Switching & protectionBench electronicsUnplug and unscrew
Signal relay
Also called telecom relay, 5 V relay, small relay.
A small relay from a board: 5 V or 12 V coil, contacts that switch an amp or two. Fine for logic-level loads. Not a motor relay just because it clicks.
If you are new to this category
A switch opens or closes a circuit. Some switches are for your finger. Some open when they get too hot and stay open. Those do not reset. A relay is a switch that a small current turns on so it can control a larger current. Use the rating printed on the part.
Click it and listen. Then use a meter on continuity, which beeps when the contacts touch. Do not guess that a small switch can run a motor.
Common is the moving contact. NO means normally open: that leg connects only while you press. NC means normally closed: that leg is connected until you press. Click the button while the meter is on continuity and write down which beep matches which leg.
Mains voltage. Line voltage, and the DC bus just after the rectifier, both kill. A disconnected cord is not a discharged supply.
| What to look for | Usual figure |
|---|---|
| Coil | Printed on the top. 5 V coils are often about 50–120 Ω; 12 V higher. A diode across the coil on the board is the flyback diode — keep it or fit one. |
| Contacts | Read the stamp. 1 A at 30 VDC is a different part from 10 A at 250 VAC. |
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
- Isolating a small load
- Reversing polarity at low current
- Learning coil drive
Leave this one when
Contacts are black from arcing, or you only needed a transistor.
Buying a new one is the better idea when
A relay module with screw terminals, if you are not ready to drive a coil and a flyback diode.
How to tell if this one still works
Coil resistance, then rated coil voltage. Listen for a clean click and beep the contacts. A weak click and high contact resistance means worn contacts.
Tools you will want: Meter, Coil supply.
Words on this page
Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.
- flyback
- The voltage spike a coil makes when you switch it off. A diode across a relay coil absorbs it so the spike does not kill the transistor.
Where people actually find it
1 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.
| From | Call | How many | How often | Where it sits |
|---|---|---|---|---|
| Refrigerator | Bonus | 1 | usually | Start relay on the compressor. |
- In this particular machine
- A PTC or a current relay. Useful only if you understand compressor starts. It is not a general relay.
- Where to look
- Start relay on the compressor.
- How to get it out
- Unplug it. Do not unseat the compressor pipes.
- The condition to expect
- PTCs fail open and are a common fridge fault.