 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Microcontroller and PC projects : VideoMite 1A...
  Page 3 of 6   |
|||||
| Author | Message | ||||
| paceman Guru  Joined: 07/10/2011 Location: AustraliaPosts: 1329 |
Looks good Grogs - nice spaghetti  |
||||
Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
Yes, it was very yummy. Thanks for asking. Smoke makes things work. When the smoke gets out, it stops! |
||||
MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
Looks good!!. Only one thing, why is that resistor on the back side. It should be possible to put it on the front. Just put the resistor 'over' the solder blob islands. Microblocks. Build with logic. |
||||
boss Senior Member  Joined: 19/08/2011 Location: CanadaPosts: 268 |
@Grogster Nice board. I didn't see the schematics bud I can't find polarity protection diode on the board(Schottky is the best choice) which is highly recommended. Bo |
||||
| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 4034 |
A protection diode - is that in case someone connects + and - the wrong way round? Does anyone really do that? Heck I write software and even I've never done it! John |
||||
| boss Senior Member Joined: 19/08/2011 Location: CanadaPosts: 268 |
@JohnS I did the same. Then I accidentally grabbed power supply with minus on center ( some manufacturers use this trick to prevent you to buy the cheap one) and I sent my MM to the silicon heaven. Replacement of damaged CPU chip is delicate task. Bo |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
@ boss - Have a look back through the rest of this thread. The diode was originally in there, but has been removed for reasons that you will discover, if you have yourself a little read. 
@ JohnS - Please clear out your PM inbox.
@ TZA - If you put the resitor on the top side over the pads, you won't be able to bypass it if the resistor is in place. I'm happy enough with the singular resistor on the bottom copper side. It will be there permanently, and to bypass it if not needed, you just bridge the solder blob point on the top copper layer.Smoke makes things work. When the smoke gets out, it stops! |
||||
| boss Senior Member Joined: 19/08/2011 Location: CanadaPosts: 268 |
@Grogster O.K. it is your decision Bo |
||||
bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2949 |
Grogster, I had a look at the Specs of the LM3940 and it claims good 3v3 regulation down to 4.5V input so your 1N5819 could still be an option, I suggest to use the 1N5817 which claims a much lower forward voltage drop (I have no experience with this one though) If it can be fitted and someone wants to feed a `marginal voltage' in then they can always short out the diode.. Up to you.. Although I have never EVER seen a 5v plug pack (I can tell you I have seen literally hundreds of them) that has a -ve centre pin (many 9 and 12V do/have done so.) Regards, Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
The only problem I have with the diode at this point, is we are right on the border-line for the lowest input voltage to the 3940(but may still be OK). I don't like running things at the limits - a recipie for problems. With respect to voltage regulators, I always aim for reasonable headway with their input voltage vs the minimum input voltage they want - Vin = Vout + 2.5 kind of idea as a minimum. But LDO regulators don't need as much head-way in that respect because of the very fact that they are LDO... I will give this a little more thought. I could include the diode and bypassing pads, as you say - that would restore the diode as boss was suggesting, and also allow bypassing if need be. I will have a little tinker with the layout, and see what I can do - real-estate is starting to get expensive on this layout now.  Smoke makes things work. When the smoke gets out, it stops! |
||||
| boss Senior Member Joined: 19/08/2011 Location: CanadaPosts: 268 |
@Grogstear Then use 3Amp schottky with cathode on 5V and Anode on ground. And you have protection with zero Voltage loss. Bo |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
Yes, but that would short out the power-supply via the diode if the polarity was incorrect. Very hot, possibly explosive meltdown of the diode.
I suppose if the diode was beefy enough.... Probably why you suggest a 3A diode, but the short-circuit current could be quite a bit more then 3A, depending on what power-supply was feeding it. Still, something to think about. Smoke makes things work. When the smoke gets out, it stops! |
||||
| boss Senior Member Joined: 19/08/2011 Location: CanadaPosts: 268 |
@Grogster Not really, modern switched PSU have short connection protection and limit short circuit current to 10% max PSU current. And you can always install a small fuse in series with 5V input. In the case wrong polarity the fuse will blow and the precious circuit will protected. Even better will be if you replace Schottky with 2W (5.6V) zener and you'll get double protection - polarity and overvoltage. Bo |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
You have a point about modern switch-mode PSU's built-in short circuit protection... I like your zener idea. Will see if I can squeeze something in. Smoke makes things work. When the smoke gets out, it stops! |
||||
| boss Senior Member Joined: 19/08/2011 Location: CanadaPosts: 268 |
@Grogster Perfect. Don't forget the fuse. On ebay they have lot of miniature fuses just for bargain price. Bo |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
Zener fitted. By fuse, I assume you mean polyfuse yes? Smoke makes things work. When the smoke gets out, it stops! |
||||
| boss Senior Member Joined: 19/08/2011 Location: CanadaPosts: 268 |
@Grogster Good point. I though ordinary fuse, because of smaller serial resistance, but the 200-300mA polyfuse should work very well. |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
I'll see what I can squeeze in there.
Most of the polyfuses I have used are SMD though, but I ASSUME you can get them in more standard thru-hole flavours. ...but then, we all know what assumptions are the mother of all, don't we!(rhetorical)  Smoke makes things work. When the smoke gets out, it stops! |
||||
| boss Senior Member Joined: 19/08/2011 Location: CanadaPosts: 268 |
@Grogster Yeah, you are right. There is one could fit to your design. http://www.ebay.ca/itm/50pcs-0-4A-72V-400mA-PolySwitch-Reset table-Fuse-Poly-Switch-Fuses-Polyfuse-/191074814189?pt=LH_De faultDomain_0&hash=item2c7cf248ed Bo |
||||
| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9586 |
Yeah, that's the kind of thing I was thinking of. I will see if I can squeeze another 5mm footprint in there somewhere(will have to move a couple of other bits a little). Naturally, you could just not install these bits if you felt that the protection was not required. I will post an update image later. EDIT: I still have concerns about this concept. Allow me to share my thoughts: At 5v, a 5v6 zener will not have sufficient reverse-bias voltage to cause it to breakdown and any current limiting via plug-pack or poly-switch will be ineffective, and therefore the 5v reverse polarity will still be applied to the circuit, likely causing fatal damage before any type of crowbar reverse protection can take effect. I think the best idea, is to go back to the series diode we have talked about before, but I will hunt out a Schottky diode with the lowest forward voltage drop I can find. Smoke makes things work. When the smoke gets out, it stops! |
||||
| Page 3 of 6 |
|||||
 Print this page |
 |
The Back Shed's forum code is written, and hosted, in Australia. | © JAQ Software 2025 |
