Determine switching frequency of photocoupler

[Andrew]

Hello,

I recently tried to use a Sharp PC3Q67Q photocoupler in my design. It
is digikey part number 425-1360-1-ND.

I am trying to use a 0-5V PWM input at 10kHz to the diode through a 500
ohm resistor (also tried 200 ohm resistor, same results), and have the
transistor output switch 12V.

The trouble I am having is that the PWM signal shows up across the
diode (pin 1, ref pin 2), but the output is strictly 12V. It seems
that the photocoupler is not able to switch at 10kHz? It does "blip"
and "dip" at a frequency of 10kHz, indicating that it is trying to
perform a square wave. This was with a 100k pulldown resistor. With a
lower pulldown resistor (3k) I was able to really see the output of the
photocoupler try to get to a square wave. The rise time is decent,
although not nearly square (roughly 6uSec), but the fall time is very
very slow. This causes the output to not be a square wave at all.

I'm not sure how exactly to solve my problem. It seems that the
rise/fall rate of the output transistor is too slow? I forget the
exact fall time value that I measured, but it was approximately 30uSec
(larger than the claimed 18uSec). I suspect this is because the 3k may
be still too large of a pulldown resistor? I did try a 1k before the
3k and that was worse. It was still not a square wave, and did not
work as well as the 3k. The claimed rise/fall times from the spec
sheet of the photocoupler were with a load of 100 ohms. I could try
this, but I am wondering if it is even worth it. Is it possible to
achieve a nearly perfect square wave on the output side of this
particular photocoupler?

So, basically, if not, I am wondering how to select a photocoupler that
is able to switch at this frequency? I COULD use a regular transistor
if I had to, but I would like to use a photocoupler still. At first I
had just looked at the max rise/fall times as being 18uS/18uS
respectively, and figured that this would allow a maximum operating
frequency of 27.77kHz. If this were, true, I wouldn't think I should
be having a problem switching at 10kHz.

Does anyone know why I am having my trouble, and what I might do to
learn more about selecting these products in the future?

Oh yeah, each output is driving 2 IRF1405 (IRF1405-ND) MOSFETs, well,
the gate of them anyway. That is what the pulldown resistor is for. I
did disconnect the gate from the circuit and just did a pull-down
resistor to ground with the same results, so I doubt this is the
problem!

Thanks alot for any suggestions,
Andrew

 

[[email protected]]

Your load resistor is much too high.

The data sheet - at http://www.smd.ru/files/upload/1241/ru/optsharp.pdf

specifies the 4usec typical 18usec maximum rise and fall times with a
100R load resistor and a 2mA current - thus at a voltage swing of 0.2V.
The capacitance and the Miller capacitance of the output transistor is
obviously pretty high, which is common in cheap opto-couplers.
Typically they use a Darlington-connected pair to boost the output
current.

You can get much faster opto-couplers. The Agilent HCPL-2400 - for
which they claim 40 MBd operation - has been around for many years.
Back around 1984 we had a bunch of these on a board to isolate a 12-bit
wide parallel bus, operating at rates of up to 10MHz.

I got stuck with the job of getting the link to work again after some
idiot in marketing had promised the customer that he could stick 18
meters of ribbon cable between the TTL drivers and the board carrying
the opto-couplers, and was able to rework the boards so that we
guarantee that the system would work with worst-case tolerance
components over the full commercial temperature range.

A few years later, one of the boards did stop working properly when it
got hot, but the offending HCPL-2400 didn't match the data sheet and
was sent back to the Agilent (then Hewlett-Packard) Quality Control
department. Dropping a new part into the socket solved the problem.

Farnell also list "high speed"opto couplers from Infineon, who only
claim 5MBd operation for their "logic output" parts, which are even
more expensive than the Agilent parts, at about $7 in small quantities.
I've never had an opportunity to play with them.

 

[John Popelish]

Hello,

I recently tried to use a Sharp PC3Q67Q photocoupler in my design. It
is digikey part number 425-1360-1-ND.

I am trying to use a 0-5V PWM input at 10kHz to the diode through a 500
ohm resistor (also tried 200 ohm resistor, same results), and have the
transistor output switch 12V.

(snip)
This coupler has no external connection to the photo transistor base,
so you have no way of bleeding the stored charge from the base to
trade off transfer current gain for speed. The best you can do is to
load the transistor with a low value of load resistance.

But even as low as 1k, the total of rise, fall, storage and delay
times amounts to about 24+24+2.2+6us=56.2us The frequency that just
allows time for all that is 17.8kHz. That doesn't leave much more for
signal swing at 10kHz, but you might just get away with it.

The key to pushing the speed involves minimizing the voltage change
across the photo transistor, so that the internal capacitances are
hardly involved. You might connect this transistor between the 12
volt supply and the base of another transistor, with the emitter of
that transistor at zero, and a low value resistor between base and
emitter. That way, the photo transistor has only to produce a .6 volt
swing or so to switch the second transistor. An even faster circuit
uses the photo transistor as one side of a differential amplifier pair
(or uses two of the photo transistors with their LEDs driven
alternately). But you still need additional parts to produce a 12
volt swing. Probably the simplest would involve a comparator.

If you look for a substitute device, look for one that brings the base
of the photo transistor out to a pin, so you can load the base to
emitter with a resistor. That will shorten both the storage and fall
time quite a bit, while slightly slowing the delay and rise times. It
will also lower the current that can be switched by the photo
transistor, so you may still need the second transistor.

You can also change to an internally shielded logic driver output type
coupler, since these generally have much faster switching speeds.

 

[[email protected]]

Hello,

I recently tried to use a Sharp PC3Q67Q photocoupler in my design. It
is digikey part number 425-1360-1-ND.

I am trying to use a 0-5V PWM input at 10kHz to the diode through a 500
ohm resistor (also tried 200 ohm resistor, same results), and have the
transistor output switch 12V.

Use two of the couplers. Invert the drive signal so that you have two
phases 180 degrees apart. Use a 200 ohm resistor between each drive phase and
the input diode on each coupler. Connect the two outputs in series between the
12 volt supply and ground. Take your output from the common connection between
the couplers. This is called a "totem pole" connection.

Somebody will probably try to warn you about "shoot-through glitches".
Since the output switches are current limited, there will be no glitches.

Jim

 

[Fred Bloggs]

The key to pushing the speed involves minimizing the voltage change
across the photo transistor, so that the internal capacitances are
hardly involved. You might connect this transistor between the 12 volt
supply and the base of another transistor, with the emitter of that
transistor at zero, and a low value resistor between base and emitter.
That way, the photo transistor has only to produce a .6 volt swing or so
to switch the second transistor. An even faster circuit uses the photo
transistor as one side of a differential amplifier pair (or uses two of
the photo transistors with their LEDs driven alternately). But you
still need additional parts to produce a 12 volt swing. Probably the
simplest would involve a comparator.

Nice suggestions- but add a little non-linearity and that part will
easily handle 10KHz...even allowing for 5nF Ciss on that monstrous MOSFET.
View in a fixed-width font such as Courier.

..
.. +12V
.. |
.. -------- +------+------+
.. | | | |
.. LOGIC | | | |
.. IN | | / /
.. --- -------- 10K 1K
.. / /
.. MOSFET -> | ON | OFF | \ 10V \
.. | zener |
.. | / |
.. +--/\/\--+--------+ +-----|>|-----+-----> GATE
.. | 220 | | | / |
.. | / | | 1N5240B |
.. | 2.2K | | c |
.. | / | | |/
.. | \ | +-----|>|---|2N3904
.. | 0.022U | | PC3Q67Q | 1N4148 |\ MOSFET
.. +---||---+ --| - O.C.- c - e
.. | | --- |/ | |
.. c | / \ ~ ~ ~ | | | |
.. 4.7K |/ | --- |\ | |
.. LOGIC>-+--/\/\-+-|2N3904 ---|--------- e - |
.. INPUT | | |\ | | |
.. +--||---+ e +5V +------+------+-----> SOURCE
.. 100p | |
.. --- 12V COM
.. ///
..
..

 

[Fred Bloggs]

Nice suggestions- but add a little non-linearity and that part will
easily handle 10KHz...even allowing for 5nF Ciss on that monstrous MOSFET.
View in a fixed-width font such as Courier.

.
. +12V
. |
. -------- +------+------+
. | | | |
. LOGIC | | | |
. IN | | / /
. --- -------- 10K 1K
. / /
. MOSFET -> | ON | OFF | \ 10V \
. | zener |
. | / |
. +--/\/\--+--------+ +-----|>|-----+-----> GATE
. | 220 | | | / |
. | / | | 1N5240B |
. | 2.2K | | c |
. | / | | |/
. | \ | +-----|>|---|2N3904
. | 0.022U | | PC3Q67Q | 1N4148 |\ MOSFET
. +---||---+ --| - O.C.- c - e
. | | --- |/ | |
. c | / \ ~ ~ ~ | | | |
. 4.7K |/ | --- |\ | |
. LOGIC>-+--/\/\-+-|2N3904 ---|--------- e - |
. INPUT | | |\ | | |
. +--||---+ e +5V +------+------+-----> SOURCE
. 100p | |
. --- 12V COM
. ///
.
.

But for a PWM app- he can expect something like +/-20% deviation about
50% duty cycle- may or may not be so good to use in that respect...

 

[Andrew]

Thanks for all your posts. I was hoping to find a direct 16 pin smd
replacement that would work (or change load resistor), but it seems
that a redesign is needed.

It is not imperative that logic side and 12V side remain isolated, so
with that in mind, would I be better off (speed wise) just to use
regular transistors instead of an optocoupler?

Also, John, would you mind telling me how you calculated the rise,
fall, storage, and delay times you posted?

Fred, your redesign seems to make some sense, but if you wouldn't mind
taking a few minutes to explain what the parts do and why, I would
appreciate it! I can figure out alot of it, but some of it I'm not
quite sure why it is necessary, and I don't like to utilize designs
without knowing the "why" behind them.

Thanks again guys!

 

[John Popelish]

Also, John, would you mind telling me how you calculated the rise,
fall, storage, and delay times you posted?

I read the data off the data sheet graph. By the way, all those times
refer to saturated switching. If you use the coupler in an analog
design (LED always on, but varying in intensity, transistor always on,
but varying in conduction, followed by a comparator or other i bit A/D
converter), none of those times apply, directly. The linear process
is much faster than saturated switching.

 

[Andrew]

Thanks John, I wasn't familiar with how to read that graph and that the
values stacked, but now I see exactly what you mean.

Any thoughts on using a regular transistor instead of an optocoupler in
order to boost speed?

 

[Rich Grise]

Thanks John, I wasn't familiar with how to read that graph and that the
values stacked, but now I see exactly what you mean.

Any thoughts on using a regular transistor instead of an optocoupler in
order to boost speed?

As long as the grounds don't have to be isolated, then that's the
logical thing to do.

Good Luck!
Rich

 

[Fred Bloggs]

Fred, your redesign seems to make some sense, but if you wouldn't mind
taking a few minutes to explain what the parts do and why, I would
appreciate it! I can figure out alot of it, but some of it I'm not
quite sure why it is necessary, and I don't like to utilize designs
without knowing the "why" behind them.

If there is no reason for isolation, then you don't need it. Ideally,
you have a common-emitter transistor amplifier, base driven by
phototransistor current source, and nonlinear negative feedback through
the zener diode. Negative feedback generally means a portion of the
output is fed back to oppose an input signal causing the output to
change. But since the zener only conducts at the end operating points of
Vce=0.6V and Vce=10V, there is no negative feedback while the transistor
is transitioning between those two operating points, making the
transition fast(er). When the phototransistor is off, the 10K resistor
supplies enough current to the transistor base to drive it to operating
point Vce=0.6V. When the phototransistor is turned on, it will shunt the
10K resistor current away from the base causing the transistor to
conduct less so that Vce rises. Vce continues to rise until the zener
diode conducts reverse current into the phototransistor collector of
large enough magnitude to prevent the phototransistor from saturating.
The output then stabilizes at the Vce high point where the sum of zener
and 10K resistor currents equals the phototransistor sink current. You
can figure out what happens when the phototransistor turns off. In each
steady state operating point condition, the junction of zener anode, 10K
resistor, and phototransistor collector, remains at ~1.3V, and this
makes for fastest phototransistor speed.

 

[Fred Bloggs]

I read the data off the data sheet graph. By the way, all those times
refer to saturated switching.

Nope- the test circuit was a 100 ohm resistor, 2V collector supply, and
2mA collector current switching. They have another graph showing the
four tx's as a function of collector resistance under same conditions.
The worst is td, the delay time, and this accounts for time it takes
that weak photon controlled Icbo to charge the b-e junction to 0.6V or
so. ts corresponds to base saturation charge bleed down into active
region, tf and tr have usual meaning.

If you use the coupler in an analog
design (LED always on, but varying in intensity, transistor always on,
but varying in conduction, followed by a comparator or other i bit A/D
converter), none of those times apply, directly. The linear process is
much faster than saturated switching.

The only problem there is an 8:1 variation in CTR at any fixed current
in the usable range of 1ma to 10ma. It seemed easier to reduce td by
injecting a large transient current into the diode.

 

[Fred Bloggs]

Thanks John, I wasn't familiar with how to read that graph and that the
values stacked, but now I see exactly what you mean.

Any thoughts on using a regular transistor instead of an optocoupler in
order to boost speed?

View in a fixed-width font such as Courier.

..
..
.. ------
.. | |
.. LOGIC | |
.. IN | |
.. --- --------
..
.. MOSFET -> | OFF | ON |
..
.. +12V
.. |
.. +------------+-------+
.. | c |
.. / |/ |
.. 2.2K +--|2N4401 ===
.. / | |\ 0.1u
.. \ | e |
.. | | | |
.. +-------+ +---------------> GATE
.. | | | |
.. c | e |
.. 4.7K |/ | |/ | MOSFET
.. LOGIC>-+--/\/\-+-|2N4401 +--|2N4403 |
.. INPUT | | |\ |\ |
.. +--||---+ e c |
.. 100p | | |
.. +------------+-------+--------> SOURCE
.. |
.. ---
.. ///
..
..
..

 

[Andrew]

There really isn't a need to isolate them. I just liked the fact that
the optocoupler was a single IC chip as opposed to multiple
transistors. I just thought it would be fast enough!

At any rate, I guess I will switch to no isolation with a design like
Fred suggested.

I have a few more questions though, if anyone is still following...

This logic driving MOSFETs is for an H-bridge motor driver. There is a
Motorola HCS12 microprocessor providing the logic PWM driving signal,
and the MOSFETS are IRF1405 (large), digikey part IRF1405-nd.

The remainder of this h-bridge design is simple 4 of these MOSFETs set
up in a typical h-bridge. Two drains are tied to 24V, their two
sources are tied to the drains of the other two MOSFETs, whose sources
are 24V ground (system ground for 5V, 12V, and 24V). The motor output
is taken between the common source-drain connections of the MOSFETs.
These MOSFETs do have built in recovery diodes.

The logic is continually driving all four MOSFETs. One of 3 scenarios
is possible. The upper left and bottom right on (motor "forward") in
this case the upper right and bottom left are off by driving zero pwm
signal, the upper right and bottom left are on (motor "backward") in
this case the upper left and bottom right are off by driving zero pwm
signal, or all four MOSFETs are off by driving zero pwm signal. There
is one PWM signal and one "direction" bit from the processor, which
other hardware assists in creating the PWM inputs to each gate. The
opposite corner gates are tied together (upper right and bottom left,
and upper left and bottom right).

I have been testing this circuit using the original optocoupler with a
3k pulldown into the gates of the MOSFETs. The signal into the
optocoupler is clean, the signal out (going into the gates) is more
sine wave like than square, and is the part I will be cleaning up with
Fred's suggestions, but even with a sine wave it will turn a small
unloaded motor in each direction properly. Everything looked pretty
good.

When I hooked the driver up with the same setup into a bigger motor,
which had a small load on it, everytime I had the processor send the
signals to turn the motor in one direction or another, the processor
would reset, and the driver would not output the correct signal. I
haven't had a chance (yet) to do further testing to find out why it is
resetting, but I'm having some trouble coming up with initial guesses.

One problem that I did notice when looking at the gates of the MOSFETs
when driving them, is that the MOSFETs that *should* be off (by driving
zero PWM signal into the optocoupler diode) are seeing some noise (up
to nearly 5V) at the 10kHz frequency that the opposite MOSFETs are
being driven at. The gates of these "off" MOSFETs are connected to the
collector of the optotransistor (which should not be conducting), and a
3k resistor to ground. This should mean they are grounded, yet they
are seeing this noise. This made me think that for some reason
something is going on with my ground, and this could potentially cause
the processor to resetting. I did not see resetting on testing the
small motor with no load, but on the larger motor, I did.

What could the problem be with my design? Unfortunately the 5V, 12V,
and 24V commons cannot be isolated, and up to this point with many
other components, there has not been any issues.

An additional problem with the h bridge design is that when the MOSFETs
are suddenly turned off when the motor is on and moving a load, the
motor's back EMF will need to be handled. I have no determined exactly
what the effects of this voltage are in this design, but I will need to
in the future. First thing is to get the processor to stop resetting.

Sorry for such a long post, and for asking some silly questions (or
having silly problems, if they are), but I am pretty new to the EE
scene and many "tricks of the trade" I have yet to learn

 

[Andrew]

I just tested my h bridge again. Just as I suspected it appears that
the ground is very noise. With the scope probe "grounded" at the 24V
ground (right at the battery), and the probe at the ground closest to
the processor I am seeing alot of noise, only when driving the motor.
If the motor is disconnected, I do not get the noise.

If it is applicable, the noise signal is at about 8Hz, with like 13% of
the pulse very rapidly going from ~5.5V to ~-4V, the rest of the pulse
being ~0V. So basically 16ms of rapid 5.5V to 4V switching then 109ms
of 0V (which it should be). The processor does not like it when this
happens, of course, and resets.

I cannot figure out what is causing this!!

I did switch the pwm duty cycle from roughly 50% to 100% so it is no
longer PWM, and I received similar results. The motor did, however,
attempt to respond correctly although from the sound of the motor it
was receiving a very noise signal still. This is the setup I used to
measure the ground noise.

Any thoughts on why I am receiving such terrible ground noise?
Everything looked like it would work so well on paper (and still does
haha)!

 

[John Popelish]

There really isn't a need to isolate them. I just liked the fact that
the optocoupler was a single IC chip as opposed to multiple
transistors. I just thought it would be fast enough!

At any rate, I guess I will switch to no isolation with a design like
Fred suggested.

I have a few more questions though, if anyone is still following...

This logic driving MOSFETs is for an H-bridge motor driver. There is a
Motorola HCS12 microprocessor providing the logic PWM driving signal,
and the MOSFETS are IRF1405 (large), digikey part IRF1405-nd.

The remainder of this h-bridge design is simple 4 of these MOSFETs set
up in a typical h-bridge. Two drains are tied to 24V, their two
sources are tied to the drains of the other two MOSFETs, whose sources
are 24V ground (system ground for 5V, 12V, and 24V). The motor output
is taken between the common source-drain connections of the MOSFETs.
These MOSFETs do have built in recovery diodes.

The logic is continually driving all four MOSFETs. One of 3 scenarios
is possible. The upper left and bottom right on (motor "forward") in
this case the upper right and bottom left are off by driving zero pwm
signal, the upper right and bottom left are on (motor "backward") in
this case the upper left and bottom right are off by driving zero pwm
signal, or all four MOSFETs are off by driving zero pwm signal. There
is one PWM signal and one "direction" bit from the processor, which
other hardware assists in creating the PWM inputs to each gate. The
opposite corner gates are tied together (upper right and bottom left,
and upper left and bottom right).

I have been testing this circuit using the original optocoupler with a
3k pulldown into the gates of the MOSFETs. The signal into the
optocoupler is clean, the signal out (going into the gates) is more
sine wave like than square, and is the part I will be cleaning up with
Fred's suggestions, but even with a sine wave it will turn a small
unloaded motor in each direction properly. Everything looked pretty
good.

When I hooked the driver up with the same setup into a bigger motor,
which had a small load on it, everytime I had the processor send the
signals to turn the motor in one direction or another, the processor
would reset, and the driver would not output the correct signal. I
haven't had a chance (yet) to do further testing to find out why it is
resetting, but I'm having some trouble coming up with initial guesses.

One problem that I did notice when looking at the gates of the MOSFETs
when driving them, is that the MOSFETs that *should* be off (by driving
zero PWM signal into the optocoupler diode) are seeing some noise (up
to nearly 5V) at the 10kHz frequency that the opposite MOSFETs are
being driven at. The gates of these "off" MOSFETs are connected to the
collector of the optotransistor (which should not be conducting), and a
3k resistor to ground. This should mean they are grounded, yet they
are seeing this noise. This made me think that for some reason
something is going on with my ground, and this could potentially cause
the processor to resetting. I did not see resetting on testing the
small motor with no load, but on the larger motor, I did.

What could the problem be with my design? Unfortunately the 5V, 12V,
and 24V commons cannot be isolated, and up to this point with many
other components, there has not been any issues.

An additional problem with the h bridge design is that when the MOSFETs
are suddenly turned off when the motor is on and moving a load, the
motor's back EMF will need to be handled. I have no determined exactly
what the effects of this voltage are in this design, but I will need to
in the future. First thing is to get the processor to stop resetting.

Sorry for such a long post, and for asking some silly questions (or
having silly problems, if they are), but I am pretty new to the EE
scene and many "tricks of the trade" I have yet to learn

You might get some very useful suggestions on how to improve this
circuit if you post a schematic to alt.binaries,schematics.electronic
or to a web page and provide a url. Without seeing that schematic, my
first question for you is what kind of bypass capacitor arrangement do
you have directly across the power nodes of the H bridge?

 

[Andrew]

I posted a schematic of the h bridge as it is now here:
http://www.geocities.com/hiddenassassins/h_bridge.JPG

I know there are probably many ways to do alot of what I am doing in
the circuit, and my way may not be the most efficient or anything, but
unless it is the cause of problems, I need to focus on what the real
problems are!

The PROX stuff is for proximity sensors, normally closed. They are not
tripping so assume them to be closed switches. All jumpers are in
place. The reason the GND24 is jumped to GND was a quick fix to adjust
trace widths. These grounds could have been connected directly. Same
deal with the "current sensing" jumpers. I have tested the circuit
with these current sensing jumpers removed also, which should
theoretically remove any current limiting, and the results were the
same.

Basically, with the current sensing/limiting I was hoping to amplify
the voltage across the current sensing resistors R13 and R14 in order
to input this into the comparator who's output turns the pwm signal on
and off (rapidly under overcurrent condition). R16/R18 vary the limit.
This was just something I thought of quick and probably is a stupid
way to do it, but it worked in another similar design, so I kept it (at
least the comparator part, I did not need to amplify anything in the
previous design).

As you can see, it is not necessary to use the optocoupler, but it was
easy to use a single IC for this, and I thought it would work. The
schematic indicates PS2501-4, but in fact I am using the Sharp PC3Q67Q
..

Also note that R1-R8 indicate 100k but are now 3k.

Hopefully the schematic is not too hard to follow.

Any thoughts on the ground issues I am having? I just can't get the
idea out of my head that somehow the MOSFETs are to blame, like
something with the gate sourcing some current to ground, or something??

 

[Andrew]

Forgot to mention 10uF capacitors from CURR_SENSE_1_A to ground and
from CURR_SENSE_2_A to ground and from COMPIN1 to ground and from
COMPIN2 to ground. These helped clean up the current sensing. The
voltage output of the op amp does indeed rise as the current does, and
does trip the comparator which does turn the pwm off. The whole thing
seems to work ok, just forgot to mention I added those caps on.

 

[John Popelish]

I posted a schematic of the h bridge as it is now here:
http://www.geocities.com/hiddenassassins/h_bridge.JPG

Excellent. A thousand words saved.

I know there are probably many ways to do alot of what I am doing in
the circuit, and my way may not be the most efficient or anything, but
unless it is the cause of problems, I need to focus on what the real
problems are!
Understood.

The PROX stuff is for proximity sensors, normally closed. They are not
tripping so assume them to be closed switches. All jumpers are in
place. The reason the GND24 is jumped to GND was a quick fix to adjust
trace widths. These grounds could have been connected directly. Same
deal with the "current sensing" jumpers. I have tested the circuit
with these current sensing jumpers removed also, which should
theoretically remove any current limiting, and the results were the
same.

Basically, with the current sensing/limiting I was hoping to amplify
the voltage across the current sensing resistors R13 and R14 in order
to input this into the comparator who's output turns the pwm signal on
and off (rapidly under overcurrent condition). R16/R18 vary the limit.
This was just something I thought of quick and probably is a stupid
way to do it, but it worked in another similar design, so I kept it (at
least the comparator part, I did not need to amplify anything in the
previous design).

The big problem with your current sense design is that you have not
taken the ground path resistance and inductance into account so your
sense amplifiers are amplifying that path voltage drop (in response to
the large and fast currents through the bridges). First, you can
greatly reduce the high frequency content of the current in these
paths by giving it an alternate, low inductance path. This involves
connecting a low internal series resistance and inductance capacitor
directly between, for instance, the drains of Q1 and 2 and the bottom
of R13, to act as a local power supply. You can enhance the effect of
this capacitor (keep more of the high frequency current in this local
loop) by adding some intentional impedance in the +24 volt line. A
small inductance, for instance. You should also return the high
current lines back to the 24 volt supply, and not pass these currents
through any lines that include the signal path.

You also need to bypass the 5 volt supply right across the sense
amplifiers, comparators and TTL logic chips.

But there will always be some instantaneous difference between
different parts of the ground system. So you can make your sense
amplifiers more noise immune by configuring them as subtracters and
subtract the voltage at one end of the sense resistor from the voltage
at the other end. Your sense amplifiers are programmed for a gain of
250. Add a similar pair of resistors to the non inverting inputs,
with the 1k resistor going to the top end of the sense resistor and
the 249k connected from the non inverting input to the local signal
ground used by the comparators (which I see are referenced to ground
at the moment, indicating a current trip of zero). then reconnect the
1k that is shown grounded to the grounded end of the sense resistor.

This implies that there are two connections to each end of these
resistors. One that carrys the current and one that connects to the
sense amplifier but does not carry current. If you are working with a
plug board, I suggest you solder an extra lead onto each end of the
sense resistors, so you can use separate sockets for the current
carrying path and the signal path, so the contact resistance of the
socket is not included in the sensing operation.

Once you get the ground noise problems under control, you will be in a
much better position to deal with comparing various ways to drive the
bridge mosfets.

As you can see, it is not necessary to use the optocoupler, but it was
easy to use a single IC for this, and I thought it would work. The
schematic indicates PS2501-4, but in fact I am using the Sharp PC3Q67Q
.

Also note that R1-R8 indicate 100k but are now 3k.

Hopefully the schematic is not too hard to follow.

It is very helpful.

 

[Andrew]

John, thanks for your quick reply!

I'm not sure how to quote your text so I'll just write what you wrote
and respond under it...

"You also need to bypass the 5 volt supply right across the sense

amplifiers, comparators and TTL logic chips."

I'm not sure exactly what you mean by this?

Also, do you suspect that current sensing in this fashion is the cause
of my ground noise problems?

When I was using a small motor (I guess the noise wasn't enough to
reset the processor), I varied the current by varying the load, and the
current sensing seemed to work the way I thought. I will try some of
your suggestions to clean it up, but I only needed the current limit as
a very crude way to protect the motor from drawing too much current.

I thought that by removing the jumper from CURR_SENSE_1_A to
CURR_SENSE_1_B and from CURR_SENSE_2_A to CURR_SENSE_2_B I was
virtually eliminating this part of the circuit from causing any
problems. The only thing left after doing this was the 2milli-ohm
current sense resistor in series with the motor, which shouldn't have a
noticeable effect.

Again, the problem is only when a motor is connected. With no motor,
everything *looks* ok. The "large" motor is about 316milli-ohms and
like 80mH. I'm not sure the specs of the smaller motor. In both cases
I suspect there is noise, but it only resets the processor with the
larger motor.

Unless you meant that my ground noise issues are likely caused by the
current sensing stuff (even with the jumpers removed), do you have any
further ideas on what the problem could be?