#6 Single P.B. toggle (flip-flop)?
Max.
.

Would you re-post your solution.
The one where the two coils are kind of in series.
I can never find it when I need it.
Some of the ones google brings up don't work
Should be a sticky.

 

Just come across: "ladder logic". Maybe a useful source of ideas.

The only thing is Ladder is based on Boolean arithmetic and you can do many things in R.L. that is impractical with physical relays.
The basics are there though.
I should qualify that a little as early control systems were all relay logic before the advent of PLC's.
Max.

I feel there should be a distinction made between "PLC ladder logic" and "Ladder logic." Although usually when someone says "ladder logic," they are referring to a PLC, but this is not always true, and Ladder logic was around before PLCs; just a style of drawing relay wiring. PLCs were modeled on the existing convention for describing control circuits, in a way that electricians could understand. You can draw almost any physical relay control circuit in ladder style, and most machinery OEMs that continue to use relay control for their machines, continue to use ladder diagrams. But there is no guarantee that if you drew and wired a physical relay ladder logic circuit and drew the and programmed the exact same thing in a PLC programming SW that the PLC would behave the same way as the physical circuit, due to the way a PLC processes the info (scan, write at end of cycle, etc.)

This isn't meant to "teach you up" Max, I know you know more about this than I do; this is meant for Richard and the general public.

 

The OP was questioning it, not me.
Post#5.
Max.

Sorry bout' that Max...

eT

 

I feel there should be a distinction made between "PLC ladder logic" and "Ladder logic." Although usually when someone says "ladder logic," they are referring to a PLC, but this is not always true, and Ladder logic was around before PLCs;
.

True, the originator of the PLC, Dick Morley designed it to replace the older mechanical relay logic of the time and to be able to display it in the same format as a hardwired schematic, especially for the shop floor maintenance crew.
Also most PLC manuf allow programming to be done in a form of Boolean mnemonics as well as 'Ladder' and switch back and forth.
Incidentally poor ol George (Boole) came up with Boolean Arithmetic before there was a real use for it until the computer came along.

Would you re-post your solution.
The one where the two coils are kind of in series.

This one? These are the relays and values I used.
Max.

 

This one? These are the relays and values I used.
Max.
Attached Files
File Type: pdf FlipFlop.pdf (164.9 KB, 9 views)

I spent a bit of time going over this and I don't see how it works. Can you explain it?

I was looking at it and thinking - I don't see how relay B ever would get energized, and I don't see how relay A would ever latch - so I thought I would translate it to a "quasi-ladder" diagram (this is a perfect example of a relay circuit that doesn't want to be drawn in ladder) since that's more familiar to me. I figured in the course of translating it I would figure it out, but I didn't. Looking at my "ladder" diagram, I'm just as confused as looking at your diagram. Maybe I screwed up the translation, let me know. I'm sleepy.

 

The one I remember was different.
I remember not being able to draw it in PLC because one relay was common to each leg.

Something you can't do in PLC ladder is a standard motor starter. It has the overload relay in the common side.

Also don't remember other components.

It should be in Max's posting history sometime after I showed up.

 

I'm wrong.

Same one.

http://forum.allaboutcircuits.com/showthread.php?t=95136

Then I'm thinking of another!

 

Must be one I posted.

Does this work?

 

Must be one I posted.

Does this work?

Short answer: yes

long answer:

The following assumes the power supply voltage is twice the rating the coils and 
not letting the smoke is dependent on not holding the button down

t0 no current flows
t1 pb1 is pressed but not yet released:
   current flows through d2.1, PB1, d2.2, d1 coil - d1 is energized with twice the rated voltage
t2 pb1 is pressed but not yet released:
   d1.1 closes, creating a ground loop through d2.1, pb1, d2.2, d1.1, d2 coil
   d2 does not energize
   d1 is still energized with twice the rated voltage
t3 pb1 is released:
   ground loop eliminated, half power supply voltage is applied across d1 coil, 
      and half across d2 coil, through d1.1, assuming d1 stays energized (probable)
   d2 energizes
t4 Both relays are energized and latched through d1.1, and will remain so
   d2.1 and d2.2 have changed state
   
t...
t...+1 pb1 is pressed:
       a loop is created from +V, d2.1, pb1, d2.2, d1.1, d1 coil
       potential difference across d1 drops to 0
t...+2 pb1 is pressed but not yet released:
       d1 deenergizes and d1.1 opens, double coil voltage rating is applied to d2
       d2 remains energized
t...+3 pb1 is released:
       all paths through d2 are cut
       d2 deenergizes
       d2.1 and d2.2 return to initial state
       system is reset to t0

 

I spent a bit of time going over this and I don't see how it works. Can you explain it?

It just works.

Beginning in the condition in which the relays first powered and the capacitor discharged, the following is the sequence.

(a) Closure of the input sw operates relay A the through the top NC contact of relay B charging the capacitor through the bottom NC contact of relay B .
If the time required to charge cap is considered negligible, the minimum closure time of the PB is equal to the operate time of relay A.

(b) Opening the input sw releases relay A, allowing the capacitor to operate relay B. The input switch must remain open for a time no less than the release time of relay A plus the operate time of relay B.
At this point relay B is latched up through its middle NO contact and the top NC contact of relay A.

(c) The next closure of the PB supplies current to both relays through the top NO contact of relay B.
An input SW closure of sufficient duration to operate relay A breaks the latch up of relay B while not allowing the capacitor to charge.

(d) Opening the PB de-energises both relays returning the system to its original condition.
The result that relay B alternates position with each successive opening of the PB.
The additional contacts on relay B control independent circuits as needed.
The sequence of events that operate relay B (a) & (b) requires more time than is needed to return the system to its original condition (c) & (d).
The operation can vary due to speed of relays and value of capacitor, if the value shown do not work consistently, there may have to be a different selection of values.
Capacitance should be adjusted to the minimum value that will reliably operate relay B, insufficient capacitance will operate relay B intermittently or not at all.
In actual practice, there is no perceptible delay on relays operating.
Hope this helps.
Max.

 

Something you can't do in PLC ladder is a standard motor starter. It has the overload relay in the common side.

It is something you should not do now in hard wired systems.
It was commonly done at one time but now is frowned on and may even be in the current NEC or NFPA79.
But there was a concern that if the coil on the grounded neutral side or wiring to the O/L's were to develop a ground, it would render the O/L's inactive.
So the current thinking is to place them in the supply side to the coil for this safety reason.
Max.

 

Short answer: yes

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I may play with some bosch relays later. They're pretty voltage tolerant.

It may be that the switch from full 12v on one to series connection gives a little boost. I believe they hold in down to a few volts.

Would be nice if 12v relays would work on a 10 v system. Have my doubts also.

 

I may play with some bosch relays later. They're pretty voltage tolerant.

It may be that the switch from full 12v on one to series connection gives a little boost. I believe they hold in down to a few volts.

Would be nice if 12v relays would work on a 10 v system. Have my doubts also.

I suspect that most relays would hold in down to half voltage, but circuit demands d2 to energize on half voltage, assuming your supply matches the coil ratings. This is why I assumed supply should be double the coil ratings.

 

It is something you should not do now in hard wired systems.
It was commonly done at one time but now is frowned on and may even be in the current NEC or NFPA79.
But there was a concern that if the coil on the grounded neutral side or wiring to the O/L's were to develop a ground, it would render the O/L's inactive.
So the current thinking is to place them in the supply side to the coil for this safety reason.
Max.

I agree but I am still seeing brand new motor starter packages coming with OLs on low side .

 

Got it, thanks max. Very clever!

 

I agree but I am still seeing brand new motor starter packages coming with OLs on low side .

I did a little research a while ago to find out how this came about, the only explanation I found that evidently it was claimed to make the wiring easier in a remote starter units for some reason, but I know for sure it got carried over to large enclosure control systems for a reason I could never fathom?
Max.

 

I did a little research a while ago to find out how this came about, the only explanation I found that evidently it was claimed to make the wiring easier in a remote starter units for some reason, but I know for sure it got carried over to large enclosure control systems for a reason I could never fathom?
Max.

I believe the rationale is this: if the OL contacts were on the high side, and a short to ground were to occur (likely) between the OL contacts and the coil (unlikely, but possible I guess), it could weld the OL contacts closed. Then you have no OL and you don't know it. But by putting them on the low side, there is no chance of them being welded, thus ensuring that the overload will always be able to do its job (ok, not always, but usually)

 

Actually the actuator of the contacts are not in series with the coil but in series with the motor, on the older thermal type and the now common Electronic type, the (old) heaters or electronics open an aux contact that is the one shown in series on the neutral side.
It is common of course to use the N.C. aux contact in series with the coil, in the case of PLC systems I also use the other N.O. contact, if available, as an input and open the coil output string as an added precaution.
Apparently I am not the only one confused by the reason.
http://www.electriciantalk.com/f28/motor-starter-27934/
I did not find anything in the NFPA79 that I have, but I remember reading a technical article in one of the Major Electrical Magazines by a Engineer that gave many current reasons for placing the contact in the live side of the coil.
It is one I have adopted for some time now anyway.
Max.

 

I found this out there from NEC

2005 N.E.C. 430.73
Where one side of the motor control circuit is grounded, the
motor control circuit shall be arranged so that a Accidental ground in the control circuit remote from the motor controller will
(1) not start the motor and ,
(2) Not bypass manually operated shutdown device or automatic safety shutdown devices.


430.31 (Specifies intended uses of Overload devices.)

But evidently there is an exception where the contactor or relay is in the same enclosure as the safety device or O/L.
Which makes no sense to me?
Max.

 

Remember that 120VAC (or grounded common) controls was not always the norm.

Years past, it was common for an enclosed M.S. package, to come with line voltage coil, prewired to 2 lines and holding contact. Ready to install start-stop buttons in cover or adjacent.

For automation, an aux. control relay was used rather than a coil change.

Yet that doesn't explain the "why".