routercnc_MK3

[routercnc]

Got one of the drivers wired up for power and hooked the linear supply direct to the mains feed (just to check operation)
The driver green led came on and got 72V across the power terminals. Then switched off to see how long it would take to drain the caps. It was dropping just under 0.5V per second so took a couple of minutes to drop to a low level (obviously will be quicker with all three drivers). But strangely it stopped at just under 4V and stayed there. I waited but it did not go any further. In the end I held a 10K resistor across the cap terminal to drain it right down. Why did the voltage stop? Must be something in the drivers that stops drawing current at 4V?
This means there will always be a bit of energy stored in the supply long after it has been turned off. This bothers me a bit so I may add a bleed resistor permanently. Anyone else checked if their power supply drops to zero?

[Clive S]

You will find in reality in use that with the motors connected it will drop in seconds. I personally don't use a bleed resistor. ..Clive

edit A 10K will do nothing to drain it down even from 70V it will only draw about 7mA

[JAZZCNC]

Your bothering over nothing. Get it built and working stop freting over shit that doesn't matter. 4v won't even tickle.!!

[routercnc]

Hi Clive, Jazz,

OK thanks for the info I've stopped worrying now . . .

[routercnc]

Whilst waiting for the various bits to arrive I've turned my attention to the wiring I'll need for the proximity switches. I've kept this job until last as I knew there were going to be problems.

The prox switches are SN04-N. Brown wire (10-30V) , blue wire (0V), black wire (signal NPN NC).

I rigged one up on the bench (at 12V for now), with a 700ohm resistor between brown and black, then checked the voltage between black and blue. Got 12V or 0V when a metal plate was present or not.


All good, but my BOB is 5V inputs only. So I've thought about the options:

1. Upgrade to 24V logic BOB ! Maybe when funds permit but not now

2. Use a pair of resistors as voltage dividers
I think can work out the ratios OK (top one ~4.5 times the bottom one, giving ~5V tap out in centre?)
But what actual values to use to control the current into the BOB?
What is the acceptable current into the BOB - I've seen a USB board quoted at 7mA . . .
How do I calculate the current my resistor selection will allow into the BOB?
I've got a feeling I need to know the resistor value inside the board . . .

Here is a diagram I created showing one option (adapted from a great post by Irving long time ago):


Here is another option:


But I have a feeling I need to do / know this:



3. Use an opto-coupler module

I bought this out of curiosity (£1.32 all in) - just got to wait for it to arrive in a couple of weeks:



http://www.ebay.co.uk/itm/3911024978...ht_5545wt_1109

I asked the supplier about max input voltage and max output voltage but after several days no reply. In the advert they suggest an ideal use is using 3V systems to control 24V systems but I want to go the other way around.
They sell sauce bottles, phones and underwear so I guess they don't make them themselves and probably have no idea!!

Any thoughts on the above options and questions?

Thanks

[Graeme]

The output of an NPN switch does not have to go to the supply voltage. There is no need for additional opto-couplers or voltage dividers. A NPN output is simply just a switch to 0V.
Depending on the inputs to your BOB you have two simple options. If the input is connected to an opto-coupler then connect the anode to the BOB supply voltage (5V) and connect you proximity switch output to the cathode (may need to add a series resistor if BOB does not have one). If the BOB has a logic type input then simple connect the proximity switch output to the input. You may need to add a resistor between input and BOB supply (5V) if one does not already exist on the BOB.

[HankMcSpank]

A NPN output is simply just a switch to 0V.

The datasheet is a bit scant... I'd verify that there aren't any pull-up resistors attached to the output of that NPN switch (which would see the supply voltage on the output if the switch is in the open state)....therefore to test, just provide it with its supply voltage & see what voltage is on it output (if any)