Can I use a mosfet to switch roughly 30A @ 12-15V

I can't figure out why you would need that honking 3W pull up resistor that's going to get so hot. If the FET is being used like an ON/OFF switch, the turn on time is not critical at all. If it takes ten milliseconds to turn on or off, who cares?

Click to expand...

Do you know how an HID ballast behaves if it's current limited on startup? Honest question.

With 1k and a gate capacitance of 9.3nF (worst case) the time constant will be 9.3µs.

If the MOSFET will pass the full current at a gate voltage of 4.7V then the turn-on time will by only 4.65µs; a mechanical switch will take much longer to turn on.

speakerguy79 said:

Do you know how an HID ballast behaves if it's current limited on startup? Honest question.

Click to expand...

Not exactly, because I haven't read the profile spec for their power converter. But, if it truly needs that 100A initial surge previously mentioned, it's not going to be able to start anyway. If it has some kind of soft-start or inrush limiting, then the turn on time of the FET won't matter as long as it is not ridiculously long. If the ballast power supply is normally powered from a relay, it would have to have some kind of built-in immunity in the turn on interval to accommodate relay contact bounce.

Hero999 said:

With 1k and a gate capacitance of 9.3nF (worst case) the time constant will be 9.3µs.

If the MOSFET will pass the full current at a gate voltage of 4.7V then the turn-on time will by only 4.65µs; a mechanical switch will take much longer to turn on.

Click to expand...

And the contacts will bounce for a time interval as it closes.

OK, I am gonna get the parts for both and try it out.

Would this be an acceptable BJT here

If you think so I am gonna place the order tonight and then build it once I have the parts.

Yes, that's one of the possible transistors I suggested a few posts ago.

Almost any device will work for the N type device (whether it be a BJT or a MOSFET). The P type is the one you have to be picky about.

Mkay, so you guys are right. I actually went and calculated the turn-on time and it's negligible even with the 1k resistor. I've just done a few power electronics projects where FET turn on time has been an issue, and was thinking in terms of turn-on current (12/100 = 120ma = about what you get out of a cheap FET driver).

Pedro, go ahead and just use the 1k 1/4 watt resistor. No need for the 100ohm 3W.

speakerguy79 said:

Mkay, so you guys are right. I actually went and calculated the turn-on time and it's negligible even with the 1k resistor. I've just done a few power electronics projects where FET turn on time has been an issue, and was thinking in terms of turn-on current (12/100 = 120ma = about what you get out of a cheap FET driver).

Pedro, go ahead and just use the 1k 1/4 watt resistor. No need for the 100ohm 3W.

Click to expand...

SWEET!!!! THanks so much both of you. I just sent in the order. Time to build and test. I'm gonna give all of you positive rep (as soon as I figure it out). I'll let you know how this project turns out.

bountyhunter said:

Not exactly, because I haven't read the profile spec for their power converter. But, if it truly needs that 100A initial surge previously mentioned, it's not going to be able to start anyway. If it has some kind of soft-start or inrush limiting, then the turn on time of the FET won't matter as long as it is not ridiculously long. If the ballast power supply is normally powered from a relay, it would have to have some kind of built-in immunity in the turn on interval to accommodate relay contact bounce.

Click to expand...

Honestly I am not sure, I know that some ballasts can work down to 10V or so, but no clue on initial current requirements. I am going to do some "beta testing" if you will, after I get it working on my own stuff. I am going to ask on the HIDPlanet forum if anyone with a hella gen 1 or similar REALLY old school ballast and people with old or cheap kit ballasts, would like to test it out. These should be more inefficient and require much higher starting currents. I plan on also maybe getting a couple of different lower power P-Mosfet as well to see if they can handle it.

By the way, my apologies to Hero and speaker, I hope I didn't give BOTH of you negative feedback. IF I did, I sincerely apologize.

You guys are freaking awesome. I don't know how else to say thanks to you guys. I just hope that in 20 years I could know HALF as much as you guys do.

Haha, yeah you gave me neg, but that's ok. I think both Hero and I have enough pos rep to be OK

He gave me positive but I've positived you to make up for it.

The number of reputation points doesn't matter to me, it's the comments that count.