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DNA Dan's discussion about secret drive
DNA Dan:
Is it a poor practice to use a two position switch when either position is going to partially complete a circuit? For instance if I use the SPDT switch and I have it rotating one way, once the limit switch is tripped it stops. But the switch is still in that position. It would be nice to just have this be a "horizontal" or "vertical" 2 position toggle. But that means there isn't an "off" position. Is that generally a poor practice in electronics?
DaOld Man:
--- Quote from: DNA Dan on March 12, 2011, 12:38:15 am ---Is it a poor practice to use a two position switch when either position is going to partially complete a circuit? For instance if I use the SPDT switch and I have it rotating one way, once the limit switch is tripped it stops. But the switch is still in that position. It would be nice to just have this be a "horizontal" or "vertical" 2 position toggle. But that means there isn't an "off" position. Is that generally a poor practice in electronics?
--- End quote ---
No, it is not bad practice. Using a two position SPDT switch is OK. I just tend to overkill a bit.
The advantage (probably only one) to using a 3 position switch is that if something happens and the motor doesnt turn off when it reaches it's end of travel, it would be nice to be able to turn off the motor to keep it from still trying to turn the monitor.
I dont think I mentioned it earlier, but it is highly important to have some sort of mechanical stop so the monitor disc cannot turn a full 360 degrees. Imagine what would happen if the monitor did 3 or 4 complete revolutions. Cords would be wrapping up and damage would occur to something (probably something expensive). So you need stops a few degrees past 0 and 90. Maybe at 100 and 350 degrees. These stops would be fixed in place and not turn with the monitor. The flag could be made to contact these stops after it contacts the limit switches.
The motor could set there and burn up if it is pushing against one of these stops for very long.
But if your rig is designed correctly the chances of this happening are very slim.
Some scenarios that come to mind would be the flag missing a limit switch (or falling off the disc), a limit switch mechanically failing, or limit switch getting loose and moving outside the range of the flag.
Of course if anything happens you can always kill the power supply by pulling the plug on it.
Or you could make an "emergency stop" switch that would kill power to the power supply or open the V+ to the drive.
An emergency stop method is good practice anyway, because the drive could fail and run the motor all the time. Limits or 3 position switches wouldnt matter then. (Unless the limits were set to cut the power to the motor, like the drawing with the diodes).
Good practice for an emergency stop switch is to have it where anyone can easily find it. And everyone using the cab needs to know how and when to use this switch.
DNA Dan:
Awesome. So I found the kit instructions for that other PWM board. It can be found here:
http://www.funnykit.co.kr/bemarket/shin/menual/fk804.pdf I have also seen some prices ranging from $7.99 up to ~$20. The kit is for ages 12+ if that shows any indication of my electronics skills :dunno It's odd because if you go to the futurekit homepage www.futurekit.com, the website has been hacked by someone. Hope it's still available through distributors. Anyway....
For a low cost manual setup, I was thinking the following. The PWM boad powered 9v from power supply. The other end connected to a DPDT 2-way switch, which is in turn connected to the motor with the limit switch feedback loop containing the diodes that you laid out in reply #13. According to the Bakaelectronics link I posted earlier, this board can still operate down to 5v whereas the GM2 motor maxes out at 12v. Since I will be throttling the motor down anyway, I figured I didn't need 12v but 9v would be a happy middle road for both devices to work. I will then have an emergency "kill" button on top the cab wired directly to the VDC+ side coming from the power supply. For the DPDT switch I'd used something like this:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=060-322
Or perhaps something like this:
http://www.cherrycorp.com/english/switches/rockers/rr_rocker.htm
I see they make them in double pole, but they are on/off only. Can I wire this with a jumper to make it On-On?
What about the diodes? This is totally unfamiliar territorry for me. I think the need the Schottky type but I am usure about the amps they can handle and the draw the GM2 has. Also that PWM board is rated 1.5A but the motor can draw this same amount. After accounting for the diodes is that too much being drawn off that board?
Here is some more information about the motor:
http://www.solarbotics.com/assets/datasheets/592.gif
I don't quite understand the current (stalled) figure. Is that how much current it will draw in a shorted out state? That is the only figure that's over 1.5A rating of the PWM board. Also confused how the diodes play into this.
DaOld Man:
Looks like a pretty neat little drive.
With this drive you wont need the secret drive.
I am attaching a modified drawing of how this motor would probably hook up to your DPDT switch and diodes and the PWM drive.
Note that since the PWM drive will stay on no matter if the monitor is turning or not, you may need to add a simple SPST On-Off switch to kill the power to it. This could also double as an emergency stop switch.
The switch needs to be rated for at least 2 amps.
Similar to this:
http://www.allelectronics.com/make-a-store/item/RS-188/ON-OFF-ROCKER-SWITCH-PREPPED/1.html
Notice that the DPDT switches polarity after the PWM drive. You cannot reverse the polarity of the power feeding the PWM drive, so I dont see any other way to do this.
A similar DPDT switch would be this:
http://www.allelectronics.com/make-a-store/item/RS-180/DPDT-MINI-ROCKER-SWITCH/1.html
For limit switches you could just use "Happ" type push buttons, or the micro switches that are mounted to them.
Or you could get a couple of these, which would be much better to make work with your rig:
http://www.allelectronics.com/make-a-store/item/SMS-196/SPDT-SNAP-ACTION-SWITCH-W/LEVER//1.html
The above limts are rated at 15 amps, which should be plenty good.
Diodes need to be rated for at least 2 amps.
Similar to these 3 amp ones:
http://www.allelectronics.com/make-a-store/item/1N5406S/RECTIFIER-DIODE-3A-600V-CUT/BENT-LEADS/1.html
The above parts can be found just about anywhere. You dont have to use the ones I linked to, the links are just to give you an idea. If you choose all the parts based on 2 amps or more rating, you should be OK.
Please note that you wont have dynamic braking with this setup.
DaOld Man:
--- Quote from: DNA Dan on March 12, 2011, 10:43:53 am ---
I don't quite understand the current (stalled) figure. Is that how much current it will draw in a shorted out state? That is the only figure that's over 1.5A rating of the PWM board. Also confused how the diodes play into this.
--- End quote ---
The stalled current is the amount of current the motor draws when the shaft cannot turn, due to a physical restriction.
This should be the very maximum amount of current the motor will ever draw.
If you are talking about the diodes in the PWM drive, looks like the one across the OUT terminals (D3) is to protect the transistor from CEMF when the drive switches the motor on and off. You will see this method used around most inductive devices, such as relay coils and motor armatures. When the inductive device turns off, it generates a back force (alternating voltage) while the magnetism is dying. Unloaded, this voltage can increase to several thousand volts, and even though it only lasts a few milliseconds, it can be enough time to damage silicon devices such as transistors. The diode shorts out the voltage when it is in one phase, preventing it from ever getting over ~0.6 volts. (This is the voltage drop across most diodes).
Im not sure about the D4 diodes function. Maybe it blocks capacitor C1 from discharging through TR1 and the motor?
Diodes D1 and D2 are setup to give the pulses different on and off times. (Look at the wave diagram to the right of the schematic.)
Looks like a pretty simple circuit, but if you have no soldering experience you may want to read up on it first, and practice on some scrap wire. If you dont have a good soldering iron, get one.
Also read up on using heat sinks on silicon circuits while soldering.
The heat sink can be a metal alligator clip clipped to the lead between the point being soldered and the device.
It absorbs most of the heat from the soldering iron to prevent from damaging the silicon part.
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