Hi Folks,
I am trying to solve a basic electrical problem but unfortunately my disability (ME/CFS) is acting up tonight and I am feeling decidedly 'brain fogged' and confused. Can someone please check my 'thinking' to make sure I am heading in the right direction? The problem I face is that I promised to make a number of 'trophies' for the annual charity kite festival for the kite club I belong to. The festival is the weekend after next and I need to get my system up and running ASAP if I am going to get them made. Unfortunately my original motors/board were not fast enough for my needs and I have upgraded my system to a Hobby CNC board, larger motors and a bigger power supply. I have the board built and I am awaiting delivery of the transformer so that I can build a larger power supply. In the meantime I am trying to get everything else sorted and the wiring figured out so that when the transformer arrives I can simply solder everything together.
Now, my original idea was to have an e-stop button controlling the coils of two relays, the switch would be wired NC so that it 'pulls' the contacts 'in' when in the 'up' position and when the e-stop is pressed the relays would drop out thereby killing the power. One relay would switch the 110VAC line going to the router motor and the other would switch the DC voltage going to the board and therefore the stepper motors. With the increase in DC supply voltage I am now confused as to the best method of reducing the voltage so that I can 'feed' the coils of the relays.
The information I found on the net for the relays in question (Potter & Brumfield type KRP11DG) says the coils are rated at 24VDC, with a resistance of 472 Ohms and a nominal coil current of 51 mA. My transformer is expected to put out 25VAC at 10A so by the time it has been through the bridge rectifier and smoothing cap I will be looking at 35.35VDC available to the board. In order to obtain 24VDC to feed to the relay coils can I simply insert a resistor into the feed line to the coils (if so what size/wattage) or do I need to build a voltage divider using two resistors with a 'tap' point between the two (and if so what size/wattage)?
I am rather embarrassed that I can't figure this out myselfas I am a qualified ham radio operator and used to be able to do this stuff with no problems before being struck down with this 'damn disease'!
As a side note, my new board has the ability to take an e-stop signal and feed it back to my controller (Turbo CNC). My previous board did not which is why I was killing the DC voltage to the board to stop the stepper motors in an emergency. I was hoping that TCNC would see the loss of signal on the limit switch line as a 'hit limit' and stop the program otherwise the program would continue to run even though everything else had stopped. Does anyone have any ideas on how I can supply a signal to the board with the set up described above bearing in mind the e-stop circuit is likely to be on 24VDC and I am guessing the board/TCNC is looking for a change in state on a 5VDC signal.
If I can solve these problems before my transformer arrives on Friday I might be able to meet the deadline although we do have a contingency plan to produce 'certificates' if I can't get the trophies made in time.
Any help greatfully appreciated.
Larry