I used an SSVR for another project, a blower for a bronze casting furnace, and found that the typical potentiometer range is terrible. 2/3 turn of the control does nothing, then the last 1/3 gives 100% of the range. This gave terrible fine control. I then found "Reverse Logarithmic" potentiometers where the resistance change is very high at the 'low end' and tapers off toward the high end. For the blower this made a world of difference.
Since I was already working on my electric conversion I bought 2 of these potentiomters. When I got the boil kettle done I was glad I did. My sweet spot for the boil is at about the 25% position, and I have great fine control over the boil.
I used 2 different SSVRs, and tried the 'standard' potentiometers ('audio taper' and standard taper) and both were terrible for fine control. The 'audio taper' is also logarithmic, but not reverse, meaning it changes slowly at first, then quickly toward the end. That was even worse than the standard type.
If you reverse the connections to the audio taper pot (use middle and other end contact), it will work the other way round, with a reverse logarithmic taper, and hence be better than the standard type. The only issue is that the direction of the pot will be reversed, which may or may not annoy you (I'm left handed, so I do some things backwards anyway...). You can also fake any logarithmic style curve you want by using different linear pots and divider resistors -
Secret Life of Pots.
I built a controller based on the Still Dragon kit, as I decided that the price for the kit, which includes the box (with feet), cable grips, potentiometer, label with markings for the pot, and the SSVR and heat sink, is pretty good, as it saves you a fair amount of time chasing around to get the right component list together, particularly if you need some other hardware Still Dragon sells to reduce the effective shipping cost. I can check which kind of pot it comes with later.
I didn't use the box from the Still Dragon kit, because I added a contactor and toggle switch to my controller. I also added a $17 current/voltage/power meter (
Bayite, available on Amazon) to my controller, so that I can actually directly measure the amount of power I'm applying for repeatability. Variations in line voltage can theoretically affect your repeatability, even with a PID operating in manual mode, although I doubt this is a real problem in practice (although I have seen that be a problem with roasting coffee in popcorn makers). I'll eventually use the box that came with the kit for a different project, but you could probably get my additions to the controller into the larger box Still Dragon offers as an optional extra.
With the Still Dragon kit using a 40A SSVR and a 5500W element, at a supply voltage of 242V, IIRC, ~62% on the potentiometer dial corresponds to 2400W, 4000W is about 80%, 5000W is about 90%. So there's a reasonably range of control, although it could be better. Next brew day I can make a more complete table if someone needs it. A 25A SSVR would probably give better linearity across the pot's range, but it would be operating close to the rated limit at full power. A 25A SSVR would probably be a good idea for 4500W elements though.
In practice, I can dial in power to ±50W across most of the power output range using the power meter to give feedback on where I'm set to. The boil control range on a 5.5 gallon batch in my 44qt kettle is between about 60-80% of the pot setting to go from barely boiling to very vigorous rolling boil, so I find I get pretty good control there.
