PropPlug Question
bambino
Posts: 789
I recently went back to the drawing board with an impact recorder design. It turned out the board was fine, but when mounted to the back of a LCD screen, the converter was too close to the backlight transformer. I redesigned the board and placed the converter circuit on a PCB mounted on top of the ProtoBoard.
Getting to the Point I found the converter was over heating, I did a continuity check and found a short to ground from the 5v line. I desoldered the converter and it's decoupling capacitor and got rid of the short(I thought)! If my test leads were placed black to GND and red to 5v I got an open, if I reversed them I got a short. My meter reads a short if under ~1.5 ohms.
Now for the really freaky part. When I removed my programming connection my problem goes away. Since the PCB is attached to the Protoboard by wire wrap gauge wires, I simply unbolted it from the protoboard to work on it, leaving the prop plug connected. The Prop IDE was not running during this event.
What am I overlooking here? I soldiered and resoldired all evening trying to get
rid of a short that was gone with the plug!
Does the plug have some low resistance diode between the two lines? And if so could this be placing some voodoo on my 5v accessories?
Getting to the Point I found the converter was over heating, I did a continuity check and found a short to ground from the 5v line. I desoldered the converter and it's decoupling capacitor and got rid of the short(I thought)! If my test leads were placed black to GND and red to 5v I got an open, if I reversed them I got a short. My meter reads a short if under ~1.5 ohms.
Now for the really freaky part. When I removed my programming connection my problem goes away. Since the PCB is attached to the Protoboard by wire wrap gauge wires, I simply unbolted it from the protoboard to work on it, leaving the prop plug connected. The Prop IDE was not running during this event.
What am I overlooking here? I soldiered and resoldired all evening trying to get
rid of a short that was gone with the plug!
Does the plug have some low resistance diode between the two lines? And if so could this be placing some voodoo on my 5v accessories?
Comments
Would be nice to know if my equipment was at fault.
If you leave the prop plug on there but disconnect the USB cable, does the problem go away? If it does, it sounds like some sort of ground loop or something like that.
The cap on the 5v line is(was a 10uf tatanium), but per your first thought I removed it. The problem was still there(I think, It was getting late last night). I have since replaced it with a 1uf(the minimum required) thinking it may have caused the overheating.
I have not tried disconnecting the cable from the plug(nice point). I'll try that when I get back tonight!
The other line on that PCB is 9v, and there is real estate left for a 7805 reg.
I did run analog grounds from the converter·back to the barrel jack grounds and power for the A2D amp driver back to the barrel jack Pos connector. So you may be right about the ground loop. It did work fine while it was in the breadboard, But so did the last design.
Thanks again Rokicki, I just needed someone to jog the old brain!
On with black test lead to gnd; I get infinite ohms @ vin and vdd, 6Mohms @ 5v.
On with·red to gnd; I get 0 ohms @ vdd and 5v, and 16k @ vin
Off with black test lead to gnd; infinite ohms @ vin, 500 ohms @ 5v, 342ohms @vdd
Off with red test lead to gnd;·14Mohms @ vin, 500 @ 5v, and 342 @ vdd
I am thinking now that this is normal because these values are from another Protoboard, right out of the box.·Could someone please beat me with the analog ruler on this one. How come the·0 ohms, At best I would have thought the closest to·0 ohms·one would get is while reading the reset circuit.··········
Post Edited (bambino) : 4/19/2007 12:23:11 PM GMT
Consider: what is the forward resistance of a diode? It depends on the Thevenin equivalent of the circuit
(multimeter) you try to measure it with (assuming you use a simple circuit). As I said before, measuring the
current draw of the live circuit is much more useful.
Your measurements are perhaps not totally meaningless, though. Again, 0 ohms can occur simply because
you're measuring a live circuit with an ohmmeter (which is not intended to measure resistances on live
circuits).
Let's go back to basics. You say your "converter" is overheating; what converter is that? If you measure the
current through the converter with a live circuit, we'd be in a position to say whether that's normal or not.
And saying "current draw with PC off is 14ma; current draw with PC on is 593ma" is highly useful, for
instance.
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Paul Baker
Propeller Applications Engineer
Parallax, Inc.
I see where the vdd line could be considered powered by the plug, but how is the 5v and vin affected upstream when there is no wallwart plugged in?
Thanks for your explanation here, either way I don't believe this was the problem of the failing converter. I replaced the 5v line on the pcb with a 7805 reg off of the vin. I won't get to through any juice to it until the weekend.
Rokicki,
The converter I am trying for the first time is an ltc1864 from Linear.
True, current would be nice to know, but I already had it cut out of the circuit and a new one in before that occurred to me. Will remember that this weekend if I·have made another toaster!
cable inline. (The banana plugs go into your multimeter). Once you've made this once you'll use it again and again (I frequently have it inline just
to watch the current as I try different things). If you don't have the plug/jack handy just grab a cheap extra wall wart, cut the hot wire in the
middle of the cable and hook the banana plugs directly on this break.
And yeah, the wall wart may be powering a number of regulators and so on and so forth, but that's fine.
·
RoboGeek
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"I reject your reality and subsitute my own!"
Adam Savage, Mythbusters
www.parallax.com
www.goldmine-elec.com
www.expresspcb.com
www.startrek.com
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The fact that the dtr line was active just didn't register with me when I was taking readings, or that the tx line might be being held high without me using hyperterminal or the prop IDE. In fact, this might fit in the stupidest thing I ever did with the prop thread.
Thanks Paul, Rokicki, Robogeek. I finished my honey doo list early enough last night to get in some prop time and I now have the circuit running. I will still have to convert my code to make use of the extra 4 bits resolution. It won't be too hard as my buffer was declared as longs to begin with, but the assembly routine isn't going to like it much. I may need to start from scratch on it.
One is rated at 1 amp, would two in parrellel double the current?
I have checked most of my resoures and finding a reg at exactly [url=mailto:5v@2amps]5v@2amps[/url] isn't easy! It is like everbody goes from 1.5 to 3 amps skipping anything close to 2.
I guess I'll have to try the LCD vendor, surely somebody has gone with a battery option before!
Anyway, many thanks for all the replies, the converter is running. It does a pretty good job dealing with the noise around my circuit!
If I have to climb to some high mountain in Tibet to get something to run this screen I guess I'll be climbing, but it sure would be nice to get em at the local radio shack.
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
I just printed that circuit off. It does not say how much extra current you can get, but it does look adjustable.
As many times as I have used the 7805 I had never looked at the data sheet for it. I had no idea you could do so much to it with external components. I happened to have the transistor it called for laying around collecting dust. It power the LCD just fine, except for the heat dissapation. I think I'll know try to roll my own buck mosfett to feed this circuit and maybe I won't brand myself when I go pull the transistor off the bread board.