Electrical: Voltage reducer for a 6 volt clock on 12 volts - yes or no?

[northern]

Just bought a "new in original box" 6 volt clock for the '54. Here's hoping that it works and that 60 years of storage have not hurt it.

After the conversion, this '54 Commander Starliner now runs on 12 volts. To run the new 6 volt Borg clock, is it necessary to reduce the voltage from 12 volts to 6 volts? I thought it was necessary, but found a number of posts on forums suggesting that the Borg 6 volt clocks run fine on 12 volts. This seems to be a controversial topic, with opinions on both sides. So I'm confused.

If one did want to reduce the voltage to stay on the safe side, what would be the best way to do that? Part numbers, etc. appreciated. The car now has 12 volt heater, defroster, and wiper motors. And I went with the Runtz in line voltage reducers for the fuel and temp gauges. So all the new reducer would have to do is feed the clock.

[Commander Eddie]

I'm no expert, but couldn't you just wire the clock through one of the reducers you are now using for the gauges?

[RadioRoy]

No.

The gauges take only about 135 milliamps per gauge continuously. I don't remember the exact number, but that figure sticks in my mind. A milliamp, BTW is 1/1000 of an amp.

The clock is run by a solenoid that pulls in to wind the clock. Clock solenoids take a surge of high current - several amps - for a short period of time - probably less than a second.

Finding a resistor that would work on a solenoid surge would require testing the peak current to determine its value and looking at that current draw to see if it is fairly consistent across the time the solenoid is energized.

Since the exact resistance cannot be specified when purchasing one, voltage reducing resistors generally give too much or too little, but almost never the exact amount, especially with a surge demand, like the clock solenoid.

This is probably one of the rare instances where too much is better than too little. If the solenoid gets too little power delivered to energize it, the contacts will burn. That's what kills the old clocks. When the car sits and the battery get low, there is not enough power to energize the solenoid, so the contacts burn and get a hard coating on them. If a car is driven infrequently, it's a good idea to put a heavy duty switch on the clock power lead.

You can also have the clock converted to a quartz movement, which will end all the problems associated with voltage/current requirements, plus it will keep accurate time.

If you don't care about damaging your NOS clock, then that is a different story.

[northern]

No.

The clock is run by a solenoid that pulls in to wind the clock. Clock solenoids take a surge of high current for a short period of time. Voltage reducing resistors and voltage regulators work on lower values of continuous current. Finding a resistor that would work on a solenoid surge would require testing the peak current to determine its value and looking at that current draw to see if it is fairly consistent across the time the solenoid is energized.

Voltage reducing resistors generally give too much or too little, but almost never the exact amount, especially with a surge demand, like the clock solenoid.

If you don't care about damaging your NOS clock, then that is a different story.

Roy, I do care; that's why I'm asking. Can you provide a more detailed method of doing the testing? Thanks.

[RadioRoy]

The first step is to determine the actual current drawn by the solenoid in the clock and the duration of the draw. If you have a milliohmmeter, one that can read very small resistances, you can measure the resistance of the solenoid. With that done, you know the dropping resistance required, as it will be the same as the coil. But you must measure the coil and not the resistance of the points.

Alternatively, if you have a digital ammeter that can read up to 10 amps and has a holding function or a peak function, you can actuate the clock with a fully charged 6 volt car battery and measure the current required. If you have a scope, you can see how long the current pulse is, but that is really not necessary, since this is only a single pulse application.

You want the resistor to drop 6.3 to 7 volts at the current the coil takes.

Calculate the power (voltage times current) and double that to get the wattage of the dropping resistor. Doubling the wattage will keep the dropping resistor from getting too hot.

Then check the electronics catalogs (Mouser, Jameco, Digikey) to see what you can find.

[Bud]

I used a 1 ohm 10 watt resistor in the hot lead of the clock in a 55 Ford that I had a few years ago. The car was converted to 12 volts before I bought it and I wanted the clock to work. I saw the car recently and the clock is still working. Bud

[RDWEAVER]

If I were you, after you figure out how to reduce the voltage so you don't damage the rewind coil I would place a diode in the line in order to take the current draw off the clocks points. If this is done the clock should last a very long time.

[JohnMSeymour]

Why not use a 6 volt IC (integrated circuit) regulator for the clock? it would have to be able to pass the current required for the clock solenoid. Do you happen to know what that is Roy?

[RadioRoy]

I used a 1 ohm 10 watt resistor in the hot lead of the clock in a 55 Ford that I had a few years ago. The car was converted to 12 volts before I bought it and I wanted the clock to work. I saw the car recently and the clock is still working. Bud

That's pretty much what I had in mind, as that's what I use for the solenoid in a Zenith-built 48 Ford radio running on 12 volts. It just felt like a good idea to actually do some testing and calculating this time.

[RadioRoy]

Why not use a 6 volt IC (integrated circuit) regulator for the clock? it would have to be able to pass the current required for the clock solenoid. Do you happen to know what that is Roy?

It's going to be on the order of 6 to 10 amps. These IC regulators will not work at those currents. That's the difference between a steady draw, and a pulse. To get a regulator working at those currents, it must handle the full current during the pulse. That would mean that regulator would be many times larger than one would think, like the size of half a loaf of bread, or so. A resistor of the proper value is a better bet in this application.

[RadioRoy]

If I were you, after you figure out how to reduce the voltage so you don't damage the rewind coil I would place a diode in the line in order to take the current draw off the clocks points. If this is done the clock should last a very long time.

Good point and maybe the second simplest solution. Each silicon diode drops .6 or .7 volts in the forward direction. If you put 9 or 10 silicon diodes in series with the clock lead, forward biased, and each capable of handling 10 amps, that would do the job.

[northern]

It's too bad that one of you guys is not my next door neighbor; electronics is not my thing. Can anyone make this simpler - a link to a part (or a part number, etc) that I can install on the clock hot wire? I do have a few multi-meters and other testing equipment around, but still am learning how to use them. Installing wiring, etc. is not a problem, and I can follow instructions.

Another question - Since the car was positive ground originally and now is negative ground, I assume that the connection that formerly connected to a negative source would now connect to a positive source?? In other words the "hot" terminal on the clock would the hot terminal, and the ground terminal would remain the ground terminal??

[RadioRoy]

The clock does not care about polarity. It will run on negative ground as well as positive ground. Most of the electrical devices in a 53 Studebaker do not care about polarity.

Sorry to have made this too complicated with our technical discussion. You could just go with Bud's 1 ohm, ten watt resistor idea. It's the simplest. They're generally in a gold colored extruded aluminum case with mounting holes.

I didn't find any at Mouser, but will poke around a bit.

The two of these, wired in parallel, would probably do the job.

http://www.ebay.com/itm/QTY-2-10-watt-ceramic-cement-power-resistor-2-ohm-wirewound-/160879708384?pt=LH_DefaultDomain_0&hash=item25752d88e0

[northern]

A local supplier here lists this as in stock:

NTE 1.0R 10 WATT POWER WIREWOUND RESISTOR 10W1D0

Does that sound like the right part?

I had thought of using a ballast resistor like the ones used to feed the ignition coil. But one post I found said that would not work - something about the ballast resistor not reacting fast enough to reduce the voltage for the brief clock wind cycle.

RDWeaver suggested adding a single diode into the supply line to protect the points. In simple terms, can anyone tell me how would that add protection? From the little I know about diodes, it seems that they act as a check valve, preventing current from flowing backwards. Is that a concern in this case?

[gordr]

The best way to use a diode to protect the points is to wire it across the coil, configured so that it doesn't pass forward current during the power-on part of the wind cycle. As the points open, the magnetic field built up in the solenoid coil collapses, and induces a reverse-voltage spike that can arc the points. A diode, properly connected, will shunt that reverse voltage spike to ground. Let's try this: -|<- if that is your diode symbol, as marked on the case, then the lead on the right should go to the ground side of the solenoid coil, and the lead on the left to the points side. If the diode has a black plastic body with a silver ring, that silver ring corresponds to the lead on the left in the graphic. Most small relays now have diodes built into them for this purpose.

And could I suggest using a light bulb for the dropping resistor? Light bulbs have a very high positive temperature coefficient of resistance, meaning their resistance is quite low when cold, but rapidly builds to a high value when they light up, so they work great great as current limiters. I would suggest a small H1 halogen bulb in the 30-watt range would be suitable. Should pass enough current, cold, to wind the clock, but if the points were to stick, it would limit current passed through the points to about 3 amps, which might save them from being cooked.

I would set the clock and a battery up on the bench, and try various light bulbs as resistors, until I found one that reliably wound the clock to its normal full cycle. Too much resistance, and the clock might half wind, and cycle more often. You don't want that.

Here is a link that speaks to diode protection: http://www.hobbyprojects.com/the_dio..._circuits.html