Hi, I have an old MRC Model 3 power pack lying around that has 2 terminals for 12VDC (besides the variable 12 VDC) along with the AC accessory terminals. I’ve heard that lighting accessories (street lights, signals, etc.) will last longer if run on DC rather than hooking up to the AC accessory terminals. Is this true? If so, do I just hook up my LEDs directly to the power pack or do I need resistors added to keep the LEDs from burning out immediately?
The MRC Tech 3 specs:
Unless you have 15/18 volt LEDs you will need resistors, red, green & yellow LEDs are typically 1.7 volts, white and blue are 3.3 volts.
Mel
Modeling the early to mid 1950s SP in HO scale since 1951
My Model Railroad
Unloaded power packs are usually sightly higher in voltage without a load.
Measure the voltage first. No guessing.
Below is the setup I have used for finding the proper resistor for a certain brightness and voltage. Very easy to do. I use the same meters. Been working with LED’s since 1972.
Sometimes I use a 2.5 k pot instaed of a single resistor. Measure the resistance of the pot and use the closest standard resistor. I have a large supply of resistors. Probably easier for me.
Since most LED’s are 20 ma max, I try to stick with 9 ma.
http://www.trainelectronics.com/Meter_HF/index.htm
A 1k, 1/4 watt resistor is the usual resistor to use. No calculations needed.
Might be slightly different brightness between red and green but not a problem for me.
Rich
LED says 3.3 Volts. That is the MINIMUM forward voltage needed to light the LED.
12 volts, 24 volts it does not care. It is more like a rectifier than a light bulb. As long as the CURRENT is limited (done with the correct resistor) it cares not at all about the voltage. Why you think LED christmas sets work witout transformer. Yess in series you say. but is minimal, and typical we wnat not to run LEDs in series on layout.
LION runs SIGNALS of him at 16v DC (needed extra voltage to operate relays, since rectifier in series with relay not work at 12 volts—neded for relay logic).
New LEDs of LION are still too bright, him uses lens hoods to correct this. Not all corrections need to be electrical.
ROAR
Yes, it will work, and yes, you do need resistors to keep your LEDs from instantly turning to DEDs (Darkness Emitting Diodes) but I’d like to suggest something else.
That supply will only put out 1 amp at 12 volts. That will light a lot of LEDs, but if you’re going to add incandescents like streetlamps or building lights, you will quickly run out of power.
You can order a nice hefty 5-amp, 12VDC supply from China (eBay or Amazon) for under $10, shipping included. You should go to Radio Shack (yes, there are still some of them) and get a fuse holder and a 4-amp fuse (below the supply rating) and you will have lots more power for your lighting. Save the old MRC for your workbench.
I have never liked manufacturers specs since they switched to VA, especially when they say “Total Output” when the power supply has multiple outputs. The user has absolutely no knowledge how much current is available at each output.
On this power supply calculating the “Total Output” of 125 VA using the max voltage 125 VA converts to 18 volts @ 6.9444 amps but one doesn’t know how much of the 6.9444 amps is available at the individual outputs, variable or fixed.
Assuming the 125 VA rating is the “Total Output” rating of the transformer in the Tech 3 any single output could be 6.9 amps if the other outputs are not used.
Another thing to confuse the end user and not publish the true current capability of the power supply.
They say total output because that’s exactly what it is - the limit is caused by the transformer, which is shared by the outputs. Each output does not have it’s own limit. If you are only using one output, then the total volt-amps is available on that one output
Or, worse - FEDs (Flame Emitting Diodes). [(-D]
Which Tech 3 has 125VA? According to the instructions, the Tech 3 9000 is 19VA and the Tech 3 9500 is 30VA.
Since there are both AC and DC outputs, specifying the total capacity in VA makes sense. For DC and pure resistive AC loads (like accessory light bulbs) with a power factor of 1, it’s the same as watts, but when you srat adding inductive loads like accessory motors or solenoid switch machines, it’s not - you have to consider the power factor as the peak voltage and peak power lag one another.
But that is of no real concern, the main thing is that the combined load of whatever trains you are controlling byt the throttle plus whatever accessories you hook up to the accessory outputs has to total less than 19 or 30VA, otherwise the overload light will trip.
FOr comparion, the old Throttlepack 501 specified a DC max of 13VA and an overall max of 23VA, so if you were using the full 13VA on the DC output you had 10VA left over for accessories. Or various other combinations so long as the DC side didn;t exceed 13VA (likely the selenium rectifier limit) and the total did not exceed 23VA (the transformer limit).
That 9500 at 30VA for a unit designed for Large Scale seems pretty weak. Most of the better LGB locos don’t draw a whole lot, but for 2 rail O scale - some of those dual motor Atlas diesels draw more than 4 amps, this thing couldn’t run them.
–Randy
http://www.modelrectifier.com/search/product-view.asp?ID=13208
That’s where I found the spec
Mel
Modeling the early to mid 1950s SP in HO scale since 1951
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
[quote user=“RR_Mel”]
rrinker
Which Tech 3 has 125VA? According to the instructions, the Tech 3 9000 is 19VA and the Tech 3 9500 is 30VA.
–Randy
http://www.modelrectifier.com/search/product-view.asp?ID=13208
That’s where I found the spec
Mel
Modeling the early to mid 1950s SP in HO scale since 1951
My Model Railroad
http://melvineperry.blogspot.com/
I agree…that is not even a Tech…that is a Throttlepack 9950…a very big differance to a Tech III.
Take Care! [:D]
Frank
Sorry, I plugged MRC Tech 3 into Google and it sent me to that page.
Mel
Modeling the early to mid 1950s SP in HO scale since 1951
My Model Railroad
http://melvineperry.blogspot.com/
Bakersfield, California
I’m beginning to realize that aging is not for wimps.
I use a MRC Railpower 1400 to power my signals. I don’t need a lot of sophisticated gear, as I only have 6 signals, and they are all right in front, so I chose to use a rotary switch to control each one, wired as per instructions from Tomar signals, using resistors supplied by them, and hooked up to the DC side of the power pack. This way, I can use the throttle to control brightness.
Mike.
The OP stated he had an MRC Model 3, which when googled it was a Vintage Power Pack made in Brooklyn, NY. I saw photos of them on EBay. Those were good workhorse power packs back in the day…
Neal
I believe from Your description You Have a Throttle pack MRC Model 3, which is a power pack from the fifties, and way underpowered for running very many lights and signals. It was mainly a Trainset power pack, with the older wire wound ceramic throttle which will not work very well with todays newer 12v motors. I forget the specific term used.
Also Lighting accessories, street lights, etc will last longer on DC refers to the accessories that have LED’s, rather than incandesant bulb’s running at 12v to 18v, AC or DC. And You do need resistors when using LED’s.
Take Care! [:D]
Frank
I believe this is what You have:
The MRC Tech power packs, were not even out until the late 60’-70’s,80’s.
