Using DC relays and rectifiers to control sidings

For many DC relays, you'll need the rectifier and a filter capacitor to keep them from chattering.

You just wire the power through the bridge rectifier, the DC output of the bridge goes to the relay through the switch, and the capacitor is directly across the output of the rectifier.

I would use a separate transformer or accessory tap of the track transformer. Set the voltage at 18. Hook the 2 transformer leads to the 2 leads marked ~. Across the + and - place a 1000 uf, 35 volt or more capacitor. This will provide a 24 VDC filtered supply for the relay coils. Use 24VDC relays with contact ratings of 10 amp or more. .

Here is a link that may be helpful

LINK

Dale H

 Lionel 364C switches   Huh?  what are they?

Out of curiosity, does anyone know what the current rating for one of these is?  Looks like a pretty simple slide switch that should melt your wiring before it fails, but I can't find an actual rating anywhere. 

Thanks John... wow! Does that bring back memories, I had one on my Lionel freight shed with the carts that vibrated around..

Three points:

DC relays will work perfectly well, and will not chatter, on unfiltered DC.

Why not use AC relays?

I'm sure that the current draw on a siding will not be a problem for the contacts on a 364C controller.

You're wrong about the chatter, it depends on the relay.  I added capacitors to our signal relays because of the chatter with just a rectifier.

AC relays are wound to compensate for the pulsed current (and changing polarity),DC relays are not.

The DC relay may work on pulsed DC of sufficient voltage,but such current applied leads to the current switching on and off 120 times per second of the coil power. (60 with half waved DC).

This is not good for the relay contacts. In long term applications,it could shorten contact life. A diode should also be placed across the coil in opposite direction to remove voltage spikes when power is removed.

Dale H

If you don't want to mess with individual electronic components (rectifiers, capacitors, etc.), another approach is to use low-cost eBay modules.  For example:

Targeting 12V as the DC relay voltage insures a wide selection of low-cost, hi-current relays.  In this example, the relay modules have screw terminals which can simplify hookup if soldering is not your thing.  If there's no place to plug in a wall-wart 12V adapter, you can also buy AC-to-DC adapter modules on eBay for a few dollars that convert lower voltage track/accessory AC to 12V DC.

When using a full bridge rectifier, how does one calculate the DC output voltage? Is that the 1.41 times the AC input formula or do I have that backwards or just totally wrong?

figure 40 percent increase. There is a small drop from the rectifier. Relays can take 20 percent under or over usually so voltage is not critical. Biggest mistake is people use 12VDC relays,then use a 14 volt tap with a capacitor. This puts over 18 volts into the coil. The proper input for a 12VDC relay is 10 volts. As mentioned 18 volts is good for a 24 VDC relay. Since most train transformers max out at 18-20 volts,the 24 volt coils can not be burned out accidentally.

BTW even if the circuit is half waved with a diode,you still get the 40 percent increase if the capacitor is sufficiently large.

Dale H

Stan, can you provide an ebay item number for the "adapter" shown for $.50 with free shipping?  I tried a search on ebay and couldn't find that specific item.  Thanks

I agree with Arthur.

I use AC & DC relays to control signals and track blocks on my mainlines but unless I’m missing something in this application using them to control sidings is overkill and much unnecessary expense and wiring effort.

Because of their ease of installation I started using 364C’s as toggle switches to control sidings 50+ years ago.  On my current 20-year old layout I have about 10 of them controlling sidings.  In all of these years I have yet to experience any melted contacts on my 364C control switches.

So what am I missing here?

Bill    

I had first thought of using AC relays but after a brief search I hadn't found one online

I really appreciate the variety of responses.  I'm pretty capable of dealing with anything electrical but have no experience with electronics.  Sadly several of the responses left me in the dust trying to understand what was being suggested.  Maybe I should have asked for explanations involving one-syllable words Lol.

Perhaps I should stick with simple electrical solutions as suggested by Arthur & Bill.  I was mainly concerned that the contact points in the 364C's could not handle the job for long.  I'm bad about over building and over thinking things so maybe I should reconsider and simplify things by sticking to just the 364c wired directly to the siding.

The AC relay has a single turn that acts as a secondary coil which has current in it, caused by the "primary" coil, further magnetizing the core. The copper single turn or ring is delayed in response to the primary coil's magnetization of the core such that it holds the armature in when the AC source goes thru zero voltage.

It's mechanically and electrically different from the dc relay but a rectifier and filter capacitor works quite well for using a dc relay on ac.

Thanks. I was aware of the 20% tolerance on the relays at least the 24VAC ones. My experience in working years was 24 VAC relays with 24 VAC transformers which usually put out 25-26 VAC (occasionally 27-28 VAC). Never worked much with DC directly and seldom with DC relays. If we ever did have DC relays the relays were matched to the power and usually 12 VDC, no conversion factors to deal with.

I was thinking I had read about the 1.41 multiplier somewhere, but couldn't find where I saw it. Probably here somewhere? I did not know the half wave diode would give the same increase as a full bridge. All I have to do now is remember so I don't have to ask again.

When you rectify a sine wave to charge a capacitor, the capacitor will charge up to the peak value of the sine wave. The peak value is, the square root of 2, times the RMS value of the AC voltage. That's 1.414. In the case of Legacy ac voltage of 18 Vrms, that's 25.46 volts peak.

If you draw less current out of the capacitor than you are putting in as charge, then the capacitor will remain at near the peak value. You have to use a generally large capacitor to accomplish this.

The relationship for that is C = IT/V where C is in microfarads or uF, I is in milliamps (ma drawn), T is the time between charge pulses (8.33 milliseconds for full wave and 16.67 milliseconds for half wave for a 60 cycle per second ac voltage) and V is ac volts of allowable ripple.

So if the relay draws say 60 ma, then C = (60 X 16.67)1

Or C = 1000 uF for one volt of ripple voltage using half wave charging.

For full wave, C = just half of that value capacitor needed or 500 uF for one volt of ripple. Because the charging pulses occur in half the time.

Obviously if you can stand more ripple voltage without the relay chattering, then the value of the capacitor can be less accordingly. Two volts of ripple voltage allowed will allow half the capacitor value needed in both cases, half wave and full wave.

Note that the capacitor needs to be dc rated at a little more than the peak voltage.

For example, 151763749035.

If you search for "DC female power adapter" you'll get thousands of listings...including variants with wires already attached (instead of screw terminals).  Most of these sellers also sell the 12V DC-output wall-warts themselves so the coaxial/barrel plug will match the adapter.  If you already have the wall-wart by all means measure it to be sure, but for this low-power application the most common coaxial plug type is 5.5mm x 2.1mm (outside x inside diameter).

Stan, THANK YOU!!!

No matter what the data base, the quality of the search terms is crucial--I was way off.

Found what I needed.

The problem with walwarts are they are not necessarily a filtered supply. As mentioned DC relays do not like pulsed current. Second they have little amperage capacity. They are OK to run a few things but they are not needed. DC can easily be made from the track transformer with 2 components. A bridge rectifier, and a capacitor. You only need one for the entire layout. Simple to do for $5 or less.

On my layout I use a Lionel brick to power 200 + relays. The minus of the bridge rectifier shares a common with the track common,so I can activate track side accessories easily using the outside insulated rail method.

DC relays are much more flexible and useful. If large capacitors are used,a delay off effect can be made,even timing circuits.

Here is an example of blowing a whistle in a pattern,using relays and capacitors.

LINK

Dale H

We have a few wall house  light switches  rated for 120 volts and probably 25 amps that we use for sidings .  They re cheap and not too hard to mount, they're a little on the big size though.

I can't see using a DC  relay with a BR to do the same thing as a  good quality  on/off toggle.  

First off, many thanks to "CJACK" for his very informative answer, complete with formulas comprising 'electronic math'.  Brings back memories when I went through college.  Now then, speaking of relays, both DC and AC-types, here is a little TRIVIA regarding the AC types you might run across, if not careful.

Many, many years ago, I was able to get some railroad-style relays made by the General Railway Signal Company (GRS).  They were their plug-in types.  They were removed from the intricate signal and interlocking (NX) installation belonging to the Key System Transit Lines.

Anyway, the AC-types looked really neat and impressive but I noticed that the coil voltage was rated at around 110VAC-- 100 cycle ...   Not paying much attention to THAT, I attempted to operate the thing in my garage on standard 110-VAC-- 60 cycle.

Bad move, it was like plugging in a dead-short.  WHY ??

Because of the old Inductive Reactance formula :  XL = 2 pi f L   Where XL is the value of Reactance expressed in Ohms,  pi= 3.1416,  f= frequency, expressed in CPS,

L= Inductance of coil, expressed in Henries.

The basic formula shows that if you INCREASE the frequency, the resultant XL value in Ohms DECREASES.  So-- trying to operate a 100-cycle relay on 60-cycle current is like a "short-circuit" ...

Just couldn't resist telling this story regard

ing my early day experience with AC relays.

Ken Shattock  (KRK)

OGR

LCCA

BAERA

NRHS

I think what Ken wanted to say was that the impedance decreases as you decrease the frequency from 100 Hz to 60 Hz.

If you increase the frequency in an ideal inductor, the impedance increases.

A real inductor/choke will start looking like a capacitor at some sufficiently high frequency.

Nothing. As 41woodie has also concluded, the 364C is adequate for his application.

In some applications it is desirable to place the on/off control switch remotely (dozens of feet) from the block, section, or siding.  In such cases using a low-current relay means you don't have to run bulky cabling that carries the full load.  I guess copper has come down in price as of late but it can be an issue.

If using command control, some systems send a hi-frequency signal over the track voltage. Again, if the power switch is located remotely from the point of use, the geometry of the wiring can affect the hi-frequency signal path and potentially degrade communications. 

By using low-current control signaling to switch heavy-duty relays you access a larger selection of on/off switches.  If going for a modern look control-panel, there are some neat looking switches with built-in LEDs that don't handle 10 Amps or whatever.

For those contemplating computer or automatic control, you generally have only low-current control signals (fraction of an Amp) so you would need relays to switch power to sidings/blocks.

Just some ideas to boost the self-esteem of the relay method...

Thanks cjack. I am going to have to save this thread so I can go fiddle with all this stuff on the bench. Might stick in the old memory a little better if I experiment, then again it's also good to save the info just in case.

Dale H,

I said I was aware of the 20% tolerance factor above, but the more I thought about this I think we used to use 10% either way. All our stuff was using matched voltage (xfmr to device) so it didn't matter and we never paid much attention to it. Anyway, it's good to know there is more wiggle room with the relays than I was originally thinking.

Wow, there has been a lot of information posted on this subject, some of which I understood!!  On my control board I'm using all older Lionel switches and controls because I kind of like the dated look of them.  I'm using 364c to control Log Loaders, Bascule Bridge and Coal Loaders basically anything that is a simple on/off function.  For my turnouts I'm using Lionel controls again because I like that look.

Everyone seems to agree that the 364c's can handle the draw of a short siding so I'll purchase some more and continue my wiring. My electronic wizardly will probably end with using LED's rather than incandescent bulbs, at least I understand enough to screw them into the socket Lol.

Now if I can figure out how to make my 022 turnouts quit acting as uncouplers on my post-war rolling stock I'll be happy (at least until the next problem rears it's ugliness)

If those are the ones with the electrical contact shoe, a little black tape over them works��

I think stan2004 has also mentioned this method to me before, sharing the common between AC & DC. I presume this would be no different using an adjustable AC to DC converter instead of just a plain bridge rectifier? The commons (or common & minus) don't mind sharing a return path?

Different power signals "sharing" the common (outer-rail in this case) MUST be electrically isolated from each other.  In practice this essentially means coming from separate transformers.  Consider the following diagram from another thread.

As soon as you install the bridge-rectifier to make DC, I think it's obvious that the DC- is now no longer the same signal as the AC com going into the bridge.  Thus, the track voltage AC common (lower transformer in diagram) must be a separate/isolated AC common from that powering the bridge-rectifier.

This requirement applies even if your bridge-rectifier is followed by a DC-to-DC regulator like those 99 cent modules from eBay.

The isolated AC transformer requirement can be met by using a DC wall-wart supply since the wall-wart has its own transformer.  Or, as Dale mentions he uses an independent/separate brick/transformer to drive his bridge-rectifier.

One landmine is that some train transformers which provide separate outputs for track voltage and 14V-16V AC accessory voltage may tie the commons internally. 

Michael--  You're right..  I was daydreaming but knew what I wanted to say.

Put it this way:  The Inductive Reactance (in Ohms) is DIRECTLY PROPORTIONAL to Frequency..   Lower resistance when a 100-cycle relay is placed across a standard 60-cycle line.  Hence, a VERY BIG spark !!

Ken.

OGR

LCCA

Sharing a common is no problem as long as the hots are not connected somehow. The topic was discussed a while ago here

LINK

As Stan mentions only a lead of the bridge is common,one of the AC leads of the relay coil power transformer can not be connected. This means using a separate transformer,dedicated to relay power

Dale H

Thanks Stan. That explains everything well and a picture is worth a thousand words! Just what I needed. At the end, you even answered the question that first came to mind when looking at the drawing, about using the accessory outputs of some transformers. The muddy waters of mixing AC & DC are starting to clear.

Now, I just need to check a few minor wiring details that, when wired, I was absolutely, positively certain I would remember, but I don't... 

Thanks, Dale. I am certain the hots are not connected anywhere, it's the commons I didn't document very well that need to be checked... 

Here is a bridge and capacitor mounted on a barrier strip to provide DC to an individual relay. If a separate transformer is used,only one is needed for all the DC relays on the layout. This one is good for 4 amps,enough for 80 watts of 24 VDC relays. Relays pull from about a half watt to 2 watts,depending on design.

Shown is a DPDT relay,more useful than you may think. The second set of contacts can be used for example to switch a block signal on the siding,an indication if it is powered or not.

Dale H

This info is interesting to me because it gives me another way to use the many excess AC transformers that I have accumulated over the years.

For example I purchased several 20/30A Bosch Style 12V SPDT Relays to turn on/off power to my passenger tracks powered by two PH180s through TPC400s (fused to 15.4A with PSX-AC circuit protection).  I'm going to be using a BPC to operate the relays using Legacy.

I have a MTH Z-DC1 Hobby Transformer that I was going to use for DC power (set at 12V).  The specs say it can handle 19.2VA, but I'm not sure if the DC current from the Z-DC1 is filtered.  My question is is it worth the effort to set up one of my spare AC transformers as above to provide the 12V and sell the Z-DC1, or should I just use the Z-DC1? 

You can see if the DC transformer is filtered or not measuring it with a volt meter with a peak setting. If it is pulsed current,12 volts will peak around 17. If your meter does not have a peak setting,add a 1000uf capacitor across the leads in proper polarity. If it boosts voltage from 12 to 17,it is pulsed current. You could use the transformer this way ,using the capacitor and adjusting the voltage down so it is 12 volts DC out.

Really it is personal preference which transformer you keep. A bridge and capacitor would cost $5 or less.

The automotive relays usually have 88 ohm coils. Some newer ones have a built in diode across the coil. They are fine except they consume a bit of power, almost 2 watts each. Not a problem unless you use a lot of relays. I would not put more than 14 volts peak into these coils,they are less forgiving than other type relays because of their low coil resistance. The 12 VDC relays I use are DPDT with 15 amp contacts. They have 480 ohm coils and use maybe 1/3 watt in comparison. Since I use hundreds of relays,power used is a consideration. Much less capacitance is needed for delay off circuits.

Dale H

I have always use toggle switches to control power to sidings.  I agree with WftTrains and Gregg.  capable switches are not that expensive and a lot simpler to wire.

Jan

They are. The problem is,if the layout is large,you have to have long runs of heavy gauge wire in and out of a central control panel. Not only is this expensive and tedious, it adds to the problem of voltage drop.

Secondly since the toggle is manual operation,the siding or block can not be automated for collision avoidance for example.

Dale H