# power supply

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#### Norm Woods

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I have a power supply that varies 0-20 volts. I have a device that needs 0-5 volts only. What is the easiest way to block anything over 5 volts? A diode?

You could use an overvoltage crowbar or other similar circuit.
How much current does the device require?
What is it's maximum voltage rating?
Do you need to operate down to 0V?

• Norm Woods
I have a power supply that varies 0-20 volts.
I have bench power supplies like that. My problem is my grandson likes to turn the knobs. So 5V becomes 20V.
I have a device that needs 0-5 volts only.
I am using cellphone chargers. 5V at 2A. Very simple and can not be changed.

• Norm Woods
You could use an overvoltage crowbar or other similar circuit.
How much current does the device require?
What is it's maximum voltage rating?
Do you need to operate down to 0V?
Thanks, device only requires about 200 Ma Maximum voltage rating 5v The power supply varies between 0 - 20 volts output. What I am really trying to do is keep the device voltage at the input on 5v no matter what the power supply is feeding it between 0-20 v. Easiest way without using a voltage regulator circuit.

What you are describing *is* a voltage regulator. There are many flavors, from simple to complex, but variable-in -- stable-out is a regulation function.

If you stick with linear topologies, the regulator will dissipate at least (20 - 5) x 0.2 = 3 W somewhere. The problem with a simple resistor-zener circuit is the conflict between the resistor value needed at 20 Vin and the one needed at 6 Vin. If you start with 6 Vin and have about 10 mA static current through the zener to maintain regulation, then

R = (6 - 5) / 0.210 = 4.167 ohms

So far, no prob. But when the input jumps to 20 V, the current through the zener is

((20 - 5) / 4.167) - 0.2 A = 3.4 A

The power dissipated in the zener is 3.4 x 5 = 17 W.

The power dissipated in the resistor is (20 - 5) x 3.6 A = 54 W. Ouch.

This is why a 3-terminal regulator is so common in this application. It is essentially an automatically varying resistor that always is the perfect value for the input-output voltage differential.

ak

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• Norm Woods
What you are describing *is* a voltage regulator. There are many flavors, from simple to complex, but variable-in -- stable-out is a regulation function.

If you stick with linear topologies, the regulator will dissipate at least (20 - 5) x 0.2 = 3 W somewhere. The problem with a simple resistor-zener circuit is the conflict between the resistor value needed at 20 Vin and the one needed at 6 Vin. If you start with 6 Vin and have about 10 mA static current through the zener to maintain regulation, then

R = (6 - 5) / 0.210 = 4.167 ohms

So far, no prob. But when the input jumps to 20 V, the current through the zener is

((20 - 5) / 4.167) - 0.2 A = 3.4 A

The power dissipated in the zener is 3.4 x 5 = 17 W.

The power dissipated in the resistor is (20 - 5) x 3.6 A = 54 W. Ouch.

This is why a 3-terminal regulator is so common in this application. It is essentially an automatically varying resistor that always is the perfect value for the input-output voltage differential.

ak
Thank you so much. that is what I needed. Just someone to verify for me some things. This is a very good explanation and something I can add to my learning and understanding. I try to keep updated and refreshed on things from a long time ago.

You're welcome. The vast majority of small project design is a little thinking and the correct application of Ohm's Law, Joule's / Watt's Law, and 3rd grade arithmetic.

ak

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