Adding a 24V power supply too the heatbed

Veggienuts · September 16, 2015, 11:52am

Ok I am stuck with this bit need help. Ok so here is what I have I have a 24v PSU I have a reprap power expander and the velleman 3d printer How do I go about setting it all up. From the surface it all seems easy but I am just unsure. Tell me if this is right.
I take the the 24V run a -+ to the input on the power expander, I then run a -+ from the output to the Heated bed. then finally I run two wires from the power expanders PCB pins to the velleman controller board heated bed thermistor PCB pins. Job done right or wrong???

Thanks for any help!

ichbinsnur · September 16, 2015, 12:08pm

Almost right.

The control pins of the power expander go to the HEATER (heatbed output) of the controller,
NOT to the thermistor input.
The controller mosfet then “switches” the power expander instead of the heatbed itself.

Just like in this picture, only that the picture shows a different controller board :

cheers,

Christian

Veggienuts · September 16, 2015, 1:04pm

[quote=“ichbinsnur”]Almost right.

The control pins of the power expander go to the HEATER (heatbed output) of the controller,
NOT to the thermistor input.
The controller mosfet then “switches” the power expander instead of the heatbed itself.

Just like in this picture, only that the picture shows a different controller board :

cheers,

Christian[/quote]How do I know which set going to the controller board are -+ or does it not matter? There are 4 PCB pins on the heatbed slot on the controller board which do I use?

ichbinsnur · September 16, 2015, 1:08pm

The leftmost 2 pins are +15V(mosfet output) the rightmost 2 are ground.
Should also be printed in the controller PCB.

Veggienuts · September 16, 2015, 1:16pm

[quote=“ichbinsnur”]The leftmost 2 pins are +15V(mosfet output) the rightmost 2 are ground.
Should also be printed in the controller PCB.[/quote] It is printed on the board but the plastic PCB’s cover the text, but thanks for pointing that out good to know. Do I need to use all 4 pins or would the middle 2 be sufficient? So do I not have to wire up the heatbeds thirmistor?

ichbinsnur · September 16, 2015, 1:59pm

Connecting one of the left and one of the right pins is sufficient.
There are only 4 pins to spread the load over the connector pins.
The thermistor is [color=#BF0000]not connected to the heater output[/color].
The thermistor for the bed goes to the [color=#BF0000]THERM2[/color] port on the controller.
And, yes, it must be connected.

Veggienuts · September 17, 2015, 4:59pm

[quote=“ichbinsnur”]Connecting one of the left and one of the right pins is sufficient.
There are only 4 pins to spread the load over the connector pins.
The thermistor is [color=#BF0000]not connected to the heater output[/color].
The thermistor for the bed goes to the [color=#BF0000]THERM2[/color] port on the controller.
And, yes, it must be connected.[/quote]Ok I got it all wired up, but there is a problem I think, it’s not stopping at the set temp it was set to 70c i turned it off at 95c.
Any ideas?

ichbinsnur · September 18, 2015, 11:39am

What temp reading do you get when the heatbed is cold?
Should be your actual room temp.
If it shows different temp, maybe the termistor is a different type.

Veggienuts · September 18, 2015, 12:09pm

[quote=“ichbinsnur”]What temp reading do you get when the heatbed is cold?
Should be your actual room temp.
If it shows different temp, maybe the termistor is a different type.[/quote]Says room temp before I turned it on to test it.

ichbinsnur · September 18, 2015, 2:46pm

Can you try again, to see if it stops at a higher temp?
But don’t let it go over 250.

Just to know how it reacts.

Veggienuts · September 18, 2015, 2:56pm

[quote=“ichbinsnur”]Can you try again, to see if it stops at a higher temp?
But don’t let it go over 250.

Just to know how it reacts.[/quote]will try and get back to ya.

kjriisne · July 17, 2016, 6:30pm

Which one of HEATER1 or HEATER2 is the heatbed?

Erik_M · August 15, 2016, 9:06am

You don’t need any additional components to run the hotbed on a higher voltage.

You do like this:

A - Connect minus of the second powersupply, PSU, to the minus of the board.

B - Connect one wire from the hotbed to plus on the 2nd PSU.

C - Find Heater2’s two +15V pins on the K8200 motherboard and clamp them out of the way.

D - Connect the other wire from the hotbed to Heater2’s two remaining [drain] pins.

E - [optional] Add a Heatsink to the Heater2 MOSFET.

Erik_M · August 19, 2016, 1:27pm

I have now actually done this myself - and it works marvelously!
With a dedicated 24V/8A PSU for the heated buildbed it heats up as fast as the hotends.
No - not kidding.

And as pointed out just above - you do not need any additional hardware.

Yeah! B-)

kjriisne · August 21, 2016, 9:31pm

Yep did this myself, and it heats up really fast. I have a 150w PSU for the heatbed and looking at the graph for the heatbed, warming it up to 100 desgrees is just as easy as 50 degrees. 50 degrees takes about 3 minutes and 100 takes about 6-7 minutes.

Processaurus · November 2, 2016, 10:49am

[quote=“Erik M”]You don’t need any additional components to run the hotbed on a higher voltage.

You do like this:

A - Connect minus of the second powersupply, PSU, to the minus of the board.

B - Connect one wire from the hotbed to plus on the 2nd PSU.

C - Find Heater2’s two +15V pins on the K8200 motherboard and clamp them out of the way.

D - Connect the other wire from the hotbed to Heater2’s two remaining [drain] pins.

E - [optional] Add a Heatsink to the Heater2 MOSFET.[/quote]

Thanks for the encouragement. Did a version of this today, using the onboard mosfet to switch the 24v to the heatbed. I did mine by using the PCB power barrel connector for the 15v supply, and the screw connector for the 24v supply. I cut a couple traces on the PCB to isolate the Heater 2 (heatbed) power input, to change it over to the 24v. I connected the barrel connector over to Heater 1 with a short piece of medium gauge wire. This way everything on the board is running on the 15v, except the heater 2 (+) connection is connected to the 24v supply. The 24v supply (-) is connected to ground, so when the mosfet turns on, the heatbed (-) is connected to ground, and the 24v runs through the heatbed, thru mosfet, to ground.

This way runs the LED for the heatbed off the 24v supply. I changed my LED resistor, R15, to 3K, but I’m not sure this is necessary, the original resistor is 1.8K, probably within safe limits for the LED current, as tiny as it is. I like the LED not being run off the 15v supply, because LED’s don’t like have voltage reverse biased, I’m not sure what the reverse breakdown voltage of our LED is. But, even if it died, there is the 1.8K resistor in series, so, even if it fails short, the +24v and +15v is only connected together with a 1.8k resistor, no safety issue for the rest of the electronics, just that LED.

Erik_M · November 2, 2016, 12:17pm

I got worried and read your description to fast, sorry.
It is just one thing that worries me.

You say you connect 24V to the “barrel connector”.
That is minus of the motherboard…

Just a moment and I will make a schematic.

Erik_M · November 2, 2016, 12:43pm

This is an n-channel MOSFET, thus power is supplied from the low side [minus], not from the high side [plus].
The correct way to connect this is this:

The left lamp is driven on 24V, the right one at 15V.
Both controlled by the microcontroller [at 5V] driving their respective n-channel MOSFET’s.

The correct way to do this is to…
…leave the n-channel MOSFET as it is, keep the two minus marked pins going to the heatbed.
…totally disregard the two pins marked “+”.
…connect the 24V minus to the 15V minus. Preferably on the motherboard.
…connect the 24V plus to the second two wires to the heatbed.

Do not connect any plus 24V to anywhere on the motherboard.

Processaurus · November 2, 2016, 7:56pm

Yes, Schematically it is pretty much the same, I modded the pcb to connect both the power supplies, just because I wanted to be able to disconnect the 24v power supply without desoldering it, to make it easier to service the PCB. Also I was worried about not reverse biasing the LED. There were space considerations too, no room for an extra pcb.

You could accomplish the same thing with an inline connector, or a little daughter board with screw terminals. Anyone who doesn’t do electronics work and is not confident cutting traces and rerouting their PCB should do it off the board, and not worry about the LED, it is probably fine. I was very careful, and cut the traces in two places apiece, and checked all of the traces I cut with a continuity tester, to make sure they weren’t connected together still, by a sliver of stray copper.

If you do it offboard, extra ground connections are scarce on the pcb, but you can get your ground connection by stacking both power supply (-) wires in the pcb (-) screw terminal, or by desoldering the barrel connector, and using one of the holes connected to ground.

It works great! My heatbed reaches 55° C in 3 minutes!

Erik_M · November 2, 2016, 8:53pm

The ground [minus] of both power supplies shall be connected, exactly where is not critical.
So there is no problem with that. It is probably better, for some obscure electroll reason, to have them both at the motherboard thou.

I am more worried about how, and that, you connected 24V to the motherboard.
But as long as you know what you have done and it works fine - good! B-)

And I am happy that someone else also have a quick and effective heatbed now.