Ohm’s Law for Cooker Repair: How to Calculate Element Resistance

Why does a cooker repairman need to know Ohm's Law? Because it allows you to predict if a heating element is "healthy" before you even turn the power on. In this guide, I’ll break down Voltage, Current, and Resistance in plain English and show you the one formula you'll use every day.

---

The "Big Three" Terms

1. Voltage (V) - "The Pressure": In the UK, this is roughly 230V AC. It’s the force pushing electricity through the wires.

2. Current (I) - "The Flow": Measured in Amps. This is the amount of electricity moving. If this is too high for the wire, the wire melts (or the fuse blows).

3. Resistance (R) - "The Restriction": Measured in Ohms (Ω). This is how hard the element is working against the flow. This is what you measure with your multimeter to find a fault.

---

The Magic Formula: V = I × R

To find the expected resistance of a cooker element, we use the formula: R = V² / P (where P is Watts).

Example: Testing a 2500W (2.5kW) Fan Element

• Voltage: 230V

• Power: 2500W

• The Math: (230 × 230) / 2500 = 21.16Ω

• The Result: If your multimeter reads 21Ω, the element is perfect. If it reads "OL" (Open Loop), the element is snapped inside.

---

Why Watts Matter

You'll see elements rated as 1800W, 2000W, or 3000W.

• Higher Watts = Lower Resistance = More Heat = Higher Amp draw.

• Lower Watts = Higher Resistance = Less Heat = Lower Amp draw.

Safety Tip: Never replace a 2000W element with a 3000W element unless the wiring and switch are rated for the extra current!

---

Summary Table for Cooker Repairs

Component	Typical Wattage	Expected Ohms (approx)
Fan Element	2000W - 2500W	20Ω - 26Ω
Grill Element	1800W - 3000W	18Ω - 30Ω
Oven Light Bulb	25W	2000Ω+

Voltage, current and resistance.

Voltage is the power of the supply and can be 6v 12v or even 230v in a home. 

Current is the measurement of the amount of electricity a load is taking the larger the load the larger the current and also the larger the wiring needs to be , Amps and be AC alternating or DC direct current, Usually 230v AC in domestic homes.

Resistance is any restriction the electricity current has to face when travelling through the circuit and can be the wire and connection or terminal and the load itself will have a determined resistance than can be measured in Ohms.

Hertz , the way a generator works it will create AC alternating current this means it reverse direction of current, the amount of cycles per second between Positive and Negative is know as hertz in the UK its set a 50hz or 50 cycles per second.

Power  Is a measure of the electrical energy used by a component measured in watts. 100 watt light bulb, 1800w or 1.8Kw element. 

You need to know a little about these in order to understand how to fault find, its best to watch youtube videos and animations to understand this but its all relatively straight forward.

Use the triangle to calculate any of the needed value divide blow into top and Multiply the lower.

Here is an example - The resistance of a heater element is 20 ohms the measure voltage and UK domestic voltage is 230v to find current devide 230 / 20 = 11.5 Amps.

Avoid the "Zero" Trap: Top 3 Multimeter Dangers

⚠️ SAFETY FIRST: EXTREME DANGER

Using a multimeter incorrectly on 240V mains can lead to electrical shock, fire, or the meter literally exploding in your hand. A multimeter is a safety tool, but only if you understand its settings and limitations. Always double-check your dial and lead positions before touching a live circuit.

Safety with electrics is obviously important , a multimeter can aid with safety but there are some dangers and its important to use the multimeter properly, in the correct setting with the correct test leads.

Here are three biggest safety issues when using a multimeter on electrical systems

1) Having meter in wrong function selection , this will give an eroneous measurement in the video below i test a live electricasl outlet in DC instead of AC and its shows a zero reading.

2) I then show another zero reading with the meter leads not correctly seated.

3) leaving the meter in amps , if you then try to , measure voltage, the voltage will appear on one end of the meter leads.

Please watch then video below for a demonstration of this.

A multimeter can give you a false sense of security. If you see "0" on the screen, you might assume a circuit is safe to touch, but that "0" could be a lie caused by incorrect settings.

---

The 3 Most Common Lethal Mistakes

1. Wrong Function Selection (AC vs. DC): Domestic mains electricity in the UK is AC (Alternating Current). If you set your meter to DC, it will show zero volts even when touching a live 240V wire. You might then touch that wire, thinking it’s dead, and receive a lethal shock.

2. Improperly Seated Test Leads:

   If the probe leads are not pushed firmly into the meter, they won't make an internal connection. The meter will show a zero reading regardless of the voltage present. Always "test the tester" on a known live source (like a socket you know works) before trusting it on a faulty appliance.

3. The "Amps" Short-Circuit:

   If you leave your red lead in the "Amps" (A) socket and try to measure voltage, you are essentially touching a solid piece of wire across the live and neutral. This creates a massive short circuit. High-quality meters have internal fuses to stop this, but cheap meters can explode or melt.

Pro Tip: The Safety Workflow

• Set the Dial: AC Volts 

• Check the Leads: Black in COM, Red in Omega

• Prove the Meter: Test it on a working outlet first.

• Measure: Only then check your oven's terminals.

Unhinged: A Step-by-Step Guide to Microwave Door Removal

⚠️ SAFETY NOTE: MICROWAVE LEAKAGE RISK

When you remove or replace a microwave door, you are handling the most critical safety barrier of the appliance. Unlike a standard oven door, a microwave door is designed to contain microwave radiation.

• Hinge Alignment is Critical: If the hinges are not perfectly aligned during re-assembly, the door may not seat correctly against the frame. Even a tiny gap can allow microwave energy to leak out during operation.

• The "Mesh" Integrity: Ensure the perforated metal screen on the inside of the glass is not bent, scratched, or damaged. This mesh is what reflects the microwaves back into the cavity.

• Interlock Switches: The door interacts with safety "interlock" switches. If the door isn't aligned, the microwave might not start, or worse, it could operate with a partial seal.

• Leakage Test: After any door repair, it is highly recommended to use a Microwave Leakage Detector (an inexpensive handheld tool) to ensure no radiation is escaping while the unit is running.

Whether you're replacing a cracked glass panel or a broken handle, removing the door is a necessary hurdle. It's often simpler than it looks, but requires accessing the hidden hinge mechanism behind the outer casing.

---

The Removal Process

1. Safety First: Disconnect the power and wait. As always with microwaves, ensure the high-voltage capacitor is discharged if you are opening the main chassis.

2. Access the Hinges: Most microwaves require you to remove the outer cover (usually held by 4-5 screws) to gain access to the upper hinge.

3. The Upper Hinge: Once the cover is off, locate the upper door hinge. It is typically held by two retaining screws. Undo these while supporting the door.

4. Lift and Release: Open the door slightly and lift it upward. On many models, the bottom is a simple "pull-away" pivot pin that doesn't require screws, allowing the door to slide free from the chassis once the top hinge is removed.

There was a bottom hinge on this i undo the screws but it was not needed as it turned out 

Remove all 4 screws of cover and pull off the cover 

below is the upper hinge with 2 x screws

One the upper hinge is undone and door it pulled from catches it will pull away freely 

Microwave Door Won't Stay Shut? Replacing a Broken Latch Spring

⚠️ Safety Note: The Door Interlock

The door hooks do more than just keep the door closed; they engage the safety interlock switches. If the latch is broken, the switches won't activate, and the microwave won't start. Never attempt to bypass these switches or "tape" the door shut to make it run. This prevents microwave radiation from leaking out while the unit is in operation.

To do this open the door , then pry off the outer plastic cover to gain access to the spring latch mechanism, the latch should pull up and move to one side to allow it to bew removed you then attach the spring and refit it can be very tight so take you time and do force it.

If your microwave door feels "limp," won't click into place, or—more importantly—won't allow the microwave to start, the issue is almost always a tiny, 50p tension spring. This is a common mechanical failure that is often mistaken for a faulty door switch.

---

The Repair Process

1. The Symptom: You pull the handle or push the button, and the door pops open, but it feels "weightless" or the hooks don't retract/extend properly.

2. Accessing the Latch:

   • Inner Door Panel: On most microwaves, you need to gently pry off the inner plastic trim (the "choke cover") around the window using a flat-head screwdriver or a putty knife.

   • Be Careful: This plastic is often brittle. Work your way around slowly to avoid snapping the clips.

3. The Fault: Once the trim is removed, you will see the plastic lever mechanism. In 90% of cases, the small metal extension spring has either snapped or slipped off its plastic mounting post.

4. The Fix: * If the spring is just loose, re-hook it using a pair of needle-nose pliers.

   • If the spring is broken, it must be replaced.

   • Pro-Tip: If the plastic post itself has snapped off, you can often carefully drill a tiny hole nearby to anchor the spring, saving you from buying a whole new door assembly.

See video of this process below 

Stuck in Place: Fixing a Microwave Turntable That Won't Spin

⚠️ Safety Note: Watch Your Fingers

While the turntable motor is "low power" compared to the magnetron, the edges of the metal "knock-out" panel on the bottom of the microwave are often razor-sharp. Use pliers to remove the panel and wear gloves to avoid deep cuts.

Very easy fix you need to cut out the access panel first and then you gain access to the turntable motor ( make sure you isolate electric supply and work safe ). To replace motor pull of electrical terminals and undo retaining screw , motor comes away easy. The internal drive gear with need fitting onto the motor after replacing , when you replacethe motor access cover usually they will go in the opposite way round and secure with a screw to make safe.

If your microwave heats fine but the glass plate remains stationary, you'll end up with "hot spots" and unevenly cooked food. This is rarely a fatal fault; usually, it's a simple mechanical blockage or a small, inexpensive motor that has reached the end of its life.

---

The Repair Process

1. The "Drive Coupling" Check: Before reaching for tools, remove the glass tray and the roller ring.

   • The Fault: Check the plastic "D-shaped" coupler that connects the motor to the plate. If it’s cracked or stripped, the motor will spin, but the plate won't.

   • The Fix: These simply pull off and can be replaced for a few pounds.

2. The Synchronous Motor Test: If the coupling is fine, the motor underneath the floor of the microwave is likely the culprit.

   • Access: Most microwaves have a perforated "knock-out" panel on the bottom. Snip the tabs to access the motor without taking the whole case apart.

   • Testing: Use your multimeter set to Ohms. Touch the probes to the two terminals of the motor.

   • Result: You should see a reading (usually between 2k and 15k Ohms). If the meter shows "OL" or no continuity, the internal windings of the motor are broken.

3. The Voltage Test (Advanced): If the motor tests fine for continuity, it might not be receiving power from the control board.

   • Test: Carefully check for 230V AC at the motor terminals while the microwave is running. Extreme caution required.

Remove panel to access the motor

 one screw usually hold the motor in place 

easy to remove and replace

make sure you replace the plate drive gear after 

The Big 5: Troubleshooting the Most Common Microwave Faults

Gemini said

⚠️ Safety Note: Avoid the Capacitor

The capacitor is the most dangerous part of a microwave. It stores energy even when the unit is unplugged. Never open the case or begin testing until you are certain the capacitor is discharged.

 ** Warning avoid the capacitor inside these and do not open case with it plugged in**

When a microwave fails, it usually boils down to one of five common components. Knowing how to test these allows you to pinpoint the "broken link" in the circuit without guessing.

---

The Repair Checklist

1. The Main Fuse: The most common failure. Use your multimeter to test for continuity. If it’s blown, something caused it to "pop"—often a faulty door switch or a shorted capacitor.

2. Thermal Cut-Outs (TCOs): These small sensors monitor the heat of the magnetron and the oven cavity. If they detect overheating, they open the circuit. Test for continuity; if they're "open," replace them.

3. Door Micro-Switches: Most microwaves use three switches to ensure the door is securely locked. They can fail mechanically or electrically. Test each switch in both the open and closed door positions using a continuity setting.

4. The Magnetron Heater: The magnetron is the heart of the microwave. You can test the heater terminals for continuity (should be a very low resistance) and ensure there is no continuity between the terminals and the metal casing (which would indicate a short).

5. High-Voltage Capacitor & Transformer: * Capacitor: Test for capacitance (e.g., 1 microfarad). DANGER: Ensure it is fully discharged before touching.

   • Transformer: Test the primary and secondary windings for continuity to ensure the high-voltage side is getting power.

First most common problem is the fuse test for continuity 

Then i would test the thermal cut outs 

Another issue could be the heater on the magnetron 

The doors usually have 3 micro switches 

and can be normally open or closed switches

so connect meter set to continuity and test by 

opening and closing the door

Test capcitor this one is 1 micro farad and it tested perfect

extra one test the primary and secondary windings of transformer.

See video of this below 

Sparking and Arcing: Diagnosing Microwave "Lightning" very easy £5 fix

⚠️ Safety Note: Don't Let it Burn!

If your microwave starts sparking, stop it immediately. Continuous arcing can cause permanent damage to the magnetron (the most expensive part) or even burn a hole through the metal cavity wall, rendering the unit unsafe and non-repairable due to radiation leakage.

This piece of card is a mica wave guard very cheap to replace or try to clean it first with soapy water and let it dry before refitting and testing microwave again. The new piece will need cutting to size so keep you old one for a template.

Few things are more alarming than seeing sparks and hearing a loud crackling sound inside your microwave. This "arcing" is essentially electricity jumping through the air, but it usually doesn't mean the appliance is a write-off. It’s often caused by a simple maintenance issue or a cheap, replaceable part.

---

The Repair Process

1. The Waveguide Cover (The #1 Culprit): Look inside the cavity for the small, cardboard-like rectangle on the side wall. This is the mica sheet (waveguide cover).

   • The Fault: Over time, food splatters onto this cover. Because food contains carbon, it absorbs microwaves, heats up, and eventually burns. Once it carbonizes, it becomes conductive and begins to "arc" to the metal frame.

   • The Fix: Remove the cover. If it has a burnt hole or dark carbon marks, replace it immediately. These are inexpensive and can often be cut to size from a universal sheet.

2. Exposed Metal/Paint Damage: If the internal paint has chipped or bubbled, the bare metal is exposed.

   • The Fault: Microwaves reflect off the walls, but if they hit a sharp, exposed metal edge, the concentrated energy can cause a spark.

   • The Fix: Sand down the area and apply specialized Microwave Cavity Touch-Up Paint.

3. Burnt Magnetron Cap: If the arcing continues after replacing the waveguide cover, the "antenna" of the magnetron behind it may be burnt.

   • The Fix: This requires opening the chassis (Safety Warning applies). If the metal cap on the end of the magnetron is pitted or burnt, it needs replacement or the magnetron itself may be faulty.

It covers the below magnetron gap

Buy these guards here in Ebay UK

See video below of this full process