Peter wrote:
I need 4 isolated outputs operating on 24V. Outputs will be changing state 24 x 7 for many years. My thoughts were opto isolator transistor drivers.The intended controller is a MS120.
This controller is going into an industrial situation and will be critical to monitoring production. Do you have any reliability recommendations?
Reply:
Hi Peter,
You didn't mention what the isolated outputs are to drive. Assuming its something with a bit of power, I suggest you look at DC solid state relays. Google for "DC solid state relays"
As to reliability, the most important things are:
- Isolate electronics from nasty environmental factors like heat, vibration, dust, humidity (condensation) and electrical abuse ("sparks and spikes").
- Make sure any nasty aspects of the load are properly taken into account. That means you must understand what the load is and does.
- Motors produce inductive kick-backs
- Motors have high inrush currents
- Solenoids (anything with a coil!) have inductive kick backs.
- Some loads look like large capacitors. From time to time we have someone use a solid state switch to directly control power to a device like a battery powered transistor radio. The problem here is that the radio will contain maybe 1000µF across its battery terminals. That will look like a dead short when power is first applied.
- Motors produce inductive kick-backs
The most important factor in component failure is temperature. As a rule of thumb every 10'C rise in temperature will double the failure rate, i.e. probability of a component failure in a given time.
One other failure mechanism is wear-out of Flash memory components. In SPLat that means how many PermStore, ShadowWrite or NVEM writes your program does. The SPLat Knowledge Base describes the endurance for each controller product, for example the section "MS120: Non-volatile memory".
By the way, do not automatically assume that solid state is always better. Good old fashioned relays still have a place. In your case, SS will be best because at 100 operations a second a relay will wear out mechanically in weeks or months. However, in general a relay is much more tolerant of an occasional overload that would kill a solid state switch instantly. An example of this would be a jammed motor drawing 20x its normal running current. The relay could take it on the chin until the fuse blows, while a transistor provides perfect protection for the fuse! When we design solid state DC motor drives we include current sensing and through hardware, firmware or SPLatWare turn off the drive if it senses an overload.
I hope this helps.
David
