Re: Relay circuit
Trevor Rymell said:
This is very kind of you. The coil resistance is 171.1 ohms. Do the other resistor values need changing for a 12v supply also?
The calculation as follow: the relay is 5V and has a resitance of 171 ohms. Therefore the coil current is I=V/R = 5/171 = 29.2mA. When you connect the coil with a series resistor across +12V, the same current should flow. There is 5V across the relay means the rest is across the resistor, which is 7V. Thus the resistance is R=V/I = 7/29.2mA = 240 ohms.
Trevor Rymell said:
Is it theoretically correct that any voltage could be used to say, drive a relay or LED directly as long as there's adequate current limiting?
Yes, as long as the voltage is higher than the LED minimum voltage(2V for red, 3~4V for white) or the relay operating voltage.
Trevor Rymell said:
The circuit I'm using is the one right at the end "Gas Gauge - Final" which was drafted for me by Audioguru. The system is pretty much complete but now I just want to add an additional "low fuel" alarm hence the transistor switch and relay.
This is the bit that is worrying. You said "I'm now running it from 12V..." but the circuit design by audioguru is for working at +5V. There is a voltage level shifting required in this case and the previous relay circuit I posted
will not work.
I still need you to describe how your circuit is powered, especially what is the voltage of the LED common rail(red arrow) as shown in the image. Is it +5V or +12V?
Trevor Rymell said:
I should say I've had an enormous amount of help from members on this board as you will see. As I didn't want to keep pestering the same people, I posted a new query about the LED voltage.
Think of it this way. If people don't want to answer your question, they just do nothing. If some forum users is clocking up thousands of postings, they are prepared to "see you through" with your problem, so don't worry about it.
Trevor Rymell said:
Your help is very much appreciated. This is a huge learning experience for me. I'm too old to think of sitting in a classroom to learn electronics. I'd much rather be learning while trying to make something I actually need (I have much more expertise with metal machining). Of course, there's enormous gaps in my knowledge and I have to ask for help.
Nobody is too old to learn new things. I always learn something myself by answering posts. Sometimes I have to go back to check with a textbook to avoid giving wrong advice so it is also good for me too.