Just out of interest, can multiple contacts be wired together, ie a 4 pole with contacts rated for 10VDC@10A wired in series to make effectively a single contact of 40VDC@10A? Or would teeny tiny difference in switching speeds cause one set of contacts to close first and take the full brunt before the other contacts caught up, leading to melting, fusing and the opening of portals to alien dimensions?
I ask because one of the plans had a relay for the bleed resistor on the 70V circuit, but I've not seen many (well, any) relays with 70VDC contacts.
Edit: Thinking about it, voltage increases in series - if it's in series then it doesn't matter if they all close at exactly the same time or not.
Last edited by Rogue; 20-09-2012 at 11:15 AM.
Voltage on relay contacts is generally not important, its the current carrying capacity. If you need more current then you wire the contacts in parallel. The reason why contacts have a lower DC voltage rating is to do with the arcing that happens when the contact opens. Generally the contact gap is very small, maybe <0.5mm or so. With a DC current the arc ionises the air in the gap and causes a plasma to develop. If the voltage is too high this plasma will continue to pass current until the contacts have opened sufficiently that the voltage cant sustain the plasma, which seriously shortens the contact life.
One solution to the arcing is to put a snubber across the relay contacts - a capacitor and resistor in series - the idea being that as the contact opens the capacitor charges up thus drawing current off the contacts and stopping the arc forming until the contacts are far enough apart to prevent the arc being sustained. This alows the contacts to be used at a higher voltage at the rated current.
This isnt a problem with AC contacts as the voltage returns to zero every cycle thus quenching the arc, thus they can be rated at much higher voltages.
Now, in your situation, using it for the bleed resistor isn't a problem. The current passed is tiny (30mA) so the voltage rating of the contacts isn't an issue as very little arc will be generated.
Incidentally, where did you get the 10v from? If you look at graph H55 on the spec for those relays, you will see that the 4PCO version is rated at 30v 6A. At 70v its derated to 0.5A... but thats only to meet the contact lifetime figures at 1800 cycles/hour. If you switch a dozen times a day it'll be a lot!!! On the AC side graph F55/1 shows that at 900VA the contact life is 200,000 cycles at 1800 cycles/hour so its not going to fail in your lifetime (about 45years at 12 cycles/day) lol.
Last edited by irving2008; 20-09-2012 at 12:30 PM.
While I wanted to keep everything from the same source for convenience, it looks like RS was my next stop.
24VDC DPDT, contacts rated to 250VAC @ 7A / 125 VDC @ 7A (RS Online 699-6869)
As far as I can see, this relay should work in any of the proposed situations that involves relays powered from the 24VDC system. As DPDT it is straightforward to wire it as normally open/closed as required.
As far as mounting goes... I could hot glue the relay somewhere and fix wires directly to the contacts. This would mean glueing it upsidedown or on its side. For various reasons I don't really like this idea. There are are DIN sockets available (699-6881) that should be much easier to fix somewhere, and would give me screw terminals to work with. While it adds on another £3 per relay I think that it might save me a bit of worry.
Now I'm up to (in effect) £10 per relay. Hmm.
Let's look back at Irving2008's previous post with drawings. In the second drawing, Relay 1 is handling the AC current and needs to be suitably higher spec. Relay 2, however, is only handling 24V. A 24VDC DPDT relay rated to 220VDC @ 2A (619-3013) is a much more comfortable £1.78 + VAT. Seeing as the transformer isn't even supplying 2A in the first place, presumably that would be sufficient?
Since I started to write this, I note that Irving2008 has posted again. If "..the current passed [for the bleed resistor] is tiny (30mA) so the voltage rating of the contacts isn't an issue as very little arc will be generated", then would the 2A relay above also work in that situation?
Yes. The originally spec'd relays (Rapid 60-1310) were fine for both positions.. I would socket mount them.
The RS699-6869 is overkill with the inbuild LED etc.
The RS619-3013 would be fine for Relay 2, but as a PCB mounted device its hard to use.
If price is critical then consider using Rapid 60-1667 in sockets. While these are only rated at 240vAC/5A and 600VA for the switching rate you will use them for they'll be fine and if you're paranoid you could wire the contacts in pairs.
As for the graphs, the issue for me is translating what I can see into usable information and that is where my lack of knowledge shines through. For example, you turn the supply on and run it for an hour. For that hour you're drawing a fairly constant 5A. My understanding was that, while the contact only switches once, it still has to carry 5A for that hour. My choices were made on that basis. Clearly that basis was wrong which is leading me to make some less-efficient choices.
Never being one to rush things, I've now got a toroidal transformer sitting on the bench ready to get wired up!
Specifically, this: Toroidal Transformer 625va 0-50v 0-50v --- link to Data Sheet PDF
I understand from the Data Sheet that:
- on the Primary side, I want to wire the Grey and Violet wires together, thereby wiring it in series to handle 230Vac from UK mains,
- the Blue wire is now 0V and the Brown wire is now 230Vac, then
- on the Secondary side, I want to wire the Orange and Black together, then the Yellow and Red together, and
- the Orange/Black is now 0V and the Yellow/Red is now 50Vac.
Wiring it in parallel will give me 50Vac @ 12.5A.
After being rectified this will give me 70V @ 12.5A (less voltage drop across the bridge rectifier)
Is anyone happy to cast their eye over the datasheet and confirm that I'm connecting up the right wires before this goes anywhere near the mains? The instructions look straightforward, but I've said that about lots of things that later went horribly wrong
The original drivers I was looking at from Zapp are now discontinued. The alternative (DM856) has a max supply voltage of +80, which I understand to be a comfortable margin for this PSU. The closest driver current options, however, are slightly different - 3.8A or 4.3A, whereas my steppers are 4.2A. The old PM752's would provide 4.02A as the closest fit. Is there a significant enough difference in the current provided that would make it worthwhile hunting for different drivers, or is 3.8A fine? These will now be driving an X2, which I'll be using as a stepping stone to my initial DIY design.
The good news is that I should be finished by 2016, possibly late 2015 if I pull my finger out...
15-08-2013 #69Originally Posted by Rogue
I got around this by rectifying the two windings separately then combining them at the DC side.
Just a thought. ..Clive
Last edited by Jonathan; 15-08-2013 at 09:25 PM. Reason: Fixed quote formatting
Is it better to match up the driver output with the motor rating exactly, or leave a bit of a margin?
Last edited by Rogue; 15-08-2013 at 09:14 PM.
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