Duet circuit diagram

Like (many) other people I am having power supply transient problems with my Ormerod RS#343.

Firstly I would like to get a copy of the circuit diagram if it is not commercially confidential. With this I can apply extra decoupling appropriatley

Secondly I wonder if anyone has used an external relay and PSU to run the bed heater. This sounds retrograde but it sure would work!

Any help appreciated
Dennis

Hi,

I don't know if its already done by Duet firmware, but I will suggest Software developer can use PWM output to switch on and off the MOSFET for heater, to act as soft start by ramping its output . That might reduce surge current on power rail.

The schematic is here.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
tru168
I don't know if its already done by Duet firmware, but I will suggest Software developer can use PWM output to switch on and off the MOSFET for heater, to act as soft start by ramping its output . That might reduce surge current on power rail.

The extruder heater is PWM, the bed heater is on/off. Currently, the mosfet turns the bed heater off very fast, which gives rise to large but very short transients. I've no evidence that these are affecting the operation of my Duet, but I am planning to add a 100 ohm resistor in series with the power mosfet gate to slow down the turn-off. Until this or something like it is done, I don't think using PWM on the bed heater would be a good idea.

I've had no problems with print stopping since I modded my board (see the thread on 3.3V supply quality) - but I only had a print stop once on me before that. I have just had a problem whereby the Z homing stopped working properly until I reset the board, but I think that is more likely to have been a firmware issue.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Hi TRU168,
What you say is true, but I want to separate out power switching from logic signals absolutley!
With PWM you will get the same transients far more often - bad.
I also am dubious about putting the thermistor signal in the same wiring loom as the fat amps, I see why it is done but
it does not help, it probably gives a route for good noise injection into low level logic.
With a circuit diagram I can sort this easily

Thanks
Dennis

Quote
DennisCowdery
I also am dubious about putting the thermistor signal in the same wiring loom as the fat amps, I see why it is done but
it does not help, it probably gives a route for good noise injection into low level logic.
Dennis

I don't have any worries about running them in the same loom. The circuit diagram shows a 10uF capacitor across each of the two analog inputs, which is more than enough to deal with any likely noise pickup, whether inductive or capacitive.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Hi DC42,
Many thanks, I can now attempt to make the system stable enough to lay some plastic! Nothing but frustration so far

Regards
Dennis

Hi dc42 (again)
If transients are created when switching the base heater, which they are, then switching it more often won't help.
That is why I want to separate the two functions with a relay.
Also - putting thermistor wires in the same loon as the fat amps is not a good idea.
Look for smoke over Reading Berks!!

Many thanks
Dennis

In case you are interested, here is a summary of mods I made to my Duet board (probably voiding the warranty in the process):

1. Connected a 470uF 25V capacitor in parallel with C3 (I would have preferred 1000uF but it wouldn't fit). This is an attempt to improve the tolerance of the board to power brownouts, by keeping the 5V supply up a little longer if the 12V supply briefly disappears (and if the bed heater is on when the power flickers, the 12V power will go down very quickly).

2. Connected a 10nF capacitor in parallel with R60. This is recommended by the manufacturer of the switching regulator chip to improve transient response. May not be needed in this application, but it's an easy mod.

3. Connected a 1uF ceramic capacitor on the underside of the board between the +ve bed connector and the -ve Vin terminal. This is my attempt to make up (in part) for the lack of local decoupling in the bed heater circuit.

4. Connected a 100 ohm resistor in series with the bed heater power mosfet gate, and another in series with the hot end heater power mosfet gate (this was the most difficult mod to do). This is to increase the mosfet turn-off time from <20ns to >200ns, to reduce the size and sharpness of the power supply transient when the mosfet turns off.

Edited 1 time(s). Last edit at 12/31/2013 10:59AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
DennisCowdery
Also - putting thermistor wires in the same loon as the fat amps is not a good idea.
Dennis

LOL .. Luckily there is 12 volts up on the hot end for the fan so it would be easy to add a buffer amp up there on the hot end I guess.

regards
Andy

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Ormerod #318
www.zoomworks.org - Free and Open Source Stuff

Dennis

I have not read every word of this thread but I agree with you completely. The driver output for an external Mosfet is provided. There is a separate 4 wire plug from PSU to power board, this would be ideal to run the bed heater with no sharing of supply or common right back to the psu. The only issue in my mind is if the driver and ground are taken to an external mosfet on the power board then you are linking the commons again and closing a ground loop. The best solution would be to use an opto isolator and another driver transistor. Alternatively leave the ground open and let the drive current return through the psu which is not very elegant in terms of what it might pick up.

Rory

I've not had any problems that I can attribute to the bed heater sharing the same PSU as everything else, other than that I can hear the fan pitch drop very slightly (about a semitone) when the bed heater comes on. The PSU that came with my kit is an Alpine 550W. In principle, I agree that a separate PSU for the bed heater, or separate wires to the same PSU, would be a good thing. But I've not found it necessary on my build.

If you do use a completely different power supply, then I agree that opto isolation for the gate drive would be advisable.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

DC

Interesting that you have an Alpine, mine is ACE I had assumed everyone had the same. I think they are the same just supplied with a different badge like Austin and Morris used to be in some rotten best forgotten motoring years.

I agree with what you say but I just prefer to separate big currents when possible,

Rory