Diode Pin what happens when a pin diode is used at high frequency?
In my profession as an avionics Communications and Navigation Engineer, we use pin diodes in VHF and UHF radio circuits. When the diode is subjected to very high frequencies typically in the 30 to 300 Megahertz range it acts like a capacitor. These devices are typically reverse biased in circuit and are used as filters to filter out unwanted harmonics in the circuit, in this role they are ideal in the tuning circuit of a radio control module to control the transmission frequency of the radio and make it easy to tune the transmitter to different frequencies.....
I hope this was enough information....good question though....
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driving stepper motors problems?
I am using an at89c2051 to drive a stepper motor (h546 from sanyo denki). The motor is connected to an l293d which is directly connected to pins p2 to p5 of the mcu. However I am having a big problem with the motor - when the motor is spinning, sometimes if i try to read some input pins these are read as high even though physically these are grounded. I am thinking that the problem could be from noise coming from the motor and somehow propogating to the mcu - do you think this is the case? If so I might try using some photocoupler isolator such as the pc845. Moreover since I am not using any protection diodes could this be a problem of back emf? But the l293d should have these diodes internally, right? Or could it be that this problems is because my motor requires more current than the l293d can source? if so what can I do?
thanks
The optocoupling won't solve your problem I think. But I assure you it CAN be solved, if you pay attention to three things: current loops, spike control and coupling elimination. In a previous life I worked on daisywheel printers which use steppers, solenoids, motors and low-level optical resolvers - all routed together in flexible cable. And it all worked fine.
First, you need to isolate the stepper driver on its own supply loop. Star-tie its power and ground back at the supply, separate from the MCU. You can also filter the motor supply to catch differential and common-mode noise.
Next, you can also slow down the edge rate to the driver using an RC damper. This will reduce the spikes. Very important. Also consider using a lighter "hold" voltage when the stepper is parked vs. when it's operating. This will save considerable power.
Also, it looks like they recommend additional freewheeling diodes in the data sheet (link below). These will help catch the back-emf locally closer to the motor. I'd say it's a good idea (I've always seen them used.)
Finally, examine the routing of your sense wires. Are they subject to noise coupling from the motor wires? If so, you can reroute them or shield them. Ground them back at the MCU and also provide their supply from the MCU too. This prevents forming another current loop.
Another measure you can use is current-mode or differential signaling to reject common-mode noise. You will need some kind of buffer for that.
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