Another capacitor discharge for O27 switch question

Chris,

I have 8 1122e 027 switches on my layout. I've done alot of modifying of them as well as replacing ones that don't snap as well as I want or create rough rides for some of my engines. I tapped into them to operate on fixed voltage, I've added the cap system as well as modifying 022 switch controllers to work with them (I hate the 027 versions). Fixed voltage is definitely the way to go with these switches to insure good "snap" (especially if you run conventional). I put the cap system in because the solenoid buzz these things create is very annoying and will potentially burn them out if a train is parked on a switch. I just went down to the layout to time the recharge time. I found that after 3 seconds I could operate the switch. 4 seconds gives good snap, but they'll pivot on less time. Keep in mind that that recharge time is necessary not only when you operate the switch, but also even when the train passes over it as the anti-derail triggers a discharge. In looking at your layout, I think the key part of it to time will be when the consist is in the "eight" and passes the switch on the upper left (time it from there as it passes up from the left side and then down through the loop and up to your mainline and goes past that same switch. The length of the consist will be the key here as the last car is still triggering discharge. Hope that helps.

Roger

The capacitor charges very fast.  I use a small resistor to slow things down slightly to prevent arcing. 

Here is throwing Atlas machines with a toggle.

Dale H

Thanks for timing the reset time, that gives me a good ballpark.  I'll run a few laps and see if I'll be within those parameters.

CK,

I timed them again today and it's actually closer to 2 seconds where they'll work after a train passes. Not good "snap" at that point, but switched. 

-Roger

A smaller charge resistor will charge them a bit faster.

Should charge up less than a second with an 18 ohm resistor. You can use less,down to 1 ohm.

Dale H

Thanks for the updates.  How do you do the math to figure this stuff out?  In other posts, I've seen recommendations to use 1N4001 diode with 4700 uF capacitors with 100-ohm resistors (or alternatively a #53 bulb in lieu of the resistor that somehow works as a variable resistor).  Dropping the resistor value from 100-ohm down to 18-ohm seems like a huge gap to me, but I don't know the formulas.  About all I know is P=IV and V=IR.

Thanks,

Chris

Dale or John,

I use the same system as Chris outlined above....the 4001 diode and 4700uf cap and a #53 bulb. Are you saying to take the bulb out and put in an 18ohm resistor? Can you give more specs on that for order purposes. RadioShack doesn't have 18 ohms. I guess Digikey or Mouser, but would need more specs.   What is the downside of taking that bulb out and putting in the tiny resistor?

Roger

The choice of the resistor is really somewhat flexible.  The real intent of the resistor is to simply reduce the short circuit current of charging a totally discharged capacitor to a reasonable current level.  A 10 ohm resistor, for instance, would limit the peak current of a 14 volt switch supply to a couple of amps, that tapers down as the capacitor charges.  The peak current is simply the supply voltages time 1.4 to account for the peak voltage seen through the diode.  For 10 ohm resistors, I'd probably recommend a 3A diode for the circuit.

Roger

I think the formula is 1.1 times R times C

So 1.1 X 100 ohms X 4700/1000000 equals about a half second. This should be fast enough. If not a smaller resistor can be used such as 33 ohm or even less. You can also up input voltage,which will allow the capacitor to have more energy when discharged.

Perhaps one of the engineers will verify the formula.

I do not have the circuit you are using in front of me. See my previous post link for the one I use. In that the resistor is used to prevent arcing on the switch contacts. It is not in the circuit when the capacitor discharges. The coil in the switch machine I Imagine is around 100 ohms

Dale H

John,

What is the advantage of using your setup rather than the #53 bulb in there in place of a resistor? And are there disadvantages for taking the bulb out?

The circuit I used (suggested by Bob Nelson on another Forum) is very simple and has been meeting my needs (although not lightning fast). I need to know what advantages there would be to swapping my system out. 

The lower value resistor will charge the cap faster, that's really the difference.  The bulb will have a much higher initial resistance and thus a slower charge for the cap.

Radio Shack carries a 10 ohm, 1W resistor. Is that going to be sufficient?

I think I'll try this on one switch later and compare how they work.

Should be plenty good, the charge is brief, most of the time the resistor has plenty of time to cool down.  The peak current only flows for a very short time before it tapers way down.

Gunrunner is correct-- the bulk of charging/discharging current/voltage level occurs during the first part of the charge/discharge cycle-- about 2/3's of the input voltage. the capacitor will be 90%+ charged/discharged after 4 time constants-see Dale H's post above for example---

AAMOF, you'll drop to about 1/3 the current after one time constant.

A 10 ohm resistor and a 4700uf cap would be basically fully charged in about 1/4 of a second.  Certainly fast enough.

Wow!  You guys work fast!

Dropping the resistor value may really help out on smaller/shorter layouts like mine.  Thanks for sharing your knowledge. 

Is it not this resistor that limits the current to the switch motor coil when a locomotive is left sitting in the nonderailing feature?  By reducing the resistance there will be more heat generated in the coil?

That's correct, and since 10 ohms results in a 1/4 second response, I'd probably recommend something like 47 ohms.  That would give you maybe 1/4 amp through the coils when not switching and someone is parked on the switch, certainly sufficient to prevent any damage.

I didn't consider the power dissipation of the resistor in that scenario, an obvious oversight on my part.  I take exception as I don't use these switches.

A 47 ohm resistor would probably want to be a 10 watt unit for this application, and if you choose to go for a 100 ohm resistor, 5 watts would be sufficient.  That would preclude excess power being dissipated in the resistor or coils.

I'm assuming the resistance of the coils is pretty low, 25 ohms for an 1122.  The impedance at 60hz would be greater, but I don't know exactly what it might be.

John,

I just swapped out two of my switches and the charge time is near instantaneous. So, I'm real happy with that. But then I read your last post and I'm not quite understanding something. The 10 ohm resistor is too small?  I sat a train on the switch and felt the resistor....got a little warm, but nowhere near hot. Are you saying that the coils in the switch will get hot and I should use a bigger resistor?

John,

I just went back downstairs and opened a switch and felt the coils while I had a couple of cars sitting on the switch. I felt zero heat. 

It really depends on the resistance of the coil, but I suspect you should at least go to a 47 ohm.  Of course, the best test is to take the cover off the switch and monitor the temperature of the coils after parking there for a while.  If they don't get hot, you don't have a problem.

I walked away and you slipped that reply in.

If the coils don't get hot at all, it's probably not a problem.

John,

I'm going to sit a train on both switches and feel the coils in both (I only felt one of them). As that one didn't even get warm, I'm thinking I'm ok. But if it does, is the 3 amp diode ok for a 47 or 50 ohm resistor. Radioshacks 50 ohm is .5W. Is that big enough. I'm still hoping  I won't have to bother with it.

Roger

Same here.....there's a big delay in emails sent and posts posted. 

The coils don't appear to be heating at all. No perceptible change in temp that I can feel. 

So, for Chris.....3amp diode, 4700uf capacitor and a 10ohm 1W resistor and it's good to go. And the switches pivot almost as fast as you can toggle the controller. 

-Roger

I'd probably use a larger wattage resistor.  Assume the AC impedance of the coil is twice the DC resistance I measured, that would be 50 ohms, or roughly the same as the 47 ohm resistor.  We're supplying it with half-wave DC with a simple diode, or full wave (120hz ripple) for a full bridge.  If you assume at least half the voltage is dropped across the resistor, that would be 1.3 watts, allowing for a reasonable overhead, I'd still want a 5W resistor here.

I can't explain why you don't see more current to heat up a 1/2W 47 ohm resistor, I tried it and it gets pretty hot, and I'm using a 2W resistor.

John,

Neither the resistor nor the coils get hot. The resistor stays lukewarm.....maybe body temp. I've already changed out 6 of them. If I notice any heat, I can easily swap out the resistor.

I'm curious what the resistance of the switch coils are.

Tell me how to measure it and I'll try it.

John,

I think I figured out a way to measure it. To convert all of my switches to fixed voltage, I Dremeled a window into the bottom directly over where the 2 coil wires connect to the hot lead. I separate them from that place, connect a wire to them and run it to the outside of the switch. I took out a couple of my "rejects" (ones that either don't snap well, or create rough rides or shorting problems with my modern stuff) and connected one lead of my meter to that wire and the other lead to  several diff. things. The two outer terminal on the switch as well as outer rails. At the terminals I got anywhere from 9.3 ohms to 9.6. At an outer rail, I got as high as 10.6, but I'm figuring there's more resistance factoring in the rail. Definitely not 25.

-Roger

I believe that I read in one of Bob Nelson's post that the resistance was 14 ohms.

The only switch I have is an old 1022, and I measured 25, but that was not at the coil.

If you have a reasonable amount of voltage there, I can't imagine how a 14 ohm coil and a 10 ohm resistor on DC aren't getting pretty warm!  What's the DC voltage you measure out of the diode that charges the capacitor?

Not sure how to connect the leads to measure that, but I tried it several ways and got the same results.....one lead of the meter on the wire out of the diode, the other lead on the capacitor. Got 17.6V.  I also tried measuring the DC off the switch terminals (with a car on the switch. Got 17.3-17.6.

The AC out of the transformer is around 16V. So I'm not sure why I'm getting a higher DC value.

John,

I had a consist sitting on a switch while I wired up the other two switches. Coil temp.....room temp. Resistor.....body temp.   So....perhaps my measured values of 9.6V are closer to actual?

Weird stuff, John.......One of the resistors was hot. Don't know why it didn't get hot on other switches. The coils on that switch, though.....were not even warm.  So, I guess we better go with the bigger resistor as you suggest. I'll have to swap them out. 

Will this one be ok?   http://www.radioshack.com/prod...sp?productId=2062292

That one is fine.