Re: Motor current measurement

If you want to see what's going on, you need to use the scope. The
current waveform will be a sawtooth as the current ramps up linearly
to the set value, which turns off the L298. The current then decays
exponentially for a fixed time (set by the L297 - or equivalent -
oscillator frequency), when the L298 is turned on again. That is of
course, if you are using the L298 in chopped mode, not DC.

A DVM is useless for seeing the peak value, though of course a DVM in
the power supply lead to the L298 will show the average (or RMS)
current your PSU is delivering, and this will depend upon chopping
frequency, among other things.

The L298 is usually used with a current sensing resistor between the
emitters and ground. The sensed voltage is fed to the PWM chip (L297
commonly) to turn off the current when it reached peak. For the
L297, the sensed voltage for turnoff is the same as the ref voltage
you feed to it (not so for some other chips), so you need to set the
ref voltage amd the value of the sense resistor in conjunction.
E.G., I use a 0.1 ohm resistor and a 280mV ref voltage to set the
peak current to 2.8A (I am using a pair of L298s, to get to this
current level). Anyhow, you should check the ref voltage and the
sensed voltage, in case you have any nasty gound-induced noise
problems: 280mV is not a lot when 2.8A is getting switched (3
times!) very close by. A scope across the sense resistor will
measure the current, but be careful of ground problems there too. A
grounded scope and a grounded power supply can cause noise problems
that totally lie about what is really happening.

BTW, my L298s driving 2.8A don't get too hot either, though it
depends totally on the heatsink area. The NEMA 23 motors get warm
also, but not hot. At 1A peak, the most the L298 will be dissipating
will be 8W peak, maybe much less average.

Hope this helps,

Jed NF1Z

--- In CAD_CAM_EDM_DRO@y..., "Ian W. Wright" <Ian@i...> wrote:
> Thanks,
>
> It is the fluctuating nature of the current at the output that was
> concerning me. I do have an old, simple scope - I haven't used it
in years
> so I guess I'll just have to get it out and dust it off and try to
remember
> where the on-switch is!
>
> The driver boards I'm playing with are the 'modular' design ones by
(Hans?)
> which have a screw pot to adjust the current. I have them coupled
up to some
> 4volt 1.1amp motors and I'm feeding them with 36volts. I set the
sense
> current up to what I think should be 1amp output and everything
works OK but
> neither the motors or the L298s get even remotely warm. I know this
is a
> good thing but it made me wonder whether I am actually getting the
1amp
> output and whether the motors should be giving me more 'push'.
> I was a bit wary of just disconnecting one motor lead and putting a
> multimeter in series in casied the chip..
>
> Best wishes,
> Ian
> --
>
> Ian W. Wright
> Sheffield, UK
>
>
> The following added for automatic email harvesters!
> abuse@w... abuse@p... abuse@b... abuse@y...
> UCE@F... abuse@t... abuse@f...
>
> ----- Original Message -----
> From: "Jon Elson" <elson@p...>
> To: <CAD_CAM_EDM_DRO@y...>
> Sent: Friday, November 22, 2002 6:38 AM
> Subject: Re: [CAD_CAM_EDM_DRO] Motor current measurement
>
>
> >
> >
> > Alan Marconett KM6VV wrote:
> >
> > >Hi Ian,
> > >
> > >A current probe for a 'scope will do the job. Or put small value
> > >resistors in series with the leads, and 'scope across them.
> > >
> > >Alan KM6VV
> > >
> > >
> > >"Ian W. Wright" wrote:
> > >
> > >
> > >>Hi,
> > >>
> > >>This may seem like a daft question but how does one measure the
current
> > >>going to a stepper motor when setting up a driver board
(L297/298
> > >>combination)?
> > >>
> > >>
> > The L298 is a full-bridge driver, if I recall. You don't have any
> > grounded lead to use in
> > that case, so it becomes a fully floating measurement. A DVM can
be
> > used pretty easily
> > for that, although some DVMs are very sensitive to pulse-like
signals
> > and may give
> > inaccurate or wildly fluctuating readings. The current sense
resistor
> > is from the junction of the
> > lower transistors to ground, so one end of it is grounded. That
is the
> > best point to measure,
> > BUT, you have to measure the peaks, not the average. That
generally
> > requires a scope.
> > What you will see is short bursts of motor current between longer
> > periods of no current.
> > When the transistors are on, you see the motor current. When the
> > transistors are off, you
> > see no current.
> >
> > Jon
> >
> >
> > Addresses:
> > FAQ: http://www.ktmarketing.com/faq.html
> > FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> > Post Messages: CAD_CAM_EDM_DRO@y...
> >
> > Subscribe: CAD_CAM_EDM_DRO-subscribe@y...
> > Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@y...
> > List owner: CAD_CAM_EDM_DRO-owner@y..., wanliker@a...
> > Moderator: jmelson@a... timg@k... [Moderator]
> > URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
> >
> > OFF Topic POSTS: General Machining
> > If you wish to post on unlimited OT subjects goto:
> aol://5863:126/rec.crafts.metalworking or go thru Google.com to
reach it if
> you have trouble.
> > http://www.metalworking.com/news_servers.html
> >
> > http://groups.yahoo.com/group/jobshophomeshop I consider this
to be a
> sister site to the CCED group, as many of the same members are
there, for OT
> subjects, that are not allowed on the CCED list.
> >
> > NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY
POSTING THEM.
> DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> > bill
> > List Mom
> > List Owner
> >
> >
> >
> > Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
> >
> >
> >