Current Chopper for Heater bed?
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People are rightly nervous about over volting the heatbed, but we are doing something similar to our steppers every time we switch the duet on. Their rated 100% voltages are well below the typical 12-24v VIN, and we accept the electronic's ability to keep it all safe. Could we do somthong similar for the heater bed?
Running 3 P3Steel with Duet 2. Duet 3 on the shelf looking for a suitable machine. One first generation Duet in a Logo/Turtle style robot!
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It is already being done (in a way) on every heated bed through PWM or bang-bang control but because of safety implications you should match the voltage on the heat bed to the supply voltage just like you do on heaters for the hot end.
There is no need to go to the complications or cost of a full current control (as compared to PWM) -
That's open loop control though. I'm on about closed loop control, similar to how the steppers are peak current controlled.
Running 3 P3Steel with Duet 2. Duet 3 on the shelf looking for a suitable machine. One first generation Duet in a Logo/Turtle style robot!
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Regards safety implications that is easily delt with with overtemp switch, thermal fuse, and fusing so that 100% draw would blow the fuse. Equally a crowbar circuit could be built in with a capacitor that slowly discharges feeding from the PWM. If the circuit PWM locks on, the voltage raises above a threshold drops the crowbar and blows the input fuse.
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@DocTrucker said in Current Chopper for Heater bed?:
That's open loop control though. I'm on about closed loop control, similar to how the steppers are peak current controlled.
I guess my understanding of 'closed loop' is different from yours. Just like a stepper driver controls current by switching off drive when a set current has been reached and turning drive back on when current falls below the set point, the heated bed is turned off when a set temperture is reached and is turned back on when the temperature falls below a set level.
Agreed on the thermal fuse (it surprises me that it isn't standard). An overtemp switch is just a self-resetting thermal fuse. Your regular (based on current) fusing and crowbar suggestion does not account for the fact that you have a lengthy time of full power operation during heat-up of the bed. If you limited current in that phase of operation you would just end up waiting much longer to reach operating temperature. You do not have that particular phase of operation in a stepper motor.
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@jens55 Potentially talking crossed purposes. I intended to infer the current control of the heater is open loop, rather than the temperature.
I'm raising the thought that over-volting heaters may be safe, (or at least as safe as our current over volting of the steppers) if the current was limited to the same peak current as that permissible when the heater is running on the rated voltage. Currently if you wish to limit the current in the heater you limit it with a max PWM parameter. The proposed system would work like the current choppers on stepper drivers and may potentially add the possibility of a few safety circuits that kill the heaters really quick if the control signal does get locked on.
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Ahhh, now I see. What kind of a heater are you proposing though? I have seen heaters rated at 12 and 24V DC as well as 120V and 240V AC. Are you aware of some other kind of heater readily available ?
I am having troubles understanding what you want to improve over how things are currently done. -
@DocTrucker said in Current Chopper for Heater bed?:
People are rightly nervous about over volting the heatbed, but we are doing something similar to our steppers every time we switch the duet on. Their rated 100% voltages are well below the typical 12-24v VIN, and we accept the electronic's ability to keep it all safe. Could we do somthong similar for the heater bed?
There are some major differences:
- Stepper motors have high inductance, and this smooths the current. This smoothing makes the RMS current very close to the average current. This allows you to protect the circuit with a fuse. Heaters have low inductance so there is no significant smoothing. If you run a 12V 10A bed heater at 24V using 25% PWM to get the same power, the RMS current (which is what the fuse sees) will be 1.414 times the average current. So you 12V 10A bed heater needs at least 14A fuse, offering less protection
- Stepper motors are controlled by hardware, not by firmware that is way more complicated and doing a lot of other things at the same time.
Duet WiFi hardware designer and firmware engineer
Please do not ask me for Duet support via PM or email, use the forum
http://www.escher3d.com, https://miscsolutions.wordpress.com -
@dc42 Thanks, I need to look into the effect of the inductance a little more. However (this post is in the hardware wishlist section) I'm thinking about the effect of using a hardware current chopper circuit on the heater, not a fixed PWM. I guess low resistance inductors could be added to the circuit if needed.
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@DocTrucker said in Current Chopper for Heater bed?:
@dc42 Thanks, I need to look into the effect of the inductance a little more. However (this post is in the hardware wishlist section) I'm thinking about the effect of using a hardware current chopper circuit on the heater, not a fixed PWM. I guess low resistance inductors could be added to the circuit if needed.
Again I ask - what is it that you wish to improve over how things are done now.
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@jens55 I'm considering running a 12V heater in boost mode for faster warmup without exceeding thermal limits of the alloy mk3 plates and potentially allowing a simple circuit that trips supply if the duty exceeds a preset limit. However, the talk of inductance and current rise time is food for thought with regards to how accurate a current chopper would be and also reducing noise on the system. The heat bed is electrically noisey. If I put my hearing aid on loop mode I can 'hear' the noise from the heater.
I've got plenty to think about now thanks, and this has helped me clarify my thoughts a little.
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One final comment on faster heatup - go 120V AC. I did and I love it!
Of course you need to replace the heating pad and you need a relay (SSR recommendedl) and an over temp fuse is recommended.
With a Creality CR10 the warm up time was excessive and maximum temp was limited to about 60C if I recall correctly. It took something like 30 minutes to get there ! I now enjoy heatup to 60C in maybe 2 minutes. Waiting drove me insane before the upgrade.
You can run an SSR directly from the old heater output that used to power the 12V bed.
Good luck!
