Basic mosfet switch question.

[Greg G]

In this schematic, provided by Jimmy M, would this circuit drain the batteries when the light is not in use?

I'm considering building a mosfet circuit like this to use in a compact M@g where I discard the original bulky switch and use a microswitch to operate the mosfet.

Thanks.

 

[JWP_EE]

The type of FET used in that circuit has an off leakage of about 1uA (.000001A). If they were 1Ah batteries it would take over 100 years to drain them.

 

[JimmyM]

What battery pack and bulb are you using? What are your size constraints?

 

[Greg G]

The light will consist of 4 Cree Q5's in series driven at 1 amp, a Shark driver, and a Modamag 8aa battery pack. The tube is actually a NASCAR Fade "drop" that came from Mac, so the switch portion of the body is gone. I'm making a custom head for it.

Which mosfet would work for this application? Are the IRF510's from Radio Shack up to this?

Thanks,

Greg

 

[JohnMuchow]

The IRF510 will work in that setup but will get pretty hot. The on-resistance is about 0.54 ohms when cool and about 0.75 ohms as it gets hot. This means there will be about 0.75W of heat (at 1A) for that little MOSFET to dissipate. You might need a small heat sink.

Does Radio Shack carry the IRF540N? It's a very common MOSFET and will only need to dissipate about 40mW when passing 1 amp. No heat sink needed.

John

 

[Greg G]

I was planning to use Arctic Alumina epoxy to glue the mosfet to a Led Zep magsink.

I am very open to using a different mosfet than an IRF510. I've read in other threads where Jimmy was looking at all the various ones to find one that required very low current on the gate.

Thanks.

 

[JohnMuchow]

One great feature of MOSFETs is that they need only the tiniest amount of current through the gate to turn on....they're voltage controlled.

International Rectifier divides their MOSFETs (broadly, very broadly) into two groups, IRF and IRL. The IRF MOSFETs like to see about 10V at the gate to turn on hard. The IRL MOSFETs like to see about 5V at the gate to turn on hard. You can often use either one for higher/lower voltage applications, but those are rough guidelines. Dozens of other things to deal with for higher current, higher speed application though.

With 9.6V (nominal) available and only 1A of current flowing, you can use just about any MOSFET in a TO-220 case (like the IRF510) for your application.

I checked the RS web site and it looks like they only carry the IRF510. That will work, but if you want, PM me your address and I'll send you an IRF540N. It will run at room temp and will work great. I use different MOSFETs now for various stuff and don't need it. :)

John

 

[Greg G]

John, what do you think about his mosfet? It has a 1.7 mOhm Rds.

https://ec.irf.com/v6/en/US/adirect/ir?cmd=catProductDetailFrame&productID=IRFP4004PBF

 

[JohnMuchow]

It will do everything you need but it's in a big TO-247 case and costs a lot more than the IRF540. It's overkill IMHO, but if it's easily available and can fit, it's a fantastic choice. The Rds-on and voltage ratings are no problem.

John

 

[Greg G]

I was just curious about it. It's a big old honker. 1/2" wide and 3/4" long just on the body part.

I plan to use the one you're sending.

Many thanks!:D

 

[JimmyM]

One great feature of MOSFETs is that they need only the tiniest amount of current through the gate to turn on....they're voltage controlled.

John

They require almost no current, Nano Amps, to stay on or off. But require AMPS of current in a short spike to turn on or off quickly. In high current applications, a fast turn on or off is critical. In lower current situations, it's not as critical depending on the FET. A tiny SOT-223 FET would need fast turn on and off to suvive a couple of amps, while a big one (TO-247, etc) can do 2 amps all day in is linear range.

 

[JohnMuchow]

They require almost no current, Nano Amps, to stay on or off. But require AMPS of current in a short spike to turn on or off quickly. In high current applications, a fast turn on or off is critical. In lower current situations, it's not as critical depending on the FET. A tiny SOT-223 FET would need fast turn on and off to suvive a couple of amps, while a big one (TO-247, etc) can do 2 amps all day in is linear range.

I agree except that an application involving amps, or even hundreds of amps, can actually benefit from a slow turn on or off. It all depends on the application.

For high-frequency work, speed is everything and the less time you spend in the MOSFET's linear region the better. But, I build electronic loads from 100W to the multi-kilowatt level with MOSFETs that operate in their linear region and up to hard-on. When switching a 1,000A load off, it's much better to do so slowly to reduce the turn-off spike induced by the load or the inductance of the wiring and to minimize EMI. Each MOSFET is driven by an op-amp that only has a 30mA short-circuit current rating with an R-C filter on the gate to slow down the current flow, and thus the switching time, even further.

John

 

[JimmyM]

I agree except that an application involving amps, or even hundreds of amps, can actually benefit from a slow turn on or off. It all depends on the application.

For high-frequency work, speed is everything and the less time you spend in the MOSFET's linear region the better. But, I build electronic loads from 100W to the multi-kilowatt level with MOSFETs that operate in their linear region and up to hard-on. When switching a 1,000A load off, it's much better to do so slowly to reduce the turn-off spike induced by the load or the inductance of the wiring and to minimize EMI. Each MOSFET is driven by an op-amp that only has a 30mA short-circuit current rating with an R-C filter on the gate to slow down the current flow, and thus the switching time, even further.

John

Don't they heat up an aweful lot? Sure, with proper heat sinking, provided you stay under the curve of safe operation, "slow" is a matter of degree.
How big are these loads? I would imagine they are quite heavily heat-sunk. (is that even a word?). I understand exactly what you're saying, and with respect to inductance it makes perfect sense. Those hard turn-on turn-off current transitions are hell on inductance. But would the short wiring in a mag-mod and the filament of a bulb be a significant inductive load? I ask because I'm curious, not because I'm trying to poke holes in your reply. Which I think is quite sound.

 

[JohnMuchow]

Don't they heat up an aweful lot? Sure, with proper heat sinking, provided you stay under the curve of safe operation, "slow" is a matter of degree.
How big are these loads? I would imagine they are quite heavily heat-sunk. (is that even a word?). I understand exactly what you're saying, and with respect to inductance it makes perfect sense. Those hard turn-on turn-off current transitions are hell on inductance. But would the short wiring in a mag-mod and the filament of a bulb be a significant inductive load? I ask because I'm curious, not because I'm trying to poke holes in your reply. Which I think is quite sound.

Hi Jimmy,
No worries, I actually appreciate it when someone finds a problem with something I've brought up. It means I'll learn something new and that's always a good thing. And you brought up some good points earlier. :)

Regarding those loads...yup, they can get REAL hot. :huh:
Getting rid of the heat is the hardest part of the design and I have to pay very close attention to the safe-operating area specs and temperature ratings. The loads I've designed span a wide range, from 20W to 20,000W. A new product I've just recently released for limited distribution is rated for 500W (400W continuous), scalable to any power level, and is used to extend the discharge capabilities of a CBA or other discharger/analyzer. Finding the proper component/fan/heatsink combination let me take it from a shoebox sized monster down to something only a bit bigger than a CBA. But, that took almost two years of testing to do. :rolleyes:

IMHO, the inductance of the wiring in a mag-mod isn't that large but, IIRC, the faster the turn-on/turn-off is the more of an effect any inductance has. So I guess it all depends? I'm just glad that all my circuitry runs really, really slow and that I don't have to deal switcher power supplies and high-frequency stuff. Saves a lot of my neurons from being smoked. :p

John

 

[JimmyM]

IMHO, the inductance of the wiring in a mag-mod isn't that large but, IIRC, the faster the turn-on/turn-off is the more of an effect any inductance has. So I guess it all depends? I'm just glad that all my circuitry runs really, really slow and that I don't have to deal switcher power supplies and high-frequency stuff. Saves a lot of my neurons from being smoked. :p

John

You and me both, John. My first attemp at building a PWM regulator for Mag lights was based on a 40kHz regulator (TI 5001). My GOD the ringing was awful. Most of the stuff I do now is low frequency. 175Hz for the JM-SST and ~250Hz for the HRDC (or PhD of you choose)

 

[Daravon]

Question:

I understand that biasing the gate + turns the mosfet on. the 4.7 resistor to knock the voltage down makes sense because the gate can only take so much voltage, but what's the 10k for?

 

[Mr Happy]

I understand that biasing the gate + turns the mosfet on. the 4.7 resistor to knock the voltage down makes sense because the gate can only take so much voltage, but what's the 10k for?

...to reduce the voltage. The two resistors act as a potential divider that reduces the voltage by 10/(4.7+10) or approximately 2/3.

A single resistor will not reduce voltage unless a current flows through it. The gate of the MOSFET draws no current, so unless the 10 k resistor is included no current will flow through the 4.7 k resistor, and therefore the full battery voltage will appear on the end of the 4.7 k resistor at the gate.

 

[Daravon]

Of course. I always forget about that with high-impedence circuits. I forgot to ground the pot on an audio amp once for the same reason, of course it didn't work because the input stage of the amp is like infinite impedence.

 

[Greg G]

So...riddle me this Oh Wise Ones.......

If let's say an IRF mosfet likes 10 volts at the gate to turn it on "hard", should we be checking the voltage at the gate and adjusting the resistor feeding the gate accordingly depending on our battery pack?

 

[Mr Happy]

If let's say an IRF mosfet likes 10 volts at the gate to turn it on "hard", should we be checking the voltage at the gate and adjusting the resistor feeding the gate accordingly depending on our battery pack?

Not necessarily. The short answer is that there is no real point reducing the voltage applied to the gate unless it is too close to the maximum permitted. So for instance, if we look at the data sheet for the IRF540N we see under "absolute maximum ratings" that the maximum gate-to-source voltage is +/-20. So unless there is a danger of approaching or exceeding 20 V (for this specific MOSFET) there is no need to reduce the gate voltage to a lower value than the battery voltage.