Premium Member Paul Bruening Posted September 24, 2008 Premium Member Share Posted September 24, 2008 Hal, John or any takers, I've seen push button potentiometers (like these:) http://cgi.ebay.com/Potentiometer-1-meg-2w...p3286.m20.l1116 side by side on Bruce's and Clive's controllers. How does that work? Do you solder them in series? Does this increase the resolution of the resistance? I've seen them as 2 digit beside 2 digit. Do you have to have the same Kohm rating for both? I'm trying to increase the resolution of control on my 24V PWM for my Mickie scan head. That's why I've asked in the T&DI forum. Link to comment Share on other sites More sharing options...
Premium Member Hal Smith Posted September 25, 2008 Premium Member Share Posted September 25, 2008 Hal, John or any takers, I've seen push button potentiometers (like these:) Do you solder them in series? No, the two sections are internally connected in series. Does this increase the resolution of the resistance? Yes, for the one shown the left hand switch is switching ten increments of 100k ohms and the right side is switching ten increments of 10k ohms. I've seen them as 2 digit beside 2 digit. Probably two separate pots. Do you have to have the same Kohm rating for both? See above I'm trying to increase the resolution of control on my 24V PWM for my Mickie scan head. If 1% is fine enough. Bourns makes three section versions of these pots where the resolution is .1% Have a look at page 4 of this .pdf to see what's available. http://www.e-sonic.com/aboutus/cat/D/digital.pdf On the controllers you mention, they're probably using digital encoding switches, not pots, for control of phase lock loops. Both of them look the same on the outside. For instance: Clive uses two-section and a three-section digital encoding switches side by side on his Milliframe controllers. He's using them to program digital counters to divide the Milliframe's internal crystal frequency down to the appropriate output pulse frequencies to control the speed of his crystal motors as well as the Arri motors, etc. his gear works with. Link to comment Share on other sites More sharing options...
Premium Member Paul Bruening Posted September 25, 2008 Author Premium Member Share Posted September 25, 2008 Thanks, Hal. That makes more sense. Link to comment Share on other sites More sharing options...
Olex Kalynychenko Posted September 25, 2008 Share Posted September 25, 2008 Hal, John or any takers, I've seen push button potentiometers (like these:) http://cgi.ebay.com/Potentiometer-1-meg-2w...p3286.m20.l1116 side by side on Bruce's and Clive's controllers. How does that work? Do you solder them in series? Does this increase the resolution of the resistance? I've seen them as 2 digit beside 2 digit. Do you have to have the same Kohm rating for both? I'm trying to increase the resolution of control on my 24V PWM for my Mickie scan head. That's why I've asked in the T&DI forum. O... If you wish use of 1Mom 2W potentiometer for control of speed of DC motor, Sorry, but, this is impossible. If you need technical consultation, can send me direct email , with more detailed information about your idea. What kinds of type of motor have of your scan head ? DC with PWM control ? If your scan head have digital feed back, need increase of resolution of feed back sensors. Need stude of principle of control and schematic. Link to comment Share on other sites More sharing options...
Premium Member Paul Bruening Posted September 25, 2008 Author Premium Member Share Posted September 25, 2008 Well, you know, I'm broke and trying to solve normally pricey problems. I've got this ginormous Mitchell motor running 24V DC, 1,800 RPM, geared down to 24 FPS. I'm trying to down-run it to match my Canon XL2 in 24P. That means reducing the Mitchell scan head down to 23.976 FPS. That puts the motor shaft speed down to 1798.2 RPM. I'm using junk parts to do this. I've got an old 24V DC (27.5V actual) power supply or a variable voltage power supply (115V AC in/ rheostat variable DC out) to drive a PWM with a 5V, 22Kohm single turn pot already soldered in. I'm planning to replace that pot with a 25 turn 20 Kohm potentiometer which I got for a whole dollar. I'm groping in the dark. Yet, I'm hoping I'll end up with enough control resolution to get this head to match the XL2. I've got a $10.00 hand-held tach to check general speeds but only the XL2 can provide, cheaply, the exact measure of speed. Link to comment Share on other sites More sharing options...
Olex Kalynychenko Posted September 26, 2008 Share Posted September 26, 2008 Well, you know, I'm broke and trying to solve normally pricey problems. I've got this ginormous Mitchell motor running 24V DC, 1,800 RPM, geared down to 24 FPS. I'm trying to down-run it to match my Canon XL2 in 24P. That means reducing the Mitchell scan head down to 23.976 FPS. That puts the motor shaft speed down to 1798.2 RPM. I'm using junk parts to do this. I've got an old 24V DC (27.5V actual) power supply or a variable voltage power supply (115V AC in/ rheostat variable DC out) to drive a PWM with a 5V, 22Kohm single turn pot already soldered in. I'm planning to replace that pot with a 25 turn 20 Kohm potentiometer which I got for a whole dollar. I'm groping in the dark. Yet, I'm hoping I'll end up with enough control resolution to get this head to match the XL2. I've got a $10.00 hand-held tach to check general speeds but only the XL2 can provide, cheaply, the exact measure of speed. If you think about value of speed 23.976 fps, you need crystal sync speed system and motor must have feed back sensor. This must be closed PID PWM speed control system. The speed control systems without feed back sensor do not have fixed speed, the speed will drift. The speed of motor will depend from working load. Link to comment Share on other sites More sharing options...
Premium Member Paul Bruening Posted September 26, 2008 Author Premium Member Share Posted September 26, 2008 Hey Olex, I was kinda' hoping the massive size of the motor might override temperature, friction and mag/film weight factors. I can pre-run, thereby, warm-up the rig. I can steady the 115V voltage coming into the system. I can put a long length of timing leader at the head of each roll to calibrate the system to each roll. But, I suspect that you are right. 23.976 is such a precise speed that even a slight drift will be a problem. I guess my assumption that this can work comes from an experience I had 25 or so years ago. I was working for the local university's football team as a film/video/lab/editor/do-all guy. The coaches wanted a big pile of B&W, 16mm film converted to SVHS. Rank Cintel prices made the project costs absurd. Telecine was too expensive as well. So, I borrowed a variable-speed, 16mm Eiki projector from another department, projected it against a white wall, pointed an SVHS camcorder at the wall, hand calibrated the film to the 15fps speed needed to precisely match the 30fps of the camcorder using the simple speed pot on the projector and got steady speed results throughout the entire project. It was as easy as falling off a log. I'm hoping against all good sense that this project will turn out that easy. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now