Crydom CMX100D10 Solid State Relays
Alexander (Sandy) Hapgood
Posts: 360
I just got some of these relays from DigiKey yesterday.
I applied 3.3V to the control pins and measured the control current at 6.7 mA so driving them directly from a prop pin should be no problem. My test circuit was applying 3.3 volts to the control pins and switching 3.3V to drive an LED on a PPDB with the power pins. They're a little expensive but perfect for switching 28.5V DC solenoids with 33V TVS diodes across them.
Anyway, if cost is no object then these guys are just the ticket!
Sandy
I applied 3.3V to the control pins and measured the control current at 6.7 mA so driving them directly from a prop pin should be no problem. My test circuit was applying 3.3 volts to the control pins and switching 3.3V to drive an LED on a PPDB with the power pins. They're a little expensive but perfect for switching 28.5V DC solenoids with 33V TVS diodes across them.
Anyway, if cost is no object then these guys are just the ticket!
Sandy
Comments
While I do not doubt they work and work well I just use tiny dual N channels in SMD8 packs to drive various large 24V solenoids. I use SI4906 at well under a buck.
I was going for overkill and I think I succeeded! :-)
In my first post I did invoke the "if cost is no object" clause.
Sandy
Well, as you can see from the posts that I am not trying to compare with isolated, it's just that isolated is not really needed in a lot of installations as careful wiring prevents ground loops anyway. The emphasis I placed was on the size of the thing too as all too often we see MOnSterFETs being used and they are totally unnecessary. Anyway it is very easy to achieve isolation as one way is with a $6 digital isolator chip with built-in isolated supply Total cost even with a more expensive MOSFET < $10 or around $3 if I use optos and the load supply.
The conditions that can cause faults may not always be possible in a specific application but the ground loops are still present. It is wise to consider this potential for every design, it can eliminated many headaches later on.
OP says he can drive relays easily if cost is no object, as perhaps he may have had problems in the past and wanted a sure-fire guaranteed cost-is-no-object solution. I drive big clunkers all the time and I never need isolation but experimenters with little experience may like the OP's solution though, if cost is no object that is. If they can't afford it they can use my solution but if they run the power ground through the Prop chip then I'm afraid my solution (and many others) will never be the right solution for them.
Count yourself lucky. It's a very real problem.
If you'd sugested the combined transistor and opto-couplers first off that would have been a reasonable comparision. That is important because isolation can be very important.
I can get lucky sometimes too but if I get lucky all the time it might be due to something else, and I feel that careful design has a place in there somewhere. Just because the magic smoke escapes and the part doesn't work anymore doesn't mean we can blame it on voodoo, it's what "you do" that's to blame.
If you'd sugested the combined transistor and opto-couplers first off that would have been a reasonable comparision. That is important because isolation can be very important.