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How to choose an SSR to drive your heated bed

By SublimeLayers Tuesday, March 8, 2016
Many folks are driving their heated beds with and SSR in order to get better heater performance using higher amp and/or voltage power supplies. Unfortunately, not all SSRs are created equal and you could actually have a fire if you choose the wrong one. I know, it happened to me. Here's what I wrote back in 2014 when this happened:
I had been running for over a year with my 24V supply hooked directly to RAMBo and then to my Onyx. When I rewired my Rostock a couple of weeks ago top permanently use my Azteeg X3 Pro I decided to "do the right thing" and use an SSR to control the Onyx. It had been working for a couple of weeks and just now, I noticed that distinctive "magic smoke" smell. Then I heard sparks so I immediately unplugged everything. When I opened the bay the smell was bad and the Fotek SSR-25 DD was extremely hot - way too hot to touch. The wires in and out were cool but the SSR was melting down. I've not experienced this with an SSR before and I got the 25A version as I thought that was plenty, maybe not. 
It turns out that simply looking at the peak amperage the SSR can handle is not sufficient. The issue is many inexpensive DC-DC SSRS have a high voltage drop which results in them getting hot, VERY hot. Even a heat sink with a cooling fan is not always sufficient. So be safe and look for a DC-DC SSR with a low voltage drop. Luckily, a good, affordable option exists - the Auber 100A #MGR-1DD80D100.  Here is the product description:
This SSR is specifically designed for low voltage DC applications such as thermoelectric cooler (TEC or Peltier cooler), LED illumination and low voltage DC heater. A unique feature of the design is its extremely low on-resistance (only 0.006 ohm@ 70A, 10V). The main benefits of using this design are1) Low heat generation from the SSR. Removing the heat generated by the SSR itself can be a headache when high current is passed. 2) Low voltage drop. There will be negligible voltage drop at the SSR. In comparison, other DC SSRs may have 1 - 3 volt drop with a 12Vpower supply.  The voltage drop can become a significant issue when low voltage DC power source is used.3) Low cost. The price of this SSR is much lower than other same current rating DC SSR with higher voltage rating. In addition, you will also save the cost and space for a heat sink.The maximum working voltage is 80VDC.Since the heat generated by this SSR is about 10 to 100 times less than other DC SSRs with higher voltage ratings, there is no need to usea heat sink when load is a 12VDC 10A TEC or heater. When used with our HS25 heat sink, it can handle a 60A load. When used with our HS25ET (rated for 25A AC SSR) external heat sink, it can take a 80A load.  And when used with our HS40ET (rated for 40A AC SSR) external heat sink, it can handle a 100A load.  
I use these exclusively on all of my 3D printers and they have been performing very well. I do have them mounted on a heat sink (no fan) and they are not even warm to the touch.

25 comments to ''How to choose an SSR to drive your heated bed"

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  1. Yes it was. Verified genuine. I even filed a report with Fotek so they knew of the issue. The published specks fort his Fotek clearly list the large(ish) voltage drop. This is perfectly fine for many applications but not for controlling a high current draw heated bed. I did not know about this particular issue at the time - I thought all SSRs were created equal. I know better now!

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  2. VERY good information. I am enjoying your coreXY build immensely, also.

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  3. Thanks. As soon as I lock down the design, I'll be posting the CoreXY build and info here. Stay tuned!

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  4. This comment has been removed by the author.

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  5. Thank Michael for this info, I'm getting this SSR right away.

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  6. So weird, I didnt delete that comment, just changed my nick :P

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  7. Unfortunately I didnt read this till today and just purchased the Fotek 100A dc to dc ssr....I will be feeding it with the Meanwell 750 watt and using a Duet Wifi for my own Delta design.....Should I worry with the larger rated SSR or do you think I should be fine? Thanks as always for your input Michael

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  8. The only issue is the Fotek has a large V drop across and can heat up - especially if it is not an authentic product. I'd really encourage you to bite the bullet and get an Auber.

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  9. OK Auber it is, Michael do I just wire the Onyx version7 like the Rostock manual states or do I have to change anything since I am feeding it 24v instead of 12v? Also shouldn't I be able to connect the Duet Wifi heater output with light gauge wires to the input of the SSR and then use heavier silicone 12 gauge stranded to the Onyx and the Meanwell PS? Thanks as always.

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  10. Hey Jim, it is wired differently - that's the purpose of the SSR. Otherwise you do wire up the Onyx (LED, thermistor, etc) like the manual. And yes, you can use lighter gauge wires from Duet to the SSRs exactly as you state.

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  11. Sorry I should of reread my post...yes, I know the ssr connections, I was, as you answered wondering about the LED, thermistor and such.....so thanks once again. I think I read somewhere where you get your heavier gauge wire but cant locate it for the life of me. Do you mind sharing where you get yours?

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  12. Jim, I get it from this eBay vendor: http://www.ebay.com/itm/HobbyStar-16AWG-Black-Silicone-Wire-RC-hobby-lipo-motor-US-SHIP-1ft-16-gauge-ga-

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  13. Michael ...only reply if you have the time. First could you draw me a crude diagram of where you put the lower gauge wires vs the lighter gauge so I am confident in wiring it correctly using the Auber ssr and also off subject and to verify....I received a 12" square of the 3m adhesive and I should tape the whole bottom of the Onyx (minus the center of it of course) to the top of the special Michael Hackney snowflake plate? Sorry to be a pain.....Jim

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  14. forgot to say....using a 12volt meanwell and a 24 volt meanwell for the heat bed.

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  15. Jim, I'm traveling so can't take photos of my printer. I'll do a rough sketch tonight.

    Yes, I cut a hole (I recall about 4" diameter) in the middle of the 3M adhesive to clear the wiring soldered to Onyx then attached to the snowflake. You want to do all the solder joints first.

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  16. great and thanks.....man I am envious of you and that Palette ;)

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  17. I was planning to upgrade my Rostock V3 with Duet Wifi to use 24V exclusively. Your other article shows you would have it completely 24V as well so I was wondering if I am getting the right parts. My goal is to print ABS, Nylon, and other high temp exotics.

    - Mean Well SP-750-24
    - Auber SSR mentioned above
    - Berd Air 24V
    - 24V 40 Watt E3D Heater Cartridge
    - Inline 150C Thermal fuse
    - 20Amp Inline Fuse

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    1. Djarmag, The MW SP-750-24 is a great supply with plenty of power.

      These need to be 24V components
      heater cartridge
      hot end heat exchanger fan (you missed this)
      part cooling - either 24V fan or Berd Air as you list

      There is no need for the Auber SSR, the Duet Wifi can handle the current to the heated bed. I run my V3 with 24V and no SSR.

      -Michael

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    2. Would there be an issue with direct power to the Onyx Rev 8 heat bed? Also, would a thermal fuse be needed as added security?

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  18. No, SeeMeCNC ships the Rostock Max v3.x and the Artemis with direct connected Onyx to Duet WiFi. Safety features are always a good idea. That said, I don't know of anyone who uses thermal fuses on a heated bed. They might exist but I don't know of any.

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    1. I measure 1.2 ohms at the pads on the Onyx Rev. 8 so with 24V, that would be 20amps. Would it be necessary to lower the power supply to 21V to be safe?

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  19. That would be a good question to ask SeeMeCNC. I don't recall what the Rev 8 was spec'd for.

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