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Forum Index : Microcontroller and PC projects : VideoMite 1A...
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bigmik Guru  Joined: 20/06/2011 Location: AustraliaPosts: 2949 |
Grogster, Looking at the specs the 3A 1N5820 seems to have the lowest forward drop, starts at about 0.22V and at 1A its drop is 0.35V (@25 Celsius) This is slightly better than the 1N5817 which starts at about 0.2v and is 0.45V at 1A Bare in mind that most 5V supplies pump out around 5.1v- 5.15V anyway you should be well and truly safe. Note also the 1N5820 may have fatter legs than the 1N581x family as it is 3A instead of 1A.. Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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boss Senior Member  Joined: 19/08/2011 Location: CanadaPosts: 268 |
@Grogster Zenner act as regular diode if plus voltage connected to Anode and minus on Cathode so forward drop Voltage is ~0.7V. Don't worry this circuit I used at least 100x. So if you connect +V to center pin the zenner open ~ at 5.6V and if you connect opposite +V to the ground zenner acts as regular silicon diode so it open at ~0.7V. Bo |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
I have found a candidate - an NSR0320 , which has a 0.24 forward voltage drop @ 10mA current, and only 0.3v drop at 100mA, which is not too bad. About $1 each. SOD-323 SMD part though, but as with my other board, I am happy to fit this part for anyone who does not want/like SMD soldering. The other option would be to leave fitting up to the builder, and provide some blob-pads to bypass the diode(and reverse-polarity protection). Smoke makes things work. When the smoke gets out, it stops! |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2949 |
Grogster, This is starting to bog down too much IMHO.. You are suffering from the `design by committee' syndrome.. You are over complicating things... If you use a 1N5817 (or 1N5819), it would not drop enough to drop enough to affect the LM3940 unless the 5V was below specs. It will also keep a 100% through hole board. If the case is that the 5V input is too low then they need to replace the diode with a wire link.. If you can provide shorting pads for the diode all the better. Regards, Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
OK. 5819 with blob pads to bypass if you want. Everyone happy? (rhetorical!!!!!!!) Smoke makes things work. When the smoke gets out, it stops! |
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MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
My opinion about a diode. Leave it out. The reason is that when you specify a DC voltage of 5v,that is what needs to be connected. You have 5v terminals on the board that will not be 5v.Less then ideal. I know it is for protection against wrong polarity and if you feel that is necessary then it might be better to start with a higher voltage and put a 5v regulator on board. It will also increase the change of someone having an adapter lying around that will work with the board. Use the USB as the entry for a 5v. Anybody using the board can always decide to leave out the 5v regulator and just bridge the vin and vout pins. In that case the user of the board is smart enough to connect the board properly. Which should be to build the board, apply power, check the voltages on the pins from the chips and if all that is in order mount the chips. As for the resistor on the bottom. What I meant is that you can place that resistor very close to the solder blob islands. Right under them or next to it. There is room enough and traces will be short. Now get that board out of the door, because it was already good from the start. :) Microblocks. Build with logic. |
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CircuitGizmos Guru Joined: 08/09/2011 Location: United StatesPosts: 1427 |
Can you add an 8088? I want to run visicalc. And a circuit for a radio. I want something to listen to while I'm running visicalc. [insert wink here] You can always put in holes for some parts and just not populate them... Micromites and Maximites! - Beginning Maximite |
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G8JCF Guru Joined: 15/05/2014 Location: United KingdomPosts: 676 |
IF one has to have reverse polarity protection, then a PMOS FET is the best solution IMHO - truly insignificant voltage drop - see http://www.ti.com/lit/an/slva139/slva139.pdf and also http://www.irf.com/technical-info/designtp/dt94-8.pdf However I do tend to agree with @BigMiK and @TZ, that reverse polarity protection for this board is probably over-engineering for the kind of users who will make use of it. Peter The only Konstant is Change |
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| WhiteWizzard Guru Joined: 05/04/2013 Location: United KingdomPosts: 2932 |
I'm with Peter on this one - to protect against reverse polarity then use a MOSFET. Cheap, insignificant voltage drop, and work very well. Only drawback are that they are typically SMD, WW |
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BobD Guru Joined: 07/12/2011 Location: AustraliaPosts: 935 |
Page 83 of the November issue of a well known electronics hobby magazine which is published monthly in OZ, has a circuit showing reverse protection with a MOSFET. It won't connect the ground unless the polarity is correct. It's an smd device but the legs are well spaced. |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2949 |
I can see Grogster now, 
Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
Where the hell did you get that photo of me, you rotter? @ CG - Might have room on the bottom layer for a quad-core BGA you could work with. Seriously though, thanks for the suggestions. It is all good to some extent, it is just hard to come up with something everyonr likes, so I guess the rule there is to not try to hard in that respect. Take on everyone's suggestions, but do what YOU feel is best - it is YOUR board. That kind of logic, if you see what I mean. As a final word on this, I do like the MOSFET idea, and have never ever thought of that, and this WILL be how I do things like this in the future because of the super low S-D voltage drop across a MOSFET - excellent idea, and the only annoying thing about that, is that I had to be told and did not think of it myself. Smoke makes things work. When the smoke gets out, it stops! |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2949 |
Grogs, I hijacked one of those Mites in high altitude balloons that EDU001 sent up. 
Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
*Huge grin*  Smoke makes things work. When the smoke gets out, it stops! |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2949 |
Grogster, It is a bit ironic that of all of the comments throughout the various design stages of your wonderful board, the most interaction, and active discussion, has been about the power input, don't you think. Mick Edit*** I agree, the MOSFET input protection looks fantastic. Will have to keep that idea in my head somewhere for future reference. Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
Yes, and to some extent - rightly so. Reverse polarity protection via a diode was something that most of us leart as one of the very first steps in electronic design. In my case, it was back in the days of Dick Smith's Fun Way Into Electronics series of books, with volume one being just a square of wood with lots of screws and washers - that was your prototype board.
Even on those boards though, Dick Smith extolled the great virtues of using a series diode to protect against reverse polarity. It's still good design even today, but I think that, as pointed out by other members, the kind of people building these boards should know enough of what they are doing to NOT kill the board by reverse polarity. The reason I have entertained this so much, is that I myself HAVE accidentally killed things that did not have reverse polarity protection. All it takes is a moment's lack of attention in connecting something up..... However, I will now be studying(and using) the MOSFET idea from now on - all voltage-drop issues essentially become a non-issue with this method - fantastic stuff.  Smoke makes things work. When the smoke gets out, it stops! |
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| G8JCF Guru Joined: 15/05/2014 Location: United KingdomPosts: 676 |
I was wondering why nobody else had suggested the MOSFET technique ! The only Konstant is Change |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
No-one had thought of it! 
A great idea, that one, so I will be making use of that in the NEXT board design. Smoke makes things work. When the smoke gets out, it stops! |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
How about a litle Zener diode humour at this point: 
Smoke makes things work. When the smoke gets out, it stops! |
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| viscomjim Guru Joined: 08/01/2014 Location: United StatesPosts: 925 |
Once, I plugged in a project using wrong polarity. My bad. Never happened again. Kind of like touching a hot burner on a stove. This board is awesome as is. Zeners, diodes, Mosfets, etc. you will never get this board out based on if someone can't hook up a power supply correctly. Your board is awesome as is. I was ready to buy one or two a few posts ago. Send that thing to Shenzhen and carry on. I want one... |
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