servo control vs stepper control

[JAZZCNC]

I wasn't suggesting he drive the CNC M/C side of the BoB from the PC. Only the PC side of the optos would come from the HDD 5v. I'm sure your PC wouldn't want you to put 24v or more up its paraport. As I stated the EL817s provide the isolation; on this BoB inbound and outbound, so your e-stop and limits etc. can go up to, say, 30v.

Ok don't want to send this thread off course too much but think best to clear this up so not to confuse.!!

Yes I under stood what you meant but don't think you did me.? I wasn't suggesting sending 24V up the PP's chuff.!!

The 24V supply has multiple uses.!!. . . It's sent thru the limits Switch's and E-stop to help with switch bounce and noise. This is done in conjunction with 24V relays, so it powers them has well.
These control the 5V signals from the BOB, they also act has safety for other things killing power to anything attached etc, enabling a hardware driven fully safe latching E-stop system that can only be restarted when in a safe condition and the master relay engages even then this only engage after pushing a reset button.
If any of the safety switch's are tripped or some things in an unsafe condition then nothing happens.! . . . . Also In this case and the point of my comment meant to also use it to power the BOB and therefore save messing about with the 5V from HDD.

[Peter.]

So from that I gather that whatever voltage I use to power the BOB, that's the signal voltage sent to the drive? Will that BOB linked in post 3 definately run safely off 24v?

[irving2008]

So from that I gather that whatever voltage I use to power the BOB, that's the signal voltage sent to the drive? Will that BOB linked in post 3 definately run safely off 24v?

The problem I have with running this BOB at 24v is this:




The output circuit has a pull up to 'Isolating + supply'. Without knowing the value of that resistor you can't be 100% sure its not going to fry. In the manual it shows 'Isolating Supply +5v'so I would assume they are sized for 5v and not 24v supply...

[boldford]

The problem I have with running this BOB at 24v is this:




The output circuit has a pull up to 'Isolating + supply'. Without knowing the value of that resistor you can't be 100% sure its not going to fry. In the manual it shows 'Isolating Supply +5v'so I would assume they are sized for 5v and not 24v supply...

They are 1K.

[irving2008]

They are 1K.

Ok, so 5mA collector current on 5v, 24mA on 24v.. so it wont fry :)

EL817 optos are rated 35v, 50mA so should be ok.

However, as I recall the diode has a 330R series resistor so forward current is about 10mA and those devices have a transfer ratio of about 1.1. So with 10mA in the diode the collector current is going to struggle to be more than 11-12mA which means the output volts wont drop below about 12 - 13v, so though it won't fry its not going to work. No good on 24v :(

[JAZZCNC]

So from that I gather that whatever voltage I use to power the BOB, that's the signal voltage sent to the drive? Will that BOB linked in post 3 definately run safely off 24v?

I wasn't referring to that particular Bob or any Bob really and so obviously it would need to support 24V but most decent Bob's feature a range of input voltages.

I wouldn't use a cheap BOB with quality Servo's and to me this BOB isn't really a quality Item, it doesn't have a Charge pump for one which is nice feature and it looks a bit small and cramped.
Cheap nasty BOB's are a pain in the arse and a large source of problems and they don't always show has plain "works or doesn't" so can cause any thing from weak signals to dropped signals to slow acting Opto's etc all of which if your unlucky to drop for it will drive you crazy. .

[boldford]

Ok don't want to send this thread off course too much but think best to clear this up so not to confuse.!!

Yes I under stood what you meant but don't think you did me.? I wasn't suggesting sending 24V up the PP's chuff.!!

The 24V supply has multiple uses.!!. . . It's sent thru the limits Switch's and E-stop to help with switch bounce and noise. This is done in conjunction with 24V relays, so it powers them has well.
These control the 5V signals from the BOB, they also act has safety for other things killing power to anything attached etc, enabling a hardware driven fully safe latching E-stop system that can only be restarted when in a safe condition and the master relay engages even then this only engage after pushing a reset button.
If any of the safety switch's are tripped or some things in an unsafe condition then nothing happens.! . . . . Also In this case and the point of my comment meant to also use it to power the BOB and therefore save messing about with the 5V from HDD.

I understood you perfectly well hence the reason why I said "I'm sure your PC wouldn't want you to put 24v or more up its paraport",
Whatever vendors BoB is used, assuming it is capable of complete opto-isolation, and you wish to maintain it, the PC side will almost certainly need to find a logic 5v since most paraports can usually sink far more current than they can source.
I realise I mistakenly suggesting the e-stop be routed back into the PC - Bad move to rely on S/W for that function!. If I did mislead anyone I apologise.