Stuck in the database of BZX85C5V1 Zener Diode

Thread Starter

woon_h88

Joined Mar 25, 2009
49
Hi,

Im trying to build a simple voltage divider but also act like a regulator to supply to a load (consists of a DIY voltage meter with a random load) with around 5V, 500mA as shown below. The incoming supply is from a 230V to 12V step down transformer and went through the rectifying and smoothing process. But can supply up to 1A.
upload_2016-3-15_1-9-44.png
I was trying to pick a RS and zener diode for the circuit, and i saw this BZX85C5V1 Zener Diode (http://docs-asia.electrocomponents.com/webdocs/0029/0900766b800297e6.pdf).

I was confused on the part at iz mA = 200mA. Does it mean that the diode can regulate up to 200mA? or the diode take up 200mA and the rest of the current go to the load side?

Thank You
 
The Iz is the maximum continuous current that can be run through the diode at any time.

Multiplying Iz and Vz for the listed diodes yields a power of around 1 Watt for each diode, which is close to the maximum power rating of the diode. (power = voltage * current)
 

crutschow

Joined Mar 14, 2008
24,045
You can't use that zener for your circuit unless the load is always connected, otherwise the diode will blow.
For a 500mA load it's much better to use a series regulator IC such as the LM7805.
Note that it will dissipate up to about 4.5W at 500mA output so it would need to be mounted on a small heatsink.
 

Thread Starter

woon_h88

Joined Mar 25, 2009
49
oh...Guess like a bad news. But what if i use it to supply my fix load (DIY voltage meter, relay, LM324) ?..Will the diode still blow?
Im not allow to a LM7805 for my project..Only discrete component for the regulator part.

Also im thinking is it ok to use a capacitor with different voltage rated in a same circuit?
 

crutschow

Joined Mar 14, 2008
24,045
Sounds like homework.

So you select a value for Rs that allows for the 500mA load current plus some current through the zener to keep it in regulation (say 10mA≥≤200mA) with a voltage drop across the resistor of the maximum input voltage minus the minimum output voltage.
Note that the zener you selected will fail if the 500mA load is removed.
It that's not acceptable then you will need a 3W zener with a proper heatsink.
Also Rs will be dissipating at least 4.5W.

If you can also use a transistor then a better solution might be to use an NPN power transistor, such as the old 2N3055, as a series regulator.
Connect the collector to the source and the emitter to the output.
Connect a 4.3V zener from the base to common (the transistor base-emitter voltage will provide the extra ≈0.7V needed).
Connect a resistor from the collector to base to supply the minimum base current required by the transistor (look in the data sheet for the minimum hFE or β value) with a 500mA load plus a few mA for the zener current.
The transistor will need to be on a heatsink.
 
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