Re: L297/L298 And a "correction"

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Lester Caine <lester@...>
wrote:

>
> Wayne C. Gramlich wrote:
>
> > To the best of my knowledge the LMD18245 is just a
> > single H-bridge. For a bi-polar driver, you would need
> > two of these. Thus,
> >
> > 2 x L298 (Jameco: 245403; $2.95 for 1) = ~$6
>>
> Wrong - you only need one L298 + the output diodes and
> large current sense resistors, against 2 LMD's - parts
> cost comes to about 5GBP

I was correcting Alan's arithmetic where he was trying
to state that 1 LMD18245 did the same job as 2 L298's
wired together as 3.5A H-Bridges. Not true, 2 LMD18245's
are needed.

> > versus:
> >
> > 2 x LMD18245 (Digikey: LMD18245-ND $13.02 for 1) = ~$26
>
> The GBP price each is approaching 12GBP so we are looking
> at 24GBP but you do not need any of the output circuitry
> that you would include with the L298. I've just kitted for
> a batch of the EMS driver kits and even bulk buying the chips,
> nearly all the cost is the 2 chips :( The only slight plus
> is that the pic is cheaper than the L297 ;)

The true advantage of the PIC over the L297 is that the
PIC + DAC allows for microstepping.

> The main difference is however in the fact that the L298 is
> only 2A and needs to be kept below 46V and 36V seems a safer
> limit when using a raw DC supply, while the LMD18245 will
> handle 3A at 55V and is quite happy on a simple 50V supply.
> And of cause the LMD18245 microsteps!

First off, a single L298 can be hooked together to provide
an H-bridge of 3.5A. Thus, the L298 can do 3.5A at 48V and
the LMD18245 can do 3.0A at 55V. Neither 48V nor 55V are
convenient to produce. A 24V transformer produces ~35VDC
and a 36V transformer produces ~62VDC.

> The 160Ncm motors, which are a nice size to work with and a
> good price, run at 2.5A, so while the L298 will drive them,
> the LMD18245 will give them a lot more poke. Next step is to
> see exactly what the performance difference is :)

I'm not interested in the IPM drag races that people seem
to engage in on this mail list. Most people run their machines
at speeds and feeds that are way below the maximum preformance
of their driver boards. The maximum length of my Sherline
table is 13 inches -- I do not need or want 60 IPM rapids.
I see no reason to pay extra $$$ for an unneeded feature.

A 3 axis board built out of L298's capable of running
3.5A per axis at 48Vs would have the following major
parts count:

6 L298 $3 $18
6 Heatsink $1 $6
48 Schottky Diodes $.25 $12
6 Current sense Res. $.50 $3
6 Comparators $1 $3
1 DAC $5 $5
1 Microcontroller $6 $6
====================================
$53

A 3 axis board built out of LMD18245's capable of running
at 3.0A per axis at 55V would have the following major
parts count:

6 LMD18245 $13 $78
====================================
$78

I stand by my assertion that a system build around
the L298 is still cheaper than one built around
the L18245. Yes, the L298 design has a higher part
count, but the total parts cost is less.

-Wayne