Why Use a Capacitive Discharge Unit you ask... aka/snap swich's

[Rangerover]

I melted my 4th and last snap switch about a month ago, and it also took out an Atlas Relay due to stuck Atlas slide switch's.  No more I installed a Capcitive Discharge Unit. I have converted some (12) to Tortoise within the last year, but I have a total of 48 turnouts. The last picture is the unit I purchased for under $35.00 delivered in 3 days and can throw 10 switch's at once. That's a lot of power and it works great easy hook up. Here's the company I purchased from:

http://www.miniatronics.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=M&Product_Code=PDC-1&Category_Code=B_4&Product_Count=1

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[jward]

capacitor discharge units can also be built easily by those so inclined.

one warning, capacitors can hold a charge for long periods of time before it bleeds off. before working on any capacitor discharge unit, take a set of test leads and jumper the + and - outputs together to bleed off any charge that may be there. otherwise, you run the risk of shock should you accidentally touch ANYTHING connected to the + output. do not forget to remove the test leads before you power up the unit, and preferrably before you connect it to the dc power source. connecting this unit to a power source should be done last, after everything else is connected to the unit.

btw, i noticed in the photo that your capacitor discharge unit has a rectifier diode, and would thus be suitable for either ac or dc input.......

[Jim Banner]

It looks like Circuitron is producing the "good kind" of capacitor discharge machine, the ones that have fast recovery thanks to a large transistor in the circuit.  The output of the transistor charges the capacitor.  When the output is shorted, or nearly shorted, as by a solenoid coil, the transistor is turned off and the charging current to the capacitor falls to a very low value.  This means the solenoid gets all the energy that was stored in the capacitor but no more.  The transistor turns back on and the capacitor recharges very quickly when the solenoid is disconnected from the capacitor (as by releasing the push button.)  The advantage is two fold - one is the fast recycle time, the other is that neither the capacitor discharge machine nor the solenoid can be damaged by the button sticking or being held down too long.

The simplest home built capacitor discharge machines charge the capacitor through a resistor.  If the resistance is too large, the recycle time is unacceptably long.  If the resistance is too small, the charging current, which flows all the time, may be large enough to damage the capacitor discharge machine, the power pack running it, and/or the solenoid it is driving.  The usual compromise gives a charging time of 10 to 20 seconds.

There are also capacitor discharge machines designed to work with toggle switches.  These are generally used one circuit per turnout.  While that may seem extravagant, the ones that include connections for signals and for frog control can be quite cost effective.  And we haven't even got started on the ones for computer and/or DCC control.

Jim

[Rangerover]

btw, i noticed in the photo that your capacitor discharge unit has a rectifier diode, and would thus be suitable for either ac or dc input.......

According to the directions, no, AC only and it should not be used for slow motion machines such as Tortoise. Of the CDU machines I've seen this is rugged, heavy duty stuff.

Paragraph from the directions or an Advantage for using this particular model PDC-1.

"The unit automatically shuts off if a control button, turnout motor or switch is jammed. There is no danger of blowing the motor as is often the when a large transformer is used (25 volts) as a multiple power source. You need only a small inexpensive transformer or power pack as the power source."

[jward]

i am going to get technical here......

regardless of what the directions say, connecting a dc power supply to a c-d- unti with a rectifier diode shouldn't be an issue. granted, it's not what the circuit was designed for. but it will work.

diodes are electrically like a one way street. regardless of what you power a diode with it is only going to pass current one way. hook the dc up to it in the wrong polarity, no current flows. reverse the polarity and the diode conducts, taking of course a small amount of voltage for its trouble. the voltage lost to the diode is going to be less than one volt. the capacitor will charge to the voltage of the power supply minus the voltage lost to the diode.

on ac, which fluctuates between plus and minus, the diode conducts on the positive half of the cycle. the capacitor will charge to the peak of the ac voltage. this will probably be a higher voltage than if you'd run the circuit on straight dc, and for a dc power supply the diode is unnesecessary. but things should work either way.

as a side note, diodes can also be used to run a tortoise motor off ac power.....

[Jim Banner]

Circuitron may have good reason for saying ac only in their manual.  One possibility is that they do not want 50% of the people who try to run their CD machine on dc sending it back as defective.  A single diode implies half wave rectification which further implies that if a dc source is used, it must be hooked up with a particular polarity.  The other polarity will not work.  This is different than with a circuit that uses full wave rectification.  It can use dc of either polarity as long as the current rating of the bridge rectifier is adjusted accordingly.

Another possibility is that with half wave rectified ac, the transistor spends the other half cycles turned off.  This lowers the heating of the transistor and can lower the manufacturing costs by eliminating a heat sink.

These are just a couple of examples from the designers' bag of tricks which might affect his choice of ac or dc to power the unit.

Having beaten that to death, does anyone want to take a guess on why Circuitron included a second diode on the other side of the board?

Jim

[Jim Banner]

While a 1.5 volt battery can charge a capacitor to more than 200 volts, it takes a great deal of circuitry between the battery and the capacitor to do this.  Capacitor discharge machines used in model railroading have no such voltage boosting circuitry in them.  The maximum voltage they can charge to is 1.414 times the rms ac voltage.  For a typical 16 volt supply, that would be about 25 volts.  Not much of a shock hazard there.

Discharging a large capacitor, even at low voltage, through a ring or metal watch band can give you a burn.  But at 25 volts, a large capacitor would be the size of a soup can and rated in farads, not micro farads.  My guess is that the capacitor in Rangerover's machine probably has a rating like 2000 microfarads, 35 volts.  Even if you charged it up to its maximum voltage, it would still contain only about 1 watt-second of energy.  Not much of a burn hazard there.

The third hazard is explosion.  This can happen if you accidentally reverse the connections on an electrolytic capacitor.  Then they can go off like a rifle shot and fill the room with shredded aluminium foil.  This is actually a greater hazard than shock or burn if you are building your own capacitor discharge machine, particularly if you have a weak heart, or your wife doesn't like the way you decorated the house with tinsel dust.

At this point, I could tell you how we used to toss charged capacitors to one another in the electronics lab, but perhaps I had better not.

Jim

[OkieRick]

Would it be overkill to place one of these vunits inline with a Digitrax DS64?  This would be for DCC control?  I ask because my layout in progress consists of Atlas Custom-Line turnouts and Remote switch machines. I'm sincere in my query.

http://www.digitrax.com/prd_statdec_ds64.php

Jim, I love your stories with happy endings.  I imagine we all can remember where we were when we discharged our first capacitor. i was reaching inside a Zenith tv. The shock didn't hurt as much a ripping my arm open on the flyback transformer when I jerked.

Thanks-

[Jim Banner]

I think you would be better off to use the DS64s as is.  I don't think the output transistors are big enough to repeatedly handle the large rush of current.  Besides, when set for dual solenoid machines, they are quite reliable about cutting off the current so that there is no chance of a melt down.

One thing you might consider when installing your DS64s is running a separate DCC bus just for them.  I don't know if they are as bad as the DS52 can be but I have run into some that would refuse to function if there was a train near the turnout.  What was reccommended to me was to use a separate booster and its own bus but I found that just the separate bus, run all the way back to the track booster, cured the problem.  If you are using PM42s or similar power management units, connect the turnout bus directly to the output of the booster.  That way, if someone over runs a switch and shorts things out, the PM42 will not remove power from the turnout control as well as the tracks.  This means you can still throw the turnout to solve the problem.

I too remember an encounter with a picture tube.  I was cleaning up my shop and picked up a dead one to throw it out.  When my forearm touched the anode ring, I found it was still charged after being out of the set for more than a year.  Mr. tube, meet Mr. cement floor.  Mr. Jim, meet Mr. broom and Mr. dustpan.  Actually, I was pretty lucky not to get hit by flying glass.  This was in the days before the safety glass was bonded to the front of the tube.  I have always suspected that when the front of the tube hit the floor, the gun assembly kept on going so that the back of the tube collapsed downward even as the face was imploding upwards.  The angels were watching over me that day.

Jim

Jim

[Rangerover]

You guys are really something I appreciate your knowledge of electronics and you left me in the dust. I too appreciate your experience with old TV picture tubes and I did know about the capicitors in them and other electronic devices, grounding out to discharge before working on them. In my youth, I was about 30 years old, I had a small farm with horses for my wife and myself and Shetland pony's for the kids add to that a 2500 lb bull and a bull calf. I installed an electric fence, well one day I forgot the fence was on and grabbed the wire early in the morning with dew on the ground, lets just say I'm lucky I'm alive, it knocked me to the ground and I had to change my underware. The capicitor in that was huge and with household current, I remember the label said "brush burner", and it did, it would burn the brush that grew near it and touched the wire. On rainy nights it sparked all over the place.

Another time I was about 14, back in the 50's, I learned that old tv picture tubes had a steel jacket in the rear of the tube. Back then we burned our househole garbage in the back yard, there was a discarded tv, Philco my dad tossed a few days prior.  Well I shot at it with a 22 rifle, after about 6 or 7 rounds I heard a bullet ricochet over my head and it went through the kitchen window. I learned a big lesson that day too! No I didn't get beaten, my dad was more surprised than I was. He was with me at the time. We both learned not to shoot at tv picture tubes after that, my dad was a cool headed knd of guy too.

Now more on this unit I bought just for insight, on the instructions it says this:
terminals..."The pair marked "input" go to any 16 to 25 volt AC source. A spare n scale power pack is an ideal source, attach to auxiliary ACC terminals on the power pack."

It goes on to say this later in the literature..."recommended is the 16 volt ac terminals of any power pack up to 25 volts from any small transformer.the available power is proportional to the voltage squared. In other words, increasing the input from 16 to 25 volts AC increases the power into the switch machines by 3.2 times. Use this factor only if you need it, because of the extra vibration given to a single motor when multibles are not switched."

I have it hooked up to a cheapy Rail Power transformer putting out 16 volts to the unit. Just to show how much it stores I can throw 4 Peco switch motors at precisely the same time with no problem using telephone wire from the slide switch machines to the switch motors. Now if you're familiar with Peco twin solenoid motors you get my point.

[jward]

4 motors with one shot? that's quite a kick.