I’m attempting control a CATO SR-R2510 Solid State Dimmer with a DigiPot (MCP41100) using SPI, and have not been having much luck.
I am using a recently purchased Pi 3B+ and have set up the MCP41100 on a breadboard. I’m running a spring boot (java) web service on the Pi and using HTTP requests from a web browser to send commands via SPI to control the resistance between one of the digipot’s resistor array output pins and the wiper.
This seems to be all working fine, measuring the output resistance with a multimeter I can set the output on a scale between 334 ohm and 72 K ohm by setting a number between 0 and 255 on the SPI interface.
I am using this to control the temperature of an element (on a coffee roaster), but when I wire the MCP outputs to the SRR the temperature is not changing at all.
When I use the SSR with its supplied manual potentiometer, which sets a range from 3 ohm to 94K ohm I get a measured temp drop of around 15 degrees for roughly the same change in resistance on the inputs.
This is probably not strictly speaking a Pi-related question, as everything up to the output on the DigiPot seems to be working fine. The basic issue seems to be that the electronically generated resistance is not behaving the same as that set on the manual potentiometer, and I am at a bit of loss to understand why this might be.
Any suggestions greatly appreciated.
The data sheets for the DigiPot are here. I got the SSR at an electrical retailer and I have not been able to track down much detailed information on it, but it’s this one.
Hi Mark,
I can’t give to accurate of an answer without a datasheet for the SSR, but it looks like you need 25-250V to control the output from the SSR. The Raspberry pi can output 5V. My guess would be that it works manually but not controlled by the raspberry for this reason.
I hope that helps! Share your finished project with the forum!
Thanks for the quick response Stephen. Yes it is slightly puzzling. The manual controller is a simple variable resistor, so I was hoping that the DigiPot would be equivalent. For some reason it is not.
There is another version of the SSR that is controlled directly by a voltage across the control terminals, but this is definitely controlled by changes to the resistance across the control terminals.
I have changed tack here. I haven’t been able to get the DigiPot working with the dimmer so am now using a relay across the inputs of the SR-R2510. I can either open-circuit or short the control terminals of the SRR2510, turning it on or off respectively. I can now control the heating element using something more like a duty cycle. It’s not the fine grained control of the 256 steps I should get from the controllable resistor but it should work.
Glad to hear you found a working solution. I’m sure there is a DigiPot out there that will work with your setup. If you can find one with a datasheet I could be more helpful troubleshooting.
