Switching transistor question

FET. An IRF7331 specs at 0.045 Ohms @ Vgs= 2.5V. Good for 5.6A

There is a Zetek part which is even more appropriate, but I don't have the part number handy. We have a similar circuit that uses the above with no problem.

Take a look at this high current, high gain, low saturation voltage transistor:

It will drop about .05 volts with 500 mA collector current and 2 mA base current.

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I'd go for a FET.

You may want to drive the gate with a slowly rising signal rather than just whacking it. There is always a possibility that the device being fed by the fet will latch up IF it is normally turned on with a switch on the device itself. In a sense, you are turning on the device in a manner the designer didn't expect.

Typically a chip road test will catch such problems, but not every manufacturer tests their chips in such a manner.

Snip

Yes, a mosfet seems tailor made. Most of the ones I have handy require about 4.5 volts to turn on - which was what was stopping me from using them. Way lower drop than any bipolar so it is the obvious winner. But the ones I found were $2.50 each / $9 shipping. I paid $9 for the cameras I'm using. One of my criteria is to keep the cost low because there's a good chance I won't recover all the cameras I deploy.

The Subconscious Engineering Department must have been working on this problem overnight, because this morning I have a slick elegant solution that doesn't require buying any parts. The picaxe 08M has a PWM output function programmed into it, actually has two. I can use that to toggle a voltage doubler with a few small caps and diodes and switch the FET on. Eureka!

I'm using only two, out of five, I/O pins. So there's still hope for using a servo to pan the camera and passive IR to trigger action. (something I may want to add once I get one or two doing what I want).

I am taking advantage of the fact that the internal processor for the camera is turning the camera on when batteries are inserted - That way I can over-ride the 40+ second automatic power down the camera has and only keep it on for 15 seconds.

As for the idea of ramping up the power to the processor inside the camera (that what you were saying?) that seems like a bad idea. For a time - the supply would be sitting on the cusp of good/bad power. Anyhow it appears as if the camera designers already thought of that - they monitor battery charge and if the power comes on with a low battery the camera doesn't come on but just alerts you that the battery is low. There appears to be about a half second or so when this is going on during power up - screen takes awhile to initialize and you can't snap a picture until it is done.

Another thing that makes me skeptical - I used a few surplus remote controls to provide a signal for break beam infrared detectors - cheaper and easier than building the signal source - the ones I was using definitely didn't like coming up slowly - they would latch up. (I had some idea to provide power during momentary outages via a "super cap" which, when it charged, kept the supply from coming on quickly and that caused a latch up)

That is true but it was not "latch-up." Almost every imaginable power reset architecture malfunctions with slow voltage turn-on. But these maximums are on the order of 15ms... The 500mA load is about the magnitude where you may have to start thinking about exceeding the VGS threshold by a safe margin, otherwise the RGS,ON may be much higher than you think. I wouldn't use a 4.5V threshold in a doubler app powered off 3V.

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Latch-up is more likely from a fast rising voltage. I'm thinking of a few hundred microseconds, not miliseconds. Yes, part of the road test for chips is a slow turn on, i.e. a few hundred miliseconds. You really should be able to apply DC at any voltage and no have the chip latch or behave strangely. The problem with your design is that it is probably full of cheap ass Chinese parts, so the parts themselves may not be very robust.

The power fet turn on is a real life problem. Some designers don't trust the shutdown pins n chips, and like to waste money by using series P-fets because it is something they control. ]In all fairness, perhaps not every chip in the system has a shutdown pin, so they decide to just power off entire sections of circuitry with the pass fet.

Latch-up testing, generally only done on the initial design, is done with the power supply applied. That is, the chip is powered and you push/pull the pins other than supply and ground. Thus most QA procedures don't test powering up the chip via a pass fet.

Tell me more (I'm opinionated, obstinate, stupid (at times) bullheaded, intractable, etc.) but this is something I haven't encountered.

And would 6 volts for what is supposed to be a 4.5 (max) turn on good enough? (for a 30 amp mosfet pulling less than an amp?)

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Micro seconds? I'm thinking PWM output toggling a voltage doubler with a filter cap and bleeder resistor. I worry that it will be too slow to turn on.

Yes it is full of cheap ass Chinese parts - I wouldn't be trying this with expensive parts.

My picaxe, if I understand it correctly must be totem pole outputs. My camera trigger is a .1 uf ceramic with no bleeder (just trying to eliminate some reverse current so went with a cap instead of cap/bleeder or open collector). come to think of it, you make it high it sources and make it low it sinks.

I suspect you are talking about some more proprietary knowledge from working or designing these cheap Chinese camera things - I'm just tinkering and trying to build something to satisfy my curiosity, learn, and just maybe wind up with something with market value. I do know its been fun so far.

I do want to build the best device I can within financial limits.

Now you seem to be arguing against slow power up.

The camera comes up 100% of the time when you slide the batteries in and close the contacts on the batteries - one of those pull out and hinges away battery compartment with the series connection for the batteries on the sliding portion.

This thing is probably relatively immune to contact bounce or garbage power. My mosfet is bound to be better than sliding metal contacts, wouldn't you agree?

Right now I can barely lift my arms from kayaking - the camera and breadboard were taped to the dash on my truck to watch the boat ramp - maybe tomorrow I'll fool with driving mosfets with the pwm output. Now it is time for homebrew beer and "zoning out."

Amazing perfroamcne! JP thanks for the link and insight inot this part! I did not know they beat vsat down sooo low!

Marc Popek