Connecting 24VDC controlled SSR to Duet 3 6HC with 48VDCin

ov_darkness

Hi,

I'm looking for a solution to a problem:
I'm building a powerful machine. Bed heaters will be 2.4kW, so I had no choice but to use an SSR.
A second similarly powered heater will be used to heat up the chamber, so 2nd SSR will be needed.
They will be powered from separate 230V AC phases.
All of the electronics will be powered from the 3rd phase.
Most SSR's are controlled by either 24VDC or 4-32VDC, but I plan to power Duet 3 with 48VDC.

Can this be done, or should I buy (much more expensive) SSR's that can accept 48VDC?
Or maybe should I use other outputs for that?
Which ones?

jay_s_uk

@ov_darkness use a fan output and set it to 12v

ov_darkness

@jay_s_uk It's a solution, but there are only six fan outputs available.
If I take two for SSR's, and I have some fans and WC pump and CPAP blower to control, I'll run out of outputs very quickly.

Alternative solution: I can buy 3HC board and power it with 48VDC (I need 48VDC only for X and Y motors), and power 6HC with 24VDC. This will be complicated and expensive, but probably doable (if you can run CoreXY with the accessory board without a problem and you can connect 24V and 48V powered boards without problems).

jay_s_uk

@ov_darkness you could also use a 24v PSU to supply the +ve to the SSR and then connect the -ve from the SSR to the heater output. just make sure you tie the PSU -ve outputs together

another option would be using something like a sammyC21 and a custom board to add some outputs and some thermistor inputs

o_lampe

@ov_darkness I'd use a simple voltage divider ( two identical resistors) and use a 48V IO-pin.
That costs next to nothing and is perfectly safe, since the SSR input pin doesn't draw much current (optocoupled maybe 2mA; YMMV)

achrn

If the behaviour of the SSR is well characterised on the data sheet, can you not just drop voltage with a resistor?

The data sheet for a typical SSR I have in front of me says the control input is current limited with a current of between 10 and 14 mA. So if there's a (say) 2k4 resistor added in series, the resistor will drop between 24V and 33.6V, leaving between 24V and 14.4V across the SSR, which is within the 4 to 32V range the SSR expects.

Admittedly the resistor is dissipating about 0.5W in this case, so needs speccing appropriately.

Or am I be elecronically naive?

ov_darkness

@jay_s_uk
As overcomplicated as my idea might seem, I'd love to keep it as simple as possible

tecno

@ov_darkness

A simple STEPDOWN adapter

dc42

@ov_darkness here are some possibilities:

Use a high current output and a series resistor as @achrn suggests.
Connect the SSR between +5V and the PS_ON output.
Connect the SSR between an IOn_OUT pin and ground. Depending on the SSR there might not be quite enough voltage to trigger it, because the voltage is only 3.3V and there is a 470 ohm series resistor.
Connect the SSR between LASER/VFD and ground.

ov_darkness

@dc42
Thank you, that's actually a lot of options!
The power on threshold is 4V for this SSR.
Maximum power draw is 0.6W

As I'll need 3HC board nevertheless, is it possible to run X and Y motors from 48V powered 3HC and use 6HC with 24V for the rest?
I'd not have the issue then, and also powering 12/24VDC outputs as fans and LED's would be a lot easier.