SSR __VA Pot value verses input voltage

[Yummyrum]

The word around here is that when running a SSR_VA (VR) style controller that the Pot value should be halved when using 110V to what it should be when using 220V .
IE , if the SSR requires a 500K pot for 220 volts , it should use a 250K pot for 110V.

The question was raised about whether this was actually true fact or Bullshit .

So here you all go .

I got a Variac that can adjust the voltage from 0-275V and fed it into my Fotek SSR 40 VA controller that has a 500K pot .

As the variac can only handle about 2 amps , I used a 100W incandescent globe as load .
I then hooked up my CRO to the output ( across the load ) via an isolation transformer 240v-24v AC

I then adjusted the Variac to 110v and marked on a piece of paper the positions of Maximum , 50% power and minimum power .
(Interestingly , finding the minimum power where the SSR just begins to fire is actually quite tricky and you have to adjust down to it not up to it.)

I then repeated for 220v .

[Yummyrum]

Here are the results .
You can clearly see that the range of the 110v is much less than the range of the 220v

So at minimum power setting , the resistance of the pot ( disconnected from the SSR to measure) was about 180K at 110v

The resistance of the pot was about 400K at 220v at minimum power setting

Incidentally , the 500K pot I had measued closer to 600k

So the it would indeed appear true that you need to halve the pot value ( than you would use on 220v ) when you are running on 110v if you want maximum adjustment range on your Pot

Now just in case you are wondering is all this pointless because I tested it with a 100w load , well , this crossed my mind too so I compared on 230v straight into the wall socket the globe and a 2400w element and both gave the same results so I am confident that load has no bearing on the results .

Of interest , you may have noticed that at maximum setting , there is still a small reduction in power . Thats normal of these style of phase controller . Fit a bypass switch across the SSR if you want 100% full power .

Also notice that the iconic smooth sine wave in reality these days has a flat top on it . 40 years ago it looked like a sine wave but as the ever increasing use of switchmode and inverter technology has arisen , so has the distortion to the mains supply as peak current pulses are drawn .

[shadylane]

Thank You Rummyum

[NineInchNails]

Well done! Thank you.

I've put together a couple PID controllers 110V as well as 220V, but went with the potentiometer that was recommended without really knowing why or if were truly appropriate. They all work, but it's nice to know that it is actually correct.

[jimmayhugh]

I built a dual controller in a single box:


I'm using 40A SSRs, as I prefer to understress my components whenever possible.

I used 470k and 47k pots in series to allow coarse (on the left) and fine (on the right) settings.

The inputs and outputs are L6-30 sockets and plugs:



I use them with both 120vac and 240vac elements, and have modified power cables for each situation.

[MtRainier]

That's great that you did the experiment and show your work.

I experienced this myself. I bought the Still Dragon controller kit. It comes with all of the parts including a 500k pot and is intended to be used with 240VAC. I put it on my 120VAC circuit and was surprised that my pot only worked over the second half of the range. I emailed them and they said to use a 250k pot if running it on 120VAC, but that they don't really sell the kit for that and I was on my own using it.

It works great.