I'm working on a project to use an Arduino to control an automatic watering system via solenoid valves. Since I have three voltages / ranges I need to deliver to various parts (24VDC to the main valve, 12V to sub-valves, 5 to 20 V to the 'duino), I was planning to use PCB-mount transformers to step down from 120V/60Hz mains. Are there any recommendations for sourcing these transformers at reasonable cost for delivery to the US (specifically central NC)? I can find wall-warts and plug adapters all over the place, even chassis-mount transformers, but not PCBs so much...
Depending on how many ma you need? It might make more sense to power the whole lot with a single 24v dc supply and use on of the many methods of dc to dc voltage regulation from there. Bringing mains power onto a custom design carries a lot of risks and design challenges to the table.
How much current is needed? I read your desription as DC voltage is needed. Transformers will step down the 120V AV, but then you need to convert it to DC.
Something like this could take care of all those for you. http://www.mpja.com/12V4A-and-24V6A-and-5V1A-1ASB-Supply-Refurbished/productinfo/19149+PS/
Power the Arduino directly via the +5/Gnd connections on the arduino power header.
The solenoid valves are the big users, each one draws between 1.2 and 1.5 A of current, and I need to keep them energized for extended periods of time (I'm using NC valves so that I don't run up a huge water bill if there's a short). I know I'll need additional bits to convert AC to DC, and I know I'll have to be careful about bringing in mains power.
All the more reason to use a pre-built switching power supply.
I'm going to assume the solenoids aren't particularly bothered by unregulated DC. (Correct me if I'm wrong.) That means you can settle for rectified and filtered DC. Regulation is harder to do for high-current applications. Save that for the parts that care -- i.e., the micro.
For several amps, forget PCB-mounted transformers. Look for a chassis-mount E-I or toroidal (if you have the cash). If you get a center-tapped transformer, you can get -12, 0, +12 and end up with 2x 12v supplies, and by reaching across both, a 1x 24v supply as well. You'll have to double up on the filter caps (one bank for the + rail, and one bank for the - rail), but caps are cheap. Hopefully it's clear, but just in case it's not, the "negative rail" is just another 12v rail with inverse polarity. You will need to watch what you use for ground / 0v.
I would use a switching DC-to-DC regulator (I'm working on plans for the MC33063, which seems easy to use) to bring one of the 12s or the 24 rail down to 5v for your logic circuits. With some agile MOSFET goodness, you can do all your high-voltage switching and whatnot.
Not that it needs to be said, but as always, be very careful with mains stuff. Measure twice, cut once. Fuse the input. I highly recommend a Variac to bring your mains AC up volt-by-volt, watching for smoke. Test just the transformer/rectifier, then add your caps and test again. When you're confident that's all safe, add your DC switching and finally your loads.
I sort of had the idea that PCB transformers weren't going to cut it, but I was hoping... oh well. I'm going to be using relays to switch the 24/12V for the solenoids, controlled by the 'duino. The solenoids are just fine with unregulated power - they're designed for use in underground sprinkler systems.
IMHO, relays are the foulest of electronic devices. A solid-state transistor will be much more reliable, switch faster (not that it's likely to be of concern), won't fail mechanically, and won't tarnish and cause intermittent or unreliable connections.
You can get high-current MOSFETs for less than an equivalent-rating relay as well.
Do what you'd like, but just food for thought. XD
Are solid-state relays more or less evil than solid-state transistors? As you might be able to tell, I'm something of a newbie here, but I'm having fun with it. Just trying to not get in too deep too fast.
Most irrigation valves of the "Home" variety and all the (commercial ones I worked with were either latching bi-polar or two winding DC operated (9 to 18Vdc)) or were of the common 24V AC variety I don't think I ever saw a 24V dc solenoid as all the irrigation clocks (timers) used Triac's for switching AC power. From 1000 valve golf courses to front lawn irrigation. This goes for 3/4" valves to 24 inch valves...