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Faster switching speed from mosfets in existing circuit/PCB
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spec:
As mentioned in reply #29, attached below is a schematic for a notional circuit for your camera strobe light.

If you favor this approach just say and I will do a schematic suitable for prototyping.

Or if you have any questions or would like any modifications also let me know
blazini36:

--- Quote from: spec on December 26, 2018, 05:35:38 am --- :) Mistakes, boobies, cock-ups are all part of development. Big errors don't matter, it is the nasty pernicious little errors, that are not obvious, that bite you in the arse and cause the most problems. My favorite errors are the ones that only show at customer demonstrations.

Thanks for info.

There is just one thing though. It is not possible to isolate the camera strobe output from the 5V/24V DC supply because it is a switch to 0V, but I do understand about the need to protect expensive cameras (I have one  :)).

I have sketched a design for driving the LEDs using a power PMOSFET and four strings of LEDS. When I have put it into EAGLE I will post it for discussion. The 5V supply is not used, only the 24V supply.

--- End quote ---

Maybe isolation is the wrong word, seems to me that optos offer better protection from shorting the signal side on the load side if components fail.

I have the components and boards for my opto setup coming in so I'm going to test that and see how it works out. If it's too slow I'll take ya up on the offer to help me out with a power mosfet schematic.

thanks bud
spec:

--- Quote from: blazini36 on December 26, 2018, 11:39:03 pm ---Maybe isolation is the wrong word, seems to me that optos offer better protection from shorting the signal side on the load side if components fail.
--- End quote ---
If you feel happier with an optocoupler that is fine. But I would not say that an opto is any safer, per se, than a non opto with built in protection. Opto couplers can fail too. :)  A high speed optocoupler could be added to the circuit of reply #30 though.


--- Quote from: blazini36 on December 26, 2018, 11:39:03 pm ---I have the components and boards for my opto setup coming in so I'm going to test that and see how it works out. If it's too slow I'll take ya up on the offer to help me out with a power MOSFET schematic.

thanks bud
--- End quote ---
No sweat. Be interesting to find out how well the opto relay works.
nick_d:
How about using an N type power MOSFET which is cheaper, and driving the gate with a P type small signal MOSFET. This will reduce the gate capacitance shown to the camera down to picafarads and you can arrange things so that the gate of the power MOSFET gets pulled up quickly but pulled down by a resistor such as 330 ohms. The only drawback to this is the power MOSFET would heat slightly during turnoff, as it would become resistive during the time to discharge the gate capacitance (about 1nF) through the 330 ohms (about 330 ns). A fast opto can be inserted too, they have open drain outputs so would behave pretty much like direct connection to the camera. This arrangement would need the 5V supply though. Probably a 7805 is the cheapest way.
cheers, Nick
blazini36:

--- Quote from: nick_d on December 29, 2018, 08:33:16 pm ---How about using an N type power MOSFET which is cheaper, and driving the gate with a P type small signal MOSFET. This will reduce the gate capacitance shown to the camera down to picafarads and you can arrange things so that the gate of the power MOSFET gets pulled up quickly but pulled down by a resistor such as 330 ohms. The only drawback to this is the power MOSFET would heat slightly during turnoff, as it would become resistive during the time to discharge the gate capacitance (about 1nF) through the 330 ohms (about 330 ns). A fast opto can be inserted too, they have open drain outputs so would behave pretty much like direct connection to the camera. This arrangement would need the 5V supply though. Probably a 7805 is the cheapest way.
cheers, Nick

--- End quote ---

The opto-mosfet(SSR) is turning out to be too slow. It requires too much on-time to drive the output. I've been reading u on mosfet characleristics and I've pretty much come up with what you mention here, minus the opto. The LED array is drawing a little under 1.2a so I'm thinking I can use one of those 6pin N/P mosfet arrays for convenience. I'm looking into gate drivers as as was mentioned before as well. The board has a 5v switching regulator so it shouldnt need the 7805 right?
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