External 5v Source

javitopia

Just think of the mosfet (the device controlling the pin) as a switch between the pin and GND.

When you issue an M80, the switch closed, so there is a path to GND.

When you issue an M81, the switch opens, so there is not path to any of the power levels.

As far as i know, you can't change this behaviour (use m81 to "close" the pin) so if you need to control a device that is active high you will need external circuitry like a 7404).

What you can do, is changing the default state. If you want your duet to power on as soon as it is supplied power, just add M80 to your con fig.g

If you want your duet to start powered down (i mean the ps_on signal) you don't have to do anything, but you should add M81 in your con fig.g, so the power control appears in your duet web control interface (in RRF3 the ATC power control block won't appear until you have issued a m80 or m81 command)

and lastly @oliof YES if you feed 5v via the ext 5v header you have to remove the int_5v jumper.

GoremanX

@achrn Woohoo! That worked. Ended up going with a 100ohm pull up resistor, and it works great. Anything higher would cut the voltage down below the 4v threshold for the remote control. The draw from the mosfet doing the switching on the power supply is negligible, so my tiny 1/4 watt resistor isn't being stressed at all. I'll wrap it in heat shrink tubing and call it done.

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A Former User

@GoremanX said in External 5v Source:

my tiny 1/4 watt resistor isn't being stressed at all.

unless I'm missing something it will be when ps_on is pulled to ground? 100 ohm and 5v = 250mW no?

GoremanX

@bearer oohh, maybe. Good call. Technically that's exactly its capacity and it hasn't been an issue so far, but it might be in the long run. Could try 220 ohm instead and that would be half its capacity. 1kohm was definitely too high, I was getting just over 3v and it wasn't enough to turn off the power supply.

A Former User

(just keep an eye on the temp)

GoremanX

@bearer switched to a 220 ohm resistor. The voltage to the ps_on pin is now 3.7v, but that seems to be enough to turn off the power supply despite being under 4v. This setup works great. Obviously I'd rather have M80 and M81 work in reverse, somehow, but this will do.

A Former User

I think that is the feature on the to-do list to allow inverting the pin, so M80 and M81 will work as expected.

achrn

@GoremanX I don't think reversing M80 / M81 helps.

With it reversed, the pin will be low when the Duet wants power to be off, and floating when the duet wants power to be on. That doesn't help you.

At the moment you have a PSU that switches on when a pin is pulled low, and you have a Duet that pulls a pin low when it wants a PSU to switch on. That sounds like a match to me. Why do you want one of them reversed?

A Former User

@achrn said in External 5v Source:

At the moment you have a PSU that switches on when a pin is pulled low,

he said the opposite in his last post? which i assumed is what all the fuss with the meanwell psu's a while back was all about as well

GoremanX

@achrn With M80/M81 reversed, I can connect the + (plus) from the remote control to the EXT_5V pin, and the - (minus) to the ps_on, and then switch the power supply off by switching its - (minus) between pulled low and floating, no pullup resistor needed. In fact, this already works when I wire it up that way. M80 turns off the power supply and M81 turns it on (which is backwards)

achrn

@GoremanX Thanks, I understand now.

GoremanX

@dc42
Out of curiosity, how much current can the ps_on pin sink? Am I stressing that circuit by running 23 mA through it almost constantly? I'm trying to find a spec for it, but I'm failing

A Former User

23mA? Thats nothing (0.023A). Don't recall of the top of my head which type of connector it is, but iirc 6 amps if its the big type used for motors on the Duet3 and 2-3 amps if its the smaller type used on the Duet2. So in any case you're at or below 0.1% of its rating with 23mA.

Edit: its the mosfet; right. Ehm, max raing is around 4A. But should probably stay below 2A without cooling.

GoremanX

@bearer I vaguely remember reading something about 200ma, but that may have been for drawing power from the 5v_ext pin (which is essentially what I'm doing, and shunting it to the ps_on pin)

A Former User

That is indeed a different question than how much current the ps_on pin can sink.

On the Duet3 I guess if you don't supply external 5v you'll be pulling internal 5v through a 0.25w 220R resistor.

As long as you don't exceed 5v you can't exceed 50% of that 0.25w rating so not an issue, but sounds like you'll be loosing a lot of your 5v across that resistor.

On a Duet 2 you can't source any current from that pin, ints purely an input afaik.

Nuramori

@javitopia said in External 5v Source:

Just think of the mosfet (the device controlling the pin) as a switch between the pin and GND.

...

and lastly @oliof YES if you feed 5v via the ext 5v header you have to remove the int_5v jumper.

Is there a danger by not removing the int_5v jumper if you supply 5v to the external header on the duet3? I think I had it that way (jumper removed), then one day things stopped working until I jumpered it again. The supplemental PSU (meanwell rp65A) does 12v and 5v and works fine.

dc42

I am late to the party, but unless you are powering LED strips or other high current 5V devices from the Duet, then I suggest you power the Duet 5V from the Pi 4 using the 5V SBC->Duet jumper, and use the official Pi PSU or similar to power the Pi. The Duet only draws around 200mA from 5V.

GoremanX

@dc42 That's what I ended up doing and it works really well

dc42

@GoremanX said in External 5v Source:

@dc42
Out of curiosity, how much current can the ps_on pin sink? Am I stressing that circuit by running 23 mA through it almost constantly? I'm trying to find a spec for it, but I'm failing

PS_ON can sink about 1.5A, same as the fan outputs; but it doesn't have a flyback diode built-in.

GoremanX

@dc42 Perfect, thank you