Zener Diode Circuit Trick — Ammeter Reading in 59 Sec! 🔥

❓ Question

In the following circuit, the reading of the ammeter will be ?

(Take Zener breakdown voltage = 4 V)

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🖼️ Question Image

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✍️ Short Solution

This is a classic JEE Zener-diode regulator problem.
No tricks, no confusion — bas Zener ON/OFF logic + Ohm’s law 🔥

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🔹 Step 1 — Identify Zener Condition (MOST IMPORTANT 💯)**

Given:

• Supply voltage = 12 V

• Zener breakdown voltage = 4 V

Since:

$$12\text{ V}>4\text{ V}$$

👉 Zener diode will be in breakdown (ON) mode.

📌 Golden rule:
When Zener is ON, it clamps the voltage across it to Vz.

So,

$$\boxed{V_{\text{node}} = 4\text{ V}}$$

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🔹 Step 2 — Voltage Across Load Branch

The 400 Ω resistor + ammeter branch is parallel to the Zener.

Hence:

$$V_{\text{across 400 Ω}} = 4\text{ V}$$

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🔹 Step 3 — Ammeter Reading (Direct Calculation 🔥)**

Current through 400 Ω resistor:

$$I = \frac{V}{R} = \frac{4}{400} = 0.01\text{ A} = 10\text{ mA}$$

📌 Ammeter is in series with this resistor,
so it reads the same current.

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🔹 Step 4 — Check Consistency (Zener Validity Check 🧠)**

Always verify that Zener really can conduct.

Current from supply through 100 Ω resistor:

$$I_{\text{total}} = \frac{12 - 4}{100} = \frac{8}{100} = 0.08\text{ A} = 80\text{ mA}$$

Out of this:

• Load current = 10 mA

• Remaining current = 70 mA flows through Zener

👉 Zener is safely conducting, so assumption is valid ✔️

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✅ Final Answer

$$\boxed{10\ \text{mA}}$$

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⭐ Golden JEE Insight

• Zener ON ⇒ voltage fixed at Vz

• Load current depends only on load resistance

• Series resistor controls excess current, not voltage

🧠 One-line memory:

Zener ON = Voltage fixed, current adjusts