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galapogos

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  1. I took the easy way out and simply placed 2 schottky diodes on the 2 voltage sources to prevent any back flow of current. It seems to work, except my voltages are down to ~4.75-4.85V. My device has a 5V tolerance of +/-5%, which puts me on a borderline case. I'm wondering if there are any low drop out schottky diodes that have a lower forward voltage? I'm currently using a Zowie MSCD202 that has a Vf of [email protected]
  2. Sorry, that was what I meant by positive or negative polarity. Positive meaning the center pin is positive and negative meaning otherwise. It seems that most AC adapters out there have positive center pins.
  3. What about AC adapters with 3 pin plugs? Is the DC ground still isolated from the AC wall socket? Anyway, does it even matter? Will breaking the GND on the USB side cause it not to supply power to the circuit? Coz I did some googling and it seems that positive polarity AC adapters are a lot more common than negative ones, so it'd be good if I can just break the USB power by disconnecting it's GND.
  4. Hmm, I thought the adapters are pretty much standard. Is one more prevalent than the other? Also, I don't think switching the GND would work right? Since the USB ground is from the PC, which is from the wall socket, which is where the AC adapter GND is from too, so they sorta share a common GND.
  5. OK, so I tested 2 of the enclosures I have here that have DC jacks. They all have 3 pins, so I assume they're switched jack. I found 2 of the pins that are shorted with a multimeter, and then I plugged in the DC jack(unconnected to the mains of course) and tested for continuity on those 2 pins again. Doh, they were still shorted. So either the 3 pin jacks aren't actually switched jack, or the enclosure maker simply shorted the 2 on the PCB side both to GND, which means they're still shorting both power supplies rather than switching them. Anyway, the center pin(pin 1) is positive, so the 2 pins that are shorted are GND. This contradicts what you say about "usually the positive is switched, not the ground."? Does it matter which one is switched? Would it still break the circuit if the USB's GND is disconnected from the circuit GND?
  6. OK, in that case pin 1 has to be Vcc then. I'll pull out my multimeter tomorrow, test it out, and report back! Thanks! :)
  7. Makes sense. I assume pin 1 is Vcc and pins 2 and 3 are GND? Hence the USB Vcc is actually shorted with the DC jack Vcc but the USB GND is disconnected from DC/system GND when a DC plug is inserted, hence disallowing power delivery from the USB connector?
  8. OK, that schematic looks like most I can find on DC jacks. How exactly does the switch work? I thought pins 3 and 2 are just shorted together.
  9. As far as I can tell, there's no switch. The circuitry for these cheap enclosures are pretty simple, and the PCB is small with very few components. What would happen when they are shorted? Worst case, would something blow up? Also, how's this for a solution? I'm thinking of either 2 FETs or BJTs with low Vce(sat), one for each power source, but opposite polarity. The base/gate would be controlled by the 5V from the DC jack, so that whenever there's power from the DC jack, the DC jack FET/BJT is switched on and the USB one is switched off. Would this work or am I crazy?
  10. Hi, I'm wondering if it's possible for a circuit to have 2 different power sources for 5V? I've seen many USB bus powered drives that normally use the USB 5V voltage to power the drive, but they often also have a DC jack. All of the ones that I've seen have both the Vusb and the Vdcjack connected to both the drive's 5V and the circuit's 5V input into the LDO regulator, i.e. both 5V sources are shorted. When an AC adapter isn't being used, this isn't a problem since there's only 1 5V source(Vusb), but when an AC adapter is used, isn't this potentially dangerous due to differences in voltage levels between the 2 5V sources, as well as 1 source overpowering the other? What are the chances of things blowing up? Thanks.
  11. Thanks, but due to the existing design I have to use a P channel transistor. Is a low RDS and Vth all that I should be looking at? I guess I just need to know some guidelines on what parameters are important when choosing a transistor. I might even go for a PNP BJT since they seem to be more recommended.
  12. Hi, I have an application where I have to switch on/off 2 3.3V low power devices(~50mA max combined) with a 3.3V MCU signal. I am currently using a TP0610K that I have lying around with the following connections. Source - 3.3V Gate - 50K pullup to 3.3V, and MCU control signal Drain - 1M pulldown and device Vcc This seems to work, but according to the TP0610K datasheet (http:// www.vishay.com/docs/71411/71411.pdf), 3.3V doesn't even begin to bring the fet into saturation mode. Here are the relevant specs of the TP0610K (at least those that I think is relevant): Vgs(th) = -1.0/-3.0V min/max Id(on) = -50mA (Vgs = -4.5V, Vds = -10V) Rds(on) = 10ohm (Vgs = -4.5V, Id = -25mA) *According to the Rds(on)-Vgs graph, Rds(on) rises sharply at about Vgs = 5V Now, while the switch seems to work now, there are a couple of things that worry me. 1) The Id(on) of 50mA is probably lower when Vgs, Vds = -3.3V, and this may be a problem since my devices consume about 50mA max 2) Rds(on) rises almost to an asymptote when Vgs < 5V With that said, I looked at a few other mosfets that I could drop into my existing design, and came up with the following: Fairchild FDV304P - Vth -0.86V, Id -460mA, Rds(on) [email protected] Vgs Fairchild FDV302P - Vth 1V, Id -120mAm Rds(on) [email protected] Vgs ON NTR1P02LT1 - Vth -1V, Id -1.3A, Rds(on) curve bends at -4V Vgs, 0.19ohm-2.5V Vgs ON NTR2101P Vth -1V, Id -3.7A, Rds(on) [email protected] Vgs ON NTR4101P - Vth -0.72V, Id -3.2A, Rds(on) [email protected] Vgs Vishay Si2323DS - Vth -1V, Id -4.1A, Rds(on) [email protected] Vgs Vishay Si2301BDS - Vth -0.95V, Id -2A?, Rds(on) [email protected] Vgs There are a few more, but I just want to make sure if I have the right idea. Basically what I'm thinking is that I need a mosfet that has the following characteristics: 1) low threshold voltage(all the above have Vgs(th) >= -1V 2) Sufficient Id(-120mA and less should be sufficient?) 3) Low Rds(on)(single digit, or even <1ohm shouldn't cause any noticable voltage dip at device Vcc) Some of the above mosfets are for logic rather than power switching, but I'm guessing that as long as the current capacity(Id) is sufficient, it shouldn't matter? Is there anything else I should worry about? Thanks!
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