Capacitors store energy even after power is off. Touching charged parts can lead to shock, burns, or damage to parts and tools. The goal here is a practical process you can follow every time you need to make a charged part harmless.
What a safe discharge requires
Three things matter: removing sources of supply, proving the part is dead, and giving the charge a controlled path out. That means lockout, a meter check, and a resistor that limits current to a sane level before any shorting stick goes near the leads.
| Type | Typical Risk | Safe Discharge Approach |
|---|---|---|
| Small electrolytic on gadgets | Low energy, sharp spikes | Clip a 10 kΩ–100 kΩ, 1–2 W resistor across leads, wait, meter to verify |
| Motor run (AC, 250–440 V) | Higher energy, steady bite | 20 kΩ, 5 W resistor with insulated leads, meter until <50 V |
| Photo flash / strobe | High pulse current, arc risk | 50–100 kΩ, 5–10 W resistor, shield eyes, meter, then shorting stick to bond |
| Power supply DC link | High voltage and energy | 20–100 kΩ, rated 5–20 W per bank, meter, then bond to ground |
| Microwave oven cap | Deadly voltage | Do not attempt without training; service tools and bleed leads required |
Discharging a capacitor safely: step-by-step
1) Isolate and lock out
Open the disconnect, pull the plug, or trip the breaker. Apply a tag and a lock so no one can re-energize the circuit while you work.
2) Prove it dead with live-dead-live
Test your meter on a known live source. Measure the part you plan to touch. Test the meter again on the live source. This simple habit catches dead batteries, broken leads, and wrong ranges.
3) Check the starting voltage
Measure the voltage across the capacitor and from each lead to the chassis or ground point. Note the value and polarity so you can judge the decay later.
4) Pick a resistor that limits current
Start with a value that keeps initial current under a safe limit: I = V/R. For 400 V, a 20 kΩ part starts at 20 mA. Power in the resistor at the start is P = V²/R; at 400 V and 20 kΩ that is 8 W, so choose a part rated well above that spike, such as 15–20 W or two resistors in series or parallel.
5) Connect with insulated leads
Use a discharge tool or a pair of clip leads that keep hands away from metal. Place the resistor across the terminals. Keep your other hand away from the chassis to avoid a current path through your body.
6) Wait for the RC decay
Voltage across a capacitor follows V(t) = V₀e−t/RC. Time to reach a target voltage is t = RC ln(V₀/Vtarget). With 470 µF at 400 V and 20 kΩ, RC is 9.4 s and dropping to 50 V takes about 19.6 s. Larger banks and higher values need longer waits or lower resistance with higher wattage.
7) Verify with a meter
Remove one probe from the resistor and read the voltage. Only proceed when the reading is below your site limit, often 50 V DC or less.
8) Bond with a shorting stick
After the resistor step, touch the leads with a shorting stick that has a built-in resistor and an insulated handle. Leave a grounding clip in place to prevent recharge from dielectrics or nearby wiring.
9) Re-test before hands go in
Check again across the terminals and to ground. Meter leads, gloves, and eyewear stay on until work is complete.
10) Keep it discharged while you work
Leave a bleed resistor or a ground clip across the part during service so any recharge bleeds off quietly.
Safe ways for discharging a capacitor at home
If you’re replacing a motor run cap on an appliance or an HVAC unit, cut power at the breaker and confirm zero volts first. A 20 kΩ, 5 W resistor with insulated clips works well for units in the 250–440 V range. Keep the resistor on the leads for at least ten to fifteen seconds, then meter the part. If the reading stays above 50 V, wait longer or use a lower resistance with a higher watt rating.
Avoid the bare screwdriver trick. Metal-to-metal shorting can launch molten specks, stress the foil, and weld the tip. The resistor step gives a controlled release, then the shorting stick bonds what remains.
Choose the right resistor and wattage
Pick resistance so the first current is in the tens of milliamps, not amps. Pick wattage so the body of the part can survive the first seconds. Initial power is highest at the instant of contact and falls as voltage decays. Wire-wound parts on ceramic bodies handle heat well and come with long leads that keep fingers away.
Use this quick math: at the instant of contact, Pstart = V²/R. A 330 V photo flash with 100 kΩ makes 1.089 W at the start, so a 5 W part runs cool. A 400 V DC link with 20 kΩ makes 8 W at the start, so a 15–20 W tool is a good pick. If heat rise worries you, split the load across two resistors.
Resistor selector quick guide
| Use Case | Suggested Resistor | Estimated Time |
|---|---|---|
| Gadget cap, 100 µF at 25 V | 10 kΩ, 1 W | RC = 1.0 s; to 50 V target not needed; to near zero in a few seconds |
| Motor run, 40 µF at 370 V | 20 kΩ, 5 W | RC = 0.8 s; to 50 V ≈ 5.3 s |
| Photo flash, 120 µF at 330 V | 100 kΩ, 5 W | RC = 12 s; to 50 V ≈ 25.7 s |
| DC link, 470 µF at 400 V | 20 kΩ, 15–20 W | RC = 9.4 s; to 50 V ≈ 19.6 s |
Safety gear and setup that pay off
Rated gloves, eyewear, and a face shield stop tiny arcs from becoming injuries. Insulated tools and clip leads keep your hands off metal. Keep one hand in a pocket when you can. Clear the area of bystanders. Post a notice so no one powers the circuit while you work.
Know when to stop
Some parts live at kilovolts and hold enough energy to punch through air. Microwave oven caps, TV sets with CRTs, laser supplies, high-power inverters, and motor drives can cross that line. If you lack training and rated gear for those systems, do not proceed. Take the unit to a qualified tech.
Why bleeder resistors matter
Many power caps ship with built-in bleeders. These parts bring the voltage down after the unit goes dark. Aging, heat, or a failed lead can stop that path. Treat every part as live until your meter proves it safe, then add your own resistor or ground clip while you work.
Common pitfalls and myths
Relying on an indicator lamp
LEDs on a board can go dark before charge is gone. They draw only tiny current and tell you little about what sits on a big can. Trust your meter.
Using too low a resistance
A small value makes huge current and heat. That can scorch boards, blast tips, and crack parts. Use math and stay with values that keep current tame.
Skipping the ground clip
Dielectric soak can bring voltage back after you remove the load. Leave a ground clip or a bleed path in place during service and re-check before hands go near the leads.
Close variation: how to discharge capacitors safely in appliances
Appliance caps are often metal cans mounted in tight spaces. Pull the disconnect, wait a minute, then test. Use a 20 kΩ, 5 W resistor with long insulated leads. Clip it across the terminals, wait, and read the meter. If you see any climb after disconnecting the resistor, re-apply the resistor and bond with a stick before you touch wiring.
Math notes you can trust
RC time and target voltage
Pick a target such as 50 V DC. Compute t = RC ln(V₀/Vtarget) with R in ohms and C in farads. Add margin for meter lag and soak. If the job has many caps in parallel, base your math on the sum of their values.
Power and energy
Instant resistor heat at contact is P = V²/R, while stored energy in the cap is E = ½CV². Both guide your choice of power rating and wait time. Heatsinks on power resistors keep bodies cooler during long bleeds.
Trusted rules backing these steps
Work practices in OSHA 1910.333 require stored energy to be released and high capacitance parts to be grounded when they present a hazard. NFPA 70E explains how to set an electrically safe work condition and test for absence of voltage with a live-dead-live method. Tool makers such as Fluke describe safe discharge with a 20 kΩ, 5 W resistor before testing.
These sources set clear expectations and match the steps in this guide line by line across many industries.
Series banks and hidden charge sources
Large supplies often use many caps in series to handle higher voltage. When that happens, charge can divide unevenly. Bleed across the whole bank does not guarantee each can is safe. Place the resistor across the bank first, then use a second set of clip leads to touch across each unit one by one. Read the meter at every step. Some film types can regain a few volts after you remove the load; the ground clip prevents that surprise.
Make a simple discharge tool
Parts
One power resistor sized for your jobs, two well-insulated leads with strong clips, heat-shrink, and a handle. Many techs mount the resistor inside a length of fiberglass tube with a banana jack at one end and a hook probe at the other.
Build steps
Crimp or solder the leads to the resistor, cover the joints with heat-shrink, and fix the body inside the handle so no metal is exposed. Label the value and wattage on the handle. Add a third wire with a ground clip if you want a tool that bonds the unit after discharge.
Use
Clip ground first, then touch the hot side. Watch the meter. When the reading falls under your limit, slide the hook probe into place for a final bond and keep the ground clip on while you work.
Prevent recharge from nearby circuits
Wiring that runs near a charged bank can couple a small charge after you finish a discharge. So can high-impedance paths through control boards. The fix is simple: leave a low-value bleed part across the leads or keep the ground clip in place until you rebuild the circuit.
Storage, labels, and handover
If you remove a cap, short the leads with a wire link before you set it on a bench. Slip heat-shrink or tape over the link. Mark the can with the date and the words “DISCHARGED & SHORTED.” Bag the part so no one touches the terminals by accident. If a job crosses shifts, write the status in the work note so the next person does not repeat the same steps.
Keep spares of resistors in labeled bags.
Extra checks for high energy banks
Before hands go near bus bars, set up barriers and a clear work zone. Keep metal jewelry away. Stand to the side instead of in front of parts when you make contact with a stick. Use a meter rated for the category of the installation and leads that match that rating.