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A discussion group for the PICMicro microcontroller. Also called the Microchip PIC, this list is dedicated to the use and abuse of this fine, simple, microcontroller. Close to topic posts are welcome, ie. general electronics.
motor driver circuit - Vecheslav Silagadze - Mar 29 11:20:00 2004
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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&xAPID=1994&DI=1034&SU=http://hotmail.com/enca&HL=Market_MSNIS_Taglines |
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Re: motor driver circuit - Eirik Karlsen - Mar 29 12:03:00 2004
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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, --
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Re: motor driver circuit - Bennet Williams - Mar 29 15:48:00 2004
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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&xAPID=1994&DI=1034&SU=http://hotmail.com/enca&HL=Mar ket_MSNIS_Taglines |
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Re: Re: motor driver circuit - Eirik Karlsen - Mar 29 16:15:00 2004
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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-- ******************************************* VISIT MY HOME PAGE: <http://home.online.no/~eikarlse/index.htm> LAST UPDATED: 23/08/2003 ******************************************* Best Regards Eirik Karlsen |
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Re: motor driver circuit - Bennet Williams - Mar 29 17:26:00 2004
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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 |
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Re: Re: motor driver circuit - Eirik Karlsen - Mar 29 17:28:00 2004
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Bennet,
I'll mail it to you off-list Bennet Williams wrote: You're right. I missed the EMF part. --
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motor driver circuit - Vecheslav Silagadze - Mar 30 10:50:00 2004
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Re: motor driver circuit - Eirik Karlsen - Mar 30 12:32:00 2004
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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, --
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Re: motor driver circuit - Bennet Williams - Mar 30 16:47:00 2004
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Re: motor driver circuit - Chad Russel - Mar 30 18:05:00 2004
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If you want a nice clean square wave, drive a resistor. Theoretically the inductive kick back is infinite. I don't think that 20 khz is fast enough to need external diodes. I have found transzorbs at 1.5X the operating voltage and 2X the motor power to be fairly bullet proof protection for everything. Chad --- 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 > <<< snip >>> ===== My software has no bugs. Only undocumented features. __________________________________ |
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Re: motor driver circuit - Vecheslav Silagadze - Mar 31 0:59:00 2004
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Re: Re: motor driver circuit - Eirik Karlsen - Mar 31 4:55:00 2004
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The spikes are really only a problem if the driver is failing. Even the
simple 1N4000 should easily
clamp the spikes. But if the main problem is lack of torque I suggest you look elsewhere...the spikes most likely has nothing to do with it. Do expect 100% torque at 100% PW. Do NOT expect 50% torque at 50% PW. Depending on frequency and motor inductance the PW and current is not linear.... To get 50% torque it is not uncommon to need 80-90% PW. Steppers are basically 'current driven', so as RPM goes up so does the voltage.... to get good speed and torque from a common '5V' stepper you may need 20-40V ! Vecheslav Silagadze wrote: "A much better choice is the National LMD18201 MOSFET H-bridge. It can-- ******************************************* VISIT MY HOME PAGE: <http://home.online.no/~eikarlse/index.htm> LAST UPDATED: 23/08/2003 ******************************************* Best Regards Eirik Karlsen |
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Re: motor driver circuit - Bennet Williams - Mar 31 13:33:00 2004
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OK. I can see from this and some other post that there is a misunderstanding about what a fast recovery diode is. The "fast recovery" refers to the diode's ability to switch from reverse bias (which is what it is in when the PWM output to the motor is high) and forward bias (which is what it is in when the inductive kick-back is being absorbed by the power supply). This must happen very fast in a PWM motor driver, regardless of the PWM frequency. The time between when you are driving current through your motor coil and when you shut off that current is very short indeed. The diode must switch between reverse bias and forward bias very fast so that it can pass the current that suppresses the inductive kick-back voltage. If the diode is too slow to recover, you will get a voltage spike before the diode switches to forward bias. Sorry for the long narrative. I hope it helps the group understand PWM circuitry a little better. On your other zero torque issue - it is common to see almost no torque down around 30% duty cycle. The last gearmotor that I used did not even start to rotate until about 50% duty cycle. When it did start rotating, it had a high torque. > I am getting a voltage spike way over my Vcc actually - nearly 2xVcc If your circuit is correct, you can see that this voltage will never exceed a diode forward drop voltage above Vcc IF your diode recovers fast enough. > in fact. The motor still runs, but it gives pretty much zero torque > at around 30% duty cycle. > I will assume it is the clamping diodes that are causing the problem > and look for faster recovery ones (even though at 20kHz I can't > imagine any kind of diode won't switch fast enough... the 1N5819 I'm > using should be more than adequate.) Yes it should. If you try new diodes, try the 1N5822. I've used these with great success (basically the same as the 1N5819 but with more forward current and fwd surge current - this might make the difference). I've checked the circuit many > times over and I'm certain that the topology is fine. > > Vecheslav Silagadze |
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Re: motor driver circuit - Vecheslav Silagadze - Mar 31 19:38:00 2004
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> On your other zero torque issue - it is common to see almost no > torque down around 30% duty cycle. The last gearmotor that I used did > not even start to rotate until about 50% duty cycle. When it did > start rotating, it had a high torque. That's interesting. Why is the power being eaten up like that? Conceptually it would seem that at 30% duty cycle I should be getting 100% torque with 30% speed because I'm delivering 30% of the power I would be delivering at 100% duty cylce. But I guess this is one more of these places where theory meets reality :) I will try a fast recovery diode, but I'm basically limited to what the local Sayal Electronics carries - from what I remember the "best" diodes they had were the 1N5819s. It's unfortunate, but it seems that the datasheet on the 1N5819 doesn't specify the recovery time (the L298 driver I'm using asks for <200ns...) Vecheslav Silagadze |
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Re: Re: motor driver circuit - Vasile Surducan - Apr 1 2:13:00 2004
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On Thu, 1 Apr 2004, Vecheslav Silagadze wrote: > > On your other zero torque issue - it is common to see almost no > > torque down around 30% duty cycle. The last gearmotor that I used > did > > not even start to rotate until about 50% duty cycle. When it did > > start rotating, it had a high torque. > > That's interesting. Why is the power being eaten up like that? > Conceptually it would seem that at 30% duty cycle I should be getting > 100% torque with 30% speed because I'm delivering 30% of the power I > would be delivering at 100% duty cylce. Conceptually wrong. |
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