WHAT??? What is this "nfet" thing of which you speak? Is this some new type of semiconductor device I've never heard of or encountered?

You're right. I meant to say j-fet ... would that be the correct term for, say, a 2N2222 transistor?

 

As for the 1M resistor, you're right again, of course. I had completely forgotten about startup conditions.
But, it would also depend on how the fet's gate is being driven, which is not mentioned in the diagram.

 

You're right. I meant to say j-fet ... would that be the correct term for, say, a 2N2222 transistor?

No, a 2N2222 is a bipolar junction transistor (specifically, an NPN), or BJT, with collector, base and emitter. A JFET is a junction field-effect transistor which, like a MOSFET, is a voltage-controlled device with a drain, gate and source.

 

As for the 1M resistor, you're right again, of course. I had completely forgotten about startup conditions.
But, it would also depend on how the fet's gate is being driven, which is not mentioned in the diagram.

Ummm... no. I cannot imagine any gate drive method or set of conditions which would require such a resistor in order to function. Keep in mind, a resistor that large cannot possibly play any part in obtaining fast turn-off of the MOSFET; to do that, it would have to be, at most, a few tens of ohms for it to quickly bleed off the stored gate charge.

The 1 MΩ resistor is there only to keep the MOSFET's gate from ever floating.

 

I do, sometimes... and it gives me a headache when that happens... ugghhh

Just be happy that you don't live in a 50Hz line frequency area. When I lived in Europe, the fluorescent light flicker drove me crazy. It was directly responsible for three additional voices in my head!

 

Most people don't see the flicker of 60 Hz fluorescence bulbs.
John

I do, sometimes... and it gives me a headache when that happens... ugghhh

Neither here nor there, but, don't fluorescent lights flicker at 120 (or 100, depending upon where you are) Hz?

 

Neither here nor there, but, don't fluorescent lights flicker at 120 (or 100, depending upon where you are) Hz?

Yup.

 

Neither here nor there, but, don't fluorescent lights flicker at 120 (or 100, depending upon where you are) Hz?

Good point

 

No, a 2N2222 is a bipolar junction transistor (specifically, an NPN), or BJT, with collector, base and emitter. A JFET is a junction field-effect transistor which, like a MOSFET, is a voltage-controlled device with a drain, gate and source.

arrrrgggghhh... I hate it when that happens ... just shoot me, would you ... I need to pay more attention to detail ... thanks for correcting me. I just hope the TS reads through all these posts so he can have the correct picture of how things are. Thanks again!

 

So, regarding transistors, there also are BJTs (in both NPN and PNP presentations), JFETs, Mosfets, nFets, pFets, Triacs, Diacs, SCRs, UJTs, photoFETs ... did I miss anything? ... oh yes! there are also BLTs, FGS and Jucy Lucy's ... right?

Wait... those are sandwiches, not transistors ... I think we're done here

 

So, regarding transistors, there also are BJTs (in both NPN and PNP presentations), JFETs, Mosfets, nFets, pFets, Triacs, Diacs, SCRs, UJTs, photoFETs ... did I miss anything? ... oh yes! there are also BLTs, FGS and Jucy Lucy's ... right?

Wait... those are sandwiches, not transistors ... I think we're done here

And don't forget the three fundamental capacitor types: TLCs, MSCs, and BFCs.

 

BTW, be aware that directly paralleling LEDs, with out some means of ensuring current sharing (e.g., ballast resistors), can lead to uneven brightness among the LEDs unless they are pretty closely matched for forward voltage at the intended ON current.

Btw, the LEDs should have current-limiting series resistors. It is inadvisable to connect the LEDs directly in parallel unless you can be sure they have evenly matched forward voltage values.

There have been a few different mentions of not putting the LEDs in directly in parallel, so I'm thinking about changing to an integrated LED driver with a boost converter such as this one (http://www.ti.com/lit/ds/symlink/tps61169.pdf). The only potential problem is I won't be able to put all of the LEDs in one series string. Can I use this with multiple parallel strings of LEDs?

 

You can put them in parallel strings but they may not all be at the same brightness level.
If so, then you could add a small resistor in series with the higher brightness string(s) to help balance that but the resistor value(s) would need to be determined on an experimental level.

 

There have been a few different mentions of not putting the LEDs in directly in parallel, so I'm thinking about changing to an integrated LED driver with a boost converter such as this one (http://www.ti.com/lit/ds/symlink/tps61169.pdf). The only potential problem is I won't be able to put all of the LEDs in one series string. Can I use this with multiple parallel strings of LEDs?

Probably, provided you've made sure that all of the LED strings have the same forward voltage drop (or nearly the same forward drop) at whatever current level you intend to drive them with. But if the LEDs are poorly matched, such as from being from different manufacturers or even from different production runs with the same manufacturer, you could end up with very different brightness levels in the different LED strings.

One workaround to this brightness difference problem is to put "ballast" resistors in series with the strings, sufficient to drop a volt or two; this will tend to equalize the currents running through the strings.