Gen 7 v1.2 - Pololu issues. (ie. Fire)

Heya,

This is our first attempt at building a 3D printer, like many on this forum.
We are putting together a Prusa Mendel using Gen 7 v1.2 board and pololu drivers.
We are using ReplicatorG v0029_r2 - settings: machine type "Prusa Mendel", connection via FTDI/usb.

We are currently testing the stepper motors and the pololu drivers - initially we were only getting a motor twitch (no rotation). We managed to get a motor to jog using the control panel in replicatorG, however the pololus have always been running extremely hot. The second attempt at testing failed to move the motor and the third attempt resulted in fire - a burned out pololu.

What we are hoping for is a way to diagnose these issues with the pololus and stepper motors and ultimately get this board printing. (The rest of our machine is assembled and ready to go!)

Photos of our board are available here:
[s239.photobucket.com]

Any assistance would be greatly appreciated.

Cheers,

Trendstate

If they're running extremely hot, then you need to add a heatsink and/or turn down the current. When you get them hot enough, the thermal protection will make them skip a lot. There's a tutorial on adjusting current here:

[reprap.org]

If you're running >1A, w/o a heatsink they'll definitely overheat.

Even with this precaution, I had one catch on fire the other day, too. My guess is that it's because I left the VMOT powered up while the VDD was turned off by mistake.

Heya thanks for the reply.
Does this mean that we have to calibrate the pololus?
We're beginners to the electrical side, would you mind stepping us through the link provided?

Thanks

> "catch on fire the other day,"


I have fired all of my ready made Pololu steppers because of one simple reason:

All pololus that I have buyed from the web marketplaces have equipped with 16 volt capacitors in stepper power input (biggest capacitors on board, possible 4.7uF, I quess).

They burst into flame at about 19-20 volt.

If using 12V PSU it still CAN be possible to have spikes at that level! Change the capacitors to 35 or 50 volt models, they do exist in market, but are more expensive tahn 16 models.

I am using 24V PSU to prevent loosing steps (and 10 ohm resistor in hotend).

Too often folks advice to increase stepper current to prevent loosing steps, the right way is to increase feeding voltage, because many motors sold in web have far too high inductance and that needs voltage to achieve enough current with high steping rates.

That is why I use 24 V PSU and I need only quite tiny current set without lost steps and still be using over 10 kHz steping rate (about 200 mm/s max feed rate).

Quote

All pololus that I have buyed from the web marketplaces have equipped with 16 volt capacitors in stepper power input (biggest capacitors on board, possible 4.7uF, I quess).

Original Pololus or StepSticks made by some hobbyist? You should send them back for refund.

Quote

Too often folks advice to increase stepper current to prevent loosing steps, the right way is to increase feeding voltage, because many motors sold in web have far too high inductance and that needs voltage to achieve enough current with high steping rates.

A lot of different motors are available and 12V are just fine to drive motors specified for 3 or 4 volts. It's true more current increases tourque and more voltage is needed only if you can't reach sufficient current.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Traumflug Wrote:
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> volts. It's true more current increases tourque
> and more voltage is needed only if you can't reach
> sufficient current.

Yes, that is basic knowledge in not running static world, which is not a problem for anybody.

But to know the real chopper controlled current of fast running motor, you should measure the coil current while motor is running at high speed stepping rate (with oscilloscope).
Stepper motors do have back EMF as other motors and that definitely will reduce coil current. Motor inductance value will tell that.

So, we are still in situation that high speed missing steps are not eliminated by increasing current setting, but increasing the feeding voltage. Increasing the desired current setting level above the maximum achieved level actuallay eliminates the correct microstepping operation.

Overvoltage feeding is just the reason for chopper regulated stepper topology at all.
In real word stepper driven machinery business, supply voltage can and should be as high as 10-70 times the nominal motor voltage.

3-4 V motor with 12V can not reach significantly high stepping rates with full torque regardless of the steady state current setting level.
Pololus current setting (Vref) does not inform you anything about the actual current slope endpoint level on each step.
And that endpoint level is what really causes the running torque.

Low inductance motors with about 2V nominal voltage does exists and they are well OK for 12 supply. Seach eBay for example.
That is seen on what users have told: somebody can drive reprap with 0.4A setting fast and smoothly, others may loose steps until max current fries pololus to smoke.

And of course I did not return the pololus. New 4.7uF/35V SMD capacitor price is less than 0.04 eur each, so it is no idea of sending them back. Even that Allegro chip is quite easy to change - although I have never managed to break any one of them. Power supply sequence does not harm the chip, Vmotor can be without Vcc.

I just want to inform that many pololu clones on the market may have unsuitable components at least higher operation voltages. And many reprap controllers developed by hobbyists does not have any protection circuits for overvoltages on their power supply lines. Some zeners or varistors shoud save most burned chips.
Sadly I do not know origin of my units, but I really hope that they were not original ones. They looks exactly similar than burned ones on this topic.

Edited 1 time(s). Last edit at 03/03/2012 07:17AM by mikropehmo.

Hello.

It sounds like mikrophemo is talking about clones, not boards actually produced by Pololu. Pololu uses power-input caps with voltage ratings well above the 35V maximum motor supply voltage.

- Ryan

Edited 1 time(s). Last edit at 03/05/2012 06:15PM by RyanTM.

Yes,

That is right. Definitely it was not original Pololu. Thanks for making it clear for everybody.
One example again that original products on the market are mostly good guality and too many clones may be scap.
And that scap is for nothing: price difference between 16V cap and 35 or 50V cap is just some cents!

Luckily, in my case, that scap were repairable quite easily, but sadly for others, it may be allmost impossible to find out SMD capacitors voltage rating only by looking the capacitor.

Because I am professional electronics design engineer, I am allways interestesteted in knowing why a product fails just for
getting and collecting knowledge, how to make it better.

And for helping others to prevent burning chips (which should never happens without direct short circuit):

First tests could be preferably done with a suitable resitor in series with stepper voltage supply line. From few ohms to 10-20 ohm resistor is OK. Current setting to minimum and then add it gradually. At some point (WERY LOW speed jogging mode) steppers should start to jog. It motor only turn forward and backward with ticking sound, one coil may be open or wire sequency is wrong.
Testing speed must be low, only ticking and rumbling type noise. If motors are whistling with high pitch when commanded to jog, that means that your step rate is too high for some program setup based reason.
In that case you may increase current too much waiting the motors to start, which will never happens anyway. If short circuit, that cheap resistor may burn protecting your more expensive drivers.
So stat with only few steps per secong speed. Then you'll see quite easily, if one coil is open or coil wire secuence is wrong.
And test all motors with that single driver first, then you'll be more sure to focus possible problems.

K