Re: [CAD_CAM_EDM_DRO] Re: Question re: about current control schematic

grantfair2001 wrote:

>Jon-
>
>Sorry, last message got sent prematurely. I am willing to look for
>other circuits - any suggestions? I am interested in more current than
>what some chips like the LM298 (2 amps, I think) can provide.
>
>

Generally, a circuit which has 2 full-bridge drivers, with independent
current
sensing and PWM control of each one works best. You can use discrete
MOSFETs, they come in small packages, and there are drivers for them.

>And since I have the parts for this circuit, I might as well make one,
>more because I have invested the time on the PCB layout. I do have a
>use for chopped current control for a unipolar motor although I
>understand some of the shortcomings of this circuit which you outline.
>I don't know why it will be terrible at high speeds, can you explain?
>
>

The circuit in that drawing has no provisions for reversing the current in
the windings rapidly, so it takes a long time to switch the magnetic field
in each phase. Also, when one phase switches, it disturbs the field in
the other phase, since the current is set for the total of phase A +
phase B.

>I am confused about the use of diodes to deal with the current in
>coils which are turned off. I understand the need to put a diode
>accross a relay coil because of this. In the configuration shown will
>a diode between ground and the other end of each coil do what is
>needed? I have seen other arrangements (I think) but have not
>understood the reasons for one approach over another.
>
>

What you want is to recycle the spikes from the leakage inductance of
each coil back to the power supply. So, a diode from T1-T4's collectors
to the other end (common) of the coils, with the cathode facing the
common is what you want. I suspect the transistors will be destroyed
very quickly without this. To speed up the reversal of the phases, it
may be best to put a small resistor (perhaps 10 - 100 Ohms) in series
with the diodes.

Jon