DCC Railways

: : > : > :> I do thanks, unlike you, your description of a CDU, : > :> which is laughable. : > : : > : Lets hear yours, if you think you can do better. : > : : > : Here's a good walk-through : >
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> : where you will see that any sensible CDU design : > : includes the current limit on the recharge side. This : > : > Does it, looks to me that it limits current draw on the recharge : > side *when the CDU is discharging*, thus protecting the : > recharging side from possible overload. : : Doesn't that amount to the same thing!!!!!!
No, and you seem to have a sticky key on your keyboard.
: : This type of CDU is only of any use at all if there is significant : voltage drop in the wiring to the point motor when it operates, or the : power supply is not up to supplying the full current required by the : motor. Otherwise after initial switch on the capacitor does nothing useful. :
Thus it would be bloody useless on a layout were one might be operating multiple numbers of solenoids, sometimes simultaneously (via a route setting matrix for example) and at other times sequentially - the latter being the more usual.
: : : > : limits the *power* but allows the capacitor to charge : > : to almost the full voltage (hence the same *energy* : > : stored). : > : > A CDU has to recharge in seconds, that is because the next : > operation could (and probably will) be in seconds, the larger the : > CDU is (thus the more work it can do, either bigger or a greater : > number of solenoids) the MORE current it will draw during this : > recharging phase -from the power bus in this case- and as the : > OP's problem appears to be a lack of available amps... : : No, the amount of current drawn will be dependant entirely on the series : resistance of the motor. As long as there is sufficient energy in the : capacitor to throw the point a larger capacitor will be no better, other : than being able to throw another motor without re-charging.
Exactly, so unless the capasitor is overly large the unit must charge quickly, or the next operation might not happen, these units are not about limitting current draw during charging but current avalible during discharge.
: : > : > All a CDU is designed to do, to use a figure of speech, is to : > give a "good kick up the arse" to the solenoid - it is not : > designed to limit power draw during recharging because that is : > not the point of the unit, or shouldn't be... : : Yes, it must otherwise you will overload the psu.
Which, unless one designs the unit not to (as mentioned below, in which case the solenoid will fail to operate and the unit will be prevented from recharging) that is exactly what will happen if the power supply or the unit is not of sufficient capacity, A CDU is no substitute for an inadequate PSU!
: : : > : > :> Next you'll be suggesting perpetual motion... : > : : > : Why would I need to do that? : > : > Because that is what you are suggesting, clue there is no such : > thing as a free lunch, if you double the voltage, you need to : > double the current as there is little point in having mega volts : > but no amps [1], think about it... One of the reasons why the : > national grid distributes at a HIGH volts/watts and then converts : > down to *lower* volts/watts (ultimately to 240v, typically, for : > UK domestic use), have it your way -with your magical "voltage : > doubler"- and the national grid would distribute the street level : > supply @ ~ 110v and then up-convert to 240v at the intake of the : > end user. : : You miss the point doubling the voltage will double the current through : the motor (Ohm's Law), that will kick it harder. Because of the short : pulse the motor is not overheated by the higher voltage. :
No, I get the whys and wherefores thanks, what I'm saying is that in doing so (stepping-up) you need to increase the input current used, always far more efficient (power use wise) to step-down.
It would actually make more sense for these point solenoids to be made to work at a (continuous rating) of ~ 6 to 8 volts and then power them for short periods at 12 to 16 volts, without any need for a CDU.
Reply to
Jerry
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Look at the "current blocking" design "A resistor to limit the maximum charging current to a reasonable level".
A better example would be
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where you can see the classic two transistor current limit circuit.
Stating the obvious.
The amount of current it draws during the recharge phase is under control of the CDU designer. It's total charge that matters, which is proprtional to both current and time.
Yes, it is designed to discharge very quickly into a low impedance load such as a solenoid point motor. That discharge takes of the order of milliseconds.
The whole point of a CDU is to buffer the main supply from these very large, but brief current pulses. To give a "discharge" that could not be supplied by the main supply. A *well designed one* does that by recharging more slowly.
As an example A few amps for a few milliseconds into the solenoid can be replaced by recharging in a few tens of milliseconds (10x the time) at a few hundred milliamps (1/10 current).
No it iosn't. You really do need to go back to basic theory.
Doubling the voltage in the charging circuit quadruples the energy stored in the capacitor so you can give an even bigger kick to the solenoid. The discharge current will initially be double due to simple application of ohms law. The charging current and charging time are, again, under control of the designer.
Completely different, and irrelevant to this discussion, scenario. High voltage is used in the grid to reduce resistive losses.
WTF has this got to do with CDUs and point motors?
Or vary the recharge time.
MBQ
Reply to
manatbandq
Not quite, it also limits the recharge current.
It has 2 very useful benefits: once discharge the current flowing is limited so it is less likely you will burn out a point motor; Once discharged the current flowing is limited and you will minimise damage to the switch contact when breaking the current flow [1].
MBQ
[1] Connecting a solenoid to a power supply through a switch means that the full current determined by the solenoid coil resistance all the while the swutch remains closed. This is a DC current. Breaking the current by opening the switch leads to arcing between the swicth contacts which damages the contacts. Given the cheapskate nature of a lot of modellers I can guarantee many people are using switches that are not really up to the job.
Reply to
manatbandq
The capacitor is discharged through the solenoid, *very* quickly, regardless of its size. The recharge will still be quick, even if it is an order of magnitude or more slower than the discharge time.
If you have no current limit circuitry then all you have is a power supply connected to a capacitor connected through a switch to a solenoid. What do you think happens when you close the switch?
Bingo! That's *exactly* what a CDU is for.
Think of a how a toilet flush works. The cistern fills relatively slowly. Flush, and it empties very quickly to wash the turds away. Wait for it to refill, repeat...
The flush lever is the point operating switch. The cistern is the capacitor. The turds are the point blades. The sudden deluge of water is the current pulse through the solenoid. The inlet is your "inadequate" PSU. Oh, hang on, so long as you don't need a s**t more often than the cistern can fill then it's perfectly adequate.
MBQ
Reply to
manatbandq
: > Does it, looks to me that it limits current draw on : > the recharge side *when the CDU is discharging*, : > thus protecting the recharging side from possible : > overload. : : Look at the "current blocking" design "A resistor to : limit the maximum charging current to a reasonable : level". : : A better example would be
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where you can see the classic two transistor current limit circuit.
So there is an alternate power path to by-pass the CDU should the capacitor(s) be discharged? By limiting the charging current one also limits the total current draw through the unit at all times and thus acts as overload protection. A CDU doesn't need a 'trickle' charge, it needs to recharge as quickly as possible, ready for the next operation (or should do if of decent design).
: > A CDU has to recharge in seconds, that is because : > the next operation could (and probably will) be in : > seconds, : : Stating the obvious.
So why do you claim otherwise, with your 'trickle charge' effect!
: The amount of current it draws during the recharge : phase is under control of the CDU designer. It's total : charge that matters, which is proprtional to both : current and time.
No, only time is in the hands of the designer, current is a fixed known, and unless you are prepared to allow the unit to fail completely due to no fail back should the capacitor be discharged even time isn't really in the designers hands ('time' is governed by the track plan and or control panel design).
: The whole point of a CDU is to buffer the main supply : from these very large, but brief current pulses.
Utter claptrap. A CDU is not like a cars ignition system, it's more like the cetral locking system, think about it....
: You really do need to go back to basic theory.
Stop talking about yourself MBQ, you do not have a clue as to how and why a CDU is used/works.
: Doubling the voltage in the charging circuit quadruples the : energy stored in the capacitor so you can give an even : biggerkick to the solenoid.
Yes, but you don't get that effect for free, which is the point, the OP doesn't have the spare amps to use in that way.
: The discharge current will initially be double due to : simple application of ohms law.
No one is disagreeing about the discharge or current provided.
: The charging current and charging time are, again, : under control of the designer.
Again, only time is, assuming s/he is willing to trade reliability and/or usability.
: > national grid distributes at a HIGH volts/watts and : > then converts down to *lower* volts/watts (ultimately : > to 240v, typically, for : : Completely different, and irrelevant to this discussion, : scenario. High voltage is used in the grid to reduce : resistive losses.
Not at all irrelevant, to your claims that one can step-up voltage/current without penalty.
: > It could be done but it would actually be more dangerous, : > one of the reasons why the USA use duel phases @ 110v : > (with a centre tap return) to allow the provision of a SP : > 220v high power supply for cookers. water heaters etc. : : WTF has this got to do with CDUs and point motors?
It doesn't, directly, it was in reply to your claim that one can magically get more current from using less, if that was possible don't you think that every electricity utility company in the world would be doing so! Lets use another example, most older CRT TVs and computer monitors have very high power consumption figures, not because the electronics on the PCB need such a supply but because the CRT needs a high voltage/current supply that is produced by stepping-up the input 240v supply, you seem to be claiming that one could design a CRT PSU that provides the same constant high current output power supply but which draws a very much lower current on the intake side - no doubt you have also been out to another one of your free lunchs today...
: > You're missing the point MBQ, but no surprise there, : > yes these units exist but they actually draw more : > current *for the same work*, OR, draw the same : > current but do less work. : : Or vary the recharge time.
Only if you trade reliability/usability, I want to be able to switch more than a couple of points sequentially at a time, as do most I suspect! Yes your suggestion might work in a route setting matrix, given a suitably large CDU but even then one would have to wait before switching the next route!...
Reply to
Jerry
: > [Exactly, so unless the capasitor is overly large : > the unit must charge quickly, or the next : > operation might not happen, these] : > units are not about limitting current draw during : > charging but current avalible during discharge. : : If you have no current limit circuitry then all you have : is a power supply connected to a capacitor connected : through a switch to a solenoid. What do you think : happens when you close the switch?
If you had a PSU of sufficient power you wouldn't need to have a capacitor in the circuit, well not connected as in a CDU, so your point (pun not intended) is what, exactly.
: > A CDU is no substitute for an inadequate PSU! : : Bingo! That's *exactly* what a CDU is for.
Oh right, so you are now claiming a solenoid can't work without a CDU?! Duh, you really are a dimshit MBQ, perhaps if you took your nose away from Google and actually did some modelling for a change...
I have been at this game for the last 40+ odd years kiddo, never have I used a CDU when powering such solenoids, although I have also never used the same PSU as the traction current is obtained from, especially the sort of model railway PSU common way back when CDUs started to be used...
Reply to
Jerry
The point is that we would not be discussing CDUs if the power supply were so over specced (compared to what could be used with a CDU) to the extent that it could power a solenoid once a second, say.
Don't try putting words in my mouth. I'm claiming that a CDU allows the use of a power supply that would otherwise be inadequate to switch a solenoid motor. It's quite a simple concept
MBQ
Reply to
manatbandq
My keyboard is working pewrfectly !!!!!!!!!!!!!
snip>
No, you are missing the point, if there is little resistance in the wire and the psu is capable of supplying the peak current required then a CDU will not improve things.
If that is not the case and, for example there is significant wiring resistance, then you could just add the capacitor without the series resistor. As long as the wiring between the resistor and the motor is *short & low resistance* you will see the same effect as having the resistor there *except that there will be a large inrush current to re-charge the capacitor, which will be limited only by the wiring resistance and the psu o/p capability.
You example of throwing multiple points from one of this type of CDU is a red herring since for one CDU to operate many points the wiring from the CDU to the points *must* be low resistance and because of multiple points it cannot be close to all of them. So you might as well throw away the CDU and just make sure that you have low resistance and a psu that can provide the peak current.
Of course the above does not apply if the CDU runs a higher voltage that the normal rating of the motor, and provides a short pulse at a higher voltage. In that case more series resistance can be tolerated.
:
No exactly the opposite if you are talking about DC, stepping down normally wastes the excess voltage as heat, very inefficient, unless you use some form of switching regulator. If you are talking about AC & a transformer, or voltage doubler, there is no significant penalty as conservation of energy applies, less any small transformer loss, whether you are stepping up or stepping down.
That is no different to them operating at 12 or 18V and using 24V pulse!!!!!!
Regards Jeff
Reply to
Jeff
That is a fair point.
Jeff
Reply to
Jeff
Correct, nor does there need to be.
I wouldn't call it a trickle charge. The circuit is designed so that the current is sufficient to recharge the capacitor in time for the next operation. That minimum re-use time will be a design parameter.
I don't. All I have claimed is that the recharge time can be longer than the discharge time. It will still "recharge in seconds".
Use of "trickle charge" is *your* terminolgy. It suggests too low a recharge current, to me, so I don't use that term.
Charge or discharge current? Fixed by whom?
The time before the CDU can be used again is a design parameter, and needs to be less than the time between operator interaction with the control panel. Once that is determined you can calculate the maximum charge current required in order to ensure the CDU is recharged sufficiently quickly.
So why bring it up?
That's claptrap.
LOL!
By this point the discussion had moved on to a general discussion of CDU operation. The OPs situation is not relevant to that discussion. It's called thread drift.
I made no such claim. [see below]
Again, I made no such claim. I claimed you can get more *energy* in the discharge pulse by charging the capacitor to a higher voltage. The higher voltage will result in a higher discharge current (simple ohms law). There will be penalties in the form of changes to the required current and/or charge time.
If you are so clever, do a SPICE simulation of a voltage doubling CDU and see how it works.
MBQ
Reply to
manatbandq
The rate at which you switch with just a psu is pretty much irrelevant, it is the peak current capability that is the limiting factor; and how much the voltage sags when you try to draw excess current. PSUs will recover to their full o/p pretty much instantly once the load is removed.
That is correct in as far as it goes, but a CDU will still fail if there is significant resistance between it and the motor. All a capacitor is doing is effectively reducing the output impedance of the source, but only for a very short period. Once discharged it will present a low impedance to the psu, and without any current limiting will initially look pretty much like a short circuit to the psu. Add to that the fact that capacitors do not like large inrush currents; that is why you will find ripple ratings on capacitors.
Jeff
Reply to
Jeff
The maximum inrush or ripple current for the capacitor is also a key design parameter if you want the capacitor to last any length of time.
Jeff
Reply to
Jeff
Good point!
MBQ
Reply to
manatbandq
: The point is that we would not be discussing CDUs if : the power supply were so over specced (compared to : what could be used with a CDU) to the extent that it : could power a solenoid once a second, say.
But most dedicated PSU's could, it is your design of CDU that can't do that, you just don't get do you, to much Googling and not enough real world experience is your problem, not helped by people who wish to play with electronics (as fun as that can be) more than they actually want top play with model trains. The problem the OP has is that his PSU can barely supply enough power for the locos never mind anything else, your idea of a CDU would actually make his problems worse because it would prolong the periods of inadequate supply, unless you care to explain were all this 'free energy' is coming from that you keep whitening on about.
The OP would have been as well to have ordered a stand alone PSU (and built a control panel) for his accessories as buy a different DCC system, other than the fact that he is well advised to move away from the basic (trainset) end of the DCC market.
: Don't try putting words in my mouth. I'm claiming that
You don't seem to know what you are claiming, that's the problem!
: a CDU allows the use of a power supply that would : otherwise be inadequate to switch a solenoid motor. : It's quite a simple concept
Indeed it is, you need a PSU that is adequate to power the solenoid regardless of if a CDU is fitted, period.
Reply to
Jerry
The point you are missing is that what is that "adequate" changes when you use a CDU.
Assume 4 ohms for the solenoid coil resistance. Assume a PSU of 16V. That requires a PSU rated at 4 Amps to drive the solenoid directly, unless you deliberately overload the PSU each time you fire point motor.
With a CDU you could easily limit the charge current to 2A and use a lower rated PSU.
MBQ
Reply to
manatbandq
Perhaps of more interest to the OP, I have 2 peco point motors throwing 2 peco points from a single output of a hornby accessory controller connected to a Select. No problem with points changing. Another point (yep), by connecting points that are likely to be changed on a single route to different accessory controllers there is less work for any single controller. Finally is a few seconds between point changes that important - lets face it how long did it take a real signalman to set up a route ?
Cheers, Simon
Reply to
simon
Sounds like the Hornby unit is well designed. Does it take power from the track or a separate input?
Exactly! A fact that seems to be lost on thise who assert we should spend more time modelling the real thing than playing with electronics.
MBQ
Reply to
manatbandq
: I think it was my comment about requiring the wiring to : be "as thick and short as possible" that set Jerry off on : one :-)
No, it's your utter ignorance which does that, and your continual refusal to accept that you are wrong -hence why you chose to take a cheap shot at me- rather than to address the points made by Jeff.
Oh well, as they say, none so blind as those who chose not to see...
Reply to
Jerry
: The point you are missing is that what is that "adequate" : changes when you use a CDU.
Err, MBQ, you are missing is the *fact* that it does not, as even Jeff has pointed out, and indeed a CDU can actually make problems worse.
Reply to
Jerry
: You example of throwing multiple points from one of this type of CDU is : a red herring since for one CDU to operate many points the wiring from : the CDU to the points *must* be low resistance and because of multiple : points it cannot be close to all of them. So you might as well throw : away the CDU and just make sure that you have low resistance and a psu : that can provide the peak current.
Err, I think that *was* my point! Lets get back the basics of why CDUs became popular in the 1960s, it had nothing what so ever to do with wiring size as most people routinely used wire more than adequate for the task, it was to (try to) prevent solenoid burn-outs due to poorly laid and/or 'sticky' points -hence my previous comment about giving the solenoid a "kick up the arse"- and as you point out to add some protection to the coil windings should the power continue to be feed to the coil due to the unit not throwing over.
: : No exactly the opposite if you are talking about DC, stepping down : normally wastes the excess voltage as heat, very inefficient, unless you : use some form of switching regulator. If you are talking about AC & a : transformer, or voltage doubler, there is no significant penalty as : conservation of energy applies, less any small transformer loss, whether : you are stepping up or stepping down.
Best you tell that to the electric utility companies then mate, they will love that revolutionary discovery! Free, or at least paid for, lunches all round...
: : > : > It would actually make more sense for these point solenoids to be : > made to work at a (continuous rating) of ~ 6 to 8 volts and then : > power them for short periods at 12 to 16 volts, without any need : > for a CDU. : : That is no different to them operating at 12 or 18V and using 24V : pulse!!!!!! :
Indeed, and no CDU in sight, except that most trainsets/layouts do not have a 24v supply, hence the suggestion of 6 to 8V coils.
Reply to
Jerry

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