motor driver circuit

Started by Vecheslav Silagadze March 29, 2004
Hello everone,

I'm looking at the output from an H-bridge to a DC motor, run with PWM, and
it looks like there is quite a bit of EMF kickback even though I put all the
requisite (schotkey) diodes in place. The motor still works, but I expect
there is a lot of energy being lost there. Is there anything I can do? any
common mistakes?
I was thinking of putting zener diodes between the motor leads to achieve
the same effect as the diodes to ground and v+ are supposed to achieve.

I know this isn't really a PIC topic, but I assumed many people on the list
have experience with the above. Plus, if it helps, the H-bridge is being run
using a PIC (and a fancy 18F458 at that.)

Vecheslav Silagadze

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There are several things you can do;
Instead of adding extra diodes, use transistors with built-in reverse zeners ( MOSFET or IGBT).
There will be some loss in these diodes in a high power application.
If you're into even higher power you could use a clever scheme and use the switches themselves
to move the back-EMF to the powersupply.

But for power 0-500W I'd go for the simplest solution and use transistors with buildt-in diodes.

Vecheslav Silagadze wrote:

 Hello everone,

I'm looking at the output from an H-bridge to a DC motor, run with PWM, and
it looks like there is quite a bit of EMF kickback even though I put all the
requisite (schotkey) diodes in place. The motor still works, but I expect
there is a lot of energy being lost there. Is there anything I can do? any
common mistakes?
I was thinking of putting zener diodes between the motor leads to achieve
the same effect as the diodes to ground and v+ are supposed to achieve.

I know this isn't really a PIC topic, but I assumed many people on the list
have experience with the above. Plus, if it helps, the H-bridge is being run
using a PIC (and a fancy 18F458 at that.)

Vecheslav Silagadze

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--
*******************************************
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*******************************************
Best Regards
Eirik Karlsen
 

It sounds like you have made a common omission. Don't forget the
noise suppression capacitors on the motor if they are not already
built in (and they usually aren't except for some modern RC car
motors). You should solder a 0.1 uF capacitor between each lead and
the case of the motor (2 capacitors total) and a 0.1 uF capacitor
between the 2 motor leads. Any ceramic disc capacitor will work, but
if you're buying new ones, try to get high-frequency ceramic caps.

BRW

--- In , "Vecheslav Silagadze"
<uwobjectivist@h...> wrote:
> Hello everone,
>
> I'm looking at the output from an H-bridge to a DC motor, run with
PWM, and
> it looks like there is quite a bit of EMF kickback even though I
put all the
> requisite (schotkey) diodes in place. The motor still works, but I
expect
> there is a lot of energy being lost there. Is there anything I can
do? any
> common mistakes?
> I was thinking of putting zener diodes between the motor leads to
achieve
> the same effect as the diodes to ground and v+ are supposed to
achieve.
>
> I know this isn't really a PIC topic, but I assumed many people on
the list
> have experience with the above. Plus, if it helps, the H-bridge is
being run
> using a PIC (and a fancy 18F458 at that.)
>
> Vecheslav Silagadze
>
> _________________________________________________________________
> MSN Premium helps eliminate e-mail viruses. Get 2 months FREE*
> http://join.msn.com/?pgmarket=en-
ca&page=byoa/prem&xAPID94&DI34&SU=http://hotmail.com/enca&HL=Mar
ket_MSNIS_Taglines


You're talking EMI, Vecheslav is talking EMF.
Your advice is sound on EMI but it does not
help much if he's experiencing fried transistors.
I've just built a 80V/40A H-bridge around a HIP4081A.
Just the HIP and 4 MOSFET's with built-in diodes,
and some circuitry for pulse-by-pulse current limit.
It drives a large 10lb 24V motor with no problems.

Bennet Williams wrote:

 It sounds like you have made a common omission. Don't forget the
noise suppression capacitors on the motor if they are not already
built in (and they usually aren't except for some modern RC car
motors). You should solder a 0.1 uF capacitor between each lead and
the case of the motor (2 capacitors total) and a 0.1 uF capacitor
between the 2 motor leads. Any ceramic disc capacitor will work, but
if you're buying new ones, try to get high-frequency ceramic caps.

BRW

--
*******************************************
VISIT MY HOME PAGE:
<http://home.online.no/~eikarlse/index.htm>
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*******************************************
Best Regards
Eirik Karlsen
 


You're right. I missed the EMF part.
In that case, make sure you are using the proper Schottky diodes. Not
just any Schottky will do. They must be the fast-recovery type, or
you will get kickback. A common choice for small motors is the
1N5822. Check out it's recovery time on the data sheet. The diodes
you are using should be close to it or you will have kickback
problems.

I'm also in the process of designing an H-bridge around a HIP4081.
I'd like to see your circuit. Could you post it? I have a circuit
design from the net, but I question the need for some of the
components like zeners to limit the MOSFET gate voltages.

BRW

--- In , Eirik Karlsen <eikarlse@o...> wrote:
> You're talking EMI, Vecheslav is talking EMF.
> Your advice is sound on EMI but it does not
> help much if he's experiencing fried transistors.
> I've just built a 80V/40A H-bridge around a HIP4081A.
> Just the HIP and 4 MOSFET's with built-in diodes,
> and some circuitry for pulse-by-pulse current limit.
> It drives a large 10lb 24V motor with no problems.
>
> Bennet Williams wrote:
>
> > It sounds like you have made a common omission. Don't forget the
> > noise suppression capacitors on the motor if they are not already
> > built in (and they usually aren't except for some modern RC car
> > motors). You should solder a 0.1 uF capacitor between each lead
and
> > the case of the motor (2 capacitors total) and a 0.1 uF capacitor
> > between the 2 motor leads. Any ceramic disc capacitor will work,
but
> > if you're buying new ones, try to get high-frequency ceramic caps.
> >
> > BRW
>
> --
> *******************************************
> VISIT MY HOME PAGE:
> <http://home.online.no/~eikarlse/index.htm>
> LAST UPDATED: 23/08/2003
> *******************************************
> Best Regards
> Eirik Karlsen




Bennet,
I'll mail it to you off-list
 

Bennet Williams wrote:

 You're right. I missed the EMF part.
In that case, make sure you are using the proper Schottky diodes. Not
just any Schottky will do. They must be the fast-recovery type, or
you will get kickback. A common choice for small motors is the
1N5822. Check out it's recovery time on the data sheet. The diodes
you are using should be close to it or you will have kickback
problems.

I'm also in the process of designing an H-bridge around a HIP4081.
I'd like to see your circuit. Could you post it? I have a circuit
design from the net, but I question the need for some of the
components like zeners to limit the MOSFET gate voltages.

BRW

--- In p...@yahoogroups.com, Eirik Karlsen <eikarlse@o...> wrote:
> You're talking EMI, Vecheslav is talking EMF.
> Your advice is sound on EMI but it does not
> help much if he's experiencing fried transistors.
> I've just built a 80V/40A H-bridge around a HIP4081A.
> Just the HIP and 4 MOSFET's with built-in diodes,
> and some circuitry for pulse-by-pulse current limit.
> It drives a large 10lb 24V motor with no problems.
>
> Bennet Williams wrote:
>
> >  It sounds like you have made a common omission. Don't forget the
> > noise suppression capacitors on the motor if they are not already
> > built in (and they usually aren't except for some modern RC car
> > motors). You should solder a 0.1 uF capacitor between each lead
and
> > the case of the motor (2 capacitors total) and a 0.1 uF capacitor
> > between the 2 motor leads. Any ceramic disc capacitor will work,
but
> > if you're buying new ones, try to get high-frequency ceramic caps.
> >
> > BRW
>
> --
> *******************************************
> VISIT MY HOME PAGE:
> <http://home.online.no/~eikarlse/index.htm>
> LAST UPDATED: 23/08/2003
> *******************************************
> Best Regards
> Eirik Karlsen
 
 

to unsubscribe, go to http://www.yahoogroups.com and follow the instructions
 

--
*******************************************
VISIT MY HOME PAGE:
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LAST UPDATED: 23/08/2003
*******************************************
Best Regards
Eirik Karlsen
 

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Thank for the replys,

I'm not using discrete FET's for the circuit - I bought a few L298's, which
are an integrated package with 2 H-bridges - 2A each. Takes up *a-lot* less
space on the board than 8 MOSFETs.
Regarding using caps on the motors - interestingly enough I tried this and
found that the motor with the caps runs much slower at the same
voltage/current. I guess this is to be expected since you're bound to have a
lot of losses through the cap at 20kHz PWM. So I just decided to avoid
putting them in at all.
Regarding the diodes, I am using 1N5819 diodes - they are 1A and looks like
they are just the lower current counterpart for the 1N5822's (which the H
bridge.)

I had a MOSFET bridge a while ago as well, before I decided to switch to the
L298, and I found that it was giving the same kind of kickback as it's
giving me now - even though the FETs had built in zeners. I don't know if
this is just something standard, or what, but have anyone looked at the
output of their bridge with a scope and confirmed a nice clean square wave?

Mine seems to have a lot of kickback on the on-off switch, and virtually
none on the off-on. I checked the diodes a 1000 times and I'm sure the
circuit is fine.

Vecheslav Silagadze
>From:
>Reply-To:
>To:
>Subject: [piclist] Digest Number 742
>Date: 30 Mar 2004 11:55:14 -0000 >There are 13 messages in this issue.
>
>Topics in this digest:
>
> 1. Re: Re: PCB direct toner transfer methode
> From: Vasile Surducan <>
> 2. Re: PCB direct toner transfer methode
> From: "Dave Mucha" <>
> 3. Re: 12F675/629 question
> From: jrem <>
> 4. Re: 12F675/629 question
> From: jrem <>
> 5. motor driver circuit
> From: "Vecheslav Silagadze" <>
> 6. Re: motor driver circuit
> From: Eirik Karlsen <>
> 7. Re :Re: serial communication framing error
> From: ""
><>
> 8. RE: Re :Re: serial communication framing error
> From: "Dave Moore" <>
> 9. Re: motor driver circuit
> From: "Bennet Williams" <>
> 10. Re: Re: motor driver circuit
> From: Eirik Karlsen <>
> 11. Re :Re: serial communication framing error
> From: "Phil" <>
> 12. Re: motor driver circuit
> From: "Bennet Williams" <>
> 13. Re: Re: motor driver circuit
> From: Eirik Karlsen < >________________________________________________________________________
>________________________________________________________________________
>
>Message: 1
> Date: Mon, 29 Mar 2004 15:52:18 +0300 (EEST)
> From: Vasile Surducan <>
>Subject: Re: Re: PCB direct toner transfer methode >oki-doki guys.
>
>First, you must be happy because are living in one (or more...) country
>where you may found everything you may need, in one store or other,
>just near your apartment doors.
>It's very difficult to understood what I'm saying if you can't experiment
>first the whole feeling on your own skin.
>Just one example: in Canada, a pack of 60 pcs Epson glossy photo paper is
>al low as 50 canadian cents. Here almost the same thing cost
> 0.6USD/1 sheet (may be read as shit) of paper.
>
>However, I don't need those. I found a good transfer paper used as support
>for Xerox glued labels. So far I'm not extremely satisfied about the
>methode. I hope you have taken a look too as I id, with a microscope (or
>some stereo lenses magnifier) at the PCB quality. It has indeed a 10 mil
>resolution, but the shape of the routes are not all very good, even the
>circuit is continuous. So I have to dig more. >best regards,
>Vasile
>http://surducan.netfirms.com >On Mon, 29 Mar 2004, rj_satterlee wrote:
>
> > Hi-
> >
> > I still use ferric chloride, and a tupperware (well one of
> > those disposable containers). The thing is, I use one of those
> > Salton Hot trays that you can find at Good Will for only a few
> > bucks. They can easily raise the temperature of the etch.
> >
> > I slightly lie. I did not get mine at Good Will or garage sale.
> > Actually, the one I use came from my Grandmother's estate, but
> > the one we got as a wedding gift still remains in pristine condition.
> > Failure to remain that way would result in my death sentence.....
> >
> > Cheers,
> >
> > Rich S.
> >
> >
> >
> >
> >
> > --- In , "Phil" <phil1960us@y...> wrote:
> > > Those pix are from 2 different boards, both with Ammonium
> > > Persulphate. Even with all its faults, I like AP.
> > >
> > > I built my own etch tank with 1/4" plexiglass from home depot
> > > and "aquarium seal" - not the best stuff but it works. it took me
> > 2
> > > attempts to get one that worked well. My tank is 8x6x1.5 which
> > holds
> > > a little more than a liter. (doing it again, I'd make it 1" wide
> > and
> > > keep it under a liter - its convenient to be able to store the
> > > etchant in a quart/liter jar.) I use a 100 Watt aquarium heater
> > and
> > > a bubbler. etch time is < 15 minutes at 100F which takes about 45
> > > minutes to reach. I can etch 5x7 boards or smaller. Total cost was
> > > about $50.
> > >
> > > Before that I used a tupperware pan but it took forever because I
> > > wasn't able to heat it easily.
> > >
> > > --- In , "jrem123" <jrem123@y...> wrote:
> > > > man, those boards are awesome. I'll have to try the different
> > > > paper. What hardware are you using to etch with? bubbles or
> > spray
> > > > or something? was it a kit or did you build it? or maybe I
> > should
> > > > be at the dys-pcb message list . . . and that other thread
> > should
> > > > be at the www.rant.com list (eh heh)
> > > >
> > > > --- In , "Phil" <phil1960us@y...> wrote:
> > > > > I found using non-porous paper to be problematic. This includes
> > > > > magazine paper, photo paper and "release" paper (which is what
> > I
> > > > > think you are talking about). The toner has no place to go and
> > is
> > > > > thus quite sensitive to pressure. I was seeing a lot of
> > > > > blotchiness. a 12 mil trace would look like a snake that ate 3
> > > > > pigs. Often blooming to 2X the intended width. SMD pads for
> > > even
> > > > > SOICs were a mess, forget TQFPs.
> > > > >
> > > > > Using inkjet paper gives excellent results and is as cheap and
> > > easy
> > > > > as it comes. My last 3 boards (3x4ish size) required NO
> > touching
> > > > up
> > > > > at all.
> > > > >
> > > > > look at the link I posted - I regularly do simple double sided
> > > > > boards. They are about 10% more work than a single sided board
> > > > > though a much easier routing job.
> > > > >
> > > > > --- In , jrem <jrem123@y...> wrote:
> > > > > > Lots of stuff out there on the net. I have successfully made
> > > > PCB's
> > > > > > with the "laser printer toner transfer method". You print on
> > > to
> > > > the
> > > > > > backing for Avery style lables, iron it onto the PCB (surface
> > > > > treated
> > > > > > with 0000 paper and rubbing alcohol) (don't touch the backing
> > > > > material
> > > > > > or the clad on the board, the oils in your skin will ruin the
> > > > > > transfer), iron the transfer on, drill (do this before
> > etching,
> > > > ask
> > > > > me
> > > > > > how I know), touch up the toner with a permanent marker, and
> > > etch.
> > > > > >
> > > > > > The toner is flaked plastic, the laser melts the plastic onto
> > > the
> > > > > > paper. When you print to the avery lable backing it doesn't
> > > fuse,
> > > > so
> > > > > > when you iron it onto the PCB it transfers to the copper.
> > > > > >
> > > > > > Works pretty good, it helps to touch up the traces prior to
> > > > > etching.
> > > > > > Don't bother buying any "special" PCB toner transfer paper.
> > > > Double
> > > > > > sided boards are probably close to unobtainable.
> > > > > >
> > > > > > Wire wrapping is easier and faster for prototyping, IMO,
> > > though.
> > > > > >
> > > > > >
> > > > > > --- Vasile Surducan <vasile@s...> wrote:
> > > > > > >
> > > > > > > Hi list,
> > > > > > >
> > > > > > > I'm interested into a viable, direct toner transfer methode
> > > > > > > PCB producing. I would be glad if someone could point to
> > > > > > > any *tested* methode. I'm interesting mostly on homebrew,
> > > > > > > without using special transfer papers.
> > > > > > > Thank you in advance,
> > > > > > >
> > > > > > > Vasile
> > > > > > > http://surducan.netfirms.com
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > >
> > > > > >
> > > > > > __________________________________
> > > > > >
> >
> >
> > to unsubscribe, go to http://www.yahoogroups.com and follow the
>instructions
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
>________________________________________________________________________
>________________________________________________________________________
>
>Message: 2
> Date: Mon, 29 Mar 2004 12:50:47 -0000
> From: "Dave Mucha" <>
>Subject: Re: PCB direct toner transfer methode
>
>--- In , Vasile Surducan <vasile@s...> wrote:
> >
> > oki-doki guys.
> >
> > First, you must be happy because are living in one (or more...)
>country
> > where you may found everything you may need, in one store or other,
> > just near your apartment doors.
> > It's very difficult to understood what I'm saying if you can't
>experiment
> > first the whole feeling on your own skin.
> > Just one example: in Canada, a pack of 60 pcs Epson glossy photo
>paper is
> > al low as 50 canadian cents. Here almost the same thing cost
> > 0.6USD/1 sheet (may be read as shit) of paper.
> >
> > However, I don't need those. I found a good transfer paper used as
>support
> > for Xerox glued labels. So far I'm not extremely satisfied about the
> > methode. I hope you have taken a look too as I id, with a
>microscope (or
> > some stereo lenses magnifier) at the PCB quality. It has indeed a
>10 mil
> > resolution, but the shape of the routes are not all very good, even
>the
> > circuit is continuous. So I have to dig more.
> >
> >
> > best regards,
> > Vasile
> > http://surducan.netfirms.com
> >
> >
>
>Hi Vasile,
>
>When I did my first laser transfer, I used the very gloss paper that
>is left over from stick on lables.
>
>This list told me that if the paper goes thru the laser, it gets
>toner and then you can iron that onto your board.
>
>No special paper, plain bond, or high clay content.
>
>I have tried magazine paper that is high clay and very shiny (glossy)
>
>The main thing is that the toner is transfered.
>
>I would recomend using one of each type you have available to see
>what works.
>
>Also, I use a manfying glass to check every trace and will touch up
>those that are not perfect. >Dave >________________________________________________________________________
>________________________________________________________________________
>
>Message: 3
> Date: Mon, 29 Mar 2004 06:05:10 -0800 (PST)
> From: jrem <>
>Subject: Re: 12F675/629 question
>
>I've used MCLR as I/O w/ the intosc on 12F629's and 675's. Didn't use
>the internal pullups or analog on any I/O. >
>
>--- Phil <> wrote:
> > I'm trying to use the MCLR pin for I/O on this device. The data
> > sheet claims to do so you clear the MCLRE config bit. When I do
> > this the device behaves very oddly (output pins go to Vdd) and it
> > almost looks like a reset that only half happened but not exactly.
> > I'm using INTOSC (with no clock out). Also, no BOD, no WDT, no LVP
> > set.
> >
> > There is a somewhat cryptic footnote in the datasheet section on the
> > MCLRE config bit that says "When MCLR is asserted in INTOSC or RC
> > mode, the internal clock oscillator is disabled." Are they trying to
> >
> > say that you can't use INTOSC and turn MCLR into a GPIO pin at the
> > same time? If so, I'm kinda pissed as that leaves me with 5 pins,
> > not 6.
> >
> > Anyone ever got INTOSC and MCLRE=0 to work at the same time?
> >
> > Given that it costs $1, I plan on using it a bunch so I guess I cant
> > be too mad ...
> >
> > Phil
> >
> >__________________________________ >________________________________________________________________________
>________________________________________________________________________
>
>Message: 4
> Date: Mon, 29 Mar 2004 06:06:21 -0800 (PST)
> From: jrem <>
>Subject: Re: 12F675/629 question
>
>I've used MCLR as I/O w/ the intosc on 12F629's and 675's. Didn't use
>the internal pullups or analog on any I/O. >
>
>--- Phil <> wrote:
> > I'm trying to use the MCLR pin for I/O on this device. The data
> > sheet claims to do so you clear the MCLRE config bit. When I do
> > this the device behaves very oddly (output pins go to Vdd) and it
> > almost looks like a reset that only half happened but not exactly.
> > I'm using INTOSC (with no clock out). Also, no BOD, no WDT, no LVP
> > set.
> >
> > There is a somewhat cryptic footnote in the datasheet section on the
> > MCLRE config bit that says "When MCLR is asserted in INTOSC or RC
> > mode, the internal clock oscillator is disabled." Are they trying to
> >
> > say that you can't use INTOSC and turn MCLR into a GPIO pin at the
> > same time? If so, I'm kinda pissed as that leaves me with 5 pins,
> > not 6.
> >
> > Anyone ever got INTOSC and MCLRE=0 to work at the same time?
> >
> > Given that it costs $1, I plan on using it a bunch so I guess I cant
> > be too mad ...
> >
> > Phil
> >
> >__________________________________ >________________________________________________________________________
>________________________________________________________________________
>
>Message: 5
> Date: Mon, 29 Mar 2004 11:20:28 -0500
> From: "Vecheslav Silagadze" <>
>Subject: motor driver circuit
>
>Hello everone,
>
>I'm looking at the output from an H-bridge to a DC motor, run with PWM, and
>it looks like there is quite a bit of EMF kickback even though I put all
>the
>requisite (schotkey) diodes in place. The motor still works, but I expect
>there is a lot of energy being lost there. Is there anything I can do? any
>common mistakes?
>I was thinking of putting zener diodes between the motor leads to achieve
>the same effect as the diodes to ground and v+ are supposed to achieve.
>
>I know this isn't really a PIC topic, but I assumed many people on the list
>have experience with the above. Plus, if it helps, the H-bridge is being
>run
>using a PIC (and a fancy 18F458 at that.)
>
>Vecheslav Silagadze
>
>_________________________________________________________________
>MSN Premium helps eliminate e-mail viruses. Get 2 months FREE*
>http://join.msn.com/?pgmarket=en-ca&page=byoa/prem&xAPID94&DI34&SU=http://hotmail.com/enca&HL=Market_MSNIS_Taglines >
>________________________________________________________________________
>________________________________________________________________________
>
>Message: 6
> Date: Mon, 29 Mar 2004 19:03:17 +0200
> From: Eirik Karlsen <>
>Subject: Re: motor driver circuit
>
>There are several things you can do;
>Instead of adding extra diodes, use transistors with built-in reverse
>zeners ( MOSFET or IGBT).
>There will be some loss in these diodes in a high power application.
>If you're into even higher power you could use a clever scheme and use the
>switches themselves
>to move the back-EMF to the powersupply.
>
>But for power 0-500W I'd go for the simplest solution and use transistors
>with buildt-in diodes.
>
>Vecheslav Silagadze wrote:
>
> > Hello everone,
> >
> > I'm looking at the output from an H-bridge to a DC motor, run with PWM,
>and
> > it looks like there is quite a bit of EMF kickback even though I put all
>the
> > requisite (schotkey) diodes in place. The motor still works, but I
>expect
> > there is a lot of energy being lost there. Is there anything I can do?
>any
> > common mistakes?
> > I was thinking of putting zener diodes between the motor leads to
>achieve
> > the same effect as the diodes to ground and v+ are supposed to achieve.
> >
> > I know this isn't really a PIC topic, but I assumed many people on the
>list
> > have experience with the above. Plus, if it helps, the H-bridge is being
>run
> > using a PIC (and a fancy 18F458 at that.)
> >
> > Vecheslav Silagadze
> >
> > _________________________________________________________________
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>Message: 8
> Date: Mon, 29 Mar 2004 10:31:24 -0800
> From: "Dave Moore" <>
>Subject: RE: Re :Re: serial communication framing error
>
>Silly, but you only mentioned two pins: RX and TX. Did you connect a
>ground
>also?
>
> > -----Original Message-----
> > From:
> > [mailto:]
> > Sent: Monday, March 29, 2004 10:18 AM
> > To:
> > Subject: [piclist] Re :Re: serial communication framing error
> >
> > hello all,
> > chad the device which i am trying to interface here is a
> > nokia mobile which speaks at a baud of 115200 and with SPBRG
> > value of 5 for a 11.0592 MHz crystal the baud comes to exatly
> > 115200.the cable which connects the mobile to my PIC directly
> > (without RS232 level conversions)as small as possible 15cms approx.
> > also as suggested by RICH . s i will check the data bits on
> > the oscilloscope and find it out.something about character
> > length was suggested .
> > could you please tell more on the character length part in
> > framing error.
> >
> > thanking u all again n waiting for an reply
> >
> > krishna
> >
> >
> > >Ensure both are using the same number of bits, 8 or 9. Check the
> > >frequency of the other device. The lower the SPBRG, the more
> > critical.
> > >Check the interface, if you are using a long cable, make sure the
> > >received signal is good.
> >
> > Chad
> > --- krishna <> wrote:
> > > hi all
> > > I am using a PIC18F452 at baud of 115200 bps.I have connected the
> > > Rx,Tx pin to a device which also transmits at a baud of 115200.I am
> > > using a 11059200 hz crystal which gives me a SPBRG value of 5 for a
> > > baud of 115200.
> > > but when the connected device actually transmits, the FERR
> > flag goes
> > > high and when RCREG is read it reads as zero only.
> > > could anyone tell me what could be going wrong and how this can be
> > > handled.I can assume that a break interrrupt has occured
> > due to which
> > > the FERR flag is high(as the stop bit is taken as low
> > instead of high
> > > ) and RCREG is read as zero.
> > > But how can this be solved ? is there any other alternative to it.
> > >
> > > waiting for a reply
> > > krishna
> > >
> > >
> >
> >
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Well....I'm not sure what you mean with 'kick-back', but if it is only a small spike observed on the scope
then I'd say 'don't worry'.  If you've made a sensible choise of output diodes then the spikes will be
limited to VCC and GND.

Vecheslav Silagadze wrote:

 Thank for the replys,

I'm not using discrete FET's for the circuit - I bought a few L298's, which
are an integrated package with 2 H-bridges - 2A each. Takes up *a-lot* less
space on the board than 8 MOSFETs.
Regarding using caps on the motors - interestingly enough I tried this and
found that the motor with the caps runs much slower at the same
voltage/current. I guess this is to be expected since you're bound to have a
lot of losses through the cap at 20kHz PWM. So I just decided to avoid
putting them in at all.
Regarding the diodes, I am using 1N5819 diodes - they are 1A and looks like
they are just the lower current counterpart for the 1N5822's (which the H
bridge.)

I had a MOSFET bridge a while ago as well, before I decided to switch to the
L298, and I found that it was giving the same kind of kickback as it's
giving me now - even though the FETs had built in zeners. I don't know if
this is just something standard, or what, but have anyone looked at the
output of their bridge with a scope and confirmed a nice clean square wave?

Mine seems to have a lot of kickback on the on-off switch, and virtually
none on the off-on. I checked the diodes a 1000 times and I'm sure the
circuit is fine.

Vecheslav Silagadze
 

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VISIT MY HOME PAGE:
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Eirik Karlsen
 


--- In , Eirik Karlsen <eikarlse@o...> wrote:
> Well....I'm not sure what you mean with 'kick-back', but if it is
only a small spike observed on the scope
> then I'd say 'don't worry'. If you've made a sensible choise of
output diodes then the spikes will be
> limited to VCC and GND.


A motor coil is an inductor and will act just like one.

Put power on it and it will resist the current by storing it in a
magnetic field. Remove power and as the magnetic field breaks down,
it will release the stored power.

Depending on the size of the coils in the motors, you would probably
get about 30% more voltage than you were supplying and could easily
get a few amps.

I'd expect way more than a blip on a scope.

In fact, when using stepper motors, the back EMF offsets a drop in
the power supply when switching from one coil to another.

I like to select the diodes by a factor of 2 regarding both voltage
and current as to what is supplied to the coil.

This is similar to relays, the colapse of the coil will deliver a
spike back into the device driving it.

Dave



Dave,
yes I'm aware of all this, infact I once exploited the back EMF to make a simple HV generator.
It ran on 12VDC and had a transistor switch a simple inductor (not a transformer).
Then rectified the spike on the CMOS drain and got about 2KV.
It was a 1200V transistor so I think the voltage were limited by the transistor's breakdown voltage!

But if you look at the L298 application note you'll se that each of the 4 outputs is clamped
by two diodes, one to VCC and the other to GND.
So if suitable diodes are selected, they are mounted in the correct direction, and the PSU
has some minimal capacitance across it, then the EMF-spikes will be limited to the
PSU rails (+/-0.7V or so).
If he is seeing large spikes then I believe he's either using one or more defective
components, or has swapped some diodes.

Dave Mucha wrote:

 --- In p...@yahoogroups.com, Eirik Karlsen <eikarlse@o...> wrote:
> Well....I'm not sure what you mean with 'kick-back', but if it is
only a small spike observed on the scope
> then I'd say 'don't worry'.  If you've made a sensible choise of
output diodes then the spikes will be
> limited to VCC and GND.
 

A motor coil is an inductor and will act just like one.

Put power on it and it will resist the current by storing it in a
magnetic field.  Remove power and as the magnetic field breaks down,
it will release the stored power.

Depending on the size of the coils in the motors, you would probably
get about 30% more voltage than you were supplying and could easily
get a few amps.

I'd expect way more than a blip on a scope.

In fact, when using stepper motors, the back EMF offsets a drop in
the power supply when switching from one coil to another.

I like to select the diodes by a factor of 2 regarding both voltage
and current as to what is supplied to the coil.

This is similar to relays, the colapse of the coil will deliver a
spike back into the device driving it.

Dave
 
 
 
 
 
 
 
 
 
 
 

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VISIT MY HOME PAGE:
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Eirik Karlsen