On 10/02/2011 01:38 PM, emc-users-requ...@lists.sourceforge.net wrote: > After having more experience with getting my PWM to VFD input working, > it comes to my mind that what is behind the VFD input is an ADC, which I > think takes a snap shot (sample/hold) at a frequency. I seem to recall > one VFD manual stating the sample frequency at something around 500 Hz, > and others in the tens of kilohertz. This seems to indicate that the > input will sample whatever voltage happens to be on the input at a > given, but regular, instant in time. With raw PWM this is typically > either 0 or 10 Volts. With filtered PWM, the voltage could be anywhere > along the charge/discharge ramps. It seems with a push/pull on the > input, it may actually make things worse by making the input more like > raw PWM. If my thinking is correct, ideally each PWM cycle would be used > as a data point to drive a stable voltage output for that particular > cycle. Even then, it might be better to block a sample that happens > right on a cycle change or filter only close to that point to smooth the > change. If you're looking for an easy way to drive both high and low through a resistor into the smoothing capacitor, I recommend a 555 timer circuit. Notice at the link below you need no extra components to make the 555 an inverting Schmitt trigger.
http://www.kpsec.freeuk.com/555timer.htm#buffer Using the opto isolator circuit mentioned earlier in this thread if you add the output transistor of the opto on the correct side of the trigger/threshold node, and a resistor to the opposite side, you can make the whole circuit non-inverting. The beauty of the 555 is if you already have the opto, the resistor, and the capacitor of the filter, one more resistor and the 555 should be all you'll need to let the PWM produce 0-10V, and most 555s are good for up to 18 volts of supply. If you need to control a 555 from a TTL output, just put an LED between the control pin and ground. that will set the trip point for an input logic high at about 1.5 volts (the voltage across the LED) and the logic low trip point of 1/2 that (about .75 volts). Luckily, these are within the standard output levels of a TTL chip. 3.3 volt CMOS logic will have outputs which work with this arrangement. ------------------------------------------------------------------------------ All the data continuously generated in your IT infrastructure contains a definitive record of customers, application performance, security threats, fraudulent activity and more. Splunk takes this data and makes sense of it. Business sense. IT sense. Common sense. http://p.sf.net/sfu/splunk-d2dcopy1 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
