24V PSU with 12V components

Donnyb99 · 16 Sept 2017, 07:22

So I've recently wired up my new DuetWifi with my new 24V power supply for a build. It's far from my first build but it's my first Duet board. I sourced many components for this build from other printers and stuff I had lying around. My bed is AC powered and switched through an SSR. My hot end (E3Dv6) has a 12V heater cartridge, my fans (1 heatsink and 1 part cooler) are both 12V as are the LEDs I've installed.

Originally I thought there was some way to pwm limit all these components to 1/4 power and simply have them run off the 24V PSU along with everything else but now I'm not so sure.

I have an LM2596 based voltage converter capable of handling a 3A load and can install it if necessary.

Can someone advise as to what the best way to get these components all working together without risking damaging them while still retaining pwm control?

Thanks in advance

deckingman · 16 Sept 2017, 07:39

TBH, as far as the heater cartridge is concerned, your best be is to buy a 24v one - they really aren't expensive. The same applies to the fans if you can source them in 24V versions. However, I have some peculiar 12v blower fans for print cooling that I can't find in 24V versions so I use a 24v to 12v dc converter. I wire this directly from the 24V supply which gives me a 12v rail. Then connect the positive side of the fans to this 12v rail and the negative side to the Duet in the normal way. I have found that I need to use a really low PWM frequency of around 10Hz but that;'s probably a characteristic of the fans. HTH

Donnyb99 · 16 Sept 2017, 16:49

Ok so 2 questions as I would like to have this build up and running before I'll be able to get 24V replacement parts here.

1. Is it not possible to pwm limit things like the heater cartridge and fan outputs? This would effectively negate the need for 24v components.

2. If I connect my buck converter and draw a 12v rail from it, could I then simply connect the +12v out to a junction strip? I could wire the +ve end of each of my 12v components to this junction and then terminate each component's ground to its corresponding spot on the Duet. Would this allow full pwm control of each component?

Thanks

fulg · 16 Sept 2017, 19:15

Limiting power to heaters via PWM is a really bad idea. If there is a hardware failure or a software bug, the heater will output four times its rated power and will likely cause things to melt and catch fire (heaterblock and wires). Limiting to 1/4 power will work in a pinch but you really should get 24V heaters if you run 24V for your own safety.

For the fans, like deckingman I also use a separate buck converter to source 12V. If you feed that +12V into the middle pin of the V_FAN voltage selection jumper (instead of using the jumper), then all fan connectors on the Duet will run at 12V even if the rest of the board is at 24V. This works because the Duet is switching on/off the GND of each fan.

Cheers,
Ben.

deckingman · 16 Sept 2017, 20:35

As Ben said, "1" is a very bad idea.
"2" is possible. That's what I said I do. It works for me as I have a mixture of 12 and 24v fans but you can also feed that 12v into the Duet as Ben said so that all the fan connectors run at 12v.

Donnyb99 · 16 Sept 2017, 21:30

Thanks for the input.

Running the +12v into the fan jumper will work great for me for my LEDs and fans.

As for the hotend, until my 24v heater arrives can I run one end through my buck converter and the other through the Duet?

deckingman · 17 Sept 2017, 07:30

If you mean run the fans via the jumper on the duet and the heater from the buck converter, I don't see why not but stand to be corrected by anyone more knowledgable than me.

Edit. Just be sure the buck converter can handle the current. Assuming the heater is (say) 40 Watt, then it'll draw 3.3 Amps, then you have the fans and LEDS so I'd have thought you'd need in the region of 5 Amps on the 12V side (but again, I stand to be corrected).

DjDemonD · 17 Sept 2017, 08:02

Sounds about right to me I'm using a 24v-12v converter 5A. Have some 24v heater cartridges now and I'm looking forward to simplifying and neatening the wiring.

Donnyb99 · 18 Sept 2017, 03:15

Ok so it turns out my buck converter can't handle the fans, lights AND the heater; the voltage on it drops to about 9.5V when attempting to heat. Is there some way to pwm limit the heater output for temporary use?

deckingman · 18 Sept 2017, 08:34

@Donnyb99:

Ok so it turns out my buck converter can't handle the fans, lights AND the heater; the voltage on it drops to about 9.5V when attempting to heat. Is there some way to pwm limit the heater output for temporary use?

Yes there is but then the heater most likely won't get up to temperature. You can add an "S" parameter to your M307 line. So for example, S0.5 will give 50% PWM. Try it and see.

DjDemonD · 18 Sept 2017, 09:22

Even if you PWM it, the total output over time may be lower but the instantaneous output is still full current and will cause the same voltage drop.

Its hard not to conclude that its best to have 24v components on a 24v machine, I didn't at first as like most of us we've come from having 12v spares lying around, or rebuilding a 12v machine, but if this isn't an option get adequate converters. at 12v for LEDS and Fans 2A will do (unless you have a lot of LED's), hot end heaters need 5A.

WalterSKW · 18 Sept 2017, 11:21

I fully support the previous recommendations and I also strongly recommend to replace the heater cartridge and not to play with 12V heaters on 24V. It's for your own safety (and possible in a worst case also that of your family) It is just not worth it.
You get one here (https://e3d-online.com/heater-cartidge-40w) at a fair price .

Donnyb99 · 18 Sept 2017, 13:09

I've already ordered the 24v cartridge. I am looking for a temporary solution until it arrives. I've gotten this figured out now thanks for everyone's input.