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shedhappens
18th Aug 2019, 05:41 PM
Hi fella's, is anyone up for solving a puzzle :D
I am hoping someone can advise me on this circuit for my cylindrical grinder.

I have a couple of questions, I have drawn a diagram in pencil of the circuit board in question and then colored the individual circuits using paint so I hope it is clear enough and understandable.

It seems to me that the orange (positive) and black (negative) circuits are the timer circuit to adjust the dwell (time from wheelhead switch contact to energizing the valve solenoid).

The large resistor mounted on the side of the control box has no wires connected and I think that it has been replaced by fitting a
5.5 M ohm resistor to the pot, but I could be wrong.

The top solenoid does not operate as the circuit is, I measured the voltage across the capacitor terminals and got a reading of about 65 vdc.

The neon tube appears to be the only negative path to energize the solenoid, how does this work? That is until the mercury switch is tilted by the solenoid below and that is energized by the relay on this top solenoid.

So it might need about 90 vdc for electron flow to create a negative circuit to the solenoid, looking at the circuit it gets the positive via the coil winding?
Is this right? The wiring looks to be ridgey didge.

So if this is how it gets illuminated it then supplies the negative to energize the solenoid?

Do I need to reduce the resistance to get a higher voltage to excite the neon for it to work?

Does the capacitor look like it would work ok given that the test reading is 10% higher that what is printed on it?

If anyone has questions or needs clarification about anything here please ask as I really would like to get this old circuit running, I have had a look and found a timer that I could graft into the circuit but that will be my last resort if I can't sort this.

cheers, shed

BaronJ
18th Aug 2019, 06:19 PM
Hi John,

A number of things stand out ! First the capacitor ! That is an oil filled paper capacitor. I wouldn't think that anything could go wrong with that, +- 10% in value is perfectly fine. A long as its clean and there are no oil leaks from any of the soldered seams, I wouldn't worry about it.

The neon ! They can only support passing a very limited amount of current and do demand a series limiting resistance. Assuming that your circuit is correct the relay coil is being used to limit the current to it. I think that it is just an indicator which will extinguish when the relay is operated.

The pot and resistors are part of the timing circuit but on your diagram don't seem to go anywhere. There would be a capacitor in there that would be charged up via the pot and resistor to create a time constant, (not a constant time). The very high values would suggest a low current circuit.

What is the black thing with 8 terminals in the middle of your diagram ?

HTH.

shedhappens
18th Aug 2019, 06:47 PM
Hi John,
The neon ! They can only support passing a very limited amount of current and do demand a series limiting resistance. Assuming that your circuit is correct the relay coil is being used to limit the current to it. I think that it is just an indicator which will extinguish when the relay is operated.
HTH.

Hi John, thanks for your reply, the neon is the only electrical (negative) path to the solenoid, there is no other way to energize the solenoid that i can see.


The pot and resistors are part of the timing circuit but on your diagram don't seem to go anywhere. There would be a capacitor in there that would be charged up via the pot and resistor to create a time constant, (not a constant time). The very high values would suggest a low current circuit.

The orange circuit/wire starts at 110 vdc then goes through the resistors and pot, then from there the reduced (about 65vdc) voltage/amperage goes to the large capacitor and solenoid. I think there might be too much resistance in that circuit to illuminate the neon? And I think that the neon needs to illuminate to energize the solenoid, as there is no other negative path.


What is the black thing with 8 terminals in the middle of your diagram ?

It is the wiring junction block, you can see the mercury switch wires attached to it

BaronJ
18th Aug 2019, 10:14 PM
Hi John,

Ahh right, terminal block, the penny didn't drop ! Give me a little time to study your circuit.

As far a the neon is concerned, if it is the return path for that solenoid, the solenoid should operate when the neon strikes. I don't think that will work, somehow.

Does the neon light up ? If so then it can only get a voltage supply via that solenoid. Yet according to your diagram that point is taken to negative via one of the mercury switches.

You have shown the wheel head switch supplying a negative voltage to the other side of the neon.

Can you confirm that the two relays (you have called them solenoids) have their contacts wired in parallel.
The bottom relay doesn't seem to have a negative connection to its coil, unless it is via the valve switch.

That capacitor seems to be directly across the entire power supply line. In which case use the negative terminal as the reference for all the measured voltages.

I'm struggling a little because there are too many points missing.

shedhappens
19th Aug 2019, 04:37 PM
Hi John,
As far a the neon is concerned, if it is the return path for that solenoid, the solenoid should operate when the neon strikes. I don't think that will work, somehow.

Does the neon light up ? If so then it can only get a voltage supply via that solenoid. Yet according to your diagram that point is taken to negative via one of the mercury switches.

I bypassed the suspect resistor and adjusted the pot to 75 vdc output and put the neon globe in.....BINGO :)




You have shown the wheel head switch supplying a negative voltage to the other side of the neon.
Can you confirm that the two relays (you have called them solenoids) have their contacts wired in parallel.
The bottom relay doesn't seem to have a negative connection to its coil, unless it is via the valve switch.

Yes that is correct, the top relay switches the negative to the bottom relay.
Remember how early English cars had a positive earth and switches operated the negative, so this machine got the same design
by the looks of it, so i guess you need to think a bit backwards :rolleyes:


That capacitor seems to be directly across the entire power supply line. In which case use the negative terminal as the reference for all the measured voltages.

It is switched by the wheelhead switch, so that circuit and the capacitor are only connected when the wheel head switch contacts the negative circuit. It looks to me that when that switch closes the voltage on the capacitor slowly rises (or quickly depending on how much voltage is coming in via the pot) and when the voltage gets to about 70 vdc the neon fires the relay, this close's the bottom relay which tilts the mercury switch and the negative circuit through mercury switch then "latches" the top relay at this point the valve lever solenoid is energized, it opens the valve and the valve switches and the head moves back and in doing so also breaks contact at the wheelhead switch.........well thats how i think it works.


I'm struggling a little because there are too many points missing.

The reference to this in the manual is "The machine is fitted with a simple timer" :D

Here is an 18 second vid of the xmas light https://www.youtube.com/watch?v=_K2WyJatl34&feature=youtu.be

I will put the control box back on the grinder and get the generator working and back on the machine then I will sort out what resistor is needed to replace the one currently on the pot.

cheers, shed

BaronJ
19th Aug 2019, 06:32 PM
Hi John,

I'm glad that you have it working. I wonder if the DC generator is actually an AC one with a rectifier to produce the DC. The reason that I suggest this, is that the old time rectifiers died quite slowly with age, the output getting less and less.

Assuming the relay coil is 4K and the supply voltage 100 volts, then V/R=0.025 amps or 25 milliampere, this is right at the limit for the maximum neon bulb current, for that type anyway. Circuit wise a little unusual.

The resistor values need to be quite high in order to achieve any appreciable time delay given the value of the capacitor. RxC=seconds

RC time constant calculator (http://referencedesigner.com/rfcal/cal_05.php)

12 Meg = 120 seconds.

Obviously I've rounded values here, but they won't be too far out.

That big resistor I would guess is a quite low value and might be something used in conjunction with a motor.

Did you manage to get a manual ?

shedhappens
19th Aug 2019, 10:43 PM
Hi John,

I'm glad that you have it working. I wonder if the DC generator is actually an AC one with a rectifier to produce the DC. The reason that I suggest this, is that the old time rectifiers died quite slowly with age, the output getting less and less.

So far I have only taken a brush cover off and it had no continuity from the brush to the commutator, so I am guessing clean the commie and maybe new brushes, there is no reg.


Assuming the relay coil is 4K and the supply voltage 100 volts, then V/R=0.025 amps or 25 milliampere, this is right at the limit for the maximum neon bulb current, for that type anyway. Circuit wise a little unusual.

I originally worked it out using an online ohms law calc at 110v / 3.9 K ohms = 0.028 A or 3.1 W, but today after finding that the neon fired up at 70 v I revisited the online calc and got this, 70v / 3.9 K ohms = 0.01795 A or 1.25 W

Do you think my figures are right? If so then things should be hunky dory.....[/QUOTE]



The resistor values need to be quite high in order to achieve any appreciable time delay given the value of the capacitor. RxC=seconds

RC time constant calculator (http://referencedesigner.com/rfcal/cal_05.php)

12 Meg = 120 seconds.

Obviously I've rounded values here, but they won't be too far out.

I will revisit your time constant calc when I get to the resistor, it is something I have never heard of so it will
be interesting, thank you for that, my plan was to get a maybe an 800 K resistor and suck it and see :D
The reason that I want to wait until I get the generator power to the control box is that the voltage might be higher or lower
than what i have now with the step down transformer and that might affect the resistor that I need.


That big resistor I would guess is a quite low value and might be something used in conjunction with a motor.

No, it only is in the pot circuit.


Did you manage to get a manual ?

Yes, I wrote a bit about it here... https://metalworkforums.com/f303/t199688-churchill-cylindrical-grinder/latest/post1954397#post1954397

Thank you for your input John :2tsup:

cheers, shed

BaronJ
20th Aug 2019, 02:06 AM
Hi John,

Sorry, I do remember reading that post. I had forgotten about it !

If you have a look at the the link I posted, it explains the time relationship R x C = seconds. Basically the higher the resistance the longer the time it takes to charge the capacitor. The neon bulb effectively sets the voltage level at which the relay will trigger. Which is exactly how using a neon as a voltage regulator works.

70 volts will be close to the striking voltage of that neon. Until it strikes there will not be any current passed through it.