Welcome to the SeeMeCNC Community

I'm completely overhauling my Rostock Max v2 from stock to:

• duet 0.8.5
  e3d v6 24v
  24V 350W power supply
  Injection Molded Cheapskates
  Tricklaser carbon fiber arms
  Heat sinks on all of the stepper motors

I'm going to use the 24v power supply hooked directly to the duet. I plan on putting a fuse inline from the duet to the heated bed, I'm guessing ~20A car fuse should work.

After reading and re-reading what I think are all of the posts about using 24V I'm still concerned about thermistors and LED's on the onyx bed. I've read where these should be updated but I haven't been able to find any specific info, such as what resistor to use on the LED or a specific thermistor to use. If someone could point me in the right direction I would appreciate it.

I've never planned on going all 24V so I haven't looked into other parts but if your bed is like mine you'll have to turn the PSU down to 19-20V for a 20A fuse to work.

It is ill advised to drive the heated bed directly from the Duet - use an SSR (an appropriate SSR as I've written about here).

Thermistors will not be a problem. LEDs will be much brighter but shouldn't have a problem (my Onyx has been running at 24V for years and the LEDs are still good). Fans are a problem, they will spin much faster and can burn out.

Use the stock thermistor for the bed and hotend. Don't worry about the LEDs. Looks like you have the hot end covered.

To make it easy I've posted on my blog How to choose an SSR to drive your heated bed. It describes the problem with cheap SSRs and offers an affordable and safe alternative.

Using an SSR necessarily requires me to use two power supplies?

Also, if I do use two power supplies can they both still be 24V?

ddpruitt wrote:Using an SSR necessarily requires me to use two power supplies?

Also, if I do use two power supplies can they both still be 24V?

No you can use one PS. The duet will power and control the SSR which you use to relay 24V to the Onyx.

It is not a great sketch but this is what I did.

Why on earth would you say it is ill-advised to use the duet to drive a heat bed directly? I would say, if you use 12v or 24v heat bed (and this is good enough for the area you want to heat) then the duet is perfectly suited to drive that. The connector is beefier than hell, there is no way it's a risk.

There's been some discussion of this at the RepRap forums. The 0.8.5 does have a much beefier terminal on it but I'd have to find a part number to look up its rating, and it's probably not as high as you think. Then there'd be a derate if you run a stranded wire directly into the terminal. The traces on the board are also a limiting factor, around 15A in the estimation of several there.

I've burnt bed terminals on a Melzi twice (I think there was an intermittent internal short in that heat bed, in retrospect). Risking totaling out a Duet would be even more painful. I'd rather run the SSR and not have to worry about it.

Would have to confirm with others, but the load on a terminal for the Onyx at 24V is near 20 Amps. While the bed current draw is intermittent, maximal load could be to much. An SSR is cheap and easy to install. Make sure the polarity to activate the SSR is correct or it will not work.

Here is a link to a low heat SSR as mhackney has detailed (http://www.auberins.com/index.php?main_ ... cts_id=288)

For smaller kossel sized beds, I do use the Duet output/connectors directly. But you can barely clamp a 16 gauge wire in them with a ferrule. For larger beds drawing a lot of current like the OPs Onyx, you need bigger wires (or several smaller) to carry the current. Even with an Orion heater I use on my Kossel 250 direct to a Duet, it gets warm enough around the terminal that I added an SSR.

Oh I kind of missed that this would be a stock onyx rev 7, and not an onyx rev 3. Nevertheless, a rev 7 would have the PWM dialed down to at least 50%. I see the risk, if the PWM/PID settings get reset accidentally. Wouldn't a 300-350 watt 24v silicone heater be fine from the duet directly?

350W at 24V would be around 15A, which most think is right at the likely limit. On the bright side, it should only be running at 100% duty cycle for a few minutes until it goes to PWM, but I still like a little more safety margin than that.

bot, I'm running a 24V 350 watt direct from Duet on a second Kossel 250 and that has been fine. But I agree with IMBoring25, I like to have a LOT of safety when it comes to things that could burn down my house. I can't tell you how alarming it was when the cheap SSR I originally used several years ago caught fire. Just thank God I was in the room to put it out. If I had run out for an errand or something I would have probably come back to a blaze. Not fun.

Also note, the Duet v0.6 terminal is only rated to 10A so I wouldn't risk running a heater without an SSR. The 0.6 is a nice little controller otherwise and if you only need 1 extruder or need to use the Duex4 is/was a great deal.

From an electrician's and wiring perspective, these electrical components would typically be within a metal housing which would limit and prevent the fire risk.

Thus, although some use these SSR's, without the metal housing (i.e. our printers), we need to be very sensitive to failure since failure is catastrophic as mhackney has discussed.

SSR and heatsink ordered on on their way, thanks for convincing me not to burn my house down.

Question: Given that it is 24V 350W 15A would it be better / easier / safer to get the Onyx Rev 3?

mine is working with a mosfet.

cheaper and cooler than a ssr

lordgort wrote:mine is working with a mosfet.

cheaper and cooler than a ssr

Don't mosfets typically fail in a short circuit? That can't be good for the Onyx, or your house.

Usually yes,

My cheapo ssrs also failed in the ON position.

teoman wrote:Usually yes,

My cheapo ssrs also failed in the ON position.

Ouch, just another reason to use a thermal fuse. Just to lazy to break mine down and install them..

In the case of a failing controller the pin goes high imp mode. for that case there is R3 (in the picture) which causes the mosfet to "shut off"

for other failure cases you really need to cook the mosfet with 260 amp.

im using this with 12v, so im fine. with 24v you should use a fuse to protect your bed... just a few pennys more


//for 12v on the controller-side R1 is 1k, R2 is 10k and R3 is 60k
//in the picture i posted a 24v controller was used

mhackney wrote:It is ill advised to drive the heated bed directly from the Duet - use an SSR (an appropriate SSR as I've written about here).

If you don't need the horsepower for the bed, would it make sense to use the heated bed output for the hotend then? Might as well use the more substantial FET and connector, right?

Well, I'm thinking that maybe it might be worth going to 24v to increase acceleration/speed of .9 degree steppers? I believe I read someone posting that the back-emf at high stepping speeds causes problems because the current rise isn't enough to counter back-emf.

Jimustanguitar wrote:

mhackney wrote:It is ill advised to drive the heated bed directly from the Duet - use an SSR (an appropriate SSR as I've written about here).

If you don't need the horsepower for the bed, would it make sense to use the heated bed output for the hotend then? Might as well use the more substantial FET and connector, right?

This is a really good question.
Is it easy to change that in the configs?

It is very easy to change that in the config.g and it is a really good idea.