Boxford TCL125. (Fresh start )

[depronman]

I've just checked both the UK and Australian RS websites and you're right. Free delivery for all online orders for those of us living down under but not for you poor souls still living in pommieland.

It’s a long way to come to Oz to get free delivery from RS . Sounds like a good excuse though...

Paul


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[John11668]

For the sort of quantities I need , ebay is sometimes a reasonable route

[Kitwn]

It’s a long way to come to Oz to get free delivery from RS . Sounds like a good excuse though...

Paul


Sent from my iPhone using Tapatalk

Any excuse is a good excuse for moving here, especially now.

[John11668]

Another lesson this evening!
I decided to take a supply to the driver board from my 5 v PSU rather than regularly falling over a Usb lead from the computer.
And I took the opportunity to make some of the other wiring a bit more compact. So I had the power switched off , and unplugged for (I think) more than 20 mins when the 24 v supply cable touched the chassis giving me a hefty flash and lightly welding itself . Maybe need to be careful of big capacitors .
I think it has taken out the driver board as the traverses dont seem to be working. Spindle controller still OK though .

Is there a way to quietly discharge the capacitor when I switch off??

[Doddy]

Another lesson this evening!
I decided to take a supply to the driver board from my 5 v PSU rather than regularly falling over a Usb lead from the computer.
And I took the opportunity to make some of the other wiring a bit more compact. So I had the power switched off , and unplugged for (I think) more than 20 mins when the 24 v supply cable touched the chassis giving me a hefty flash and lightly welding itself . Maybe need to be careful of big capacitors .
I think it has taken out the driver board as the traverses dont seem to be working. Spindle controller still OK though .

Is there a way to quietly discharge the capacitor when I switch off??

Big capacitors?, I always fit a bleed resistor across the terminals - nothing too exotic - for exactly that reason. Others here will argue against that... we agree to disagree.

[John11668]

Big capacitors?, I always fit a bleed resistor across the terminals - nothing too exotic - for exactly that reason. Others here will argue against that... we agree to disagree.

Not as bad as initially assumed .. Board IS working but amazingly the jog function works better when some "plonker " does not forget to check that the "Jog on" function is selected Dooohhhhh!

Nonetheless I do feel it would be a good idea to bleed away charge from the capacitor when the the supply is broken . What size of resistor would be approprate ??

[Neale]

I'm in the opposite camp to Doddy in that I don't use any bleed resistors. Why? Because they only matter in practice if you are running the PSU with no load. Once it's all in and working, the load on the PSU is going to discharge the capacitors anyway. If it's a switch-mode supply, then the actual capacitor value is quite small - my 24V supply feeds, amongst other things, the "Mains On" LED on the front panel and that and the motion controller between them discharge that supply very quickly - the LED goes out with no measurable delay when you hit the mains off switch. The 68V main driver supply has four stepper drivers permanently connected; even in "disable" mode when the output devices are presumably switched off, they draw power for their internal electronics and again, that's enough to bring the volts down fairly quickly.

While a bleed resistor won't do any harm, it's something else to house, it wastes power whenever the PSU is energised, and it can get hot.

There are different ways to look at the problem and estimate associated risk. For example, I don't have an interlock on my control cabinet that cuts mains power when it's opened. You pays your money and you takes your choice!

Although a bit of a belt-and-braces man, I personally stop short of the electric trouser hoist!

[Doddy]

For the benefit of others: This addresses a particular question posed, based on the OP's determination that having a charged capacitor is something that he wants to avoid and because of the observed behaviour. It's not intended as a 5 page debate over bleed-resistors-or-not.

"what size of resistor would be appropriate"

You need to solve a couple of equations...

Tau = RC

(R is the resistor value in Ohms,
C is the capacitance, in Farads
Tau is the value of time taken for the voltage in the capacitor to fall by 63% (or to 37%) of the original voltage).

So, for example a 20,000uF (0.02F) capacitor, and a 1k resistor (1000R), Tau = 1000 * 0.02 = 20 seconds to discharge a 100V to 37V. Then another 20 seconds to discharge to 14V (37% of 37V) and so on. The lower the value of resistor, the quicker the rate of discharge.

However, the lower the value of resistor, the higher the power dissipation that resistor endures whilst powered on, until the capacitor is discharged.

P = V^2 / R

(where V is the voltage across the capacitor)

Assuming your 24V supply.

P = ( 24 * 24 ) / 1000 (using the 1k resistor above).
= 0.576 Watts.

Therefore you'd need a resistor rated at above 0.576 watts - there are a 0.6W rating, then you typically find 1W, 2W, 3W, 5W etc...

Also consider then that you're dumping that power into your PSU enclosure as heat whilst the PSU is operating. To be honest, a couple of Watts is 2/3'rds of cock-all, but you need to be aware.

Myself - I have 72V DC power and, I think 10,000uF capacitors (x2, dual secondary/supply) - I really can't remember but expect I've used in the order of 2k2 wirewound 3W resistors. But that's just my choice/wet finger.


EDIT:

AHA, Neale - you posted first. Part of my rationale is that my cabinet is modular in design - as you say, if I isolate the drivers whilst I'm working in the cabinet, it's reassuring vs spot-welding the wedding ring to chassis.

[John11668]

OK points taken and it seems that no damage was actually done but the arc was a bit intimidating.
The wedding ring seldom gets into exposed positions (fat fingers) but I am never without the trusty old (and thoroughly abused )Seiko Titanium, and the idea of it being welded to the chassis does not appeal.
However I am not sure if I can accommodate one of these https://www.cpaltd.net/dynamic-braki...l?options=cart

If I choose to dispense with the old Boxford control panel then that will vacate some space where the junction strip now resides so may think about it .
Laterally thinking, might it be possible to make the E stop a changeover switch which when it breaks connects the capacitor via the NO terminal to the heater to and thus on to earth , thus avoiding having an electric heater going continually in the panel as the machine is operating??

[Doddy]

OK points taken and it seems that no damage was actually done but the arc was a bit intimidating.
The wedding ring seldom gets into exposed positions (fat fingers) but I am never without the trusty old (and thoroughly abused )Seiko Titanium, and the idea of it being welded to the chassis does not appeal.
However I am not sure if I can accommodate one of these https://www.cpaltd.net/dynamic-braki...l?options=cart

Check your decimal point.



These are 3w...or maybe 5w from the look of them, across the capacitor terminals.