Electrical – Relay Circuit Interfere with RF signal – How to improve

There are 2 devices that can generate spikes.

1. The relay coils. The diodes in anti-parallel will help suppress the spikes when the relays are turned off (The stored energy in the magnetic field is converted back into voltage/current causing a spike that may be inductively and capacitively coupled back in to the mains). Assuming these are DC relays, feed by DC this could be a good solution But may still need filtering.

2. The motor of the fan and the contacts of the relays. Here the motor can be switched on at any point in the mains cycle. If the relays close or open when the maximum current is being drawn you have a similar situation as turning off the relay coils. (The stored energy in the magnetic field is converted back into voltage/current causing a spike that is conductively and may be inductively and capacitively coupled back in to the mains) The solution is to switch at the zero point in the mains cycle. +1 @Brain Drummond comment for devices to search for that can use Zero Point Crossing ( zero crossing switch, or zero crossing SSR (Solid State Relay) )

Keeping the current loops as small as possible and moving cables away from noisy circuits can also help depending on how sensitive the audio equipment is.

After that it is inductors ,resistors and capacitors to construct filters to damp any spikes left.

The FET is a IRL3705N. It has a threshold voltage of min 1V and max 2V.

Threshold voltage is just the point at which the FET starts to turn on. To be fully turned on it needs 5V or more (see datasheet fig 1. and fig 2.). A 14.4V drill motor can easily draw over 30A at startup, so the FET needs to turn on as hard as possible to maintain a low Drain-Source resistance.

If the FET is only partly turned on then it will drop more voltage and heat up. This also applies when switching it on and off. If you are applying PWM then you must ensure that the Gate voltage rises and falls quickly. Each transition causes the FET to heat up a little. As PWM frequency is increased there are more transitions and the FET will get hotter, so the rise and fall times need to be shorter.

Your circuit shows the Gate being driven by an optocoupler switching +12V to a (1k?) resistor. With this driver circuit the Gate voltage should rise reasonably quickly (assuming the optocoupler is being driven hard enough) but will fall quite slowly due to the large amount of charge stored on the Gate. To improve the switching speed you could add a push-pull driver using an NPN and PNP transistor, like this:-

Both transistors are connected in 'Emitter Follower' configuration, so they amplify the current by about 100 times (Hfe of the transistors) and reduce switching time by about the same amount.

For efficiency the PWM frequency should be high enough (>3kHz) that motor inductance smooths out the current, but low enough to not cause too much switching loss. At this frequency the flyback diode needs to be a fast switching type, preferably a high current Schottky type to keep voltage drop down.

When the motor is a generator, the diode is initially reverse biased.

If throttle is reduced quickly then the motor will continue to generate voltage until it stops, but as that voltage is the same polarity it won't hurt the rest of the circuit. If you switch direction while the motor is spinning the diode will become forward biased and 'brake' the motor. After switching direction You should not apply PWM until the motor is stationary, else a very high current will flow through the FET. With the diode providing braking the motor should stop quickly.