Very quick circuit question

Haggisman

Haggisman

Haggisman

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Birmingham
I'm trying to hook up a couple of LED strip lights to a door switch, and just wanted to check if my circuit design will work:

I have 2 sets of LEDs, a 12v set of single colour, and a 5v ARGB set, and ideally I'd like to switch them both with a single switch, so I was thinking of putting the switch on the 12v circuit and then adding a relay to switch the 5v circuit, something like below (please ignore me not using the correct symbols :p):

9ZhPAmV.png


If there's a better way to do this then I'm open to suggestions :)

Thanks!
 
Put a resistor in series with the 5 volt LED string of ample wattage. Google on line LED resistor calculator for sizing the resistor. Then both strings can run off 12 volts
 
If you can't get your head around calculating resistor value you could buy one of these and set it to max resistance then slowly reduce it until the 5V LED's start working. Either leave in circuit or measure resulting resistance with a multimeter and replace with a fairly similar fixed resistor of suitable wattage.

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No, and I'm on a phone so can't draw it. 12v supply. One side connected to both LED strings, other side of 12v supply to switch. Then other side of switch direct to 12v string. Also that side of switch, but via the resistor or variable resistor to 5v string, observing polarity of both strings of course.

How long the string is will determine resistor wattage to handle the current, PROBABLY that variable one is beefy enough, but should certainly do for testing to ascertain value of a fixed one.

Variable: one outer pin and centre pin only used. Which outer determines whether CW or CCW increases or decreases resistance. Measure it with meter first. Set to max resistance and decrease .
 
Thanks, so like this?:

dk6mzIi.png


I don't have a variable resistor, but I do have a box full of different resistors, so would be preferable to not need to order something just to find the required resistance - I'm guessing I can just measure the resistance of the 12v strip and then use the calculator I linked above to determine the resistor I need for the 5v section?
 
Yes, that's it. Start with say around 5k Ohm and come down in about 500 Ohm steps depending on the resistor values you have. Once they light you should increase a bit until they don't and aim kind of mid way. You can't go by the resistance of the string as diodes don't work like that, you are using resistance to limit the CURRENT the string can take. Calculating it for an RGB string, especially if they are multi colour individual LED's could be tricky. I'd go for the Heath Robinson method :) HTH.
 
Chris Wilson said:
Yes, that's it. Start with say around 5k Ohm and come down in about 500 Ohm steps depending on the resistor values you have. Once they light you should increase a bit until they don't and aim kind of mid way. You can't go by the resistance of the string as diodes don't work like that, you are using resistance to limit the CURRENT the string can take. Calculating it for an RGB string, especially if they are multi colour individual LED's could be tricky. I'd go for the Heath Robinson method :) HTH.
Click to expand...

perfect, thanks for the help, appreciate it :)
 
Your welcome, just make sure whatever resistor you select doesn't get more than lukewarm. If it does buy a bigger wattage one, err on the "too big" side :) Good luck!

All this assumes the 5v string doesn't have any other components in it other than the LED's.

If so you could use an eBay buck converter to drop the 12v to 5v, they're dirt cheap.

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LuckyBenski said:
Why can't you just use a double pole switch? :confused:
Click to expand...

I couldn't find any double pole door switches :(

Chris Wilson said:
Your welcome, just make sure whatever resistor you select doesn't get more than lukewarm. If it does buy a bigger wattage one, err on the "too big" side :) Good luck!

All this assumes the 5v string doesn't have any other components in it other than the LED's.

If so you could use an eBay buck converter to drop the 12v to 5v, they're dirt cheap.
Click to expand...

200 ohm seemed to work, but it got very hot after a minute or so, so I've ordered one of those buck converters instead, cheers for the suggestion
 
Haggisman said:
I couldn't find any double pole door switches :(



200 ohm seemed to work, but it got very hot after a minute or so, so I've ordered one of those buck converters instead, cheers for the suggestion
Click to expand...
In that case using your resistor in circuit check with your meter on the 10 amp or highest current measuring range how much current the 5V chain is drawing. Sounds like a fair bit and those small buck converters have pretty limited current producing capacity. The specs, like most Chinese stuff, will be, err, pretty optimistic.


Another simple alternative is one of these metal clad resistors which can also be screwed to a metal plate as a heatsink but almost certainly will be cool without that.

 
'ang on a mo.... I'm very rusty but

I think there is no such thing as a 12 or 5v LED (We ain't talking zener diodes here).

A standard Led has a volt drop across it circa 1.7v. The current flow determines the light emitted. The resistor you choose is determined by the supply voltage and the current you want the led to carry.

Just us a single 12v DC supply and drop the current to your desired (in this case the "5v" device you mention) LED with a chosen resistor.

I guess that you may have some pre-manufactured Led with an inline resistor so the manufacturer is suggesting 12v or 5v supply?

Badly worded but hope you get the gist.
 
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Chris Wilson said:
In that case using your resistor in circuit check with your meter on the 10 amp or highest current measuring range how much current the 5V chain is drawing. Sounds like a fair bit and those small buck converters have pretty limited current producing capacity. The specs, like most Chinese stuff, will be, err, pretty optimistic.


Another simple alternative is one of these metal clad resistors which can also be screwed to a metal plate as a heatsink but almost certainly will be cool without that.

Click to expand...

I looked at the spec of the resistors I have, and they are only 1/4w, so that would do it :cry:

There are 180 WS2812B LEDs in the strip, at apparently max 60mA each, so looking at 10.8a. If my basic maths is correct that's a little over ~50W?
 
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Haggisman said:
I looked at the spec of the resistors I have, and they are only 1/4w, so that would do it :cry:

There are 180 WS2812B LEDs in the strip, at apparently max 60mA each, so looking at 10.8a. If my basic maths is correct that's a little over ~50W?
Click to expand...

Ooh er . That's a whole bunch of lighting.:).

And there was me thinking you had a couple of leds to power. I don't half feel daft about my previous post here:)
 
Baldbloke said:
Ooh er . That's a whole bunch of lighting.:).

And there was me thinking you had a couple of leds to power. I don't half feel daft about my previous post here:)
Click to expand...

Hehe, np - it's just a standard 3m ARGB LED strip, going to be going under some shelves in a cupboard, but obviously I don't want them on all the time, hence the door switch
 
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