# Snub AC inductive protection

Hello, I plan to eventually implement an Arduino to control a relay that will control an AC 100 w fan or maybe a higher wattage if possible. I know the basics of how a diode can provide an alternate path for current but I have recently read that with high voltage AC, a "snub" protection circuit is recommended to prevent arcing. I will provide a circuit below and I get that the diode provides an alternate path and the resistor reduces the current but what I don't understand is what is the purpose of the capacitor (CS)?

DC power supply, single diodes, MOSFET, and a transformer. That doesn't look like it can do anything useful to me.

AC is normally controlled through a TRIAC. No arcing as 1) there's no air gap to arc and 2) it switches off when the current is zero. You indeed may need a snubber for the TRIAC to switch off as it's an inductive load, but that's a resistor + capacitor in series, not parallel.

A flyback diode (which is what you're probably thinking of) is for DC inductive loads.

tjones9163:
Hello, I plan to eventually implement an Arduino to control a relay that will control an AC 100 w fan or maybe a higher wattage if possible. I know the basics of how a diode can provide an alternate path for current but I have recently read that with high voltage AC, a "snub" protection circuit is recommended to prevent arcing. I will provide a circuit below and I get that the diode provides an alternate path and the resistor reduces the current but what I don't understand is what is the purpose of the capacitor (CS)?

You cannot have diodes in an AC snubber, they have to work with both polarities.

The normal snubber is a resistor and capacitor in series. The resistor limits the instantateous spike voltage, the capacitor reduces the losses at the ac frequency itself (but must be large enough to handle the amount of charge
in an inductive spike - hence you have to calculate the snubber components depending on requirements).

Having a capacitor directly across a switch causes capacitive current spikes, just as much a problem as inductive voltage spikes to relay contact life! In your circuit the capacitor would discharge through the
diode and switching device stressing both components.

An RC snubber can go across the load or across the switch. With the latter you get leakage current
with the switch off, which may be a problem (for mains equipment this leakage current will normally be
in the dangerous/lethal range, note).

So its best practice to snub an inductive ac load itself, thus avoiding this off-leakage (it saves power too).
This also means the correct component values are always used since they are designed with the load.

Other snubbing techniques exist, such as just a resistor (for momentary loads like ac solenoids this
is the simplest way). Or a MOV which simply limits the voltage transients directly and has
the advantage of protecting against voltage spikes coming in on the supply too, and having no leakage.

Either way an ac snubber doesn't prevent the spike, it limits the maximum voltage to a known level
(can be several times the supply voltage peak in fact), the idea being to reduce arcing substantially and
avoid over-stressing any insulation.

thank you guys for response i believe i found a more appropriate picture

Yes, that's an AC snubber.

But the real question is: how do you plan to actually use it?

MarkT:
You cannot have diodes in an AC snubber, they have to work with both polarities.

Actually you can - and the OP's circuit ( apart from the error of having the R and C in parallel when they should be in series) is a good example. The diode means that only the dangerous positive voltage transient produced by the turn 'off' of the MOSFET is attenuated, without affecting the turn 'on', when the current rise is controlled by the inductor. The diode prevents the turn 'on' voltage transient ( which is predictable and safe) being attenuated, and so removes that energy loss , improving efficiency.

The circuit isn't symmetrical, so doesn't require symmetrical snubbing.

I've used similar topologies in SMPSU's.

Allan

If OP intends to make this an AC circuit, it'd be a good idea to use (draw) an AC power source, rather than a DC power source.

allanhurst:
Actually you can - and the OP's circuit ( apart from the error of having the R and C in parallel when they should be in series) is a good example.

Obviously the capacitor and resistor could not be in series with the diode.