Wiring a Solid State Relay to Ramps

[longjohn119]

. You also want to make sure the SSR is for DC loads, not AC. Unlike mechanical relays, that makes a difference for SSRs.

AC SSRs use triacs and DC SSRs uses MOSFETs .... One advantage of the higher current rating SSRs is if you get one rated at 100A you can ditch the heatsink for a 15A-20A heater. Frankly once you approach 20A@ 12V DC it's time to raise the voltage to 24V or go to an AC heater. Things just get too lossy, connections more problematic, etc. It's a big part of the reason I went with a 10" rather than the 12". I have my doubts about the longevity of that 12" pcb heater at 12V. Look at all the trouble they go to for the 12V high amp connections on a desktop computer with 4 parallel 12V lines to the motherboard and 2 or more in parallel to a video card

[uncle_bob]

Hi

The need for a heatsink varies a *lot* depending on which one you get. A 100A relay will heat up less at 20A than a 20A relay will. Since P = I^2 x R (most modern relays are resistive) the difference can be anywhere from 5X (straight voltage drop) to 25X (resistive). Indeed the 100A relay might dump more power at 100A than the 20A relay does, but it isn't more than 2X in most cases.(yes that messes up the numbers even more).

Bob