I have some issue with a Hikvision make DVR, where few channels showing very low brightness or no video. But few ports showing good. I have open the box and noticed resistance between working bnc input ports the good one have 75 Ohms while bad one have 0 ohm (short). Further I had removed diodes (not sure if they are TVS diodes for ESD protection) on those shorted ports & then measured resistance between ground and input pin. The result was good 75 Ohms & then connected camera input there which also worked perfectly. Now I searched that diode (component) over the internet but couldn't find anything with that label marked (6L H503). Would you please assist me what are those components as I am assuming tvs diodes.

Thanks

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(Probably because the Mosfet behaves odd when no HDD/Load is connected on its Drain. Which I didn't because of the wrong by the owner given 19V adapter. A Sata HDD drive only works on 12V and/or 5V! Because the simulation also showed that the Mosfet Drain output voltage already was about 3.49V without any decent load below about 10Kohm, even if it wasn't activated by the NPN Transistor or when the Gate wasn't attached. Which means dissipation because the Mosfet is in a wrong state between open or closed !! Where it always should show ZERO output voltage when not activated.)

Why I chose a N-MOS enhancement Mosfet in practice that never got hot but as another Tina simulation showed always switches Vin through to Molex Vout. So it is never an open connection between Source and Drain. Which also explains that the repair still was successful after removing the Short circuits on the Mainboard, but indeed only the HDD 12V on the Molex is never switched off in practice. Which I couldn't test in the end because I had none of the videocables or control cables necessary. Which was fully tested by the now satisfied customer. So I encourage anyone to use a great program like Tina 10 which now can be bought with a free upgrade to the coming version Tina 11 !

(I bought the Classic version because of its VHDL and other Language simulation enhancements and many more extra's such as the unlimited design capacity, which is not available in the cheaper/student versions).

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Good question ombra32, I previously had the same thought about the internal Diode but I am very sure it functions because a P-channel didn't. And more types of Mosfets just are not possible. Except for depletion or enhancement Mosfet types of which the enhancement types are the common used types in Digital circuits. So I probably picked the right one. And the N-channel used here probably switches open when another part of the circuitboard activates the Gate so it becomes an Isolator. The Body-diode of a P-Channel would be completely turned the other way around and that just didn't work. The Gate of the 7831 N-channel Mosfet is here higher and positive opposite Source like you said. And the Photos are proof the Device works. Without a Services Manual or Schematic of a Device it always is hard to tell how it exactly works because we therefore only repair/replace the parts that were burnt or broken. If the chosen Mosfet is still the wrong type than maybe some protection circuit now could not be working. And because the Device did work like a new one and the short circuit was removed my job was done. If you want to know more about those kind of Mosfet types look for instance here:
https://en.wikipedia.org/wiki/Depletion_and_enhancement_modes
The Customer was very happy that he got his expensive Device back working, and I haven't heard any complaint since. And if the Customer is happy so am I.

In addition to your very good question Ombra32, following link explains the differences between all type of Mosfets very well!:

https://oscarliang.com/how-to-use-mosfet-beginner-tutorial/

Depletion Mode Mosfets are normally ‘ON’ and can be turned OFF on their Gates. And that site also explains that the internal Body Diode unintentionally may allow current to flow.

The next link tells us that Depletion Mosfets are just rarely used and the rare cases when they are used:

http://electronics.stackexchange.com/questions/237566/typical-use-of-depletion-mosfet

So maybe also a Depletion Mosfet would have worked also, but just finding one isn’t easy.

Especially the SO-8 types.

Since the previous Short Circuit was removed, now maybe only the 12V regulation to the HDD Molex connector does not work, if that is why the Mosfet was used for. And that most likely is possible here with a Depletion type Mosfet.

But since the 16 Channel x264 Digital Recorder works even after the owner used the wrong 19V Power Adapter (twice), I am glad I could repair his expensive Device where others couldn’t or wouldn't!

Thank you for your very observative question Ombra32!
Thanks, Albert.

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From the schematic, if you put an-N channel mosfet on positive pole in that way, you obtain an always on mosfet due to parasitic body diode from source to drain.
Moreover, to power on an N-mos you need a gate voltage higher than source voltage, and this is impossible without a charge pump.

Are you sure all is working fine??

Regards

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Do understand that the Maximum Current possible is limited to a value as is given in the Datasheet. The maximum current for the above given AO4406 Mosfet will be around 13A continuesly, when Vgs = 10V minimum. And only very quickly pulsed the Mosfet will be able to switch higher Drain currents! (upto 100 Amps! at a frequency of about 1MHz). Also understand that a Mosfet only uses some energy when its state changes. Only than the Gate draws some current like changing the polarity of a small Capacitor. In fact a modern Personal Computer Microprocessor/controller has millions of these tiny Mosfets on a few square milimetres. And if they all switch extremely fast, its surface will be getting very hot because of it! Also keep in mind that Voltages and Currents higher than their maximum given values will ultimately destroy your Mosfet to Kingdom come.

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Yes of course I can Lad. I already repair about 5 years or so mainboards from different Laptops. So I know about most Mosfets because of it.
And often the 3.3V, 5V ,12V or Vin (~20 VDC from adapter) Mainboard voltages are not present. Mostly because of bad Mosfets. And if you examine the concerning Mosfet Datasheet you'll notice that they very often are placed in a SMD 8 pins (SO-8) housing.
They like the ones in the article only have 3 active connections being Gate (always pin4), Source (the three pins 1 to 3 connected) and Drain (the pins 5 to 8 connected with each other).
So if you look at these Datasheets you also will notice that there are N-Channel Mosfets and P-Channel Mosfets. For instance here for the AO4406 Mosfet:
http://www.aosmd.com/res/data_sheets/AO4406A.pdf
It is a N-channel 30V (Max voltage on the Drain-Source part of the 4406) Mosfet. Only if the Gate-Source voltage is more positive than 0 Volt, it conducts current from Drain to Source. And if you examine the curve you will see that this Mosfet will only conduct 10 Amps at a Gate-Source voltage of only 3V. And that is at a Drain-Source voltage of about 5V. And already at a slightly higher Gate-Source voltage of 4.5V and a even lower Drain-Source voltage of just about +1.2V it already conducts currents as high as 60Amps! The Gate makes it possible for the Mosfet to function or not. Like an electronic switch. The Treshold voltage tells us for instance at what Gate-Source voltage our Mosfet stops conducting and becomes an isolator. Our Datasheet shows us that that will happen between voltages from only 1.5V to 2.5V when VDS=VGS. So the higher both voltages, the higher the current through it.

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