Arduino Mega, dc barrel and VIn

I am using an Arduino Mega clone with RAMPS 1.4. I wish to increase the voltage that is fed into the RAMPS board from 12V to 13V due to the heat bed not reaching temperature, but I know this is above the recommended input for the Mega and I only have the one power supply. If I were to use a 9V line into the DC barrel on the Mega, would this switch off the V In pin or would I be connecting 9V to 13V and have a -4 voltage drop and hence current flowing in reverse? If so, could I just remove the V In pin from the Ramps board without affecting anything else? I can knock up a regulated 9V line as I have a load of 7809 regulators kicking about.

Cheers

your almost there...

Easy way is to remove D1 on the ramps, this is the diode that feeds 12v into the vin pin of the mega.. (it is under the X and Y stepper driver modules)

If you don’t remove it, then yes you connect 9v and 13v, this would be bad...


NB your fans may not like the extra voltage either...

Edited 1 time(s). Last edit at 05/17/2017 07:30AM by Dust.

Ah yes, good point about the fans. I will probably get a second PSU then purely for the bed and run that at 24V whilst leaving everything else as is.

If you do that please note the 15amp poly fuse is only rated for 16v on most ramps.

Ok, I have on order a 24V psu, a step down adjustable DC to power the arduino. I will run the hot end and fans on the 12 and the bed at 24V. I will probably drive the bed via an external mosfet and spec one up. Should I drive it directly from the arduino or from the heated bed connector?

*edit*
Actualy I have some 12.8A rated logic level mosfets in my cupboard, [www.fairchildsemi.com]. I will read up the datasheets and see if they can handle a gate voltage of 12V.

Edited 1 time(s). Last edit at 05/17/2017 10:48AM by Aethelstan.

Looks like you have a heated bed with 12 and 24V alternative. But I bet, you won't see a huge difference, when you use it in 24V connection with a 24V PSU. ( actually it's the same, except the voltage drop along wires is smaller )

You can really crank up the power by using ~14-16V on the 12V connection. But only, if you have sufficient wire gauge. ( That's the main reason for bed doesn't reach temp- issues )

According to my calculations using the stated resistances, I should get an extra 16W at 24V?

dual voltage heated beds are two resistors in series or parallel depending on 12v/24v hookup

12v is in parallel, 24v is in series

Cant remember actual values but let say its 2.2 Ω per 'resistor'

so 12v with resistors in parallel
1/(1/2.2Ω + 1/2.2Ω ) = 1.1 Ω

and 24v with resistors in series
2.2Ω+2.2Ω = 4.4 Ω

now on to wattage

for 12v

i = v/r where v = 12 and r = 1.1 Ω
= 10.9090909 amps
p = i^2*r where i = 10.9090909 and r = 1.1
= 130.90 (rounded)

for 24v

i = v/r where v = 24 and r = 4.4 Ω
= 5.45454545 amps
p = i^2*r where i = 5.45454545 and r = 4.4
= 130.90 (rounded)

Ie they are the same!

Or you if you want you can use p=v^2/r

for 12v its still 12^2 / 1.1 = 130.90 (rounded)
and 24v its still 24^2 / 4.4 = 130.90 (rounded)

The stated resistances don't quite match the parallel /series combination. I haven't measured them as I am out of the house but 12V is 1.6 ohm whilst 24V is 5.4 ohm. I trust what you are saying and it makes sense. I would wonder why the stated values are off like that?

That I cant say.... it is "fishy"

What heated bed are you looking at?

It's the mk3 aluminium heat bed. Thinking about it, 1.6ohms is 2*3.2 ohms in parallel, and 2*3.2 ohms in series is 6.4ohms rather than the stated 5.4ohms. It *could* be a typo or poor translation from the Chinese cloners. I will try measuring the voltage drop at the bed and if need be changing the cable and see what happens. I am using some army surplus heat resistant cable that was used for electric fireplaces, I assumed (silly and dangerous I suppose) that it was rated for 2kW items @240V and hence around 8.5A. I now realise there is no info on that, it just states 1.6mm. No idea if that is square mm, diameter or radius.

After typing all that out, then I think your suggestion of poor cable choice is likely to be the answer.

Chinese PCB bed heaters are often made to very poor tolerances, probably due to lack of control over the copper thickness. It's very common to get a bed heater that won't reach 100C @ 12V even with good electronics and cables, because the resistance is higher than it should be. Occasionally users get PCBs that have too little resistance, which leads to overheating electronics.

Unless the PCB heater listing you are buying from specifies a resistance or power tolerance, I advise avoiding PCB heaters. Silicone heater are generally made to closer tolerances.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I wouldn't give too much on a common multimeters ohm-readings. Especially, when resistance is so low like here...