Electric Help

hlumbard · 04-29-2009, 11:33 AM

I'm planning on building a son of fermentation chiller soon for my next brew and I'm wondering if there's some good resources for wiring help? I know the instructions say to just use a 1K ohm resistor and assume 10 mA for an LED but is all that correct? Doesn't the resistance of the fan come into play too? (For that matter the thermostat as well?) I thought the whole circuit had to be looked at.

I got a couple 80mm fans coming from newegg already and I'm getting my insulation this weekend.

**bull8042** · 04-29-2009, 12:04 PM

If you are using a 12V supply and the LED and resistor are in series, you will drop .7V across the LED since it is basically a diode. That leaves 11.3V across the resistor. 11.3V / 1000 (1k) = 0.0113A, or 11.3mA. Usually, most LEDs will handle up to 30mA, but that depends on the LED specs.
Not having the schematic in front of me, I can't tell you much more than that.

**jpc** · 04-29-2009, 07:55 PM

Quote:

Originally Posted by bull8042

If you are using a 12V supply and the LED and resistor are in series, you will drop .7V across the LED since it is basically a diode. That leaves 11.3V across the resistor. 11.3V / 1000 (1k) = 0.0113A, or 11.3mA. Usually, most LEDs will handle up to 30mA, but that depends on the LED specs.
Not having the schematic in front of me, I can't tell you much more than that.

It's been far too many years, but I recall that an LED drops about 1.2 V, not the "typical" 0.7 V of a standard diode. Of course, please don't trust my rusty memory...

Edit: ...or is it 2.2V? Either way, I'm pretty sure an LED drops quite a bit more than the typical diode. Has something to do with bandgap, I think...

hlumbard · 04-29-2009, 11:00 PM

So basically if I use a 1K ohm resistor I'm safe and don't question it. After I get all my equipment in order I'll double check everything here before I fry it. I plan on building the box this weekend and doing electric work through the next week. Thanks for the help!

**BrewBeemer** · 04-30-2009, 12:03 AM

Quote:

Originally Posted by bull8042

If you are using a 12V supply and the LED and resistor are in series, you will drop .7V across the LED since it is basically a diode. That leaves 11.3V across the resistor. 11.3V / 1000 (1k) = 0.0113A, or 11.3mA. Usually, most LEDs will handle up to 30mA, but that depends on the LED specs.
Not having the schematic in front of me, I can't tell you much more than that.

bull; on most general led's they operated at 20ma needing a 470 ohm resistor I recall with a 12 volt power supply. One must add that the power supply or cube will read higher than the rated output hence pushing the led harder making for a brighter but shorter led life span. Unless you have a selection of resistors to add in line with a digital meter your only be at a close guessing game on the led's ma draw for the given led that is to be used. Those rotary muti-resistance units from Radio Shack sure come in handy when establishing the correctly needed resistance for those scratch built projects. They are cheap and handy as hell plus come with leads and alligator clips. Man can never have too many alligator clip jumpers, mine walk away offten. I would start high on the resistance scale while reading your meters ma scale, keep reducing the resistance until you establish the maximum continuous ma rating of the led your using, do not use the maximum surge ma numbers which are a lot higher plus a shorter life span.

Off topic but I must add;
I just got home from having two discs plasma cut away from pressing on the spinal column nervers. Let the games begin as the hospital meds are starting to wear off with me finding out what post surgery pain is while it's getting worse. I've the next 3 to 10 days to look forward to with this pain plus making meds adjustments as needed.
I'm a stubborn Germam I will make it. Cheers. Carl on the mend seems sleep is my best friend while I recover and heal. Dreaming of brewing ideas helps before I zonk out.

ClaudiusB · 04-30-2009, 12:47 AM

1K resistor is a good value for a 12 V supply and 10 mA LED current.
470 Ohm is good for 23 mA LED current assuming 1.2 V LED drop.

Cheers,
ClaudiusB

**bull8042** · 04-30-2009, 12:46 PM

Quote:

Originally Posted by jpc

It's been far too many years, but I recall that an LED drops about 1.2 V, not the "typical" 0.7 V of a standard diode. Of course, please don't trust my rusty memory...

Edit: ...or is it 2.2V? Either way, I'm pretty sure an LED drops quite a bit more than the typical diode. Has something to do with bandgap, I think...

Sorry, I need to improve my proof-reading skills a little.... I left off the "1". Here is some more info:

It is safe enough to assume 1.7 volts for non-high-brightness red, 1.9 volts for high-brightness, high-efficiency and low-current red, and 2 volts for orange and yellow, and 2.1 volts for green. Assume 3.4 volts for bright white, bright non-yellowish green, and most blue types. Assume 4.6 volts for 430 nM bright blue types such as Everbright and Radio Shack. Design for 12 milliamps for the 3.4 volt types and 10 milliamps for the 430 NM blue.

....Compliments of the interwebz....

hlumbard · 05-02-2009, 12:43 PM

I have a power adapter that says 12V DC 1A. My question is after I cut the plug off do I how do I tell which is positive and negative? Does it matter? I'm thinking yes. I don't have a multimeter but I'll soon have a spare computer fan. I bought two in case I broke something.

One wire does have white dashes running along it. That's probably a clue huh?

HomebrewJeff · 05-02-2009, 01:15 PM

Quote:

Originally Posted by hlumbard

I have a power adapter that says 12V DC 1A. My question is after I cut the plug off do I how do I tell which is positive and negative? Does it matter? I'm thinking yes. I don't have a multimeter but I'll soon have a spare computer fan. I bought two in case I broke something.

One wire does have white dashes running along it. That's probably a clue huh?

Yea, as a kind of general rule of thumb, the dashed or striped wire goes to the outside. If you look at the adapter, you'll need to determine if it's center positive or center negative. The center positive markings look something like

(-)----(----(+)

..so in this case the dashed wire is generally the negative side. Again, this may not be the case with your power supply. For any type of electrical work, I'd pick up a multimeter. They are cheap, and will help to make sure you are wiring things correctly.

hlumbard · 05-02-2009, 01:22 PM

Having the right tool for the job isn't really my thing. Ha! Every time I do a project I'm always working WAY harder than I need to because of lack of proper equipment. My converter is like you said, negative on the outside so I'm pretty sure the dashed wire is negative. Thanks! I'll be sure to post pictures when it's all done.