Wednesday, December 9, 2009

Starting at 0,0,0


So I have decided to take on the project of building a McWire in order to RepStrap up to a Mendel. I'm starting to gather parts so I can start and hopefully finish building the majority of the structure over my Christmas holiday. Until then I have set out to study the various electronic set ups. All seems pretty straight forward. I did, however, find a small mistake on the PWM Driver 1.1.

In reviewing the schematic, I have found that the free wheel diodes seem to be connected incorrectly. The free wheel diodes are D1, D2 and D3 that are the 1n4004. These diodes are used to deal with inductive loads. Inductive loads such as a relay coil will want the current to keep flowing until the inductive energy within the load is dissipated. These diodes, as hooked up now, are not creating a path to deal with this energy. If an inductive load is used, most likely there will be a large voltage spike on the collector of the TIP50 transistors. V=L*dI/dT so the faster it is turned off, the higher the current or the larger the inductance the higher that spike will be.

If noise has been an issue for your system or your burning up PWM drivers this may be a major contributor.

I would recommend moving the diode or adding an additional diode. The hook-up would be anode the the TIP50 collector and cathode to Vss.


The existing diodes are fine where they are but aren't really doing anything.

2 comments:

Jonathan said...

I second your analysis.

I was confused at first since the original position seemed familiar: this is how the diodes are (and must) be placed on a h-bridge driver.

However since the other 3 diodes are missing this
don't work for single transistor setups.
Your correction will work properly.

You might want to report the problem here too: http://reprap.org/bin/view/Main/PWM_Driver_1_1#Known_Bugs

bjbsquared said...

Thanks for the Link.

The know bugs link directed me to the forums. I made a new topic under the electronics section and describing the issue.

I think that should do it.