Capacitor Discharge Circuit Question

[jprampolla]

Hi Folks,

With a traditional capacitor discharge unit for a turnout solenoid with just a resistor and capacitor, will the capacitor recharge if the push button is held down? Also, roughly speaking, how long will the jolt from the capacitor last, when fully charged, when the button is pushed?

Thanks!

Take care, Joe.

[cacole]

The push button goes on the discharge side of the capacitor, not the charge side. The idea is that the capacitor will charge up when the button is NOT pressed, and will discharge when pressed. The capacitor stores electricity so the switch machine will be given a stronger jolt than it would if it was connected directly to the power pack.

Depending on the brand of swtich machines you are using, you might not need a capacitive discharge circuit. Atlas Snap Switches, for example, usually won't benefit from one, and may actually be harmed by it.

[Leon Silverman]

The jolt length depends on the capacitance rating of the capacitor. The higher the capacitance, the longer the jolt. However, these questions are academic. If you hold the button down for any extended period of time, you will probably burn out the solenoid switch motor without necessarily depleting the charge built up in the capacitor.
The capacitor discharge circuit allows multiple switch motors to be thrown at once and , if you are using a power pack to power both your trains and the switches, avoids the momentary slowdown of your trains when you activate the switch solenoids.

[rrinker]

Ah but Atlas snap switches work GREAT with capacitor discharges. Keeps from burning them out - ESPECIALLY the N scale ones. Once i built one for my last N scale layout, I never again burned another coil.

That resistor that is on there to keep the capacitor from shorting the power supply also limits current if you hold the button down - properly done, you won't burn out the coil by holding down the button like would happen with a straight AC supply.
The downside is - the capacitor recharges rather slowly. So if you have one of those "Oh s$)@" moments, you might not be able to throw more than the one switch. This is easily rectified by the next level up in capacitor discharge supplies, which incorporate a transistor to allow faster capcitor recharge. This is the one sold by Circuitron, and is also found in the book Practiical Electonic Projects for model Railroaders - which is where I built mine from oh so many years ago. Not that anyone cares about the details, but basically the transistor switches in a smaller resistance to charge the capacitor faster, but once it's charged the transistor cuts off and once the capacitor's energy is discharged, any current flowing through a held down or stuck pushbutton is limited by the higher value resistor to a few milliamps.

--Randy

[jprampolla]

Hi Folks,

Many thanks for all the information.

So, if I increase the size of the resistor, the capacitor will still recharge, but more slowly with more resistance, but eventually up to the same voltage as if a lower rated resistor was in place.

What I want to do is achieve the same result as if I used the more elaborate circuit with a transistor so the solenoid will not get any more current after the initial jolt. In my odd ball case, If the cap takes a minute to recharge, that would be fantastic.

I think I am good to go. The arrows on the capacitor indicate the lead that connects to the negative side of the circuit?

Thanks, again! Great help here!

Take care, Joe.

[dinwitty]

mmm...slo-mo-tortoise machines... dropping an obvious hint.....

[rrinker]

Use the more elborate circuit if that's what you want. It's not really THAT much more elaborate, there's essentially only 1 more component, and you can even get that at Radio Shack. Check Rob Paisley's site http://home.cogeco.ca/~rpaisley4/CDPSU.html

You want circuit B right at the top. These are simple to build and make great first electronic projects - the only thing really able to be destroyed by ham-fisted soldering is the transistor, and even that can take more than a typical integrated circuit. The only thing is he doesn;t provide values for the parts. I'll have to go look up my copy of Practical Electronic Projects, but I think the transistor is a 2N3055 and the cap was around 2200uF 50V. I'll get the real info for you.

Yes, polarized caps have the arrow pointing to the negative lead. Do NOT hook them up backwards - BOOM! Literally. It can be dangerous.

--Randy

[jprampolla]

Hi Randy and Folks,

Many thanks for the help! I have a Kalmbach Peter J. Thorne book and have been to Mr. Paisley's site before and recently. It is a MJE3055T in Mr. Thorne's book.
The most critical thing (for me) is correctly identifying the cap's "+" and "-" leads, which I don't deal with frequently.
I think I can work out the rest by adjusting the resistor. I have 3300 mfd caps, which I am sure will work well. I will try the simple, traditional circuit first, and if I don't like it, I will go with the transistor circuit.

Take care, Joe.

[jprampolla]

Hi Randy and Folks,

Just finished my experimenting, and I am happy to report that the circuit form Thorne's "Model Railroad Electronics, Basic Concepts, etc.," page 51, fig 2, the traditional cap discharge circuit with just a resistor, diode, and capacitor, works just fine for my situation. I am using an approximately 6.5 VAC wall transformer, with cap discharge circuit using a 470 ohm resistor, 3300 mfd 16 V capacitor, with a 6 volt miniature, intermittent duty solenoid that has a 2 ohm coil. Works just fine in this initial testing. Seems that I get the one, good jolt, regardless of how long the push button is depressed. That's what I wanted so that if the button
remained depress or quickly repeatedly pushed, the solenoid would not energize again. Might even increase the ohms of the resistor later to make the recovery even slower.

Thanks again, everyone, for the kind, prompt help!

Take care, Joe.

[jprampolla]

Hi Folks,

I did add another 470 ohm resistor to the circuit, so now the recharge time is a little longer, just what I wanted.

Thanks again, everyone, for all the advice, technical help, and encourgement!

Take care, Joe.