Ahhh, wait a sec - those are all LM3914-driven instruments, not just the dash lighting, correct? If so, they're going to have to be driven in parallel.Tacho = 50 LED's
Speedo = 50 LED's
Fuel = 30 LED's
Temp = 30 LED's
Oil = 30 LED's
Battery = 20 LED's.
MOSFET Transistor Drain = Collector Gate = Base Source = Emitter
Oops! Well, that happenssorry, it's a bit late here too, somehow ended up reading about transistors thinking I was reading about MOSFETS.
Well, you have that a bit confused.ok, so i've done a bit more reading, getting my head around MOSFETs. correct me if I'm wrong, I would use a enhancement mode P-Channel MOSFET, when there is 0v on the gate, there is no conductivity between source and drain, as the gate voltage increases so does the channel conductivity until Gate = Source then there is no conductivity?
That's the rub; if Vgs was halfway between, the MOSFET would be in "linear mode", which means that it has resistance, and will dissipate power. This means heat, which you want to avoid.so if the gate acts like a cap then if the frequency of the 555 is high enough, with a 50% duty cycle, it will be as though there is 50% VCC on the gate?
Let's stick with Vgs measurements so it doesn't get overly confusing.makes perfect sense, I think...
so with the -10v is that when it passes -10v or when it is at -10v. I understood the -10v from the reference from the word go, what I am wondering is
on a 14v+ supply
when gate = 14v then it is off
when gate = 4v then it is on
when gate = 0-4v it is??? eg 1v, 2v etc or does it need to be 4v.
The problem is that a standard bjt 555's output doesn't go all the way to Vcc; it's about Vcc-1.7v. That might cause a P-ch MOSFET to be conducting in linear mode (ie: high resistance) which you don't want. That's why a driver is necessary.so pretty much what I need to do is have the output from the 555 at 4v, 100-200hz, and play with the duty cycle to set the brightness...
That's the basic idea. It doesn't have to be terribly precise. Targeting 200Hz would give you a pretty large degree of leeway for faster or slower.so, I would try to keep the frequency as close to 100hz as posible to prevent the MOSFET from heating up too much due to charge/discarge of the gate while keeping the "flicker" of the led's at a rate that is not visible.
Hardly any at all, if a decently sized MOSFET (or pair of them) is selected.how much heat would the MOSFET need to handle? I'm sure a heatsink would solve any probs with heat?
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by Luke James