Thread: Ready Steady Eddy
-
19-11-2013 #71
Please forgive my ignorance but could someone explain how this will work in basic terms? Why is Eddy's first circuit not fail safe and how does the next circuit achieve that?
Sorry, I've not had a lot of sleep over the past few nights (babies all have colds....) and I think my brain may be turning to mush.
-
19-11-2013 #72
The transistor is used as a switch. When positive current is applied to the base of an NPN it switches on. In the original circuit, the transistor is simply connected in series with the relay, so when the current is applied the transistor switches on and so does the relay. The problem is, if the signal to the base is broken (e.g. wire accidently cut), the transistor and thus the relay will switch on. We want the relay to be off in this situation, so another transistor is used to invert the signal. When the current is applied to 'new' transistor, it switches on so connects the base of the second transistor to ground, which in turn switches it off - hence the signal is inverted.
-
19-11-2013 #73
Thanks for taking the time to reply Jonathan. Very helpful. I was also having hard time understanding/visualising the "active low" part of the puzzle.
This also helped me understand:-
"If the input signal is high there will flow current through R2 and the transistor's base-emitter junction (base, not gate). This current will be amplified, and the collector current through R1 will cause a voltage drop so that the output will be low. Input high, output low.
If the input signal is low there won't be any base current, and no collector current. No current through R1 means no voltage drop, so that the output will be at +V. Input low, output high."
-
19-11-2013 #74
Yes that's what the Charge Pump relay is doing. The BOB is turning a 12khz signal from Mach3 into a 0 or 5vdc output, on my particular BOB the output of ENA is 0v when Mach3 is healthy and +5v dc when it stops.
Those look just right, I looked on Farnell and they appear to be 4 pole Change Over which is what you need if using my diagram, the 5A rating is for each contact.Last edited by EddyCurrent; 19-11-2013 at 09:39 PM.
-
19-11-2013 #75
-
19-11-2013 #76
-
19-11-2013 #77
-
19-11-2013 #78
The breakout board I'm using is this one SYSTEM1
I emailed Roy regarding the charge pump relay and he obviously suggested a transistor buffer but more interestingly the board has 'spare' 12v and 5v outputs and his suggestion was to connect a small 5v reed type relay between +5v and ENA. This has the advantage that when Mach3 is healthy ENA is at 0v therefore a potential of 5v exists between +5v and ENA making it fail safe plus it would share the load better with the 4 connected AM882's. This might be the way to go other than the reed relay does not sound too appealing although in practice one of those encapsulated type that look like a chip with about 8 legs have proved to be reliable in industrial situations.Last edited by EddyCurrent; 19-11-2013 at 10:12 PM.
-
19-11-2013 #79
With electrical systems there are generally two main circuits, the 'control circuit' and the 'power circuit'. Contactors are regarded as components of the power circuit whereas relays belong to the control circuit, it's mostly to do with the current carrying capacity and the ability to break that circuit without arcing or at least controlling the arcing. There are relays with positive guided contacts and these are for use in safety circuits, I have not come across equivalent contactors but that's not to say they don't exist. I'm not 100% on this but I think relays originated in communications areas such as telephone exchanges. Contactors can have auxilliary contacts fitted which in essence are relay equivalents and are used in the control circuits for those contactors.
Last edited by EddyCurrent; 19-11-2013 at 11:30 PM.
-
19-11-2013 #80
Just to clarify Jonathan's answer, the BOB ENable signal is Active Low i.e. enabled = 0v, disabled = 5v. In Eddy's original circuit the relay would be OFF when the BOB says Enabled and ON when disabled. But a BOB failure, a supply voltage failure, or burn out of the transistor would all turn the relay OFF, an enabled state, so not fail safe. As J says, the addition of the second transistor inverts the logic so relay is ON only when system is enabled and voltages are present.
J's later solution achieves the same result by using a PNP transistor to invert the logic.
[edit] typed this ages ago but forgot to hit submit.... :roll: and now its out of date lol
Last edited by irving2008; 19-11-2013 at 11:38 PM.
Thread Information
Users Browsing this Thread
There are currently 5 users browsing this thread. (0 members and 5 guests)
Similar Threads
-
traveling steady, turning 1/2 bar down over 10".
By graffian in forum Metalwork DiscussionReplies: 3Last Post: 08-05-2013, 07:59 PM -
NEW MEMBER: Hello all, finally ready to start a build log.
By Iwant1 in forum New Member IntroductionsReplies: 11Last Post: 25-12-2012, 06:03 PM -
eBay: Boxford 125 TCL Mach3 Ready
By rnr107 in forum Items On eBay UKReplies: 1Last Post: 25-07-2012, 07:29 AM -
Anyone have experience of ready made machines
By bogstandard in forum Milling Machines, Builds & ConversionsReplies: 9Last Post: 07-04-2010, 07:44 PM -
Windows 7 RTM Confirmed & Ready !
By Lee Roberts in forum Computer SoftwareReplies: 8Last Post: 29-07-2009, 03:33 PM
Bookmarks