bobc Wrote:
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> Heh, you guys are quick!
;)
> The ESTOP feature was a last minute addition I
> couldn't resist trying, but I did goof it up.
> Logic chips are quite cheap, and I quite like the
> idea of a hardware stop.
No problem with the idea at all! I like it.
> With a quad NAND (74HC00) and removing the pullup
> resistors R204, R205, etc it works to a point, but
> an issue I have noticed is that if there is no
> 3/5V power from the Due (i.e. Due powered by USB,
> and USB disconnected), or if Due is in
> reset/unprogrammed, but power on 12V, then the
> MOSFETs for the heaters turn on. Obviously that is
> not ideal.
No, not really.
Also of note: I now see that the FET gate drive transistors (the 2N2007's) invert the signal, so I have probably suggested the wrong pull-up/pull-down resistor setup. This means you probably want them all pulling up. Advantage is you might be able to use 1 x 8way resistor (in an SMD pack) instead of 8 single resistors.
PS: Please double check that is correct!
> I think your 74LS244 suggestion would be better
> and hopefully fix that issue. I was thinking of
> commoning some motor enables so that all the
> heater outputs are on the ESTOP.
I thought about something that would drop in replace it (eg: NAND, NOR, etc), but couldn't figure out a way to do that would handle "the Arduino isn't providing any signal" case, hence the idea of a 74LS244. There may be something better that might be closer to the mark. I'll trawl through my 74xx and 40xx data books at some stage soon.
(Yes, OMG, Someone who still has data books on hand - for cases like this they're pretty useful!)
> I think the board also needs 1 or 2 small FET
> outputs for fans etc.
Sounds good to me.
> Some other known issues:
> - JP101 pinout needs changing
I'm assuming it's back to front compared to the other power connectors?
> - need to derive a 9-10V gate voltage for 24V operation
Any idea how much current we'll need?
The IRLB8743 needs a gate voltage below 20V. So if 12V is easier to get, then perhaps we could use that? It's also 3 volts less that we need to drop (dissipate) from whatever we use to get the voltage down.
Stupid idea #1: 3 pin regulator. For 12V operation, bypass the connections (connect IN to OUT). For 24V, populate with a 12V (or even a 9V) reg, OR if you have 12V (eg: from your PSU) connect it to the OUT on the board. Have to figure out a nice way to arrange the pins (to make it easy to connect in/out) and label them though. Also leaves the possibility of confusion.
Stupid idea #2: If the current needed to drive the FET is negligible, just set up the 2N2007's drain on a voltage divider, set for 3/4 of supply voltage. For 24V, that works out to 18V (below the IRBL8743's 20V max) and for 12V, that's 9V. Maybe err on the slightly lower side in case the 24V is exceeded a bit (which still gives a reasonable gate drive at 12V - more than we get with 5V anyway). If people want to max it at 12V, they can remove the bottom resistor in the voltage divider.
Another thing of note: I notice that you're using the IRLB8743 to drive the LED, but you're getting the supply for the LED from the gate drive voltage. If someone does something stupid like soldering in the LED backwards (the markings on many SMD LED's make this easy to do), you're going to end up with supply voltage (through the load - such as the heatbed) minus the LED voltage drop applied to the gate drive voltage. I don't think this will cause too much of a problem (due to the resistor in-line with the LED) but thought I should mention it, especially if you go with the voltage divider method, as it will definitely skew things.
-------------------------------------------------------
> Heh, you guys are quick!
;)
> The ESTOP feature was a last minute addition I
> couldn't resist trying, but I did goof it up.
> Logic chips are quite cheap, and I quite like the
> idea of a hardware stop.
No problem with the idea at all! I like it.
> With a quad NAND (74HC00) and removing the pullup
> resistors R204, R205, etc it works to a point, but
> an issue I have noticed is that if there is no
> 3/5V power from the Due (i.e. Due powered by USB,
> and USB disconnected), or if Due is in
> reset/unprogrammed, but power on 12V, then the
> MOSFETs for the heaters turn on. Obviously that is
> not ideal.
No, not really.
Also of note: I now see that the FET gate drive transistors (the 2N2007's) invert the signal, so I have probably suggested the wrong pull-up/pull-down resistor setup. This means you probably want them all pulling up. Advantage is you might be able to use 1 x 8way resistor (in an SMD pack) instead of 8 single resistors.
PS: Please double check that is correct!
> I think your 74LS244 suggestion would be better
> and hopefully fix that issue. I was thinking of
> commoning some motor enables so that all the
> heater outputs are on the ESTOP.
I thought about something that would drop in replace it (eg: NAND, NOR, etc), but couldn't figure out a way to do that would handle "the Arduino isn't providing any signal" case, hence the idea of a 74LS244. There may be something better that might be closer to the mark. I'll trawl through my 74xx and 40xx data books at some stage soon.
(Yes, OMG, Someone who still has data books on hand - for cases like this they're pretty useful!)
> I think the board also needs 1 or 2 small FET
> outputs for fans etc.
Sounds good to me.
> Some other known issues:
> - JP101 pinout needs changing
I'm assuming it's back to front compared to the other power connectors?
> - need to derive a 9-10V gate voltage for 24V operation
Any idea how much current we'll need?
The IRLB8743 needs a gate voltage below 20V. So if 12V is easier to get, then perhaps we could use that? It's also 3 volts less that we need to drop (dissipate) from whatever we use to get the voltage down.
Stupid idea #1: 3 pin regulator. For 12V operation, bypass the connections (connect IN to OUT). For 24V, populate with a 12V (or even a 9V) reg, OR if you have 12V (eg: from your PSU) connect it to the OUT on the board. Have to figure out a nice way to arrange the pins (to make it easy to connect in/out) and label them though. Also leaves the possibility of confusion.
Stupid idea #2: If the current needed to drive the FET is negligible, just set up the 2N2007's drain on a voltage divider, set for 3/4 of supply voltage. For 24V, that works out to 18V (below the IRBL8743's 20V max) and for 12V, that's 9V. Maybe err on the slightly lower side in case the 24V is exceeded a bit (which still gives a reasonable gate drive at 12V - more than we get with 5V anyway). If people want to max it at 12V, they can remove the bottom resistor in the voltage divider.
Another thing of note: I notice that you're using the IRLB8743 to drive the LED, but you're getting the supply for the LED from the gate drive voltage. If someone does something stupid like soldering in the LED backwards (the markings on many SMD LED's make this easy to do), you're going to end up with supply voltage (through the load - such as the heatbed) minus the LED voltage drop applied to the gate drive voltage. I don't think this will cause too much of a problem (due to the resistor in-line with the LED) but thought I should mention it, especially if you go with the voltage divider method, as it will definitely skew things.