Calling All Engineers - ELI5 SCR vs FET in Dimmers

Hey all – I’m in need of some help understanding at a 5th grade level (ELI5 = explain like I’m 5) the difference between a Silicon Controlled Rectifier (SCR) and a Field Effect Transistor (FET) in regards to dimming.

In other words, we’re looking to replace the FET as the lead times are getting ridiculous (52+ weeks) and one of the options presented to us was the SCR method which has a 12 week lead time.

I’m sure we’ll get a good explanation, but I was just curious if anyone out there has an opinion on either or and if the SCR would be a viable method.

If you need any more information, please let me know!


I am not an engineer, but this is my understanding of the difference (I could be wrong though)

FET (MOSFET): An electronic “switch” that allows current to pass from the input pin to the output pin as long as a small amount of current is supplied on the control pin. Can be turned on and off whenever you want. Dimming is achieved by timing when it is on and off. The benefit is that you could theoretically write firmware that allows you to switch between Leading edge or Trailing edge dimming (ELV or MLV).

SCR: Similar to a TRIAC in operation. It is an electronic “switch” that allows current to pass from the input pin to the output pin when current is applied to the control pin. The SCR will then continue to allow current to pass until the input pin hits 0v (regardless of the state of the contol pin). Dimming is achieved by timing when in the AC sine wave the SCR turns on. This means that it can only do Leading Edge (MLV) dimming.


This is my understanding as well.


FETs also allow current to flow in both directions while SCRs only allow current in one direction like a diode.


Are MOSFETs a type of FET? I thought MOSFETs only allow current in 1 direction (and 2 of them are needed for AC switching).

MOSFET: Current flows in 1 direction. Need 2 of them to switch AC
TRIAC: Current flows in both directions
SCR: Current flows in 1 direction. Need 2 of them to switch AC

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Metal Oxide Semiconductor Field Effect Transistor :wink:
MOSFETs are a type of FET but are directional


Awesome guys, thank you!

Well this is certainly interesting. I’d love to understand this more from the engineers as maybe it would help with better bulb compatibility.

So, in theory, it shouldn’t be too different than our current design as our FET version is Leading Edge?

Any downsides (outside of the possibility that the FET switch could have two different firmware options)?

I read this as well, but am not sure what this means lol.

My question would be: would there be any downsides to only having a one-directional current flow vs two?

If my non-engineering brain is thinking through this, wouldn’t you only need a one-directional flow as you’re either dimming down or up? I’m sure it’s much more complicated than that.

This is what I was told last night:

I’m not entirely sure what it means, but it may possibly answer your statement?

With the MOSFET design, you may be able to add a config option to change between Leading/Trailing edge dimming (if there is room on the device). I have an Aeotec nano dimmer that works with either method (the only reason I havent switched it out for an Inovelli dimmer is because it is not compatible with my bulb, and the bulb is not replaceable)

That statement just makes me more confused.
If we choose [MOS]FET, we need 2 FET chips, and fet is chopping at the back end of AC when achieving dim. To me, it makes sense to need 2 chips because the MOSFET can only pass current in 1 direction, and AC alternates between positive voltage and negative voltage. 1 MOSFET would handle the positive portion of the sine wave, and the other (installed “backwards” would handle the negative. However, if the existing FET is chopping at the back end of the AC to dim, this suggests the current dimmer is using Trailing edge dimming instead of Leading edge.

On SCR, we need 1 chip and it is chopping at the front end of the AC. If SCRs only pass current in 1 direction, I am not sure how the dimmer would work with only 1 of them. Perhaps they are referring to a TRIAC instead of an SCR?

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Concur. I was trying to wrap my head around their statement about SCR and it doesn’t make sense.

@jtronicus and @stu1811 – thanks! I’ll ask for clarification tonight.

It could also be bc Darwyn is like me (non-engineer) and is passing on info from their engineers, so I’ll confirm with him tonight :slight_smile:

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Would switching away from MOSFETs affect the ability to use dumb/aux switches? I found a post of yours from a while back that suggests that was one of the reasons you used MOSFETs in the first place.

@Eric_Inovelli would it also impact non-neutral?

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Yes, this is interesting and hopefully we can get some clarification. Our PRD for the ZigBee switch requires dumb/aux and neutral/non-neutral, so I’m hoping the SCR method accounts for this.

All MOSFETs have a diode in parallel with the switch. This means that all MOSFETS can switch the voltage on and off in one direction but will always conduct curent when the voltage is the other direction. So, to block AC voltage which has both directions, you need to put 2 MOSFETs in series connected in reverse direction to each other.

The SCR is a switch that can only conduct in one direction. Once turned on, it will continue to conduct until the voltage across it is reversed. You put 2 SCR’s in parallel to control AC.

Right now, I can’t think of any reason why a MOSFET or SCR would work any differently when doing leading edge dimming.

You can’t do trailing edge dimming with a SCR.

The “chip” might be 2 SCR’s in a package already connected in parallel. They often come packaged like that because they are mostly used to control AC which makes it common to find them packaged that way.


Damn, super helpful – thank you!

Now to figure out the aux/dumb and/or neutral/non-neutral :slight_smile:

So much to learn.

The MOSFET or SCR is the switch or power device that turns the AC on and off. Then, there is electronics that turn these devices on and off at the right times, as well as doing all the other smart stuff. Each device type works a little differently, so it requires a different interface between the electronics and the device. I don’t know why the device type would affect the aux/dumb and neutral/non-neutral when that is done in the software and electronics.

It shouldn’t affect the dumb 3-way because that is accomplished by switching the load and traveler terminals as both travelers. This has nothing to do with the power device type used.

It shouldn’t affect the non-neutral because it seems the electronics get the power they need by never fully turning on the output. This always leaves some voltage between the line and load terminals. The electronics can tap into this voltage and use it as power as long as there is enough available. This can be accomplished with either power device type.

The above is why the dimmers can be non-neutral but the switches can’t. The switch uses a relay to connect the line and load and once turned on there is no voltage between those terminals that can be used to power the electronics.

The only one I’m not sure of is the aux switch. I really don’t know how they work or if the power device type could affect them.

It’ll be interesting to hear what the manufacturer has to say.


Wow lots I can say about this one!

SCR vs MOSFET for most home products comes down to what several are saying here. Leading vs Trailing Edge aka forward vs reverse phase control dimming is a big distinction. With SCR, leading edge dimming is the only option. With MOSFETs (and the right firmware) both are possible. The key difference is that a trailing edge dimmer will — in most cases — dim loads that are not purely resistive quite nicely. A few other components are necessary such as transient voltage suppressors and a snubber will help balance the effects of the inductance.

In short, without getting too deep into the details, if you can stick with MOSFETs there are many more options available to you for the future of the product. Having a leading and trailing edge dimmer in our walls would be an amazing advantage to properly dimming the ever increasing variety of bulbs on the market. I’m also a bit puzzled by your CM’s struggle to secure parts. There literally are zillions of MOSFETs. Many have comparable specs. Another alternative would be to use IGBT devices (insulated gate bi-polar transistors). If you are considering “switching” (…someone see the pun please…) to an SCR design there’s already some scope there. It could be better to direct that effort towards re-specifying alternative MOSFETs or some redesign with IGBTs instead.

This is not a put down of SCRs in the least. You would be able to get solid and reliable performance from an SCR design and I suspect your BOM cost might drop a little too! But in the long run? With difficult to dim LED bulbs everywhere? I’d go MOSFET if I could.


Very good information above!
To sum up, SRC will block current from any direction if turned off due to the nature of the silicon structure (PNPN structure, can only turn on if the last PN junction conducts current and can then sustain Latch Up effect until current polarity swapped. ) thus it can only do leading edge dimming because it need the AC power to switch polarity to turn off.
MOSFET is a switch which will conduct current in any direction if gate voltage is applied( For PMOS, gate voltage to be lower than source voltage by a threshold. A MOSFET has 3 terminals, Drain, Source and Gate. Source=Hot, Drain=Load, Gate=Control in this case) . And it can be turned off whenever gate voltage is changed to be the same as the source voltage. That’s why a MOSFET can be used for leading/trailing/even middle dimming because it doesn’t rely on the AC polarity to turn off. However, any MOSFET has a parasitic PN junction between drain and bulk(tied together with source internally). Thus when its drain/source voltage is reversed, the parasitic PN junction will turn on so 2 MOSFET in series is needed in this case.

One thing I would like to add to the discussion is the power dissipation on the SRC and MOSFET.
SRC will have a ~0.8 to 1.8V voltage drop when turned on while MOSFET can be very small depending on the spec (On resistance). From a power dissipation standpoint(heat generated during on state), MOSFET is MUCH better and can make your product cooler or have a higher output power rating or require a smaller heatsink.
So take 1.1V drop for easy math, 1.1V out of 110V would result in ~1% of the load power being dissipated on the SRC itself. when you have 600W load turned on, 6W heat on the SRC can not to be ignored and can be an concern from a product longevity standpoint. Keep in mind, the smart switch can be installed in an insulated wall.


Sure, but MOSFET is kind of like a resistor when on, and at 600W or 5A it could be dropping a similar voltage, especially when you consider there are 2 in series.

You have received a lot of good info already.

One thing no one has mentioned wrt SCRs. Since their dimming can only by delaying the turn on time as stated by chopping the beginning of the cycle, the load receives basically a square wave ON hit. When dimming is set to 20% or 80% it’s no big deal. When dimming is set to 50% the load sees a near instant 0 to 170v. This can cause audible ringing, excessive inrush currents, and possible reduce the life expectancy of the lights being controlled.