gents slave (unipolar)

[duncan webster 1Participant @duncanwebster1]

I think I'm answering my own question here, but if someone could sanity check it I'd be grateful
I'm building some electronics to drive a Gents slave from another clock. I've found out via Google that I need 220 mA pulses lasting 200ms every 30 seconds, and the coil resistance is 4 ohms. I have a dropper resistance, actually 3 off 82 ohm in parallel giving 27ohm effective. I'm driving it off 8 V, so I will get 255mA, actually a bit less as I haven't allowed for the FET. So far so good. Now for the question! If it was energised full time each resistor would dissipate 592 mW, but it is only on for 200ms every 30 seconds. Can I get away with 0.25W resistors?

Whilst I'm on does anyone know where I could get drawings for a silent bipolar slave? Or even a silent unipolar. I've built 2 slaves using stepper motors, but there is a certain fascination in making the whole thing using a ready made stepper is cheating!

[duncan webster 1Participant @duncanwebster1]

[Michael GilliganParticipant @michaelgilligan61133]

Not quite 'drawings' Duncan … but this should be helpful: **LINK**

MichaelG.

[John HaineParticipant @johnhaine32865]

Answer to first question, is yes. Power dissipation in each resistor will be about 1/150 times 600 mW or about 4 mW. You could even use one 27 ohm resistor as it's a preferred value.

What do you mean by a silent bipolar slave?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Haine on 27/09/2018 23:07:50:

What do you mean by a silent bipolar slave?

.

Presumably the one use in various Board Rooms, and BBC Studios

… as illustrated in the YouTube video that I linked.

MichaelG.

.

See also Fig. 2 here: http://sound.whsites.net/clocks/alternate.html

Edited By Michael Gilligan on 27/09/2018 23:34:12

[John HaineParticipant @johnhaine32865]

Or indeed in every quartz analogue clock. I was just checking I understood Duncan's question. It's easy to modify a quartz movement of the right type to disconnect the motor coil from the ic so as to drive it from a separate circuit. If you want to make one from scratch you need to make a Lavet type stepper.

**LINK**

These look fairly simple in principle if you can find or make a suitable cylindrical magnet for the rotor. I've not found any instructions or drawings for making one.

Duncan, I'd be interested in how you approached making your stepper motor slave clocks if you could give more details please?

Edited By John Haine on 28/09/2018 07:36:55

[Michael GilliganParticipant @michaelgilligan61133]

How infuriating

Grace's Guide has a picture of the movement, but I can't make out the Patent No.

https://www.gracesguide.co.uk/File:Im20150914RB-Gents1.jpg

… Any offers from those with better eyes [or perhaps better imagination]

MichaelG.

Edited By Michael Gilligan on 28/09/2018 08:24:10

[duncan webster 1Participant @duncanwebster1]

Thanks for confirming my thoughts on resistor. There must be some limit on this, you couldn't have it on for an hour then off for 150 hours. Must be to do with how hot it gets during the on time.

John, I've done the dodge of using a quartz clock movement driven from my electronics to get me going on another clock project, but I want to make my own, preferably one that moves every 30 seconds.This is shown in Michael's link, but the Gents moves 1/4 turn every step, and looks subtly different to the Lavet. I've got one of those as well, but I don't want to destroy it by taking it apart to measure everything up. Looks like I'll have to experiment, I was hoping someone would have done the spadework, I know, idle so-and-so. I've also done it the other way round and used the electronics from a quartz clock to give one second pulses to drive a normal Synchronome slave, again via some electronics

here's a couple of pictures of one of my stepper driven slaves, the brass gear is just an idler which connects the stepper to the extra gear on the minute shaft 1/48 of a rev of the stepper (little steppers are typically 48 steps per rev) moves the minute shaft 1/240 revs, ie quarter of a minute. The electronics then advances 4 steps per minute. All done with stock gears, I'm not brave enough to make my own yet, but it's on the list. On my rescued Blick, I geared it 1/48 to 1/120 and advanced every half minute. I'm planning to take the Blick apart again soon to fit microswitches to the one rev per day wheel, I'll take some pictures then

[Michael GilliganParticipant @michaelgilligan61133]

Duncan

I've found the patent: **LINK**

https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=GB&NR=604840A&KC=A&FT=D&ND=3&date=19480712&DB=EPODOC&locale=en_EP

You will see that it ony covers the 'motor'

MichaelG.

.

See also: http://pulsynetic.eu/slaves/

Edited By Michael Gilligan on 04/10/2018 09:20:19

[SillyOldDufferModerator @sillyoldduffer]

Posted by duncan webster on 28/09/2018 10:33:11:

Thanks for confirming my thoughts on resistor. There must be some limit on this, you couldn't have it on for an hour then off for 150 hours. Must be to do with how hot it gets during the on time.

…

Yes, the time factor is important. It boils down (ho ho) to how quickly the resistor can dissipate the heat. You're proposing to use resistors rated for ¼W, and I'd be very surprised if they couldn't deal with 1.4W for 0.2 seconds in every 30.

With short pulses, the duty cycle can be used to calculate the average load. With longer or more powerful pulses, the element might well heat faster than the resistor body can dissipate it. The temperature inside could reach damaging levels perhaps cracking the body or altering the ohmic value of the resistor, or even magic smoke. Depending on the resistors construction, local hot-spots can be troublesome, and I think the actual limits of a particular resistor could only be found by experiment. Compared with transistors they're tough little beasts though.

Normally in this situation the simple answer is to use a higher wattage resistor.

Dave

[GeorgineerParticipant @georgineer]

I've just dug out a Gents' slave clock from hibernation and there is some useful information embossed in the Bakelite case:

"Connect in series with a Gents' master clock.

Working Current 0.22 Amp.

Max Current 0.27 Amp.

Min. Current 0.17 Amp."

Also (scratched into the Bakelite by hand) is the resistance of 4 ohms, Ref. No. CO, and Serial No. 43248.

George

Edited By Georgineer on 04/10/2018 18:11:04

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