Converting PWM and DC motor boards to 24V?

I'm planning on converting the PWM driver to 24V. I know an EE I can pester! He told me that it should be a matter of replacing four resistors (labeled R7, R6, R4 and R5 in the schematic [reprap.org] with 1.2K ohm instead of 560 ohm, and making sure that C1 (100nF) and C2 (100uF) are rated for 30V. Sounds reasonable to me, but shouldn't T1, T2 and T3 (all of them TIP120's) be rated for 24V too? The data sheet says they're rated for 5A, 60V, so I guess that's OK. Safe to assume they'll be dissipating quite a bit of heat?

And since the gearmotors I have are 24V as well, it would be nice to drive that at 24V as well. Schematic can be found at [reprap.org] What would be involved in converting the DC motor driver board to 24V? My random guessing (well, I know SOME theory, but I'm such a noob when it comes to electronics!) leads me to think that R5 should be 1.2K ohm (power LED) and that capacitors C8, C5, and probably C6, C7 need to be rated for 30v or so.

L293D is listed as "QUADRUPLE HALFH DRIVERS" and is rated for 1A at voltages from 4.5 to 36V, so it should be fine.

The 78l05Z is listed as a voltage regulator, but that's all the information I was able to find. There are apparently a great many versions of this particular IC, and I don't know which one I'm getting.

I've already ordered the kits from the rrrf, does anybody know if the capacitors are rated properly?

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I'm building it with Baling Wire

OK, fine. Whatever. I'll do it myself.

The board (DC Motor Driver) is pretty much fine as is. Replace the 560 Ohm resistor with 1K ohm so you don't blow the LED, and you're pretty much good to go. The "100 uF electrolytic capacitor" is only rated to 25v, so the margin on that one is a little thin, but I think it'll be OK.

The real problem isn't with running it at 24V, the real problem didn't start until I though to measure the resistance of my gearmotor, and calculate the power requirement. It's 7 Ohms. At 24V, that's 3 1/2 amps! The L293DNE is only rated to 600 ma according to [focus.ti.com] !

So it's about 6 times to small for my gearmotor.

I suppose in theory I could stack six together, but I've heard that since resistance drops as temerature increases, you get a runaway effect. The transistor pulling the most current heats up, and pulls more, heating up more and pulling more current. So you need active load balancing to keep parallel power transistors from misbehaving. But that was with quite high-power stuff. I don't know if it'd apply here or not, I'll have to keep looking.

If I could use the L293, it's rated to 1amp, but still isn't big enough AND needs "external clamping diodes" which there isn't a spot for on the RRRF board.

That can't be right, 24VDC * 3.5 Amps, that's almost 100W. Maybe I should look up the motor instead of measure it? It's labeled as a PITMAN GM9232C113-R1 3000008A 24VDC 65.5:1 Ratio.

from [www.mgs4u.com] "The motors pull about . 35 amps at full rated operating voltage . They turn about 100 rpm at full voltage (24 vdc), but will run much slower at as little as a volt!" I wasn't able to find a real datasheet for this part, but similar parts where rated from .2 to 1.1 amps, so it sounds about right.

Maybe I dropped a decimal point? Maybe they did? .35 Amps would put it well within the 600Ma "absolute maximum" rating on the L293D, so that'd be nice.

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I'm building it with Baling Wire

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The L293DNE is only rated to 600 ma according to [focus.ti.com]! So it's about 6 times to small for my gearmotor.

That's a monstrous gearmotor you've got there... probably waaaaay more power than is needed for pushing filament

It is probably easiest to grab a little commercial DC motor driver... there will be loads about for pushing robots and RC cars around and the like. Cheaper and easier than trying to build your own high current controller. Google should suggest dozens for you.

You might be able to hack something together from a stepper motor controller board, but that has expensive electronics (L297) that you simply don't need for a normal motor.

I began to suspect my the gearmotor was substatially beefier than the recommended solorbotics one when I measured its resistance and calculated the power requirements. That was mostly confirmed when I saw somebody selling new ones for $150.

Still, if I set the PWM for 25% or so as a maximum I think it'll be fine. Unless the PWM pinout goes high when the arduino is reset, in which case I'd have to remember to unplug it every time I reload or reboot. And then it'd blow up the first time I forgot. Bad deal, but I can check the LEDs on the board (without the motor connected) and determine that visually. If the pins do go high during a reset/reboot, I may have to think of something else.

If I can find a way to use what I have, I will. The question is how much risk of damaging the L293D am I willing to take? If it does blow, I'll have to buy a new one, plus a smaller gearmotor.

I discussed this with my consulting EE. The foolishness of putting "don't blow up" in software was discussed. He also said that since they're on the same chip, tying the inputs together on one side, and the outputs together on the other, should work OK. The tempurature differance between spots on the same die probably won't get high enough to cause one side to blow up. Hooking two seperate chips up in parallel is out of the question though. But that would allow me to use both channels of the DC motor driver board, and run my gearmotor at almost 50%... Well, provided the pulse widths are narrow enough. Anybody know the frequecies the PWM controlling the extruder motor is set to run at in the arduino firmware?

I'll look it up eventaully, of course. I haven't gotten that far into the firmware yet though. Still working on getting interrupts working with line drawing and arc drawing with the whole midpoint circle thing.

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I'm building it with Baling Wire