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Thread: Correct electricity supply for slave clock - avoiding overheating

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

A small correction..I said in#9 that a pulse of about 200 milliseconds is all that's required to impulse a dial of the Synchronome type. In fact, because I'm currently working on a dial that requires impulses of alternate polarity which DO require a pulse of about 200 Ms I made that mistake. Checking in Hope-Jones book the pulse used to reset the gravity arm in a Synchronome - and hence drive identical dials as those we are considering - is only 60 Milliseconds long. (page 108 in my edition). It won't make much difference to the operation of the dial but it will increase the battery life a little. With a Pi driving the clock it should be easy to experiment to find the length of a reliable pulse.

2. Re: Correct electricity supply for slave clock - avoiding overheating

Originally Posted by ElectricTime
Is the use of the resistor across the coil to prevent relay contact sparking ?
Yes..It is simple and effective in the original context. It dissipates the back emf. It's worth Googling "contact arc suppression" for the pros and cons of many other methods.

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

An electrical lesson:

Coils create a current in a changing magnetic field.
When the contact start to open, the field collapses.
This tries to induce a current in the coil.
Since the points have just started to open, the current
has no place to go but the field is collapsing. The voltage
start to raise as the energy of the coil has to go someplace.
If the point could open really really fast, the stray capacitance
would eventually be enough to absorb the current and
limit the voltage.
This creates a ringing like a flywheel and spring. Once the field
collapsed the capacitor would build it back up in the opposite
direction. This action would continue until the resistance of the coil
would dissipate the energy in heat.
The rise in voltage happens in nanoseconds the point just don't
open fast enough. The voltage quickly reaches the arcing voltage
of the points. The energy is now turned into light and heat that is
dissipated quite quickly. Even hardened point will be eroded.
The resistor gives the coil someplace to dump the energy stored
in the magnetic field.
Better results can be had with a small capacitor and a resistor
but that requires selecting the right combination. You don't want
a capacitor that will weld the points on closing and you don't want
it to ring because the resistor is not of a low enough value.
As Roger has said, you want to use a lower value of this resistor for
circuits using solid state switch. The resistor has been optimized
for a contact, like a relay.
Tinker Dwight

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

Originally Posted by rogerj
Yes..It is simple and effective in the original context. It dissipates the back emf. It's worth Googling "contact arc suppression" for the pros and cons of many other methods.
I've been a big believer in using off the shelf Solid State Relays with built in Snubber networks. Had basically 0% failure, though some have leakage current which can drive you crazy at first.

5. Re: Correct electricity supply for slave clock - avoiding overheating (By: ElectricTime)

Good tip ! found some on *bay for <2\$.I have ordered one to experiment. They all seem to be rated for mains voltage but I assume they will switch low voltage DC ok. Not sure about the truth of the claimed manufacturer but worth a punt.

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

Many AC Solid State Relays are SCR/Triac based and will not switch DC properly (they rely on the AC zero crossing to shut off the switching device). There are also some that require a relatively large load voltage to trigger. You'll want to check the ratings closely when you get the device.

7. Re: Correct electricity supply for slave clock - avoiding overheating (By: flynwill)

If it is rated for mains voltage, it is most likely a SCR type and won't
work with DC.
You might get by with switching the AC to the power supply.
Tinker Dwight

8. Re: Correct electricity supply for slave clock - avoiding overheating (By: Tinker Dwight)

Originally Posted by Tinker Dwight
If it is rated for mains voltage, it is most likely a SCR type and won't
work with DC.
You might get by with switching the AC to the power supply.
Tinker Dwight
I did wonder about that..It will take a couple of weeks to come from China. If I don't report back assume you are both right and it didn't work on DC !

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

SSR relays come in lots of flavors, as long as the OUTPUT is DC it should work fine. An ac output relay typically won't turn off with DC.

10. Re: Correct electricity supply for slave clock - avoiding overheating (By: ElectricTime)

One way to limit current is with a light bulb in series. They have low resistance when cold but if the pulse lasts the bulb lights the resistance goes up.

11. Re: Correct electricity supply for slave clock - avoiding overheating (By: fdew)

Light bulbs have a really large resistance swing from cold to hot.
I'm not sure that is desired for this purpose.
It might be that the light heats too fast and not allow the coil to pull in.
Something like a DC motor might work better. On can control the time
till it limits the current by the mass of a flywheel.
Tinker Dwight

12. Re: Correct electricity supply for slave clock - avoiding overheating (By: Tinker Dwight)

I can't really see any advantage in messing with bulbs when a resistance is simple enough and there is one provided in the clocks..In the early days - pre trickle charging and wet cells - Gents sold a bell that could be put in series. As they patented the idea, Synchronome used instead a lamp in a box with a red glass. The idea was to warn of low battery voltage. The way it worked was if the current became insufficient to reset the gravity arm the pendulum assisted it on the next swing. Because of the greatly lengthened pulse the bell would ring....or the lamp would flash to alert the keeper of the clock..With a normal short pulse it did nothing. R

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

The thought is not so much to replace the resistor but to give protection
in case the voltage should stick on, from a failure of the master clock source.
A dynamic current source that can limit the current would protect the
slave coils.
The lamp idea responds too fast and is not likely to actuate the slave.
Tinker Dwight

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

Here are two things you could do to protect the clock coils:

First is to use a constant current source circuit. This would allow you to add or remove clocks from a series circuit without any re-adjustment of resistors. You can build a current source from a few inexpensive components. One of the simplest circuits uses an LM317 voltage regulator and a single resistor. About \$2 worth of parts. Do an internet search for "LM317 constant current source" for more info. Since you'll be switching an inductive circuit, a few more components may be necessary to protect the regulator.

These clock coils are not designed for continuous operation. They may overheat even with the proper current if left energized constantly. You may also want to use a time delay relay to limit the maximum pulse time. Set it to break the circuit when the delay expires. Set a delay of a few seconds. If your clock control computer locks up with the output on, the time delay relay will stop current flow when the delay expires. I have a lot of these relays on the shelf at work. They're available in various operating voltages and with fixed or variable delays.

The only problem with this solution is that these relays are fairly expensive. Last time I ordered some they were about \$100 each. Since you don't need an industrial rated relay there will be cheaper options.

Just thought of a really cheap way to fix this problem. Use a time delay fuse to protect your clock circuit. Pick a value that will blow with the normal clock operating current. With the low duty cycle pulses that are normally applied, the fuse won't blow. If the output gets stuck on, the fuse will then blow in a few seconds.

15. Re: Correct electricity supply for slave clock - avoiding overheating

To that I could add a third even simpler suggestion. Replace the battery with a capacitor charged via a resistor (from the battery). I haven't tried this but it loosely parallels the way I powered the impulse magnet in my cHipp toggle clock..This https://planetcalc.com/1980/ online calculator shows that a 3300uf capacitor charged via a 1500 resistor will be 99.2% charged in 24.75 seconds when it would be ready to generate the short pulse for the clock and dials. The short circuit current would be in the order of <20 Ma with those values. Use the calculator to work out other values..
3300uf may not be the right value but as we are discussing a diy electronic clock experimentation is in order.
A fourth approach might employ a 555 timer wired as a one shot with a timing of , say 0.5 second. Either gated with the signal from the Pi or operating a secondary switch . Homemade clocks are very satisfying projects !

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