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Voltage regulator Question

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joe_1

Member
I got two ways to get 5V from the bridge rectifier connected to 110VAC:
1-By using the LM7805
2-By using Zener diode 1N4733A

My question is which approach is better?
Thanks.
 
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BrownOut

Banned
Neither. You're bridge rectifier should be connect to a low-voltage transformer. Then you can think about filtration and regulation.
 
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joe_1

Member
The transformer I am using does step the voltage down to 15VAC, but my issue is voltage regulation which has nothing to do with the transformer.
 

EN0

Member
I got two ways to get 5V from the bridge rectifier connected to 110VAC:
1-By using the LM7805
2-By using Zener diode 1N4744A

My question is which approach is better?
Thanks.

If you want to get 5V out, the datasheet says you'll at least need 7V-20V input voltage. Also, there is a typical 4V drop across the diodes, and usually you would use a bridge. Therefore, you can use that transformer that takes 120V AC down to 15V AC, then you'll get 9V on the output. Or, you could have the transformer step down the voltage, as long as it doesn't go below 7V or above 20V. Keep in mind you'll also need a filter capacitor to smooth the ripple. I would suggest you go and look at the basic schematics for power supplies. Observe the diode bridge and the filter cap. After the cap, it goes into the regulator circuit.
 
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joe_1

Member
Thanks guys.
Actually I built two circuits, one using the LM7805, and the other using Zener diode 1N4744A. They both work OK, and I am getting 5VDC.

My question is not about how to get 5VDC; I am just wondering if the LM7805 is a better regulator than the Zener diode 1N4733A. I know the Zener is cheaper, and does not require heat sink.
 
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BrownOut

Banned
Zener based regulation is usually more precise. IC regulators deliver more current.
 

MikeMl

Well-Known Member
Most Helpful Member
Zener based regulation is usually more precise. IC regulators deliver more current.
Just the opposite. The voltage out vs load current of an IC regulator is much flatter than a series resistor-shunt zener regulator.

Isn't the 4744 a 15V zener, not a 5V one?
 

joe_1

Member
Just the opposite. The voltage out vs load current of an IC regulator is much flatter than a series resistor-shunt zener regulator.

Isn't the 4744 a 15V zener, not a 5V one?

You are right. I meant 4733A.

The reason I am posting this question is because my boss saw this zener circuit and said that zener-resistor based regulators are crude, cheap, and they wont handle situations where the AC voltage drops to 100VAC.
For some reason, I used to think that inside the LM7805 there is a Zener!
 

BrownOut

Banned
Just the opposite. The voltage out vs load current of an IC regulator is much flatter than a series resistor-shunt zener regulator.

That's why I said IC regulator delivers more current.
 
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MikeMl

Well-Known Member
Most Helpful Member
You are right. I meant 4733A.

The reason I am posting this question is because my boss saw this zener circuit and said that zener-resistor based regulators are crude, cheap, and they wont handle situations where the AC voltage drops to 100VAC.
For some reason, I used to think that inside the LM7805 there is a Zener!

Slightly more than a Zener.
 

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bountyhunter

Well-Known Member
Thanks guys.
Actually I built two circuits, one using the LM7805, and the other using Zener diode 1N4744A. They both work OK, and I am getting 5VDC.

My question is not about how to get 5VDC; I am just wondering if the LM7805 is a better regulator than the Zener diode 1N4733A. I know the Zener is cheaper, and does not require heat sink.

A Zener is a shunt regulator and is very crude for use as a regulator of voltage if you have to supply current to a load. The 7805 is far superior in general..... which is why they were invented and then replaced zener shunt regulators on nearly everything on earth. Zeners have very limited use these days. There are IC shunt regulators that are far superior and very cheap like the LM-385 for example. The voltage precision is better as well as temp stability.
 

bountyhunter

Well-Known Member
You are right. I meant 4733A.

For some reason, I used to think that inside the LM7805 there is a Zener!
There might be some Zeners somewhere in there, but only used to set up an internal voltage reference not the output voltage. That is set by a bandgap reference.
 
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