Thanks Dabbler, I'm sure there will be some interesting discussion and analysis around that schematic- on first glance it looks like a light dimmer circuit driving a bridge plus a few refinements...cool
The use of TWO fuses is very odd. And R12-R13 just
looks like a waste of energy, but might be there to save the switch contacts
from burn scars. If those components are left in place, the AC for
release will be less effective (an MOV instead of the resistors and diode
would possibly be suitable). The 3P3T switch does disconnect the DC,
which is a tough job.
R3A sets a fixed light-dimmer setting as is sensible. Triac device variation would
be the reason not to use a fixed value, and "R3" is probably the manufacturer's
tweak, removed during final assembly.
R3 is used to set the low threshold of the magnets. each magnetic chuck, even if they try to make them identical will have different characteristics in the low end. It just sets a minimum bias by forcing the phase shift to be nonzero. A good starting point is less than 10% of the pot, or about 33K. Mine is 50K.
F1 is the normal fuse you'd expect. F2 is there to prevent catastrophic failure if one of the diodes or the triac calls it quits. better to blow a fuse than all the components.
R12 and R13 is there to bleed the massive energy stored in the magnet when the switch is turned off. It is set up as a voltage divider to lessen the stress on the bleeder diode D5. With this circuit, the energy will bleed off quickly, in about 1/10th of a second on my chuck. If an operator is very aggressive it stops the extreme strain on the circuit if the stored energy is fed back into the circuit due to a fast transition from 'magnetized' and 'demagnetize'. I think R12 helps smooth the waveform during the inrush...
A quick note about the release: First it is reverse DC that causes the demagnetization, and second you need a much weaker field to demagnetize the part lest you remagnetize it with the opposite poles... You can all ways give it a quick 'nudge' to lower the force to remove it from the chuck...
I hope this makes the circuit a little clearer. This unit has been in operation since 1990 without failure, so I assume it is a good circuit.
I don't quite follow what they're doing with the triac gate circuit switching- is the triac in full on mode for release? Then what power level do they use for mag on?
Why not just leave the triac setting the same for release as when mag on? Wish I could reach through my laptop and put a scope probe on a couple points
ps I see the two fuses would not make Bob K. happy! Also no isolation
Is the 500k pot an operator adjustment or an internal trimpot?
The Triac is phase switched - the RC (R3 + C2) circuit causes a phase delay in triggering Q1 using Q2 diac for edge transition. By doing so, they change the available voltage potential (because of the Triac turning on later in the phase) Because of the inductance of the magnet, the limiting the voltage will limit how much energy the magnet will be using to hold down the part.
On release, the triac is still energized in the same way as it was in holding mode. The extra resistance in the demagnetize circuit is to lower the magnetic force a little to use less force in demagnetizing the part. You use less energy to demagnetize. If you used the same energy, it would just magnetize the part with opposite poles.
The pot is an operator control to change the strength of the mag chuck when holding small parts that you don't want distorted. That is the beauty of an electromagnetic chuck. You can choose the holding power.
That is about all the knowledge I have on the subject. Please, if there's an electrical engineer on board, maybe you could explain further/better/more accurately...
I guess I should have been more specific in my question: I understand how the phase angle control of the triac is accomplished with the diac and r/c time constant. What I don't quite get is the physical switching as pertains to the demag mode. The triac is full on, but then the pickoff from the bridge is giving reverse polarity half-wave (or half amplitude) DC to the chuck at reduced current I think is what they're doing, and D5 is actually forward biased. Personally I would have used tapering AC in some way for the demag, but apparently this works well too.
I get what you mean now - sorry to have missed it. Yes it is half-wave DC. I think you have the right of it - trying to reduce the power in the chuck during the release... There is an important omission on the diagram - the release position is momentary, spring return to 'off'. The 'on' position will stay on. From my current limited experience with the chuck, about a second on 'release' is more than sufficient to release a part.
I had proposed a circuit a few dozen postings back about one possible approach and here we have a circuit that's quite different. Fascinating, as Mr. Spock would say.
More than one way to skin a chuck. I may do yet another schematic with all the best features...
My mag chuck has one figure on the name plate stamped '66' but I can no longer make out what the value is for. Now I'm thinking it may be a de-mag voltage rating. 110Vdc operating, 66Vdc reverse for demag. Would sort of tie up with what this circuit is doing using only a half wave rectifier for the demag function.
It is pretty much similar to the curcuit I was trying to develop - triac feeding a rectifier with some reduced voltage for demag. I was trying to changing the triac triggering for the reduced voltage but the full to half rectifier is a nice approach.
Look closer I think it's the 150 ohm 1/2W resistor in the triac circuit, R7- the color code (or what's left of it) looks like brown green brown = 150
PS maybe it's not burned and it's just a shadow on the picture-
I'll still check it out before turning the machine on! Frankly there's problems with all 18 Bijur flow control devices. I may have to replace almost the entire lube system - the chuck is 2nd priority at the moment...
OK ... I finally received my order of PTCs from China (thru Alibaba). Tried one in my homemade magnetizer/demag, but initially with unsatisfactory results. So I measured the "cold" resistance of the PTC - about 20Ω. The motor stator I'm using has about 3Ω resistance, so the issue was one of impedance matching. I wired four of the PTCs in parallel and tried again. This did the trick. Holding down the switch, I can feel the "buzz" start at a good power level, then dissipate in about ½ to 1 second. It does a good job of demagnetizing things like screwdrivers. If I want to magnetize, I just blip the switch very briefly.
I think these devices will work well to control the demag circuit of a magnetic chuck. Just wire a set of paralleled PTCs in series between the demag switch and the coils. I have quite a few of these left, and will be more than happy to send some to anybody who asks (until I run out). Just measure the DC resistance of the coil and decide how many of the 20Ω PTCs it would take to approximately equal that resistance. PM me with your mailing address and how many you'll need.
PS - Don't try to test your circuit immediately after soldering to the PTCs - let them cool for about ½ hour or so, or the residual soldering heat will have them in a high resistance state. I'll let you guess how I discovered that little tidbit of info
Very nice John- so the parallel ptc group you have now is about 5 ohm cold. If you had to order them again I'm guessing you would order a lower resistance value.
Come to think of it, did anyone post the cold resistance of their chuck? Anyhow, it looks like the chuck and ptc ohms should be about equal. Good to know.
ps it might be good to point out that resistances divide in parallel = 4 X 20 ohm devices in parallel =
5 ohms. 2 X 20 ohm devices = 10 ohm and so on...
That's quite an "if" Would if I could
It took me a good deal of searching to even find any suitable PTCs. Nearly all of them have a very sharp cutoff, and are intended as thermal switches. Finally went to Alibaba, where nearly all of the vendors wanted to sell in quantities like 5000 units. I thought I was very fortunate to find the one I did ... minimum order quantity 50.
Having used 4 for my mag/demag, I still have lots left to give away.