PCB relay EMI

O

Osiris

Member
Hello,

I designed a PCB with a Teensy 4.1 and 2 miniature PCB relays type TQ2-12V. The relays are switched on by a NPN-transitor. The protection is a fly-back diode (1N4148) across the coil and a TVS-diode (SAC12) nearby across the 12VDC power supply. Since the relay can generate EMI (and maybe decrease the lifespan of the Teensy 4.1), is there a safe distance from the Teensy 4.1 or should relays not be placed on a PCB together with a Teensy 4.1? At this moment the distance between a relay and a Teensy 4.1 is 44.5 mm.

I hope someone has the magic solution.

Osiris
 
Magnetic field generated by PCB relay coil is relatively small and 44.5 mm distance is large enough to weaken it so it is safe for the Teensy not to affect lifespan. Google for "PCB relay board" and you will see many designs that place such relays near ICs.
But whenever it is good idea or not depends on what you are doing with the Teensy. If you are using ADC or otherwise dealing with small analog signal like analog audio on your board, having high current devices on same PCB may affect ADC, analog / audio performance.
What is more important is PCB layout, the width and the distance between traces and proper decoupling. That is crucial as relays will be switching potentially large current. Solid ground plane is recommended. Decoupling caps (100nF) near ICs. The diode that you have used 1N4148 is small signal. It might be too weak. And make sure that the current needed to drive the base of that NPN transistor does not exceed capabilities of Teensy (less than 4mA) - properly calculated resistor between Teensy pin and NPN transistor base is important. You might choose N-channel MOSFET instead as it doesn't require high driving current.
 
Last edited:
Thomas thank you for your reply.

I changed the 1N4148 diodes for 1N4007(1000V/1A). I do have a ground plane and al ICs are decoupled. The transistor 2N3904 can switch 54mA with 3.3 volt from the Teensy and a 10K resistor : Ib = 3.3V - Ube(0.6V) / 10kOhm = 0.27mA. Ic = Ib x hfe(200) = 54mA. The relay uses 11.7mA. The current drawn through the relaycontacts will be a maximum of 50mA.
Thank you for your considerations. I think I will move the relays out of the smal signal zone on my PCB.

Kind regards,
Osiris
 
The 1N4148 is more than capable of handling the tiny currents from those PCB relays. Random data sheet says 300mA continuous, 500mA repetitive surge. The surge current rating being the important thing here.
 
Yes, 1N4148 handles 75mA, the relay uses 12mA, not a problem. Base resistor for the 2N3904 should be about 2k2, giving > 1mA into the base. For switching make the base current about 10% of the load current. The hfe is not relevant to switching as the device isn't in active mode, but in cutoff and saturation modes. hfe only applies to active mode.
 
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