse voltage higher than the Zener Voltage, represents a very low impedance path to the current, lower than the load itself.
So it will get hot anyways. Unless the load resistence is low enough to cause a higher voltage drop enough to "cut" the zener diode.

Is this the circuit we are talking about?

-----R----+-----+
DZ LOAD
+--+--+
GND

With no load, all the current is drawn through the limit resistor and the zener. With a load the current after the limit resistor is shared by zener and load.
The zener and load become parallel thus current is shared.

 

[Mikebits]

Wait, I take my last back. The current through the zener remains the same with load or no load. Current increases in the limit resistor with a load. I had to work it out on paper. Or maybe it is late and I am confused...