Power conversionBench electronicsTest and identify

Bridge rectifier

Also called diode bridge, KBU, GBJ.

Four diodes in a package or as four discrete diodes. Turns AC into pulsating DC. The big square ones on supplies have a current and a voltage printed; the SMD ones on a dead board often do not.

If you are new to this category

These parts change wall power into the lower voltage a device uses. A sealed supply with a label is the safer kind. An open board from a lamp can still be connected to the wall on the side that looks like low voltage. Isolated means the output is not electrically connected to the wall. If you are not sure, treat the output as live.

Read the label out loud: volts, amps, and the polarity symbol. Then measure. A number you remember from a similar brick is not a measurement.

What the meter is telling you

Volts: how hard the electricity is pushing, measured with the circuit powered and the probes in parallel. Ohms and continuity: measured with the part unpowered. A beep means a connection. OL or a 1 on the left usually means open, no connection. Amps need a different socket on many meters. Using the amp socket to measure volts blows the meter fuse.

Mains voltage. Line voltage, and the DC bus just after the rectifier, both kill. A disconnected cord is not a discharged supply.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Drop	About 1.2–1.4 V for a silicon bridge at modest current. Schottky bridges are lower.
Mark	A part like KBU1010 is 10 A, 1000 V. Read yours. Do not invent the number from the body size.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

After a known transformer
Replacement in the supply it came from

Leave this one when

No marking and no idea of the current. A bridge that fails short puts mains on the capacitor.

Buying a new one is the better idea when

A bridge is a dollar. Buy it if the salvage one is unmarked.

How to tell if this one still works

Diode test: two diodes from each AC pin to +, two from − to each AC pin, the other direction open.

Step-by-step on the bench: Minimum meter checks

Tools you will want: Meter.

Words on this page

Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.

mains: Ordinary household wall power. It can kill. Unplug before you open anything that used it, and do not experiment on live wiring.
capacitor: A part that stores a small amount of electrical energy. Large ones in power supplies can stay charged after you unplug. Discharge them before you touch the leads.

Where people actually find it

3 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Hair dryer	Bonus	1	sometimes	Handle, on DC-motor models.
LED lamp	Bonus	1	usually	Input.
Seasonal light string	Bonus	1	sometimes	In the plug.

Bonus, if you are already in thereHair dryer1 · sometimes
In this particular machine
Mains rectifier for the motor.
Where to look
Handle, on DC-motor models.
How to get it out
The diodes are the part.
The condition to expect
Good.
Bonus, if you are already in thereLED lamp1 · usually
In this particular machine
Four diodes.
Where to look
Input.
How to get it out
Only if you enjoy desoldering.
The condition to expect
Fine.
Bonus, if you are already in thereSeasonal light string1 · sometimes
In this particular machine
A small rectifier.
Where to look
In the plug.
How to get it out
Only if you open the plug.
The condition to expect
Good.