I'm looking at PID devices for coffee machines. There's a tendency to use SSRs (for the regular reasons) but at the current levels (12 amps 120VAC) required these tend to be large (60mm longest dimension).

Even with my own SSRs using opto-isolators and triacs I still need a large heat sink area - especially inside a coffee machine which gets warm and won't dissipate the heat.

I'm looking at controlling 4 AC items in the coffee machine (2 boilers, pump, solenoid) and trying to stuff 4 SSRs into that space will be a challenge.

So here are my questions:

  • Am I right in concluding that 4 x EMRs can fit into a smaller space given the largish current?
  • Also if I go the EMR path, how can I extend the life of the relays?
  • Is it possible to use a zero crossover to switch the relay? Or is the switching delay too slow?
  • What other traps should I watch out for?

2 Answers 2


You can use an Opto-Triac as a cheap SSR. There are many posts here that discuss that. One of them is here.

Here is where @stevenvh builds an equivalent of a SSR.

Here is another discussion related to yours.

Another good post answered by @Anindo.

Below is one of the classic image used in most of the SSR discussions:

enter image description here

Opto-triac solution will save you space over EMR.
Opto-triacs do come in zero-cross flavors.
For a coffee machine switching delay is negligible.
"Moving parts" are the traps to avoid.

  • \$\begingroup\$ I'm just not finding that opto-triacs or ssr's are smaller than relays for high power (12 amp) applications. The heat sinks need to be large especially inside hot coffee machines. My current estimate is 2x to 4x the volume. \$\endgroup\$ Feb 5, 2013 at 5:33
  • \$\begingroup\$ @headingwest VO2223A is the same size of DIP8 and smaller than a EMR. VO2223A is well capable to switch 44W load, is a DIP package and cannot (and may not need) accept a heat sink by nature. \$\endgroup\$ Feb 5, 2013 at 6:27
  • \$\begingroup\$ Hi @Chetan, we are talking of different things. I have built my own ssr using the methods described in your links. 120VAC at 11 amps is 1300+ watts. The triac (not the opto) needs the heat sink to switch that much current. The heat sink sizes are around 20-40mm in each dimension. Plus the complication of being inside a 70 Celsius environment where heat is hard to dissipate. \$\endgroup\$ Feb 5, 2013 at 21:25
  • \$\begingroup\$ @headingwest your post did not mention power rating. Please update your post to get the right answers. \$\endgroup\$ Feb 5, 2013 at 21:33

EMR's produce heat too so dispating heat is the issue. A PID controller with a temperature senser inside the cabinet will be able to adjust the output accordingly.


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