Micro Trainer Relay
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A browser-based simulator that mirrors this board. Both relays are there, the 4-pin SPST and the 5-pin SPDT, including how 87a hands off to 87 when the coil pulls in. Energize a coil and the armature, the contacts, the lamps and every meter reading move together.

It also shows you where to put your leads for each measurement: coil voltage across 86 and 85, coil current, coil resistance with the power off, and B+ at pins 30, 87 and 87a.

Three battery conditions let you work the fault that catches students out. A worn 9 V cell still reads close to nine volts sitting there, then sags under the coil. A weak one falls below the relay’s 7.2 V pick-up voltage, so there is current flowing and nothing moving.

Switch the coil off with the flyback diode and again without it, and see the back-EMF spike that a multimeter is far too slow to catch. A separate section puts a starter solenoid on the contacts and steps through why a weak battery makes a relay chatter, one stage at a time, so you can stop on any step and talk about it.

It runs on any device with a browser. There is also a download button on that page that saves the whole thing as a single file, so it keeps working on a classroom machine with no internet connection.

Everything a student needs is printed on the board. How a relay works, why you would use one, what the flyback diode is for, and the diagnostic order to follow. It is all alongside the relays themselves, so the reference and the part you are probing are in front of you at the same time.

The terminal numbers are the real ones. 30, 85, 86, 87 and 87a come from DIN 72552, the German standard the whole industry settled on. They are the same numbers moulded into the relay in a vehicle’s fuse box, which is the point: what a student learns here transfers directly.

Low current on one side, high current on the other. 86 and 85 are the coil, and they are the low-current side. 30, 87 and 87a are the contacts, and they carry the load. A relay exists so that a small current can switch a large one, and keeping those two sides straight is most of relay diagnosis.

Battery. One 9 V cell in the printed base. Switch MASTER off when you are done and it lasts a long time. A relay coil draws far more than a logic board does, so a cell left switched on will not survive the weekend.

Voltage with the power on, resistance with the power off. Coil resistance has to be measured with MASTER off. Measuring resistance on a live circuit gives you a false reading, on this board and on a vehicle.

This is the part worth drilling, and it is printed on the board so students can follow it without you standing over them. A relay has two circuits in it, and almost every relay complaint sorts itself out the moment you decide which of the two you are chasing. The click tells you which.

• No click? Check the control side. The coil is not making a magnetic field. Verify B+ at 86 and ground at 85. If both are there and it still will not click, the coil itself is open — check its resistance with the power off.
• Clicks but the load does not work? Check the load side. The coil is fine, so the fault is past it. Test for B+ at pin 30 first, because if it is missing there the contacts never had anything to pass along. Then test for B+ at pin 87 with the relay energized.
• B+ at 30 but nothing at 87? The contacts have failed. The coil pulled the armature over and the contacts did not carry. That is a dead relay, and you found it with two measurements rather than by swapping parts.

Students can practise the whole tree in the simulator before they touch the board, and the readings will agree with what the meter shows them here.

A relay coil is an electromagnet, and an electromagnet does not let go quietly. Switch it off and the magnetic field collapses, and that collapse drives a voltage spike backwards out of the coil, in the opposite direction to the supply that was feeding it. That spike is the back-EMF.

The flyback diode across the coil gives that energy a loop to circulate in until it dies out, instead of letting it reach whatever was switching the relay. In a vehicle that is an ECU driver, and it is a great deal more expensive than the relay.

The honest difficulty in teaching this is that a multimeter will never show it to you. The spike is gone in well under a millisecond, and a meter that updates a few times a second reports nothing unusual. That is exactly why the simulator has it: you can switch the diode out and watch the spike on a trace, then switch it back in and watch it disappear.

Every board is tested before it ships. If one arrived damaged or fails in normal classroom use, email me and I will make it right.

Include your order number and a photo of the board if you can, which usually gets it sorted in one reply.

Email: scott@sbctrainingllc.com

Thank you for supporting a small shop. This board exists because relays get taught as a diagram on a slide, and then students meet one in a fuse box and have no idea which two of the five terminals to put a meter on.

I build trainers and curriculum for other systems too, including CAN bus, lead acid batteries, and series and parallel circuits. If your program needs something that does not exist yet, tell me what you are trying to teach and I will tell you honestly whether I can build it.