Zener diode ballast resistor question

In summary, the conversation discussed determining the appropriate value for a ballast resistor when using a 5V 0.5 watt zener diode in a circuit with a variable supply voltage (15V minimum to 20V maximum). The maximum zener current was calculated to be 100mA, and the suggested resistor values were 100Ω at minimum supply voltage and 150Ω at maximum supply voltage. In determining if the chosen 90Ω resistor is sufficient, one should consider the current and power dissipated in the diode at this value.
  • #1
tommoturbo
33
0

Homework Statement


Determine the value of the ballast resistor is satisfactory if a 5Volt 0.5 watt zener diode is used in the circuit

sorry can't upload a picture but basically

VS = 15v min to 20v max

R ballast = 90 ohms (series with zener)

and R load (in parralel with Zener) is 50 ohms


Homework Equations



max zener current = watts/voltage 0.5/5v= 100Ma

now I am getting a bit stuck i keep getting values of 100ohms at min Vs and 150 ohms at max Vs.

will update post later running out of time at the minute

thanks guys



The Attempt at a Solution

 
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  • #2
tommoturbo said:

Homework Equations



max zener current = watts/voltage 0.5/5v= 100Ma

now I am getting a bit stuck i keep getting values of 100ohms at min Vs and 150 ohms at max Vs.

will update post later running out of time at the minute

thanks guys

The Attempt at a Solution

Looks good, I agree with your current and resistor values. So, is the 90Ω sufficient?

If you're unsure, here is a hint: what are the current and power dissipated in the diode if 90Ω is used for the ballast resistor?
 

Related to Zener diode ballast resistor question

1. What is a Zener diode ballast resistor?

A Zener diode ballast resistor is a component used in electronic circuits to regulate the current flowing through a Zener diode. It is typically a low-value resistor that is connected in series with the Zener diode to limit the current and prevent damage to the diode.

2. How does a Zener diode ballast resistor work?

The ballast resistor works by providing a fixed resistance in the circuit, which limits the current flowing through the Zener diode. This allows the diode to operate in its breakdown region, where it maintains a constant voltage despite changes in current. The ballast resistor also helps to dissipate excess power and prevent the diode from overheating.

3. When is a Zener diode ballast resistor needed?

A Zener diode ballast resistor is needed when using a Zener diode as a voltage regulator. Without the resistor, the diode may not be able to handle the current flowing through it and could be damaged. The resistor is also necessary to maintain a stable output voltage from the diode.

4. How do you choose the right value for a Zener diode ballast resistor?

The value of the ballast resistor depends on the desired output voltage and the current flowing through the circuit. It can be calculated using Ohm's Law (R=V/I), where R is the resistance, V is the desired output voltage, and I is the current. It is recommended to choose a resistor with a slightly higher value than the calculated value to ensure the Zener diode is operating within its safe range.

5. Are there any disadvantages to using a Zener diode ballast resistor?

The main disadvantage of using a ballast resistor is that it can cause a drop in efficiency, as it dissipates power that could have been used by the circuit. Additionally, using a ballast resistor may not be suitable for high-power applications, as it can generate a significant amount of heat. It is important to carefully consider the circuit's requirements before deciding to use a Zener diode ballast resistor.

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