| Electronics > Beginners |
| How to wire this setup? |
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| james_s:
The vast majority of power supplies are isolated so that shouldn't be an issue. If you connect the ground from it to the ground on your Arduino and nothing blows up then it's isolated. |
| KL27x:
--- Quote ---If you connect the ground from it to the ground on your Arduino and nothing blows up then it's isolated. --- End quote --- Not exactly logical. Connecting the grounds together on this circuit should never cause anything to blow up whether or not either of the PSU's are earth referenced or isolated.* Because his circuit is working after getting the C and E's the right way around (and still devoid of any ground jumper), it is pretty obvious that both his PSU's are earth referenced. It's still a good idea to add a direct connection between both grounds to reduce any deviation from ground loops/leakage or whatnot. And OP, it's completely improper to draw up the schematic the way you did with a separate "USB ground" and "wall wart ground." If the two power supplies were actually isolated from each other, the circuit could not work. The transistor has two currents going on, one at 12V and one at 5V. But if you look at it, there are only 3 pins. Both currents, the Ice and the Ibe must flow through the emittor. Quid pro quo, the emitter must be connected to the negative rail of BOTH power supplies. Quid pro quo, both ground rails must be connected. Without the jumper wire, your psu's grounds ARE connected, but depending on what receptacle you plug your wall wart into, this connection could mean 10 or 20 feet or ground wire. There is going to be some voltage drop across this ground wire in your house's walls, depending on what's going on at either end of this wire. This little differential, if any, can be eliminated by adding the jumper wire locally, at your circuit. If you wanted to keep both power supplies isolated from each other, you must switch the current with some other device than a transistor. Such as an optocoupler or a relay of some sort. BTW, I went over all your problems in my post from 2 days ago. *OTOH, if you connected the positive rail of one PSU to the ground rail of the other, and "nothing blows up," then you can be sure that at least one of those power supplies is isolated. |
| Youkai:
--- Quote from: KL27x on May 15, 2018, 10:09:59 am ---You also don't have a ground connection between the arduino and the breadboard/PSU. Even if they are both earth referenced, it's probably a good idea to add a direct connection so's your voltages are more stable. --- End quote --- AH I see KL27x. I must have just completely missed this as it went over my head. I understand it better now that I have a little more context. I'll run the jumper between the arduino GND pin and the breadboard ground rail. Thanks for your assisstance everyone. I'll update again later when I have some more progress. Anybody know where/what I would search for to find a wall wart that would be more appropriate for my use? (i.e. more amps) Is there some industry term for those things that I can google? |
| KL27x:
I am partial to MPJA.com out of Florida. Overstocked and obsolete PSU's are their bread and butter. You can find what you need for very cheap, but there's a minimum shipping charge of something like 10.00 which might kill the deal, unless you need higher quantities and/or some of the other junk they sell. You pretty much need to know the voltage and the amperage you want, and you want to look at the PSU's that are cased/enclosed. Years ago I purchased a handy dual 12 and 5V brick for like 8.00 that would have met your needs, but this was probably a discontinued product. What they carry at any given moment will never be the same. Not good fora long-term volume project, but great for tinkering and one-offs. |
| KL27x:
--- Quote --- --- Quote ---Your 12V supply IS isolated? Correct? --- End quote --- What does this mean? I don't know how to answer. --- End quote --- The first important bit of almost every power supply is a transformer. The current that is induced on the other side of this transformer is completely isolated from everything else. Any potential created across the two leads of the secondary only exists between those two wires and nowhere else. In a simple DC PSU, there is a next a bridge rectifier and some filter caps. The bridge rectifier turns the AC into DC, and the filter caps smooth it out a bit. So now you have two wires with a DC potential between them, but they are still isolated from anything else. So either wire could be likened to the terminals on a battery. If you taped up one of one of the output (the positive or the negative), you could stick the other one anywhere you want, and nothing would happen. You could stick it in a toaster, you could stick it in a electrical socket, you could put it on your tongue (while standing barefoot on wet concrete), and nothing would happen. But don't do any of that. The PSU output wire won't affect the mains, but if you were not properly insulated from earth when you did it, you could be electrocuted. In most switching PSU, the negative wire is attached to the earth wire for safety reasons in case of a fault. This PSU is now earth-referenced. It produces X volts relative to earth grounding (and both rails have reference to mains live/neutral as well!). In many non-switching PSU, there is no connection. The most obvious way to tell is to count how many prongs are on the plug. 2 means it is definitely isolated. 3 means it is most likely earth-referenced, but to be sure you can just stick one probe on the 3rd ground prong and one on the negative output rail and do a continuity check. (While the psu is unplugged, of course). But your Arduino and transistor switched LED circuit, while a bit more complex in parts, is another way to confirm. :) |
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