Do I need a snubber?

Hi gang,

I am building a controller for a fan. It is a household fan similar to a box fan. This is it here.

Basically the fan will plug into my device which has a relay to control the fan and the device is plugged into the wall. Think of it as a relay in a box. The fan will be left in the on-low position 99% of the time and may be set to medium or high on rare occasion. The fan will be cycled on/off normally only once or twice a day.

The fan is 120VAC. I have measured the following on it, but I don't really know anything else.

Mode Amperage Power off resistance
Low 1.3A 30.3 ohms
Medium 1.7A 24.5 ohms
High 1.9A 18.8 ohms

I know that when you have an inductive load it is best to have some manner of snubber to handle the induced voltage when the relay opens. My first question is, since a fan like this has its own power switch, does that imply that there is already a snubber installed in the fan to keep the switch from arcing when you turn it off?

If not and I were going to add a series RC circuit across the fan motor do you guys know what would be the correct values of R and C to start with? I have researched this quite a bit and my head is swimming from all of the seemingly non-definitive data that is out there. Though I have found some examples of formulas where people calculate the correct values, but you seem to need to know the motor's inductance and stuff like that, which I just don't know.

Any insight is greatly appreciated. Please let me know if further data/explanation is needed.
Thanks.

mmitchellmoss:
My first question is, since a fan like this has its own power switch, does that imply that there is already a snubber installed in the fan to keep the switch from arcing when you turn it off?

No, they assume the switch is sufficiently rugged.

Maybe your relay is - if it's contacts are as big as the switch in in the fan. :roll_eyes:

In general - relay ... motor - you need a snubber,

Resistor and capacitor. What values? Off hand I would suggest about a microfarad, mains rated (not a DC rating) and a 220 Ohm resistor of one watt rating. Note even while it may not dissipate anything near that rating, you need the voltage rating which you do not often see specified, but is proportional to the size of the resistor.

Your question is a but confusing. I hadn't heard the term "snubber" in decades, so I am a bit surprised to see it in the Arduino forums. (A snubber is a circuit to reduce RF interference).

"I know that when you have an inductive load it is best to have some manner of snubber to handle the induced voltage when the relay opens. "

This is a flyback diode. It's also known by a half dozen other names, but flyback is the predominate useage. It's purpose is to short out the voltage generated by a collapsing magnetic field. For example when power is removed from a relay the collapsing magnetic field generates a voltage in the coil in the opposite polarity. Or when you remove power from a DC motor, the motor becomes a generator while it coasts down and the flyback diode shorts out this induced voltage. (It also dynamically brakes the motor).

Professor Google has more math about the flyback diode than you will ever need.

A flyback diode is never used on an AC load like a fan motor.

It matters not to the Arduino what the fan power ratings are- as long as the relay contacts are rated for the projected load. all the Arduino cares is that you have a flyback diode on the relay coil to protect the Arduino.

The best name is free-wheel diode, since it allows the current to keep flowing when the switch turns off, and
for it to decrease passively as the magnetic field subsides more slowly - if you've ridden a fixed-wheel bicycle and
tried to stop pedalling suddenly you'll know why a freewheeling action is gentler on the whole system, and
why the phrase "inductive kick-back" is apt :slight_smile:

A fly-back transformer has a particular role generating high voltages and the fly-back diode is to charge up a
storage capacitor from high voltage pulses - the name isn't so appropriate to when preventing high voltages
happening.

For AC an RC snubber can serve the same purpose, reducing high voltage spikes, to protect other
components. Snubbers can only mitigate the spike to a smaller level, not eliminate it.

SteveMann:
Your question is a but confusing. I hadn't heard the term "snubber" in decades, so I am a bit surprised to see it in the Arduino forums. (A snubber is a circuit to reduce RF interference).

It is also used in AC circuits to take care of residual currents. Yet another application is in conjunction with a TRIAC, to help it actually switch off as the current goes to zero (current lags voltage in inductive AC circuits). I've used this when building a circuit to control power to a 220V, 3-4A water pump. There are also snubberless TRIACs out there which don't need this.

This is a flyback diode.

That one is specific for switching of DC inductive loads. A diode across the load would act as a short half the time in an AC circuit!

@Paul__B Thank you for this information. That is what I was looking for.

Paul__B:
Resistor and capacitor. What values? Off hand I would suggest about a microfarad, mains rated (not a DC rating) and a 220 Ohm resistor of one watt rating. Note even while it may not dissipate anything near that rating, you need the voltage rating which you do not often see specified, but is proportional to the size of the resistor.

I built the 220 Ohm 1 Watt in series with a 1uF 400V capacitor. The whole thing was placed in parallel with the fan motor. This seems to work. The fan still seems to run well. One question though. After running for a while, the resistor does get hot. I don’t know how hot exactly. You can hold your finger on it, but it is borderline almost uncomfortable. Would this be considered normal for this type of circuit or is this an indication that RC components are not optimally sized?

Thank you.

1uF is a bit large, use a 0.1uF 400v instead, should be fine.

Maybe try a 120 Ohm 1 W resistor instead then. :grinning:

The capacitor (with the resistor of course) across the fan (not the switch) actually improves the power factor of the fan which is in this case minor, but in general a Good Thing.

Strictly speaking the capacitor is supposed to be rated 200 V AC (for a 120 V mains) rather than 400 V DC, but it will likely be OK.

Thanks guys. I will experiment with some of these other combinations.

mmitchellmoss:
Hi gang,

I am building a controller for a fan. It is a household fan similar to a box fan. This is it here.

That's got an induction motor, highly inductive.

Basically the fan will plug into my device which has a relay to control the fan and the device is plugged into the wall. Think of it as a relay in a box. The fan will be left in the on-low position 99% of the time and may be set to medium or high on rare occasion. The fan will be cycled on/off normally only once or twice a day.

The fan is 120VAC. I have measured the following on it, but I don't really know anything else.

Mode Amperage Power off resistance
Low 1.3A 30.3 ohms
Medium 1.7A 24.5 ohms
High 1.9A 18.8 ohms

The current taken by the fan is printed on the nameplate, that's where you look. For an induction
motor the inductance utterly dominates the winding resistance, which is thus irrelevant

I know that when you have an inductive load it is best to have some manner of snubber to handle the induced voltage when the relay opens. My first question is, since a fan like this has its own power switch, does that imply that there is already a snubber installed in the fan to keep the switch from arcing when you turn it off?

Very likely

If not and I were going to add a series RC circuit across the fan motor do you guys know what would be the correct values of R and C to start with? I have researched this quite a bit and my head is swimming from all of the seemingly non-definitive data that is out there. Though I have found some examples of formulas where people calculate the correct values, but you seem to need to know the motor's inductance and stuff like that, which I just don't know.

Any insight is greatly appreciated. Please let me know if further data/explanation is needed.
Thanks.

Its a fairly complex business. You want a relay rated for an inductive load for sure. MOVs are easier to
specify but have a limited operating life, RC snubbers need to be matched to the load, at least roughly.

Commonly called "contactor". Especially at 120V, high currents. Or use a TRIAC/SSR.

A small 220V houshold fan doesn't need too great precautions in selecting a relay, mine are rated 0.15-0.3A.