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Important to understand how to read an electrical label.

The AC voltage of the coil is both the recommended and ideal rated constant duty voltage for a relay coil. This matters because the coil generates heat in operation that must not overheat and melt the unit. So the longer the relay will be energized with the coil powered, the more important it is not to stray much from this rating. If only activated for short times and infrequently, then slightly higher voltage should not overheat (example 16V VS 12V rating). Also, DC would tend to saturate the coil thus more heating, VS AC. Again, this is for the coil.

The contacts have both a Current (Amperage) and Voltage rating they can safely and reliably switch on and off without damage. Another factor is contacts are designed differently for DC- especially higher voltage DC. This is because DC tends to create an arc as the contacts open and must be extinguished for the contact to actually be open. This is less important at lower voltages but again a design factor. AC is self extinguishing arc because the AC waveform turns off voltage potential each cycle at the zero crossing. Again, the key here is the label rating tells us about the contacts and what load they are designed and rated to switch.


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Put another way:

The rating of the coil is important in most circuits. Also AC VC DC rating of the coil is a factor to consider- it's all about overheating the coil with extended ON time. AC being alternating and thus having the zero crossing means off time the coil is not powered, where with DC current the power is always flowing thus more heat. Also, lower voltage may activate a relay coil but at some point there is slow activation or not enough power to activate reliably. So ideally there is a minimal range of voltage they should operate. The upper range of voltage is that again, more voltage is more current, and DC VS AC play into a factor at what point a given voltage along with ON time of the coil determines a practical limit.

The switch contact rating- the voltage indicates the safe maximum voltage the contacts are rated to turn on and off. Higher voltage could arc across the contacts. So effectively this is more of a maximum value than a recommended operating voltage. Again, you can run much lower voltage through a contact it's just 2 pieces of metal touching when we get down to it. Lower voltage at some point on a high voltage rated switch contact may not be self cleaning, but again, for model train discussion, nothing wrong with using 120-250V rated contacts for our typical low voltage (8-20VAC) AC functions on a layout. DC is slightly less desirable but again for the most part in the model train world, we are switching below 25VDC and the contacts are not likely to arc.

@Charly posted:

Thank you so much, great information.  I got this in box of stuff when the owner of a store I worked at passed away and I had no idea what he was planning on doing with it.

Potentially I could use this to operate a 153 block signal, yes?

Overkill, but sure, it would work. Given the size, this might have a loud click compared to a smaller relay.

Another factor to keep in mind in the block signal- if using insulated rail, then in theory, the relay coil is not activated on for long periods- think hours or days of constant ON state. If using conventional operation, track or accessory voltage you are again in that 8 to maybe 16V range of AC, so that's within reason in the 12V specification given the expected duty cycle.

The only scenario that might be a problem is a train parked constantly for a long time activating the coil, and the coil was seeing say a full 18V AC. Then you might get into that overheating- but using an accessory voltage or other lower voltage would allow near unlimited operation time without any worry.

That relay is not useful if you need a NC contact. It has two coil terminals, and two form A terminals. Whether it's AC or DC coil is not indicated. If there's a label stating the coil voltage, we can't see it.

(Form A = mAke, or NO.  Form B = Break, or NC.  Form C = one A and one B with a common, or "single pole, double throw.")

If you search on this forum for the term "relays" you will find dozens of messages recommending the exact relays to use in electric train applications.

Many of us here are very familiar with the various types and applications.

My personal recommendation is to use the 6/12/24 volt DPDT relays sold by the Altronix company.  They are mounted on printed circuit boards and use screw terminals for their connections. Google "Altronix relay RB5" for instance.

Creating a DC bus on your layout and using these relays provides a very efficient system.  There are members of this forum who create and sell relays already ready to accept either AC or DC and which will pretty be applicable to anything that you would need to do on a layout.

Last edited by Arthur P. Bloom
@Brad J posted:

“more voltage is more current,” Wrong. Voltage is inversely proportional to current. Ie: voltage goes up current goes down, voltage goes down current goes up.

Note that Vernon was discussing the relay coil.  Relay coils are NOT constant power loads...they are simply just that...a coil of wire.  So if you put in more voltage you indeed draw more current.  And in the context of O-gauge, you do need to mind the coil voltage specification if you are using a 12V AC coil relay and driving it with, say, Accessory 14V or 16V AC, or 18V AC Command AC.  You may overheat and destroy the relay at higher than nominal voltage.

OTOH you may be referring to a "family" of relays of the same part number (but different voltage suffix) where you choose the coil voltage from popular voltages such as 6V, 12V, or 24V.  In which case, yes, the coil power is constant.  For example, a family of relays may have a constant 1/2 Watt power rating to operate the relay.  So the 12V coil version draws less current than the 6V version.  And the 24V coil version draws less current than the 12V version.

JA relay

Referring to the earlier datasheet link, your last photo has a "-P" suffix in the part number which says it has upgraded contacts which means it can handle even more power.  But for an O-gauge application, even the standard part (without the -P suffix) is way more than enough power handling capability...hundreds of times more capability than what a 153 block signal would require.

But for block signal applications controlling a red light and a green light, to the earlier discussion, I still think you want an SPDT or Form 1C relay (part number starting JA1c) which will have the "extra" contact that went missing in the eBay part you mentioned.  Note that your latest pic shows a part number starting  JA1a... which means it too will be "missing" a contact.


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  • JA relay
Last edited by stan2004

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