Advice needed on capacitor discharge units

[IndyBob]

i'm finally about to start building a layout .  The design will have about 40 points overall. I have bought lots of Peco point motors, some traditional PL10 motors and some of the new twistlock motors.  i am wondering if i need to use a capacitor discharge unit with them . I am no electrician so can't build my own as many seem to do.

 

a few questions - what is everyone's experience with Peco's PL-35 CDU? how many would I need to power 40 points?

 

Ayy advice gratefully received.

 

 

[Dagworth]

I don't know the PL-35 unit but just one of any type of CDU should be sufficient, assuming all your point motors are run from one single control panel. If you have two control panels (one each end of a large station for example) then one CDU at each control panel.

 

Generally a CDU will happily fire one or a pair of point motors at a time with a couple of second interval for it to recharge ready to work again. 

 

Andi

[DavidB-AU]

You rarely change multiple sets of points at the same time. Maybe 2 for a crossover or 3 to set a route in a ladder yard. The total number on the layout isn't critical.

 

An option to buying a CDU is make one yourself. They are very easy to build and there are plenty of plans online. The number and type of capacitors you need will be dictated by the number of points you need to change at once.

 

Cheers

David

[BR60103]

The CDU does 2 things for you.

It gives a huge jolt to throw the point motors.

It allows only a small trickle of power through if the switch is left on.  I've seen situations where the switch, usually a push button, has stuck on and the point motor has overheated. (I've also seen it with a CDU, and nothing worked until it was resolved.)

Power required is determined by number of simultaneous points thrown.  Look at your track diagram to see.

Free suggestion: get all your point motors from the same manufacturing era. If they have a different electrical characteristic (not sure what it's called)  they may not all throw at once or one may never throw. (for points thrown at the same time)

 

CDU also works with a thing called a diode matrix where you push the button for the final point in a series and all the rest line up for it. This will be covered in next term's course.

 

Edited April 30, 2020 by BR60103

[kevinlms]

7 hours ago, IndyBob said:

? how many would I need to power 40 points?

 

Ayy advice gratefully received.

 

 

Generally you need one for the whole layout. However, if it's a large layout and you have two or more operators, it is probably more convenient to divide it up, so that each operator has their own and prevent them from trying to throw groups of points at the same time. They aren't expensive and the convenience is well worth the extra outlay. 

[Pete the Elaner]

A CDU also stops the switch being burned out, or corrosion on the stud & pencil.

It also allows you to run lighting from the same transformer coil. Wihout it, the lighting will blink when you throw a point.

 

With a CDU, the burst of current is very short, dropping very quickly as the capacitor discharges.

Without a CDU, when the circuit is opened again, the collapsing magnetic field around the coil produces a voltage in the opposite direction. This voltage can be much higher than what you applied to the motor in the first place. It is this voltage which causes the switch to burn out.

With a CDU, the current (after the initial burst) is very low, so the switch suffers very little damage.

 

We had a Peco CDU on a club layout & it was gutless. I really need to dissect it some time to see what components they used 

Gaugemaster's CDU is a much better product.

 

And for the club layout, I had some home-made ones in my toolbox which several of us had made in order to encourage a younger member to solder. These worked perfectly well & the layout has been using it for about 2 years now.

[Crosland]

8 hours ago, BR60103 said:

The CDU does 2 things for you.

It gives a huge jolt to throw the point motors.

It allows only a small trickle of power through if the switch is left on.  I've seen situations where the switch, usually a push button, has stuck on and the point motor has overheated. (I've also seen it with a CDU, and nothing worked until it was resolved.)

 

The cheapest of cheapskate will literally just use a capacitor (mentioning no names) as CDU, This does not protect the switch or the point motor and can lead to the situation you describe.

 

Always look for a CDU that has a current limit upstream of the capacitor. This does two things:

1. Allows much less current to flow once the capacitor is discharged.

2. Reduces the recharge current so that your power supply doesn't have an issue. This is more of a problem with moderns switch mode supplies that are very sensitive, rather than a transformer/rectifier.

[DCB]

2 hours ago, Pete the Elaner said:

A CDU also stops the switch being burned out, or corrosion on the stud & pencil.

It also allows you to run lighting from the same transformer coil. Without it, the lighting will blink when you throw a point.

 

And for the club layout, I had some home-made ones in my toolbox which several of us had made in order to encourage a younger member to solder. These worked perfectly well & the layout has been using it for about 2 years now.

 

Its more like spark erosion than corrosion. I have to polish up my screw heads on my probe panel every coupe of years. 

The original advantage of the CDU was the trains didn't stop dead when you threw a point like they did when the track power and uncontrolled power came from the same transformer.

 

My home made capacitor discharge system has worked well for around 35 years, and my motorbike capacitor electrical systems for over 40, the same caps still work well, but its over engineered with 63 (?) volt capacitors on nominal 12 volts fed from a dedicated 1 amp nominal 12 volt DC supply.  I did use a proper Lucas 12 volt capacitor with a proper Lucas 12 volt Zener diode  on a bike once and it exploded.  Fully charged Caps don't take kindly to being shorted out.  I never use anything less than 24 volt capacitors on 12 volts now.  Exploding caps are only funny in retrospect, big "Pop" and yellow liquid spraying everywhere. No idea what it was but it stained my fingers yellow.

[Pete the Elaner]

1 hour ago, DavidCBroad said:

 

I did use a proper Lucas 12 volt capacitor with a proper Lucas 12 volt Zener diode  on a bike once and it exploded.  Fully charged Caps don't take kindly to being shorted out.  I never use anything less than 24 volt capacitors on 12 volts now.  Exploding caps are only funny in retrospect, big "Pop" and yellow liquid spraying everywhere. No idea what it was but it stained my fingers yellow.

 

12v may not always be 12v. An accurate 12v AC is 12vRMS, which is about 17v peak, which is what the capacitor needs to deal with.

 

A blown cap is not pleasant is it? One of the club layouts had a flasher unit which needed something smoother than uncontrolled 12v DC from a controller, so somebody smoothed it with an electrolytic capacitor, but this didn't work, so I was asked to look.

I found the capacitor bloated & the board around it was damp, so something had caused it to explode.

My meter revealed it had been wired with no regard for colour coding, so black was positive & red was negative. Definitely not my work!

[cliff park]

A guy at work was probing in the back of a power unit and a large electrolytic exploded in his face. He was OK but until the day he retired he was known as Nanky Po.

[DavidB-AU]

10 hours ago, Pete the Elaner said:

 

12v may not always be 12v. An accurate 12v AC is 12vRMS, which is about 17v peak, which is what the capacitor needs to deal with.

 

 

I was going to mention this if the discussion went on to CDU design. The voltage rating of the capacitor needs to be at least 1.4 times the AC input voltage plus another 10% margin, rounded up to the next available voltage rating. So for 12V AC input you'd be looking at a 20V rated capacitor. For 16V AC you'd need a 35V rated capacitor. 50V would be even better.

 

Cheers

David

[WIMorrison]

Don't you mean 1.41 times the DC input voltage (which will be the RMS value)?

 

[Pandora]

As an alternative to CDU,  can we use a bank of AA size rechargeable batteries?

Edited May 1, 2020 by Pandora

[Crosland]

2 hours ago, WIMorrison said:

Don't you mean 1.41 times the DC input voltage (which will be the RMS value)?

 

 

Depends on what you mean by "DC".

 

If it's unsmoothed, rectified AC, then it depends what measurement is quoted. Safest to assume you are given the RMS value and multiply by 1.4.

 

If it's smooth DC then there is no need to worry about RMS.

 

[Brian]

On 30/04/2020 at 01:25, IndyBob said:

i'm finally about to start building a layout .  The design will have about 40 points overall. I have bought lots of Peco point motors, some traditional PL10 motors and some of the new twistlock motors.  i am wondering if i need to use a capacitor discharge unit with them . I am no electrician so can't build my own as many seem to do.

 

a few questions - what is everyone's experience with Peco's PL-35 CDU? how many would I need to power 40 points?

 

Ayy advice gratefully received.

 

 

Hi

1 or 40 points still only one CDU is needed.  You only operate one or two points simultaneously usually . Some users may move more at one time via a diode matrix, but conventionally 1 or 2 at a time is the norm.  

The original Peco PL35 wouldn't blow the proverbial skin of a rice pudding but I understand it has been upgraded now. However there are far better CDUs available and often cheaper too. Look at the excellent BlockSignalling range Link to BlockSignalling or even the Gaugemaster CDU Gaugemaster link

Edited May 1, 2020 by Brian
URL added

[Pete the Elaner]

27 minutes ago, Pandora said:

As an alternative to CDU,  can we use a bank of AA size rechargeable batteries?

 

The short answer is no. Although they both store charge batteries & capacitors work very differently.

 

The requirement is a short pulse. A capacitor discharges very rapidly through a relatively low resistance such as a coil, its voltage dropping as it discharges. After a quick press of a button, charge & (therefore voltage & current) will have decayed almost completely.

In comparison,. batteries discharge very slowly, making them more suitable for a device which you want to work consistently for a period of time, such as a motor or radio.

Edited May 1, 2020 by Pete the Elaner
spelling

[NinOz]

1 hour ago, Brian said:

Hi

1 or 40 points still only one CDU is needed.  You only operate one or two points simultaneously usually . Some users may move more at one time via a diode matrix, but conventionally 1 or 2 at a time is the norm. 

Depends on how far apart (size of layout) the points are.  I have about 10 or so CDUs, with local panels.

[Crosland]

3 hours ago, Pete the Elaner said:

 

The short answer is no. Although they both store charge batteries & capacitors work very differently.

 

The requirement is a short pulse. A capacitor discharges very rapidly through a relatively low resistance such as a coil, its voltage dropping as it discharges. After a quick press of a button, charge & (therefore voltage & current) will have decayed almost completely.

In comparison,. batteries discharge very slowly, making them more suitable for a device which you want to work consistently for a period of time, such as a motor or radio.

 

Batteries can be made to discharge quickly, often spectacularly so.

 

The real difference is in the internal resistance. Pulling a high current from a battery results in heating in the battery, due to the higher internal resistance, and a drop in the terminal voltage, again due to the higher internal resistance.

 

The following is less relevant to the original question:

 

The various forms of storage go, in terms of decreasing internal resistance: primary cells, rechargeable cells, supercapacitors, capacitors.

 

Energy density goes the other way: primary cells (highest) -> capacitors (lowest).

 

An e-vehicle might use supercaps to absorb energy during regenerative braking or deliver a short term high current pulse, but you need batteries for greater energy density and longer range.

 

You can also damage capacitors by trying to charge/discharge them too quickly. They have a "ripple current" rating to specify what they are capable of.

[Brian]

3 hours ago, NinOz said:

Depends on how far apart (size of layout) the points are.  I have about 10 or so CDUs, with local panels.

Yes of course I know that.  I was trying to answer the OP question simply!  But the number of points operated from any one panel or position and its CDU is not limited, as only usually one or two point motors only move together!  Doesn't matter if you have 1 or 100 panels that was not what I was referencing.

  

 

[Sol]

15 hours ago, Brian said:

Hi

1 or 40 points still only one CDU is needed.  You only operate one or two points simultaneously usually . Some users may move more at one time via a diode matrix, but conventionally 1 or 2 at a time is the norm.  

 

 

That is true for a one or two person layout needing only one CDU - be no good for my UK outline layout that has 5 crew  & 70 electrically operated turnouts or even Banbury Connections in New South Wales that requires 8 operators.

https://banburyconnections.weebly.com/operations.html

 

One of my modelling mates in the Barossa Valley here in South Australia will end up with well over 100 electrically operated turnouts and is using DCCconcepts CDU   https://www.dccconcepts.com/product/power-supply-integrated-cdu-unit-super-high-power/    

    and he has a large diode matrix to be able to switch 6 turnouts at once.

https://www.dccconcepts.com/manual/cdu-2-mains-powered-super-high-energy-dual-output-capacitor-discharge-unit/

 

Edited May 2, 2020 by Sol
error in number of turnouts and adding more words

[NinOz]

9 hours ago, Brian said:

Yes of course I know that.  I was trying to answer the OP question simply!  But the number of points operated from any one panel or position and its CDU is not limited, as only usually one or two point motors only move together!  Doesn't matter if you have 1 or 100 panels that was not what I was referencing.

  

 

I have routing where up to 6 points will switch in a yard simultaneously. 

Shouldn't make generalizations.

[rab]

6 hours ago, NinOz said:

I have routing where up to 6 points will switch in a yard simultaneously. 

Shouldn't make generalizations.

I suppose if we're being prototypical, and I guess depending on the era,

points should be switched sequentially rather than simultaneously.

A bobby could only pull one lever at a time.

 

Edited May 2, 2020 by rab

[Sol]

That is true - with electronics, they can be operated sequentially but depending on layout size & number of operators, there could still be a few solenoids being activated at the same time so a CDU like DCCconcepts or one normal CDU  per station is better IMO. They are not expensive.

[Brian]

8 hours ago, NinOz said:

I have routing where up to 6 points will switch in a yard simultaneously. 

Shouldn't make generalizations.

If you read my original post you would see that issue was covered with a comment to say several points can be moved at once.  However, I doubt (not tried) that the latest Peco PL35 would do such? Certainly the original version (I had one to test) struggled to move two PL10 at once.  

No further comments from me as obviously all these replies are not helpful to the OP at all. 

[IndyBob]

thanks for the advice.  Much appreciated.