LinkBack URL
About LinkBacks
Also, would chopped be to much to ask for too? Trying to balast an 80V, 7A circuit would require huge resistors and generate a huge amount of heat
| | |
My personal preference is bipolar, but it is Joes threadOriginally Posted by BCwanderer
Also, would chopped be to much to ask for too? Trying to balast an 80V, 7A circuit would require huge resistors and generate a huge amount of heat
The technology to do 80V and greater N-CHN discrete Mosfet H bridges is out there, NBD. The logic to implement a dual H bridge is NBD. Starting to insure timing and recircuilation starts to up the complexity. Physical PCB layout handling the FETs gate charge, current requirements ups the problem. Miscrostepping implementation ups the complexity. Before you know it it turns into more of a challenge than it looks like off the cuff. Not that it's impossible to do, but when you start getting into component costs, numer of components, it's an agressive DIY project. I'm currently playing with a simple implementation of a compromise which looks like it maybe a good fit for those who want the most bang for the buck. i.e. power, speed, accuracy and minimizing resonance. But just aint enough free hours and energy...............
Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
Vist my websites - http://pminmo.com & http://millpcbs.com
Sounds very good I am looking forward to seeing what comes about, Their are people that will help you out if you need it maybe a private section on your site/forums, to grant certain individuals access to that area only which are working with you to try to produce a Bi-polar drive above the 50v mark.
Joe
The ability/technology to switch the 80 volts exists. The challenge comes in when you try to any electronic swithing FAST.
One very serious challenge in switching any H bridge drive at high speeds is "shoot through" - the point in time when two conductors on the same side of the bridge are on/partially on. This is effectively a short to ground which always has a high potential for smoke emissions.
Inductive feedback is also possible but lets leave that asside for the moment.
The enemy is "time" as in time to swith the pass element (fet) on and off at the same time. Since the element can be partially conductive (hence semiconductor), this "gray" area is what causes problems.
Stray capacitance (size of traces, trace location, trace length,etc) affect the rate at which you can charge/discharge the gate. Hence even though you turned the drive OFF, the element may still be conducting enough to give rise to shoot through. Hence, you can't merely drive with an PNP/NPN totem pole - you need interlocking logic to prevent the phenomenon.
Once you get past this, you may have a suitable design as in IC's, logic, et al. It would work on a real, properly laid out PCB. Yet, it goes nuts on a breadboard.
Decision time.
It SHOULD work but doesn't - it will PROBABLY work if you had a PCB. Do you go ahead and spring for a PCB??? And the kid needs new shoes, the wife needs a new kitchen and the cable bill is late (can't lose the DSL connection).
Remember, its a DIY project and the board may cost several thousand to layout and prototype.
Imagine doing that several times before you get a production board to work.
Having been through the iteration a couple of times with very simple in comparison projects, I now see why a board with maybe $20 in parts costs $500.
Now the issue of inductive feedback....... another variation of what's transpired above...
To simplify: you can't see electrons, therefore you can't trust them. Easier than herding cats without a food dish but not by much...
So are we saying It just not possible for DIY 80v Bi-polar Drivers? or just stating what has been read.
Joe
I figured if it were easy then it would have been done a long time ago and folks like Gecko wouldn't be able to charge what they do because there would be more competition in that voltage range and their products wouldn't be so proprietary.
This is why folks like me need engineers and machinists to keep us grounded in reality. I look at something like this and say, well all the parts are there for about $20 bucks, and I can make a 50V board, an 80V should be just a small step away. But you guys are the specialists who know the subtle little problems that I don't see looking at the grand overview of things.
So it seems servo's may be the way to go in that voltage range
Steven
The laws of physics are pretty hard to refute. To illustrate, think about this for a bit:
Take a 12 volt relay, engage it and then disngage. Do so with and without a protective diode. Watch the result with a recording oscilloscope. You should see a pretty decent voltage kick so be careful.
Now imagine doing that with the multiple windings of an 80 volt coil. Imagine doing so at 20khz (1.2million times in a minute). Can you spell "light show"???
The point is not in that it can or can't be done. They said you couldn't put a man on the moon - and we did using computers not as powerful as the one being used to generate this message.
What's being said is that viability for a DIY project may not be there. The essential point in the reply was merely a reality check - something that is often readily overlooked by even the best intended DIY'er.
Layout, parts and design of a 80 volt bipolar h-bridge driver are available by looking at the DIY servo controller.
The servo board design show the details how to build a high voltage H bridge plus parts list.
http://gsst.wikispaces.com/UHUget
The H bridge design is discussed at the thread
OpenSource CNC Design Center > Open Source Controller Boards > DIY Servo controller
For a bipolar stepper design, you need two H-bridges and a stepper sequencer driver.
The high voltage Bridge driver is ready for duplication. Two H-Bridges like the servo H bridges will drive the stepper motor. The stepper interface and chopper control needs to be added.
One of the high costs are the ir2184 chips. You need 4 driver chips plus assorted support parts for two H bridges.
The H bridge driver is very similar to the Gecko drive output.
Gecko has a very compact surface mount output stage.
A DIY unit would probably be built a little larger.
Gecko drivers are very compeditively priced. They are compact and well built.
For a DIY builder here are some costs
Newark sell ir2184 Fet driver chips for over $5 each. Thats $20
Two H bridges need a total of 8 FET chips IRF740 chips cost $1.27 each
Thats $10
A microchip or stepper driver chip will cost you another $4
The board will cost $8 to $15
Add assorted capacitors, voltage regulators, current sense resistors and connectors and you have another $15
Add shipping and postage for all the parts from various suppliers Add $20
Cost quickly add up to $65 to $85 per board.
If you build 1000, you can cut the parts cost in half.
Geckos are very well priced. Plus the engineering is already done.
I think that is why there are not many competing DIY designs for bipolar stepper drivers
It is still worthwhile looking at it for personal satisfaction
Last edited by BCwanderer; 06-06-2006 at 03:46 PM. Reason: More detail
I remember seeing that about 3 months ago, but wonder how it relates tro steppers.
Joe
Could you use the programable stepper sequence driver from the PicStep board to do this?
RE: ir2184 chips. I have seen so many companies very eager to give away their chips as free samples. Get your brothers and a few uncles enlisted in a "asking for free samples campaign" and you can probably build several boards for free
Steven
PicStep vs discreet H Bridge construction The PicStep driver uses the LMD245T chip which has onboard fixed frequency chopping.
A DIY Discreet FET Bridge would need a comparator and a reference to drive a current chopping unit to enable square wave current limiting.
The Pic16F628 chip from PicStep would need additional circuitry to enable chopping.
A DIY bridge needs 8 lines to drive the 8 FETs plus step and direction inputs plus two lines for chopper sensing plus outputs to initiate phase control.
A Pic 18F452 would provide more output lines to enable control and also would be easier programming than the smaller Pic16 chips
Pic18F chips are often available on E-Bay for $5
The newer pic chips run up to 40 mhz which would run chopping to 20 khz easily
Atmel chips would work great too.
A good microcontroller coding guru would be an asset to design microstepping and fast and slow current decay.
I have thought about this a lot but my wife wants me to pick up a parrot at the airport, erect a tarp structure for the hay and lay concrete in the barn.
It is tough to make time to finish all the ideas between chores.
Posting Permissions
| | |
