Sorry to drag this through once more. You gotta understand I have YEARS of custom buildings and signals installed, so I hope you can appreciate my hesitation !!!
There’s something in the math that just ain’t adding up in my head …
As it stands, all my buildings and signals work perfect with the appropriate resistors for each of their applications all currently running on a 12 volt / 3 amp supply.
If I upgrade to the 12 volt / 10 amp supply, aren’t all those resistors I’m currently using going to be severely under-rated for the higher amperage source ??? The resistors limit the current to the LED right ? - so far, they’ve been limiting the 3 amps … won’t they be insufficient with the 10 amp source ???
Man, I just can’t bring my self to hook up those two wires !!! I’m so afraid I’m going to be stressing the LEDs that are already installed ( 22 multi-head scratch-built signals, plus a myriad of custom structure installations ). Literally years of work could be wiped out in a flash !!!
Amperage rating is the highest amperage that the supply might deliver. Circuit amperage is always determined by the voltage and resistance. At 12 volts, the power supply will deliver up to ten amps.
If your that worried, why don’t you keep the old supply running what it’s running and just add a second supply and new wiring to run anything your going to add.
I jusr reread his details. My only concern is that you don’t do what the electrical engineering professor at *&^% University did with the pencil sharpener batteries(dang funny true story and it happened TWICE)and connect the DC reversed. LEDs are not operable in reverse and you might panic when nothing lights up.
Oh, yeah, in summary of my previous posts, NO, you won’t blow anything up so YES, you may switch out your power supply. Just don’t come running into my store complaining that your pencil sharpener is running backwards, oops, wrong story, don’t panic if nothing happens when you mistakenly wire it backwards because your using LEDs and not bulbs.
No. This is a constant voltage source NOT a constant current source. It will supply UP TO 10 amps only if it is needed. It will not force 10 amps through the loads. It’s safe. Just don’t hook it up backwards or using the wrong leads.
Mark, maybe this will help you understand what is happening from another point of view…think of your circuit as a plumbing circuit. If you increase the size of your pipe diameter, and keep the water pressure the same, you can flow more water through the pipes. The pressure in this case is the Voltage in your Power Supply (12V), it has not changed. The water flowing through the pipes is the electrical current. By installing a larger Power Supply that can deliver more current, you have effectively done the same as increasing the size of the pipes, more water ( more current ) can now flow with the same Water Pressure ( Voltage supply )
If you are concerned about reaching the limit of your present PS, either place a multimeter set to AMPS scale (DC) in series with the hot lead to your accessories, or, buy an ammeter and permanently install it in your layout to monitor your use. That way, you will be able to check on just how close you are to reaching safe limit. A good rule of thumb used by the electrical code is 80% of max load current. So if your PS can supply a max of 3 amps, do not exceed 2.4 amps.
[quote user=“Mark R.”]
Sorry to drag this through once more. You gotta understand I have YEARS of custom buildings and signals installed, so I hope you can appreciate my hesitation !!!
There’s something in the math that just ain’t adding up in my head …
As it stands, all my buildings and signals work perfect with the appropriate resistors for each of their applications all currently running on a 12 volt / 3 amp supply.
If I upgrade to the 12 volt / 10 amp supply, aren’t all those resistors I’m currently using going to be severely under-rated for the higher amperage source ??? The resistors limit the current to the LED right ? - so far, they’ve been limiting the 3
I’ve read all the answers and still felt something was missing from them.
The math doesn’t change. Each resistor is limiting the current needed for its specific load. The current available from the supply is irrelevant. I mean, I hope you weren’t using 3 amps in your math for calculating the resistors. You were using 0.015 amps or something similar for the LEDs right? That LED and resistor combination will draw the same 0.015 amps tomorrow with new power supply just like it did yesterday with the old one.
Thanks for bearing with me guys - the old gray matter just ain’t what it used to be I guess. [D)]
So if I have this correct now, a 1000 ohm resistor will limit the current to the proper level for the LED regardless if the source has a 1 amp or 20 amp rating if it’s the same 12 volts.
Resistor values would only need to be changed if the voltage supply was to be altered - again, regardless the the amperage rating.
i’d go -
negative - LED - LED -LED - 600ohm resistor - Posiive 12v
/\ - LED - LED -LED - 600ohm resistor - Posiive 12v
/\ - LED - LED -LED - 600ohm resistor - Posiive 12v
Do a few rows of LEDs and tie them to the same ground llok at radio shack for pcb bradboards they’ll really clean it up. hell if you wanted to get really into it instead of using the resistors get a potentiometer it varies the resistance so you could make them brighter or dimmer
According to that calculator (im not saying ohms law is wrong) I would need a 425 ohm resistor. I used that calculator before for other led lights and it worked fine. When I put in a 12V supply voltage, with the LEDs themselves drawing 3.5V/20mA, it came up with a 425 Ohm resistor. However the next standard 5% resistor is 470 ohm (yellow violet brown). It looks like I need to go over to the electronics store to pickup a (yellow violet brown) resistor
Ok you need 20mA running through the LED.
12V - 3.5V = 8.5V drop across resistor. V=I*R. V=8.5V I=.02A that makes R = 425 Ohms.
The above is for a 12V → Resistor → LED → GND setup.
If you want three LED’s on the rail you need a Resistor equal to 75 Ohms and you set it up like:
12V → R → LED → LED → LED → GND
However, this setup may cause some of the LEDs to go bad after a bit. Safe bet is:
R = 25 OHM
12V → R → LED → R → LED ->R → LED → GND
EDIT:
Or put them in Parallel and go
12 → R=425 ->LED → GND.
R=425 ->LED → GND
R=425 ->LED → GND
In this configuration you can put more than 3 LEDS on the 12V rail.
12V → R → LED → LED → LED → GND
will NOT make one of
Mark, in a parallel circuit, the total current is the sum of the currents in each parallel branch. In a series circuit, the current will be the same value in the total series circuit.
It is the same if you switch your auto battery with a higher amp-hr type or your alternator with high output model. You are still using the same voltage lid with a higher amp potential.
In other words you can add more signals than you have now.
So just to help out the non-electrically inclined.
It was explained to me using very small words that you are to think of volts as the size of the pipe, and amperage how fast the water flows through the pipe. The flow can change, but the size of the pipe won’t.
Everything you hook up has an internal resistance (you for example). Metal, very little. Think of Amps as power ‘Capacity’ (supplied & consumed), and Voltage as the ‘Intensity’.
Your 100 watt lightbulbs consume 1A each on 100 volts (.91a @ 110v) so theoretically you can plug 15 of them into your household 15 Amp circuit before blowing a fuse - or if you didn’t have a fuse the lights would dim.
When you increase the Voltage (Intensity) your lights get brighter and motors turn faster. When you add Amperage (Capacity), you can run a lot more things.LEDs have little resistance when being lit so we add resitors. They limit current at a specified voltage, so as long as the Voltage stays the same level you’re safe.
A 10 amp supply will just power 3.33X more things than a 3 Amp. NOTE: ANY POWER SUPPLY should be fused.