Silicone + SSR + 110v AC = Awesomeness?

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gestalt73
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Silicone + SSR + 110v AC = Awesomeness?

Postby gestalt73 » Thu Dec 15, 2016 5:02 pm

Hey guys,

I'm almost ready to build a 400mm version of my SuperKossel, and trying to solve for the heated bed.
http://forum.seemecnc.com/viewtopic.php?f=41&t=5747

I've found a 350mm round silicone heater pad that works at 110ac
https://www.amazon.com/gp/product/B0153O18JE/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=A3FDQHBW7TUKTN

If I'm understanding everything correctly, I can use an appropriate SSR between the duet and the bed heater
https://www.amazon.com/gp/product/B0087ZTN08/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A1THAZDOWP300U

I'll be putting an inline fuse in as well, Is there anything else I should do to safe-ify the build?

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Re: Silicone + SSR + 110v AC = Awesomeness?

Postby Jimustanguitar » Thu Dec 15, 2016 7:02 pm

I stuck three of these on the bottom of a thick aluminum plate for my bed.
http://www.banggood.com/Car-Engine-Heat ... 19259.html

The problem is that my bed is a 6# chunk of tool plate, and it's too large of a thermal mass to get up to a decent temperature without really digging deep into the PID settings. Those heaters also have an internal 120C thermostat, so the pad gets hotter than the plate rather quickly, and they click off more than it seems like they're on.

The concept definitely works, but I need a different heater pad and/or a smaller aluminum plate.

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Re: Silicone + SSR + 110v AC = Awesomeness?

Postby joe » Thu Dec 15, 2016 7:40 pm

I am using a 120 v Silicone heater with its own temp controller and it IS awsome!

It is similar to this http://www.ebay.ca/itm/Dia-10-Round-Sil ... 3f4681179b

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Re: Silicone + SSR + 110v AC = Awesomeness?

Postby Jimustanguitar » Thu Dec 15, 2016 10:28 pm

The same company does make one that's 300mm round. I've almost pulled the trigger on it several times.
https://www.amazon.com/KEENOVO-Circular ... B00NXXJEU8

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Re: Silicone + SSR + 110v AC = Awesomeness?

Postby timskloss » Fri Dec 16, 2016 8:55 pm

What controller are you using to send pulses to the SSR?

I ask because on my new v3 with Rambo board the bed heater control circuit uses a high speed pulse train, or pulse density modulation, that may not work with an AC SSR. The reason is due to how the SSR works. On a DC SSR the SSR turns on and off within a few microseconds of receiving the signal from the controller. This is not the case for an AC SSR. Most AC SSRs made today only switch on or off when the AC sine wave is crossing zero volts. This is done to prevent power surges that can upset other electronics. So when an AC SSR receives a rising edge control signal to turn on it will actually wait until the next zero crossing of the AC waveform to turn on. On 60 Hz mains (here in US) you have 120 opportunities to switch the AC SSR on or off each second. Generally for AC SSRs I would reccommend a pulse repetition time of 1 second or more to give you enough resolution to control the heat.

Whew, ok so what does that mean? On my v3 the bed heat command signals can be pulses as short as 2 milliseconds wide. I checked with an oscilloscope. If connected to an AC SSR it will give heat but I don't think it will be very consistent because the very short pulses from the Rambo may or may not sync well with the AC waveform. This doesn't matter for the initial heat-up when full power is being commanded, but when you are controlling at your desired temperature the power to the bed may not match what the controller is trying to send.

There is a control in the Rambo firmware where you can switch from pulse density to pulse width modulation, but that could also affect the stability of the hot end controller as well because they are linked. I was about to experiment with changing the pulse mode when I found that the expensive DC SSR I purchased only had a 900 Hz switching bandwidth but the Rambo bed heating waveform was operating at a 2.5 kHz bandwidth. But I decided to give it a try anyway. If found that the Crydom SSR ($100) I have switches ON in a few microseconds, but takes about 500 microseconds to turn off. The hysteresis is consistent therefore tuning the PID settings will accommodate for the lagging turn-off time.

I am using a 24V 650W DC power supply now for my bed in the v3 and am MUCH happier with the reduced heating time and dependable hold at 90C. For safety I added a conventional Form 1A normally-open relay in the 24V power rail; the coil is powered from the 24V power supply but through a chain of thermal fuses (150C) at points where the circuit might overheat (SSR & bed). I still don't print when nobody is home, but I sleep much better at night.

Also, if you go the AC route, I suggest adding a ground fault interrupter if your outlets are not already protected. You can add it to your main circuit breaker panel, or replace the wall outlet with a GFCI, or simply purchase an in-line GFCI. These work great if you 'accidentally' leak any AC where you don't want it (like your fingers).
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Re: Silicone + SSR + 110v AC = Awesomeness?

Postby gestalt73 » Sat Dec 17, 2016 12:05 pm

Hey Tim,

Thanks for the post, you've included a wealth of information and feedback.

I'm not sure what pwm cycle and frequency is used by the Duet Wifi, but going by the LED it looks to be somewhere in the 20hz range.
I'll see if I can find any additional information about that. I didn't even think about the limits of the AC SSR.

And thanks for the tip on the thermal fuses! That was the piece I was missing. I wanted an extra safeguard against thermal runaway.

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Re: Silicone + SSR + 110v AC = Awesomeness?

Postby timskloss » Sun Dec 18, 2016 10:29 pm

If you can see the LED flashing then maybe the AC SSR will work. It won't hurt anything to try. If it holds temperature at your set point without undue oscillation then I would say you are good to go. On the v3 the bed LED doesn't exactly blink, it sort of slowly comes on and off when holding at the target temperature. Without an oscilloscope you can't tell that the pulses are really short.

The inexpensive 150C thermal fuses--I got mine from Digikey--only withstand up to 10 amperes of current which isn't enough for a v3 bed heated by 24V, so I had to add the additional conventional relay. So far it has worked well and I have had the bed up to 105C without tripping the two fuses under the bed. In your case with 120VAC you might be able to run the bed current directly through the fuses. Do you know the resistance of your bed? Also remember that the fuse case is conductive and you don't want it to short out to anything. You can wrap it in a few layers of Kapton tape to insulate it before attaching to your bed.

I attached the fuses to the SSR and backside of the bed using Permatex Ultra Copper high-temp silicone sealant which is serviceable over 300C and then covered with Kapton tape.

***AND BE CAREFUL SOLDERING to thermal fuses--remember what they are! :lol: I use a pair of needle-nose vise grips between the fuse and the wire where I am soldering to sink the heat away before it is conducted into the fuse. Check with a ohm meter afterwards to make sure you didn't accidentally trip it when soldering. OR you could just use solder-less crimp terminals
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Re: Silicone + SSR + 110v AC = Awesomeness?

Postby dc42 » Fri Dec 30, 2016 5:46 pm

I use a 230V AC 350W bed heater on my delta, controlled by Duet WiFi electronics and and SSR. See https://miscsolutions.wordpress.com/201 ... a-printer/ for details. The Duet series electronics uses slow (10Hz) PWM for the bed and chamber heaters so as to be compatible with all types of SSR.


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