🔍 Understanding RMS (Root Mean Square)

📘 Definition:

RMS (Root Mean Square) is a statistical measure used to express the effective or equivalent value of a varying (AC) waveform — especially voltage or current.

For AC signals, RMS represents the amount of DC power equivalence. It tells us the equivalent DC value that would produce the same power dissipation in a resistive load.

⚡ Formula for a Sine Wave:

RMS Voltage (Vrms) = Vpeak × 0.707
RMS Current (Irms) = Ipeak × 0.707
  

Where Vpeak or Ipeak is the maximum (peak) value of the waveform.

💡 Why Use RMS?

• It allows comparison of AC power to DC power.
• It's used in power calculations: P = Vrms × Irms
• Used in multimeters and real-world power ratings.

📏 Quick Conversion Values (Pure Sine Wave):

• RMS = Peak × 0.707
• Peak = RMS × 1.414
• Average (rectified) = RMS × 0.9
• For Half-Wave DC: Avg = RMS × 0.9 / 2

🔋 Real-World Application:

In household AC systems (like 120V or 230V AC), that voltage is an RMS value. The peak voltage is actually higher:

120V RMS → ~170V Peak
230V RMS → ~325V Peak
  

RMS is the standard for AC measurement because it represents usable, real-world energy.

🔌 RMS vs. Average Voltage in Rectified AC

Goal: Understand how to convert between RMS, peak, and average voltages or currents in a sinusoidal AC waveform — particularly in the context of rectifiers.

⚡ Step-by-Step Breakdown

1. RMS to Peak:
   Vpeak = VRMS × √2
Vpeak = VRMS × 1.414
2. Peak to Average (Full-Wave Rectified Sine):
   Vavg = Vpeak × (2 / π)
Vavg ≈ Vpeak × 0.637
3. Combining Both:
   Vavg = VRMS × 1.414 × 0.637
Vavg ≈ VRMS × 0.9

📊 Example

If you have 120V RMS from the wall:

• Peak Voltage: 120 × 1.414 ≈ 169.7V
• Average (after full-wave rectification): 169.7 × 0.637 ≈ 108V
• Or: 120 × 0.9 ≈ 108V

✅ Summary Table

Conversion	Formula	Approx. Factor
RMS → Peak	RMS × √2	× 1.414
Peak → Average	Peak × (2 / π)	× 0.637
RMS → Average	RMS × √2 × (2 / π)	× 0.9

Note: These values are valid for pure sine waves only. For other waveforms (square, triangle, etc.), the ratios will differ.

⚠️ Correcting a Common Mistake in DC Voltage Calculations

When calculating the DC output from rectified AC (such as in full-wave or bridge rectifiers), it's important to apply diode or SCR voltage drops after computing the ideal DC voltage — not before.

✅ Correct Method:

VDC = (VRMS × 0.9) − Vdrop
  

• For a bridge rectifier: Vdrop ≈ 1.4 V (two diode drops)
• For a center-tap full-wave: Vdrop ≈ 0.7 V (one diode per cycle)
• If an SCR is used in conduction, add its on-voltage (~1.2 V) to the drop

❌ Incorrect Method (don’t do this):

VDC = [(Vpeak − Vdrop) × 0.637]
  

This results in underestimating the output voltage.

🔍 Real-World Confirmation:

In testing, using the incorrect method resulted in a voltage error of −0.45 V. Using the correct approach — subtracting diode drops after calculating VRMS × 0.9 — matched the measured output precisely.

🎯 Conclusion:

This reinforces the importance of understanding when and how voltage drops affect the output in rectifier circuits. Apply voltage drop corrections only at the output stage — not before calculating the average DC level.

🔌 RMS, Peak, and Average in Rectifier Circuits

📐 Key Relationships (Full-Wave)

• Peak Voltage: Vpeak = VRMS × √2 ≈ VRMS × 1.414
• Average (DC) Voltage: Vavg = Vpeak × 0.637
• Or simply: Vavg ≈ VRMS × 1.414 × 0.637 ≈ VRMS × 0.9

🌓 Key Relationships (Half-Wave Rectification)

In half-wave rectification, we lose half of the waveform. So we divide the full-wave average by 2:

• Average (DC) Voltage: Vavg ≈ VRMS × 0.9 / 2
• Example: If VRMS = 120V → Vavg ≈ 54V (ignoring losses)

⚙️ Example: SCR as a Switched Rectifier

If using an SCR in conduction mode, consider:

• Diode voltage drop: ~0.6V
• SCR voltage drop (on state): ~1.2V
• These values must be subtracted after calculating DC output.

✅ Summary:

• Full-wave: V_avg ≈ V_RMS × 0.9
• Half-wave: V_avg ≈ V_RMS × 0.9 / 2
• Always subtract the diode/SCR drop from Vavg when accuracy matters

🔍 Real-World Forward Voltage Drops in Power Rectifiers

While the typical silicon diode forward voltage drop is cited as ~0.7 V, actual values can vary significantly based on type, construction, and current. In power applications, drops as low as 0.5 V are common.

🧾 Common Forward Voltage Drops

Diode Type	Typical Vf @ Rated Current	Notes
Standard Silicon Diode (1N4001, etc.)	0.6–1.1 V	Vf increases with current and temperature
Fast Recovery Rectifier	0.9–1.5 V	Lower recovery time, higher Vf
Power Silicon Diode (e.g., 6A10)	0.5–0.8 V	Designed for low Vf at high current
Schottky Diode (e.g., 1N5822)	0.2–0.5 V	Fast switching, low Vf, limited voltage rating

⚡ Why It Matters

• In low-voltage supplies, forward drop losses become significant.
• Lower V_f → less power wasted as heat (P = I × V_f).
• Heat sinks may be reduced or eliminated with low-V_f devices.

✅ Technician Tip:

When working with power rectifiers, check the datasheet graph for I_F vs. V_F. At 1 A, a 6A10 might drop 0.55 V — far lower than expected for a silicon diode. Don't guess — measure or read the specs!

🔌 SCR and Diode Voltage Drop Reference

⚙️ Practical Assumptions for Teaching and Calculation

Device	Typical Forward Voltage Drop	Notes
Silicon Diode	~0.6V – 0.7V	0.6V is commonly used for educational calculations.
SCR (Thyristor)	~1.2V – 1.7V (latched on)	1.2V is a solid approximation for most SCRs under load.

📘 Why These Numbers Work

• Silicon diode: Textbooks and circuit examples often use 0.6V as a baseline.
• SCR: In full conduction, the SCR acts like a diode with slightly more voltage drop due to internal structure and high current design.

⚠️ Real-World Considerations

SCR datasheets define VTM (on-state voltage drop) under specific conditions (e.g., 1.2V @ 10A). The drop can vary:

• Low current: May drop closer to 1.0V
• High current: Could rise to 1.5V – 2.0V

✅ Use in Power Calculations

Assuming 0.6V for diodes and 1.2V for SCRs is ideal for teaching:

• Demonstrating power loss (I × V)
• Understanding conduction states
• Explaining the behavior of rectified AC power using SCRs as switched rectifiers

This approach emphasizes clarity and functionality over precision — perfect for classroom or YouTube instruction.

📺 Diodes in Switch-Mode Power Supplies (SMPS)

In CRT televisions and modern power supplies, special-purpose diodes are used to handle the demands of high-speed switching and significant current flow.

⚙️ Key Characteristics of These Diodes:

• Fast recovery time: Prevents reverse current during high-frequency switching (often 15–100 kHz).
• Low forward voltage drop: Reduces heat and power loss. Many are Schottky or specially doped silicon types.
• High surge current rating: Essential during power-up or line spikes.
• Mounted on heat sinks: Dissipates heat from continuous and pulsed current loads.

🔍 Example Devices

Part Number	VRRM	IF(avg)	VF @ IF	Package
UF5408	1000 V	3 A	~1.0 V @ 3 A	DO-201AD
MB6S (bridge)	600 V	0.5 A	~1.1 V per leg	SIP-4
MBR2045CT	45 V	20 A	~0.55 V	TO-220

These diodes often fail due to overheating or excessive reverse voltage — a common cause of power supply failure. The use of Schottky or ultrafast diodes dramatically improves efficiency and reliability in modern designs.

🛠️ Technician Tip:

If you're salvaging diodes from power supplies, look for part numbers etched or stamped on the case. Cross-reference with datasheets to determine V_R, I_F, and recovery time before reuse.

See Highly Integrated TV Chassis

This is why one can't use a 1N4007 in switching circuits.

📦 Selenium High-Voltage Diode Reference

This reference compares three common selenium HV rectifiers used in CRT televisions and high-voltage circuits. These devices are low-current, high-voltage rectifiers — not to be confused with silicon power diodes.

Part Number	Reverse Voltage (VR)	Forward Current (IF)	Application	Notes
NTE504	~23,000 V	~0.2 mA	Flyback HV rectifier	Replaces GECR-6, low-current selenium, epoxy case
GECR-6	~23,000 V	~0.2 mA	Same as NTE504	Compact axial-leaded selenium stack
ECG118	~6,500 V	~0.5 mA	Focus rectifier	Used in CRT focus bias circuits

Reminder: These are not high-current devices. Despite the high voltage, low current (under 2mA) for CRT anodes.

Buy selenium Rectifiers today.

• GECR-6 Selenium Rectifier
• NTE527A HV Diode
• ECG118 Selenium HV Rectifier

🧲 Selenium Rectifier – Construction and Process

Selenium rectifiers do not use doped PN junctions like modern silicon or germanium diodes. Instead, they operate using a metal–semiconductor junction with a naturally formed barrier oxide layer. The result is a one-way conduction effect similar to a diode, but achieved through different means.

🔧 Basic Construction:

• Base Metal Plate: Usually aluminum or steel; acts as the anode.
• Selenium Layer: Thin crystalline selenium deposited on the base metal; acts as a semiconductor.
• Oxide Layer: A thin layer of selenium dioxide (SeO₂) forms naturally or through heating, creating the rectifying barrier.
• Top Contact: A metal layer (typically nickel or cadmium); serves as the cathode.
• Stacked Construction: Multiple cells are stacked for higher voltage handling, often with cooling fins.

⚡ How It Works (Rectification):

• Electrons flow easily forward from the cathode into the selenium layer and out through the base plate (anode) to the rest of the circuit.
• In reverse, the selenium dioxide layer blocks electron flow, providing rectification.
• This is a metal–semiconductor junction process — similar to a Schottky diode, not a PN junction.

🧪 Key Characteristics:

Feature	Description
Voltage drop	~1V per cell (higher than silicon or germanium)
Reverse leakage	Higher than modern PN diodes
Efficiency	Lower due to forward drop and heating
Durability	Very rugged; can withstand overloads
Recovery time	Slow (not suitable for high-frequency use)
Common Uses	Vintage TVs, battery chargers, early power supplies

📚 Why Selenium Rectifiers Use Stacking

Selenium rectifiers are built from multiple thin metal–selenium cells stacked in series. This design is not just mechanical — it improves the electrical performance in several ways:

• Higher Reverse Voltage Handling: Each selenium cell can withstand only a limited reverse voltage before leakage becomes significant. Stacking cells in series increases the total reverse voltage rating, as each cell shares part of the voltage.
• Lower Leakage Current: With more cells sharing the reverse voltage, each one operates at a lower voltage stress, which reduces leakage and improves overall rectification.
• Higher Power Dissipation: Each cell has a limited current capacity and heat dissipation ability. Adding more cells increases total surface area for heat transfer, allowing the rectifier to handle higher load currents without overheating.

Many stacks also include cooling fins or air gaps between plates to improve heat dissipation during high-load operation.

🏁 Summary

Selenium rectifiers operate through a metal–semiconductor junction with a critical oxide layer (SeO₂), not by PN junctions. Though largely obsolete today, they played an essential role in early power electronics before being replaced by silicon diodes.