I’m building a modular layout with initial plans for three and likely expansion to six modules. I plan on operating turnouts with servos using Berrett Hill (BH) products. Each 8-foot module will have up to 8 turnouts and a separate BH control base. BH recommends a separate 5VDC, 2A power supply for each control base. I could use properly sized, inexpensive wall warts but who wants all the extra cords?! What I would prefer to do is purchase a large 5V supply and have fused drops for each module. Plus, a future goal is to add building lighting effects and I would tap off the 5v supply for this purpose & use a buck converter to lower the voltage to 3.3VDC.
Looking for advice on how to do this?
One large 5VDC PS for all six modules or two PS’s, each sized for 3 modules?
Select PS at what percent of rated capacity? For three modules, I would need 6 A for the three base stations. There is 2 A peak draw at startup with 8 servos on each base station and then a much lower current draw during operation. Using 80% of the rated capacity, I should look for at least a 7.5 amp PS (6 A/0.8 = 7.5). Agree?
Recommended PS source?
Any recommendation on 2A in-line fuses (protect control base) for this application?
I have no clue about the current draw of LED lighting for a model railroad. Does anyone have a guesstimate on how much to add to the PS capacity for “future-proofing” with added lighting?
If it were me I would go with a 12 volt 15 amp or so main power source then break it down using buck converters. I started out with 12 volt 15 amp and out grew it and ended up with a 30 amp 12 volt main supply but then I use lots of incandescent GOW bulbs on my layout. (about 500 GOW bulbs)
Yikes, I had not previously checked the specs on my servos… Stall current is 1.3 Amps, too late to return…
I purchased two buck converters for my initial foray with servos. The model I purchased does a great job of down-converting voltage but has a current output range of 0 to 5A. I was connected to a 2A PS so there was no concern about providing excessive current to the sensor shield. After Mel’s comment, I looked and could find buck converters that provided a set current level but none that could set an upper limit on current and allow lower current levels.
OOOPPPPSSSS, misread the spec. For use with berrett Hill, I should purchase a buck converter that sets the upper limit at 2A.
The buck converters I use are variable current from 0 to 12 amps with fan cooling and 8 amps with out the fan. I settled on this converter for my 1.35 volts, 4.5 volts, 5 volts and 8.5 volts.
The current limiters are adjustable from 0 to max output. I run my max current at 8 amps.
The load on the 1.35 volt converter is just under 3.6 amps with all vehicle lights on.
I have very little current on the 4.5 volt, Miller Engineering signs.
The 5 volt supplies my 11 Arduinos (about 2 amps) and two converters on 8.5 volts with everything on draw a total of about 10 amps, 200+ 12 volt Grain of Wheat bulb running on 8.5 volts at about 60ma each.
I use a 12 volt 30 amp switching power supply for my main source with everything on about 18 amps.
I’m using 8 Arduino Random Lighting Controllers to randomly turn on and off 20 bulbs each in structures, an average of just under one amp each.
Over time the current consumption on a layout can really get up there. LEDs would be probably be 90% less but I like the looks of incandescent bulbs.
Mel, thanks for the info and link! You’re the best!
I admit that I find the specs confusing:
Module Properties: non-isolated step-down module, constant current and constant voltage module (CC CV), charging module Constant current range :0.2-12A ( adjustable )
I read that as saying I could adjust to a constant current of 2A to the BH control base… Provides sufficient current at startup but do I really want to send max current to the servo control base all the time? I’d say no…
ADD IN the operating specs for my servo are 6mA idle, 400 mA running, and 1300 mA stall (at locked)
Berrett Hill specifies a max of 2A per control base. I can buy 2A wall warts but prefer to purchase one or two larger supplies instead of six 2A supplies. With a larger supply, my options to control the max current are to use a buck converter that sets an upper limit (plus full range below) or add a 2A fuse.
Those are pretty hefty servos, I seriously doubt you will need 100ma per servo to operate a turnout, my servos only draw 25-30ma to move the points and 13ma is the idle current.
EDIT:
The SG90 servos start to heat up at continious 40ma. I try to keep the idle current under 20ma.
I need to do some testing. It would be easy enough to tap into the positive wire to the sensor shield. Take a reading with no servos attached. Then readings with servos idling and operating. Not sure how to do the stall current test…
constant current regulators vary the output voltage to maintain a constant current. ~10ma constant current regulators would be used with LEDs on a DC layout
a current limiter, would cause the output voltage to drop to limit the current to what it is set to, preventing damage but not necessarily providing sufficient voltage for the circuit to work
if BH requires 2A, it’s not likely to draw more than that if you use a 4A supply.
the supply does not force current through a circuit. the supply provides a voltage and the circuit draws the amount of current it needs based on the resistance of the circuit (see Ohms Law)
the servos will draw the current they require to maintain the position they are diven to. limiting the allowed current may result in the servo not holding position
while a larger servo can draw more current, that current depends on the load. if a smaller servo can handle the load, a larger servo may draw less than max current for the same load
Thank you for your discussion. I had read the spec as being an adjustable constant current (ie set at 2A). I do like the idea of using a fuse to protect my investment in the base station. I will visit with Kevin at BH to get a better idea of their specifications. The webpage says “each control base should have it’s own power supply”. Wall warts are fine for situations with 1 or 2 control bases but not realistic for larger layouts. Thanks again, have a good day!
