Goal: $300, Received: $125.00 (42%) Contribute Now
Donate whatever you can or Join the 14,000 other NAWCC members for only $80 (plus $10 for hard copy publications). Check it out here.



Page 1 of 3 123 LastLast
Results 1 to 15 of 31
  1. #1

    Default Correct electricity supply for slave clock - avoiding overheating

    Hi

    I have a few slave clocks which are fairly standard movements found in GPO clocks etc. They run on 12 volts and I've got a Raspberry Pi set up to provide the pulses at the required intervals.

    However I noticed the coil in one of the clocks gets very hot if the pulse stays on for too long - I discovered this when testing. If something went wrong in operation and the pulse remained on for too long, I'm worried the coil would overheat and get damaged.

    Is the overheating just something that happens on slave clocks if the pulse is left on for too long, or can the overheating be prevented by using a resistor? Or maybe a thermal fuse would be best to cut out the current in such a case?

    Thanks

  2. #2
    Registered user.
    Join Date
    Nov 2012
    Location
    Denver, Colorado
    Posts
    160

    Default Limit pulse duration? (By: dw2007uk)

    "GPO" ? "Raspberry Pi"? Can the pulses be limited so that they never stay on too long?
    Confucius say: "Man own clock know time. Own two, never sure."

  3. #3
    Registered User
    Join Date
    Feb 2007
    Location
    Rolling Hills Estates, CA
    Posts
    1,007

    Default Re: Limit pulse duration? (By: Robert Gift)

    Indeed I would recommend adding a "one shot" to your interface between the PI and the clock to avoid the possibility of the coil being left on too long as a result of a crash or software fault.
    Will McCown -- Rolling Hills Estates, CA

  4. #4

    Default Re: Limit pulse duration? (By: flynwill)

    [QUOTE=flynwill;1094380]Indeed I would recommend adding a "one shot" to your interface between the PI and the clock to avoid the possibility of the coil being left on too long as a result of a crash or software fault.[/QUOTE

    A 200 to 250 millisecond pulse should be all you need..at the current recommended on the back (usually) of the movement..I Probably 220 Ma. If using a 12 volt supply a suitable series resistance should be used..Most of the slave dials I've used can be pulsed with a quick dab of a 1.5 volt battery.

  5. #5

    Default Re: Limit pulse duration? (By: Robert Gift)

    Thanks for your replies.

    Quote Originally Posted by Robert Gift View Post
    "GPO" ? "Raspberry Pi"? Can the pulses be limited so that they never stay on too long?
    GPO is General Post Office which used many slave clocks and they have a fairly standard movement. Raspberry Pi is a small computer which runs Linux operating system.

    The program I have running limits the pulses to be very brief but, as flynwill recognises, there's a chance there could be a software fault which causes the pulse to stay on too long.


    Quote Originally Posted by flynwill View Post
    Indeed I would recommend adding a "one shot" to your interface between the PI and the clock to avoid the possibility of the coil being left on too long as a result of a crash or software fault.
    Do you think a resistor would be the best choice? Would the resistor get too hot though if the pulse went on too long?


    Quote Originally Posted by rogerj View Post
    If using a 12 volt supply a suitable series resistance should be used
    What sort of value resistor would you recommend?

    In the original setup with a master clock, would there have been something to prevent a failure in the master clock which could have caused a pulse being applied for too long?

    Thanks again!

  6. #6

    Default Re: Limit pulse duration?

    Quote Originally Posted by dw2007uk View Post
    Thanks for your replies.


    GPO is General Post Office which used many slave clocks and they have a fairly standard movement. Raspberry Pi is a small computer which runs Linux operating system.

    The program I have running limits the pulses to be very brief but, as flynwill recognises, there's a chance there could be a software fault which causes the pulse to stay on too long.



    Do you think a resistor would be the best choice? Would the resistor get too hot though if the pulse went on too long?



    What sort of value resistor would you recommend?

    In the original setup with a master clock, would there have been something to prevent a failure in the master clock which could have caused a pulse being applied for too long?

    Thanks again!
    In the original PO No 36 clock the pulse was generated mechanically by a count wheel turned by the pendulum. See here :
    http://www.hvtesla.com/masters/po36_countwheel.html It's almost impossible to visulalise a situation where the pulse could be prolonged.
    A series resistance is the universal method of current control for slave dials and in your case, with a 12v supply, a good starting point would be 47 ohms.
    If the switch is solid state it should be protected by snubber diode.
    Last edited by rogerj; 02-28-2017 at 11:38 AM. Reason: spelling !

  7. #7
    Registered user.
    Join Date
    Oct 2010
    Location
    Calif. USA
    Posts
    13,122

    Default Re: Limit pulse duration? (By: rogerj)

    Coils are current devices. Some have internal resistance to limit their
    current while others depend on an external current limit.
    I don't know the specifics of your clock but many slave clocks are intended
    to run on a series current loop. Each clock dropping some voltage with the
    master having a limiting resistor to adjust the loop current.
    If the output were continuous, for the suggested limiting resistor,
    with 12V across 47 ohms, it would dissipate
    3 watts. I would recommend using a 5 or 10 watt ceramic resistor ( commonly
    available ).
    That should be enough to protect the coil, should the software leave the pulse on.
    I would also mount the resistor in a location that has air circulation rather than in
    a small closed box, if you don't want to cook everything in the box.
    Tinker Dwight

  8. #8

    Default Re: Limit pulse duration? (By: Tinker Dwight)

    Quote Originally Posted by Tinker Dwight View Post
    Coils are current devices. Some have internal resistance to limit their
    current while others depend on an external current limit.
    I don't know the specifics of your clock but many slave clocks are intended
    to run on a series current loop. Each clock dropping some voltage with the
    master having a limiting resistor to adjust the loop current.
    If the output were continuous, for the suggested limiting resistor,
    with 12V across 47 ohms, it would dissipate
    3 watts. I would recommend using a 5 or 10 watt ceramic resistor ( commonly
    available ).
    That should be enough to protect the coil, should the software leave the pulse on.
    I would also mount the resistor in a location that has air circulation rather than in
    a small closed box, if you don't want to cook everything in the box.
    Tinker Dwight
    Thank you!

    Apologies for the questions (I'm new to this), what value resistor (as in number of Ohms) would you recommend for each clock?

  9. #9

    Default Re: Limit pulse duration? (By: dw2007uk)

    I assuming you realise that this type of slave clock is always connected in a series loop - and in series with the final switch. This is probably a transistor triggered by the Pi.
    As I said previously, 47 ohms would be a good starting point with one dial..If more than one dial allow 1.5 volts for each dial and subtract the answer from 12. Call the answer "x" Then....by ohms law, 12 volts minus "x", divided by .2 (amps) is the likely value. You must be sure that the pulse from the Pi is just long enough to advance the dial reliably..
    A 50-50 square wave is no good and will likely be the cause of overheating..you must use a "one shot" monostable or trim the software to produce the output pulse of about 200 to 250 milliseconds..

  10. #10
    Registered user.
    Join Date
    Oct 2010
    Location
    Calif. USA
    Posts
    13,122

    Default Re: Limit pulse duration? (By: rogerj)

    What is the supply voltage and what is the resistance of your coil on the slave?
    Tinker Dwight

  11. #11

    Default Re: Limit pulse duration? (By: Tinker Dwight)

    Quote Originally Posted by rogerj View Post
    I assuming you realise that this type of slave clock is always connected in a series loop - and in series with the final switch. This is probably a transistor triggered by the Pi.
    As I said previously, 47 ohms would be a good starting point with one dial..If more than one dial allow 1.5 volts for each dial and subtract the answer from 12. Call the answer "x" Then....by ohms law, 12 volts minus "x", divided by .2 (amps) is the likely value. You must be sure that the pulse from the Pi is just long enough to advance the dial reliably..
    A 50-50 square wave is no good and will likely be the cause of overheating..you must use a "one shot" monostable or trim the software to produce the output pulse of about 200 to 250 milliseconds..
    Thanks for explaining - I'll see what I can come up with!

    Quote Originally Posted by Tinker Dwight View Post
    What is the supply voltage and what is the resistance of your coil on the slave?
    Tinker Dwight
    The supply voltage I was using is 12 volts. I've just measured the resistance of the four slaves I've had running - one has a resistance of 9 ohms, one is 3 ohms (this one has a resistor soldered in across the terminals) and two are 1,000 ohms (these two also have a resistor across the terminals).

  12. #12

    Default Re: Limit pulse duration? (By: dw2007uk)

    Power is the Voltage Squared divided by the resistance.

    Assuming continuous current, and V= 12VDC

    for 1000 ohms P=144/1000=.144 watts
    for 9 ohms P=144/9= 16 watts (this is high)
    for 3 ohms P=144/3= 48 Watts (high and would probably overload your power supply).

  13. #13
    Registered user.
    Join Date
    Oct 2010
    Location
    Calif. USA
    Posts
    13,122

    Default Re: Limit pulse duration? (By: dw2007uk)

    The 1K one is likely to use the full voltage but the low ohms ones
    are surely current loops.
    Tinker Dwight

  14. #14

    Default Re: Correct electricity supply for slave clock - avoiding overheating

    Quote Originally Posted by dw2007uk View Post
    Hi

    I have a few slave clocks which are fairly standard movements found in GPO clocks etc. They run on 12 volts and I've got a Raspberry Pi set up to provide the pulses at the required intervals.
    Thanks
    I should have read the original post more carefully..You appear to have a collection of different slave clocks - as evidenced by the variety of measured coil resistances..maybe they are not all GPO clocks..Some pictures would help a lot..
    The GPO slave clocks I've been referring to are all like the ones I have and as pictured in this photo. Made by the Synchronome company, they measure about 9 ohms including the parallel connected resistor. Incidentally, this resistor is included to increase the life of the contacts that switch the dials. It may not be sufficient protection for solid state switches against the spikes produced by back EMF. The value of the resistance is usually between 5 and 10 times the actual coil resistance. I'm not sure exactly what resistance Synchronome would have used.
    All dials in a series loop should be the same. Dial mechs with high resistance (like 1000 ohms) must have been intended for parallel connection and clearly won't be suitable for a series loop. Parallel ccts often worked at 24 volts.
    The current limiting resistor for the loop SHOULD only dissipate power for 1/5 of a second every 30 seconds. In these circumstances a 1 watt resistor should be adequate..

    Attached Thumbnails Attached Thumbnails DSCF1966.jpg   DSCF1966.JPG  
    Last edited by rogerj; 03-03-2017 at 06:48 AM. Reason: picture too big

  15. #15

    Default Re: Correct electricity supply for slave clock - avoiding overheating (By: rogerj)

    I have to assume the clocks are not being run directly by a raspberry PI - the GPIO outputs are only 3.3 or 5V and would toast pretty quickly.

    A hardened SSR should work fine for this application - relay contacts are a different story.

    Is the use of the resistor across the coil to prevent relay contact sparking ?

Similar Threads

  1. Help finding correct spring unit for my clock please
    By LD1001 in forum 400-Day, Atmos and other Torsion Clocks
    Replies: 1
    Last Post: 05-14-2009, 06:02 PM
  2. Low Volt Power Supply for Many Clocks
    By tbonjour in forum Electric Horology
    Replies: 2
    Last Post: 12-11-2008, 06:50 PM
  3. Mech movement for slave clock
    By bob asbra in forum Clock Repair
    Replies: 9
    Last Post: 07-20-2008, 10:00 AM
  4. Correct mainspring size for grandmother clock
    By Bill V in forum Clock Repair
    Replies: 7
    Last Post: 01-07-2007, 10:09 AM
  5. Need assistance in locating the correct part needed for this clock
    By Wurlitzer in forum 400-Day, Atmos and other Torsion Clocks
    Replies: 12
    Last Post: 05-11-2006, 12:53 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •