Re: 10 amps to drive steppers

> From: "Elliot Burke" <elliot@...>
>
> I bought an old CNC mill and need to retrofit it. It has some ancient 3
> phase electronics, huge inductors, and so on to drive the steppers. That
> will all be replaced with modern electronics- the question is what
> electronics.

Is this a Bridgeport In the BOSS 3-6 range? If so I am retrofitting one now
using the Camtronics board (I hope!) If I read the BP documentation
correctly, the desired motor current is 8 amps.

> Can bigger transistors be put in the Camtronics stepper drivers to handle
> more current?

The transistors used are:

# RDSon(ohms) IDcont(A) IDmax(A) PDmax(W)
IRFZ34N 0.04 19 100 21
IRF9Z34N 0.1 -17 -68 56

This data from the DigiKey catalog.

So while there is room in IDcont for higher current at 4 and 10 watts apiece
(using 10 amps) cooling will be the issue. As the board is laid out now, the
eight transistors used for each motor are attached to a common heatsink which
is pretty large. I haven't calculated out the surface area/theta's/power
relationship, but forced air cooling is a must as the instructions that come
with the board say. It may be better in the end to remote mount these FETs on
a separate heatsink such as the giant one which already exists in the
Bridgeport's electronics cabinet. I asked Dan Mauch about running at higher
current and I think he said he had gone up to 8 amps before experiencing
problems. Also, IIRC, Dan Falck said in another post he had blown up his FETs
at 9 amps. Anyway, I'm in the process of trying this out and I'll let you
know how it goes. The only modification to the board needed to allow higher
current is to parallel the existing .1ohm sense resistors with another .1ohms
which will double the adjustment scale to 10 amps.

Wish Me Luck!

Matt