Electronics > Projects, Designs, and Technical Stuff
Faster switching speed from mosfets in existing circuit/PCB
spec:
--- Quote from: blazini36 on December 25, 2018, 04:19:04 pm ---Hi spec,
You're referring to the original schematic I posted right?
--- End quote ---
Whoops, your reply #22, with the revised circuit was not showing when I posted reply #23. Apologies. I have modified reply #23 accordingly.
Thanks for the additional information though.
spec:
--- Quote from: blazini36 on December 24, 2018, 05:00:51 am ---I bread boarded 1 "channel" of the schematic with the opto, and it seems to work well with the modified LED Board. Spent most of the day routing new boards, There's plenty of space on the PCBs for extra caps if necessary.
--- End quote ---
Hi blazini36,
just looked at your new circuit in reply #22. The architecture is much improved, but the SSR would have a turn-on delay of over 1ms at the currents required to drive the LED banks in series parallel.
https://standexelectronics.com/viewer/pdfjs/web/viewer.php?file=https%3A%2F%2Fstandexelectronics.com%2Fwp-content%2Fuploads%2F2018%2F09%2FPhoto-MOSFET-Relay-SMP-36_V01.pdf
Do you actually need isolation between the 5V line and the 24V line?
Also, am i correct in saying that you do not care how slow the LEDs turn off (within reason)?
Is it your intention to have 500mA flowing through each LED string, giving a total LED current of 1A?
blazini36:
--- Quote from: spec on December 25, 2018, 09:38:48 pm ---
--- Quote from: blazini36 on December 24, 2018, 05:00:51 am ---I bread boarded 1 "channel" of the schematic with the opto, and it seems to work well with the modified LED Board. Spent most of the day routing new boards, There's plenty of space on the PCBs for extra caps if necessary.
--- End quote ---
Hi blazini36,
just looked at your new circuit in reply #22. The architecture is much improved, but the SSR would have a turn-on delay of over 1ms at the currents required to drive the LED banks in series parallel.
https://standexelectronics.com/viewer/pdfjs/web/viewer.php?file=https%3A%2F%2Fstandexelectronics.com%2Fwp-content%2Fuploads%2F2018%2F09%2FPhoto-MOSFET-Relay-SMP-36_V01.pdf
Do you actually need isolation between the 5V line and the 24V line?
Also, am i correct in saying that you do not care how slow the LEDs turn off (within reason)?
Is it your intention to have 500mA flowing through each LED string, giving a total LED current of 1A?
--- End quote ---
Bah, why did I not notice the on time on that opto, maybe I got confused about ms vs us or something. There does actually seem to be an overlap of what the actual camera's behavior vs the strobe outputs is. Worse case scenario is that I have to rework the board for something like this https://www.opto22.com/products/dc60mp, which is a SIP version of the external SSR I originally used with the AC driver with the same specs. this switches on in 100us, yet I was able to set camera exposure at less than that. I'll have to test and see what happens, worse case is that I have to rework the board for a different switching device.
Isolation is more of a concern between the camera's GPIO and the strobe business I have going on. Plus I find the optos a bit easier to work with (speed may wind up being a concern though). It might not be a concern but the camera's are far more expensive than what the PCB components cost. Honestly all I really have to do now with the drivers out of the way is safely switch 2-3amps quickly
Turn off time isn't super important. The only reason it's switched at all is the LEDs will get super hot and waste alot of power.
The LED's can handle quite a bit more if heat is managed but 500ma per string is actually conservative for what I'm trying to do, I may actually drop the resistors down or break them into 3-4 strings to increase the voltage per string.
blazini36:
--- Quote from: spec on December 25, 2018, 09:38:48 pm --- Hi blazini36,
just looked at your new circuit in reply #22. The architecture is much improved, but the SSR would have a turn-on delay of over 1ms at the currents required to drive the LED banks in series parallel.
https://standexelectronics.com/viewer/pdfjs/web/viewer.php?file=https%3A%2F%2Fstandexelectronics.com%2Fwp-content%2Fuploads%2F2018%2F09%2FPhoto-MOSFET-Relay-SMP-36_V01.pdf
Do you actually need isolation between the 5V line and the 24V line?
Also, am i correct in saying that you do not care how slow the LEDs turn off (within reason)?
Is it your intention to have 500mA flowing through each LED string, giving a total LED current of 1A?
--- End quote ---
Actually now that I look at it again it looks like the biggest mistake I've made regarding the turn on time is the forward current of the photomosfet's LED. I chose the resistor based on the typical current listed in the datasheet. Looking at the graph it seems that if I just switch the 4.7k resistor with a 150ohm I can speed the opto up. I didn't see anything in the datasheet regarding the load current vs turn on time, can you point that out? It seems at a minimum on time of .3ms I chose one of the faster optos I could have in this package.
spec:
:) 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.
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