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Are these blocking diodes nessary?

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Here's a portion of a schematic for a HV driver (30V) on the driver side of the ULN2803

Now when the POWERFET is switched off (Open) will current flow back into the PIC output port without the 1N4148 diodes?

Question is: can I omit the diodes safely?
 

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Looks to me like you can omit them. Current should not flow back into your output drivers. Those outputs likely have over voltage clmaping diodes on them anyways if that is the concern.
 
Don't know where you found the circuit?, but I would never have even considered putting diodes there.

The only reason that comes to mind is that the original circuit may have had something else connected to the driver inputs as well? - such as manual over-ride buttons?.
 
William At MyBlueRoom said:
Now when the POWERFET is switched off (Open) will current flow back into the PIC output port without the 1N4148 diodes?

Yes. The MOSFET opens the GND connection of ULN2803(bad circuit design anyway, as one can just TRIS the port to get the same effect), as a result some current from 30V will find its way and flows into the PIC if the diodes are not there.

William At MyBlueRoom said:
Question is: can I omit the diodes safely?

No.
 
eblc1388 said:
Yes. The MOSFET opens the GND connection of ULN2803(bad circuit design anyway, as one can just TRIS the port to get the same effect), as a result some current from 30V will find its way and flows into the PIC if the diodes are not there.

Oops! - never noticed the FET there!, it never occurred to me that someone would do something so stupid! :)

Presumably it's intend to PWM all the outputs?, but whoever came up with the idea needs taking outside to have his fingers broken! :)
 
Yup, removing the ground on a chip is like painting a big red target on your back with the words shoot me underneath. You can expect the chip and the system to behave very eratically. No values from the datasheet are reliable if the chip doesn not have VCC and Ground in the specified range.
 
Nigel Goodwin said:
Oops! - never noticed the FET there!, it never occurred to me that someone would do something so stupid! :)

Presumably it's intend to PWM all the outputs?, but whoever came up with the idea needs taking outside to have his fingers broken! :)

That's why I asked, even though the ULN2803 is a transistor array and not a logic IC. I got the idea from this site.

http://www.wrighthobbies.net/bots/lamp/index.htm

The idea has been tossed. Its for brightness of a single 7 segment dispay, I can do the PWM in software.
 
I think that circuit is a little unusual myself.
In my case, I connect all "GND" pins to ground (-ve)
AND I remove the diodes (more like shorting them).
 
Hello everyone,

Just a quick comment on the LED circuit using the ULN2803 transistor array. The diodes isolate the transistor array from the Mega32. Since I am switching the ground pin on the ULN2803, the diodes prevent the chips from grounding through the MCU. Yes, switching the ground on the ULN2803 isn't proper but with the diodes in place it does work. That lamp has been running non-stop for over 2 years without a problem.

I wouldn't recommend this for other applications (certainly not commercial applications), but for this lamp it works.

The other option is to PWM the source voltage but the RGB LEDs have a common source pin so that wasn't possible.

You can slap my hand now :)

Eddy Wright
Wright Hobbies Robotics
 
eddy@wrighthobbies.com said:
Hello everyone,

Just a quick comment on the LED circuit using the ULN2803 transistor array. The diodes isolate the transistor array from the Mega32. Since I am switching the ground pin on the ULN2803, the diodes prevent the chips from grounding through the MCU. Yes, switching the ground on the ULN2803 isn't proper but with the diodes in place it does work. That lamp has been running non-stop for over 2 years without a problem.

I wouldn't recommend this for other applications (certainly not commercial applications), but for this lamp it works.

The other option is to PWM the source voltage but the RGB LEDs have a common source pin so that wasn't possible.

You can slap my hand now :)

Eddy Wright
Wright Hobbies Robotics
If the attached schematic, which I got from the ULN2803 datasheet, is correct, I see no "sneak" current path from the output back to the input when the ground pin is floating. Do you have evidence that one actually exists, or are you just adding the diodes as a precautionary measure?
 

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If the mosfet is open, and one of the PIC outputs is driven low, the "ground" pin of the transistor array is going to be sitting at .7V courtesy of the protection diode at the bottom of the ULN2083 pic. Which means 4.3V still available to turn the darlington on.

Now if you play the game by having the PIC tristate it's IO's instead of driving them low, you'll be able to do without the diodes.

But yeah - ugh.
 
hjames said:
If the mosfet is open, and one of the PIC outputs is driven low, the "ground" pin of the transistor array is going to be sitting at .7V courtesy of the protection diode at the bottom of the ULN2083 pic. Which means 4.3V still available to turn the darlington on.

Now if you play the game by having the PIC tristate it's IO's instead of driving them low, you'll be able to do without the diodes.

But yeah - ugh.
Yep, you're right. I didn't see that path.
 
Ron H said:
If the attached schematic, which I got from the ULN2803 datasheet, is correct, I see no "sneak" current path from the output back to the input when the ground pin is floating. Do you have evidence that one actually exists, or are you just adding the diodes as a precautionary measure?

Ron, I found the current path by experimentation. As hjames pointed out, the protection diode provides the path.

Eddy
 
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