Digitrax PM42 & MRC 9500 Power Supply Compatibility

[donwa2]

Greetings everyone,

 

This is my first posting, and I am curious if anyone might be able to offer any insight regarding my purchasing decisions. I have been shadowing this site for a while and learned a lot browsing through messages.

 

I own an MRC 9500 power supply (single page manual attached to provide electrical specs.) from the DC era of my layout construction. Over the last five years, I have made significant effort to bring the N-scale layout and motors up to a functional DCC system by means of a Zephyr DC50.

 

I have three reverse loops that I would like to integrate into the layout, which basically calls for a PM42. My question is; can I use the MRC9500 power supply with the PM42? If not, it appears that I need to purchase the Digitrax P-14 power supply. If I can use what I already have, that would be great.

 

Thanks for your consideration,

Don

MRC9500_Manual.pdf

[KenS]

It's hard to be sure if the 9500's AC or DC accessory outputs would be safe.  Per the manual you supplied, it appears these are rated to produce 19 VAC or 20 VDC.  The PM42, per its manual, can use a supply of 12-18 VAC or VDC.  So, on paper at least, your voltage is too high, but only just.  Those may be overestimates; accessory supplies are usually in the 16-18V range, which would be fine.  And amperage-wise, I'm sure the 9500 can easily output the needed 125 mA.

 

But since it could be producing more than the rated voltage (these things are always approximate and power packs are notoriously so) it's hard to be sure.  You could write to Digitrax and ask, but they can't tell you for certain, they can only tell you if 20V is okay or not, and if you don't know how accurate the MRC manual is, that could still be a bit risky.

 

If it were me, I'd probably not take the risk, no matter what they said. Although as I said, it probably would be fine.

 

You can use the PS14.  But since you have to cut the connector off and solder to the edge connector, you could also use this 12V 600mA power supply from Sparkfun which costs about half as much (shipping could hurt you though, that's worth checking). Note that if you end up with more than one PM42, they can share a power supply.  The PS14 can power two of them, the Sparkfun supply can power four. The 9500 could probably power more than you'd ever have (although perhaps the accessory outputs are limited to less than it's full 1.5Amps).

 

And welcome to the board, by the way.

[donwa2]

Thanks very much for the insights, Ken. I didn't know about the variations regarding power supplies, but based on the kind of electrical knowledge displayed on your website, that is rather reassuring to hear from an expert.

 

Among the other discussions I've had with folks locally about this, no one yet has flat out said, "don't do that!" Contacting Digitrax is a good idea. My take on contacting manufacturers about things like this is that they always want to talk up their own product, which makes it difficult to get a straight answer. If something should go badly connecting the MRC9500 to the PM42, then Digitrax would most likely find that reason to deny any sort of warranty return/replacement, since it's not factory. I guess I'll ask about that when I write.

 

The Sparkfun unit is incredibly affordable; and versatile. I do foresee myself needing more than one PM42, so this is really good to know. It's nice to have options.

 

Thanks again,

Don

[KenS]

I'm not sure I'm an expert, just an opinionated amateur. 

 

I haven't found Digitrax to be all that responsive when I've asked questions, but others have.  It may depend on the questions or who gets them.  I've never had to use their warranty, and I don't know if they're as good with the other things as they are with decoders, but they have a very strong reputation for standing behind their decoders, so they might replace a damaged PM42 if it looked like an honest mistake, I don't know.

 

Power packs for model trains have always been somewhat loosely specified in terms of output voltage for accessories (and for trains too, really).  In part that's because there's no standard for what accessory voltage they should output, just a convention that it's around 16 or 17 volts (and some do AC, others do DC and a few like your MRC do both).  It's also cheaper when you buy components if you don't try to select them or adjust them for a specific voltage; there's a range of normal variation, and if you just grab things out of a box it's faster, cheaper and results in each power pack being a little bit different.  Those "20V" output specs are almost certainly a worst case that comes into play only if your house current is higher than usual (it can vary from day to day or even hour to hour by nearly 10%) and the individual power pack was higher than usual. And Digitrax almost certainly has a safety margin above their rated "18V" limit. And that's why I said there was a good chance it would be fine.  One thing I like about the Sparkfun unit is that it's regulated, which means it's going to put out very close to exactly 12V no matter what.

[donwa2]

Thanks for the insights...

 

Well, I tell you what...

I went out to Radio Shack to purchase a voltmeter yesterday. Just a $20 unit, but just the same, when I tested the MRC Power Supply's Fixed DC terminals, I got a reading of 21.8v (!); when I assumed that it was going to be closer to 20v, since that's what it is rated at. This is really interesting for me to see.

 

Since the variable DC terminals were easy to get the voltmeter to hit whatever I wanted, up to 20v, it occurred to me that I could hook up the PM42 to those terminals with the throttle locked down to any voltage I wanted. But that just sounds like a silly solution.

 

If the Sparkfun unit will just do the best (and affordable) job, then I think I'm just gonna go with that... and in the meantime, do some deeper study of electricity, which is a topic that has always eluded me... let alone the depth of DCC.

[KenS]

One thing to be aware of: a power supply with no load will always be a few volts higher than one with a load (regulated supplies might not, but no-load is still an excptional case).  It's probably going to be 20v, or even a bit lower, when the supply is doing something.

 

Still, I'd assumed those ratings would be on the high side.  Twenty volts is pretty high for model train accessories.