Heatbed SSR heatsink question.

[arnoldstrife]

So I'm checking my math and ensuring I'm not missing something. So the Omron SSR Datasheet (https://assets.omron.com/m/453bbd7a38a218b5/original/G3NA-Series-Solid-State-Relay-Datasheet.pdf) and Voron build guide says the max AMP of the Omron G3A-210B-DC5 SSR is 4AMP without a heatsink. My Bed heater is a 750Watt Silicone AC Heater. I'm in US so 120Volts. so that's 6.25amps. Which means I should limit it to 60% bed heating power. to keep it under the 4amp limit.

I'm wondering then can't I add a heatsink to bring up the SSR to 10AMP limit (or 7amp because there's an asterisk that surface mounting causes a 30% derating of the load in the datasheet. I'm not sure what that means). I bought this one: https://www.amazon.com/YXQ-Heatsink-Solidstate-Aluminum-Cooler/dp/B01M8MIVJ7/ applied thermal paste and It looks better than the ones in the datasheet to meet the minimum requirements and I just need to run over 4amps for like a minute when it first heats up. Afterwards, it's comfortably between 10-30% load well within even the heatsinkless load.

So if my math is right, this should allow me to go full throttle and not limit the heat bed speed if I use this heatsink. I'm not sure if I'm missing any elements that I should be aware of or reading the datasheet wrong so I thought to double-check.

 

[vinnycordeiro]

With a heatsink (and ideally also a fan cooling it) you can absolutely crank the PWM duty cycle of the SSR without problems.

 

[arnoldstrife]

With a heatsink (and ideally also a fan cooling it) you can absolutely crank the PWM duty cycle of the SSR without problems.

Thank you!

 

[NoGuru]

I run mine at 100% duty cycle, for years, no problems.

 

[ChrisA]

So I'm checking my math and ensuring I'm not missing something. So the Omron SSR Datasheet (https://assets.omron.com/m/453bbd7a38a218b5/original/G3NA-Series-Solid-State-Relay-Datasheet.pdf) and Voron build guide says the max AMP of the Omron G3A-210B-DC5 SSR is 4AMP without a heatsink. My Bed heater is a 750Watt Silicone AC Heater. I'm in US so 120Volts. so that's 6.25amps. Which means I should limit it to 60% bed heating power. to keep it under the 4amp limit.

I'm wondering then can't I add a heatsink to bring up the SSR to 10AMP limit (or 7amp because there's an asterisk that surface mounting causes a 30% derating of the load in the datasheet. I'm not sure what that means). I bought this one: https://www.amazon.com/YXQ-Heatsink-Solidstate-Aluminum-Cooler/dp/B01M8MIVJ7/ applied thermal paste and It looks better than the ones in the datasheet to meet the minimum requirements and I just need to run over 4amps for like a minute when it first heats up. Afterwards, it's comfortably between 10-30% load well within even the heatsinkless load.

So if my math is right, this should allow me to go full throttle and not limit the heat bed speed if I use this heatsink. I'm not sure if I'm missing any elements that I should be aware of or reading the datasheet wrong so I thought to double-check.

I'm also doing the same calculations and was wondering how
(1) the designers were able to specify the SSR with no heat sink. My theory was that the 4 amp limit is not a hard limit. And they hope theplate reaches temperature before the SSR melts. This likey just by luck works.

(2) Are they really controlling a triac with PWM? It theory the shouild not work unless you can sync the PWM with the AC cycle. But I see no attempt in Klipper to do this. The SSR ONLY switches when the AC crosses zero volts and and NOT when the DC control voltage goes low. It waits until the next zero to switch. So it waits a random time between 0 and 1/120 seconds to go off or onn after the edge ofht e PWM signal. Maybe the rqndomness cancels and over time a 60% PPWM results in 60% on-time. But it wouild only work on average over time.

The usual way to control a Triac i is by "phase angle" where you control the phase of the DC control signal. This does near-perfect, smooth control. Light dimmers work this way. The light see a persistent (say) 60% of each and every AC cycle. The dimmer controls the the AC phase when the triac conducts.

I think we'd be best off with a "bang-bang" controller running at about 5 Hz or maybe even slower.

I am planning to convert my DC-powered V0.2 heater to AC and I'll run some bench experiments, not with a working printer. I can simulate the heater with an old-style hot-filament 100W light bulb. Light bulbs make great dummy loads.

My question is "What waveform does the heater see if runing on 60Hz mains with 60 or 50 Hz PWM." My guess is there will be a "beat frequency" of the difference between the PWM and AC frequencies. But I'm gussing

It is so much more simple with DC. A MOSFET conducts or not and they can switch very fast. But this is AC and a triac, not an FET

 

[Dave32]

I've been thinking the same (and I believe you have also commented in the other thread).

My best theory is that the only way it works at all, is because the Vorons have such massive slabs of aluminum for the heated bed. Basically, the thermal mass is the "capacitance" that smooths the randomness of the SSR power application. (And that randomness, in turn, comes from the fact that Klipper's PID control is by chance running at a different frequency that is not in sync with the AC line, which fortuitously permits random "hits" of the SSR to grab different portions of the AC cycle effectively creating a more uniform average power output.) It's the "swiss cheese model", but in reverse!

I do agree that bang-bang could be a better choice here. Another part of me thinks that a small circuit could sit between the controller and the SSR, intercepting a moderate rate PWM-type output, converting that to a duty cycle, and then intelligently switching the SSR with a 120Hz control loop based on a zero-crossing detector. But at least for my v2.4, I suspect the damping from the bed mass makes that unnecessary.