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12v Relay help please

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An LED is a diode that has a small fixed voltage drop. If you connect it directly to a power supply that has a voltage higher than the voltage rating of the LED then the LED will draw an extremely high current and will burn out. Most ordinary LEDs have a max continuous current rating of 30mA. They are recommended to operate at 20mA.

A red LED is about 2.0V so with a 12V supply, 10V will be across the 1.5k current-limiting resistor and Ohm's Law calculates the current to be 10V/1.5k= 6.7mA which is not very bright. For 20mA then use R= 10V/20mA= 500 ohms which isn't a standard value so use 510 ohms.

A blue or white LED is about 3.5V so the current-limiting resistor will have 12V-3.5V= 8.5V across it. The value of a current-limiting resistor for 20mA is R= 8.5V/20mA= 425 ohms which isn't a standard value so use 430 ohms.
 
Uncle $crooge: Oh yes. Totally. However, I have never gotten to grips with all the maths stuff ( as I posted earlier ) though through the years, I have learned what works and what dies a horrible, yet short life.
Where leds are concerned, I without doubt start with a 1K resistor and work from there... Though I never make anything that gets further than a prototype board, ( well,,, very rarely ), I just play with things, adapt stuff, think 'see what I did there ?' Then move onto something else.
 
Hi guys, I've built the circuit...

Is this how it should be connected?


**broken link removed**

Thanks in advance
 
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The relay must have a 5V coil to work with the 5V supply.

You are using a darlington transistor instead of a normal transistor. It has two transistors connected to make its input very sensitive.

The darlington might turn on by itself if the computer is turned off, so add another 10k resistor from its base to 0V to turn it off when it should be off.
 
From what I have learned of the ULN2803 (never heard of it before today), which is the chip on the Velleman board. It can only 'sink' current ( ie. ground it ), throughout this thread, I have assumed that the board could 'source' ( ie. supply it. ) My fault entirely. Mind you, not to worry. From what I can gather;
Whatever is present on one of the 8 screw terminals on the velleman board will be connected to the 'clamp' terminal when you click the mouse on the appropriate bit of software.
As I see it, you have the two red horizontal ( nearly ) lines the wrong way around.

Believe me, If my local Maplin branch had one of these things in stock, I'd have bought it about 3 days ago and made it, tested it and sent you exact details, diagrams, pictures, video, video in 3-D...etc

Don't worry, you wont melt your PC.

Though, thoroughly check that your relay board works first.
 
Uncle $crooge you are absolutely right. Though Nemo couldn't a BCX38B. So he's using a generic medium power standard issue NPN, basically whatever Maplin had in stock. He's just using an old diagram. :)
 
@ Houseofwax

Like this?

**broken link removed**

Quick question:

I feel I shouldn't need the ground connection where the "???" are on the picture.... is that right?

thanks (really) for all the help, just starting to understand whats happening (I think) :eek:
 
You don't need a medium power transistor to drive an ordinary relay. It won't have enough current gain like a little transistor has, so the 10k base resistor doesn't provide enough base current for it to activate the relay.

Things should be calculated to work together, not just picking an old diagram and whatever is in stock.
 
Where did that Velleman thingy come from? This thread started with a +5V signal from USB, not a relay driver driving another relay driver.

Please post a schematic of the Velleman thing.
 
As I see it yes, that is how is should work.
Yes indeed you do need the 0v connection. It all has to have the same ground reference point. Remember that you are dealing with signals at this end, two wites touching with a 'click' happens at the other.

Uncle $crooge: Maybe I simplified what I was trying to say. What I meant was that the replacement used for the one in the diagram was a suitable one for the application. The bit about 'whatever Maplin had in stock' was just me being a bit annoyed that they don't seem to ever have what I want. Ie. The Velleman board that Nemo is playing with.
 
audioguru said:
Where did that Velleman thingy come from? This thread started with a +5V signal from USB, not a relay driver driving another relay driver.

Please post a schematic of the Velleman thing.

Here you go all you wanted to know about the Velleman USB Interface (thanks to house of wax for posting it earlier in the thread)

Velleman USB Interface PDF
 
Nope. The Velleman thingy is made to drive relays. It won't drive the base of an external NPN transistor relay driver without having a pullup resistor.
 
audioguru said:
Nope. The Velleman thingy is made to drive relays. It won't drive the base of an external NPN transistor relay driver without having a pullup resistor.

Oh Poo ...... back to square one again :(

So I guess I should be making something like the diagram on page 19 of the velleman manual.... mmmmm struggling with that one a bit though, e'g the external power supply.

Thanks for the reply
 
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