v3 Maximum PSP Voltage Setting

[Christian79]

Hello Everyone,

I have just finished upgrading my old v1 to a v3 and am going through the calibration process now. As with my v1 my bed times were so slow, in fact even with the Bed Drive Max set to 255 my bed would timeout on the autotune to 70C. So I checked the PSP and it was putting out a nice 12.5v at idle with no adjustments. I know the Hotend (E3D), Rambo, and bed are ok at higher voltages so I bumped my PSP to 14.6v at idle (max setting is 14.9v). Aside from driving the fans 20% over nominal and needing to retune my PID settings, can anyone see any issues with this? As a side note my bed now goes from 20C to 80C in 8 minutes.

Input appreciated......

[Noircogi]

I don't know which onyx you have. The one which came with my Max V3 measures 0.91 ohms. You are probably getting some voltage drop across the FETs in the Rambo, but at 14.6 volts you'd be pushing over 16 amps through the Rambo FETs and fuse, which are only rated for 15A.

I had my V3 set to around 14.3V for a while (before doing a 24V bed conversion) and it would occasionally not power up correctly.

[IMBoring25]

Without running the bed across an SSR, you're probably looking at melting down the bed terminals on the Rambo if you haven't already. There have been a few who have done so without overdriving the voltage.

[Christian79]

You both make good points, I will have to measure the current through the circuit and see what the actual value is. I did not think much about increasing the voltage as the circuit is fused however, since the Rambo now only sinks the current, I am not sure if the fuse projection is Still active. Also the Rambo specs indicate it can accept 12 to 35 volts at 15A so this should not be an issue. I suspect the terminal damage is a result of loose electrical connections creating a large amount of resistance at the terminal and as a result more heat. Thank you again.

[Xenocrates]

You both make good points, I will have to measure the current through the circuit and see what the actual value is. I did not think much about increasing the voltage as the circuit is fused however, since the Rambo now only sinks the current, I am not sure if the fuse projection is Still active. Also the Rambo specs indicate it can accept 12 to 35 volts at 15A so this should not be an issue. I suspect the terminal damage is a result of loose electrical connections creating a large amount of resistance at the terminal and as a result more heat. Thank you again.

You will not be able to measure the current in circuit. It will exceed 10A, which is AFAIK, the highest current reading on a multimeter in common use, so you'd need a clip on one, which would be likely to get confused at the PWM, and potentially also by the other electrical flux in the printer. Better to measure the voltage and resistance, since you know it's DC, and unlikely to see any significant effects from the induction of the bed or other parts.

Yes, part of the issue is poor connections with meltdowns, especially in the older ones which bundled multiple wires in. However, considering what I've seen for resistances on the Rambo, it is quite possible to draw nearly 15A at 14V, which will over time cause the fuse and board to heat. It is also more likely for you to take lifespan off the power supply. They are engineered to deliver 29A~ at 12V. That's 348W, which gives you 24A at 14V, if it were one to one. Unfortunately, it isn't quite, so you may have less margin still. At 14A for the bed, plus ~1.2A per motor, times 3, 3.8A for the heater cartridge, and ~.7 for the extruder, you get pretty close to what I would consider the limit of 22A, and are probably over the limits of what it's really designed for. Note that the fuse, like the switching, is likely on the negative (For some reason I can't really fathom. It's far better to fuse and switch voltage than ground or neutral, from a safety and sanity perspective).

[Jrjones]

My inline multimeter is rated for 10A or 20A for 30 seconds max every 15 minutes... too bad I blew the fuse measuring a motor that exceeded its nameplate amperage (50amp instead of 15).
Since I got a clamp on AC/DC meter I use that for measuring current.
I just measured mine.
From room temperature, 100% power applied: 12.6V, 14.0A -> works to ~176W and 0.9 ohm resistance.
Bed at 110°C, 100% power applied: 12.6V, 11.24A -> works to ~141W and 1.12ohm resistance.

When I measured my bed resistance at room temperature, I measured 1.1 ohm.
I wouldn't increase your voltage to your heated bed until you've measured what you are drawing for amperage.

[Christian79]

I am sorry for the slow response, been a busy week at work.....

I finally had a chance to measure the resistance of my heated bed at the solder pads (@20C) and was meant with 1.2ohms. That coupled with a voltage of 14.6v would yield a current draw of about 12.2A. However, considering the other factors mentioned in this thread I reduced the idle voltage to 14v (13.97v at full load) and will leave it there for now. Surprisingly though the reduction of 0.6v cost me 2 minutes on a temp calibration from 20C to 80C....

Previous 20 to 80 => 8 Minutes
Revised 20 to 80 => 10 Minutes

Any additional information would be appreciate, thanks again guys!

[Jrjones]

Your heating time seems normal to me. I put a towel my heated bed and leave it for 10-15 minutes to get to 110°C, then another few minutes with the towel off for the temp to stabilise without the towel.