#robotics Logs

Feb 17 2012

09:12 delinquentme http://functionaljobs.com/jobs/90-ai-programmer-clojure-at-zenrobotics-ltd
09:16 rue_more ooo it looks like kicad can run my plotter
09:16 delinquentme >_< rue!
09:16 rue_more placing components is a bit more labour
09:29 rue_more hmm I cant figure out how to move a component in kicad
09:30 rue_more aha
09:30 rue_more M
09:32 rue_more now how do I route a ratnest
09:32 e_more notices he's gonna be stupidly late for
12:04 thefinal any suggestion for a modular motor controller board (if that can be stacked to control n motors would be really great) for 24v dc motor?
12:05 Tom_itx how many motors?
12:05 thefinal 1-8
12:06 Tom_itx i'd look at what mesa has
12:07 Tom_itx http://www.mesanet.com/
12:07 thefinal are these board good ones? (sorry i haven't any experience with mesa
12:07 Tom_itx they support the 'emc' project which has been renamed linuxcnc
12:07 Tom_itx so yes
12:07 thefinal oh
12:08 thefinal that's good
12:08 thefinal many thanks
12:08 Tom_itx not your average 'cheap' crap
12:08 thefinal yep i'm not looking for cheap and crap ones… but want serious one...
12:09 Tom_itx ask pcw in #linuxcnc which one would suit your need
12:09 Tom_itx doesn't appear to be there right now but he works there
12:10 thefinal ok thanks
12:10 Tom_itx i'm sure others there could help as well
12:11 KongfuPanda hello robotics people !
12:24 thefinal just wondering about some hardware i have found… what robotic people think about this ones?
12:24 thefinal http://www.solutions-cubed.com/Solutions%20Cubed/MOTM3.htm
13:07 Tom_itx pcw just joined
14:06 Norhan hi all, I need help in my report on robots and want to know the names of the most Famous scientists in the field of robotics can you help me :)
14:09 peeps[work] you guys seen this? http://en.wikipedia.org/wiki/File:Robocoaster.jpg
14:20 LoRez I've seen stuff like it.
15:07 Norhan hi all, I need help in my report on robots and want to know the names of the most Famous scientists in the field of robotics can you help me :)
15:40 Norhan hi all, I need help in my report on robots and want to know the names of the most Famous scientists in the field of robotics can you help me :)
16:24 SolarNRG Do any of you have any ideas how I can separate my alternator? HELP. I've taken out all the bolts and it won't separate http://i.imgur.com/wdDM7.jpg
16:24 SolarNRG http://i.imgur.com/errFJ.jpg
16:25 Steffanx pass
16:53 Tom_itx SolarNRG, pop it lightly around the seam with a hammer
16:53 Tom_itx it may just be stuck
16:53 Tom_itx LIGHTLY
16:54 Tom_itx also try prying between the screw mounts with a screwdriver
16:54 Tom_itx after all that's what screwdrivers are made for
16:54 Steffanx :P
16:56 Tom_itx i think i also see a dowel pin locating the halves, it may be stuck there too
16:56 Tom_itx and if that doesn't get it, get a much bigger hammer and beat the living shit out of it
16:57 SolarNRG I ended up snapping what I think is the rectifyer off the back
16:57 Tom_itx bad idea
16:57 SolarNRG Its like a plasticy thing with 3 things firmly attached to 3 wires
16:58 SolarNRG I have no wire cutters
16:58 Tom_itx yup, you can get another one
16:58 SolarNRG So I ripped it off
16:58 Tom_itx usually all you do to those is put a new diode pack on it and replace the bearings
16:58 SolarNRG Aren't the bearings magnetic?
16:59 Tom_itx why would they be?
16:59 SolarNRG because that alternator needs to turn round at one helluvaspeed
16:59 SolarNRG magnetic bearings make little resistance
16:59 SolarNRG friction losses
17:00 Tom_itx not unless it's off a bentley or some such
17:00 Tom_itx they wouldn't bother
17:01 SolarNRG there goes my idea of a steam turbine direct driving two alternators and reusing the magnetic bearings
17:03 SolarNRG So what can I do if I've trashed the rectifier?
17:03 SolarNRG I mean I want to change the windings anyway
17:03 SolarNRG So it generates power at a lower spin speed
17:03 Tom_itx go get another one
17:04 SolarNRG Get some wire cutters next time
20:38 peepsalot i'm trying to thread a rod using a die but it doesn't seem to want to cut. i think my cheapo die is softer than the rod. what would be the best kind of die to use?
20:39 Tom_itx a good one
20:39 Tom_itx taper the end of the rod to help get it started
20:40 peepsalot do they make them in carbide?
20:40 Triffid_Hunter Tom_itx: he's trying to tap a motor shaft, possibly hardened
20:40 Tom_itx oh
20:40 Tom_itx peepsalot, i haven't seen any
20:40 Tom_itx i'm sure they'd be expensive if they did
20:41 Tom_itx at least case hardened
20:41 peepsalot this die set just says it is carbon steel
20:42 peepsalot "do not use on hardened materials"
20:42 Tom_itx better listen to it
20:42 peepsalot the thought of how hard the dies were didn't cross my mind when i got it
20:42 peepsalot Tom_itx, well, i don't know if it's ahrdened or not
20:42 Tom_itx get a good greenfield die or something similar
20:43 peepsalot i'll taper the end a bit and see if that helps
20:43 Tom_itx you still might chip the teeth if the shaft is too hard
20:43 Tom_itx you're likely to just screw up the shaft if the die isn't hard enough for it
20:53 peepsalot grr, need a lathe
20:55 Tom_itx why does it need to be threaded?
20:55 Tom_itx is there an alternate method to your design?
20:55 peepsalot yeah i could put a coupler on it, but this would make the whole thing more compact
20:56 peepsalot i want to try to use it in a worm drive
21:03 pnorman if it's hardened, you're not going to be able to machine it really
21:05 peepsalot how do i know if it is
21:08 Triffid_Hunter peepsalot: if a carbide die can't make a decent dent in it, pretty sure it's hardened
21:08 peepsalot i don't have carbid die
21:08 peepsalot just chinese carbon steel
21:08 Tom_itx he's got one that says: "do not use on hardened materials"
21:09 peepsalot unfortunately carbon != carbide
21:11 Triffid_Hunter yeah I misinterpreted
21:13 peepsalot if i'm gonna bother with a coupler i'd like to have one that went up to a larger diameter. but i don't seem many rigid couplers with different sized ends. lots of flexible ones like that though
21:14 peepsalot i guess i'd have to machine that myself too
21:17 peepsalot hrm, sdp-si has some. bit pricy though
21:19 Triffid_Hunter peepsalot: yeah you pay a lot for dies that can cut hardened steel :P
21:21 peepsalot Triffid_Hunter, i mean for a coupler instead threading the motor shaft. no idea where to get hardened dies. the dies i see are all either carbon steel or HSS
21:22 peepsalot is HSS considered very hard?
21:22 Triffid_Hunter peepsalot: print a coupler :P
21:22 Triffid_Hunter peepsalot: no
21:41 pnorman If you want a hardened shaft with threading you'd generally thread, then harden it
21:42 rue_more drillrod
21:44 Tom_itx rue_more what are you working on?
21:44 rue_more waking up from the nap
21:44 rue_more going over the kicad stuff a bit more
21:44 Tom_itx i think i dozed off here for a bit
21:44 rue_more thinking what to do next
21:45 Tom_itx why are you ditching eagle?
21:45 rue_more eagle is not the future
21:45 rue_more people are gonna stop using it soon
21:45 Tom_itx what makes you say that?
21:46 rue_more the owners of eagle are greedy and its quickly losing favor
21:46 Tom_itx since element 14 got them?
21:46 rue_more how much would you pay for software to do hobby stuff?
21:46 Tom_itx i haven't looked at new versions
21:46 rue_more exactly
21:47 Tom_itx well, for cad cam i paid $8k
21:47 rue_more eagle went stagnent about 5 years ago
21:47 Tom_itx it paid for itself in a year and i've been using it ever since
21:47 rue_more your semicommercial tho
21:47 rue_more I'm talking just hobbyists
21:47 rue_more people who dont even dream they have as much as 8k
21:47 Tom_itx i know
21:48 Tom_itx i saw an opportunity and took it was all
21:48 rue_more the market goes with the hobbyists
21:48 Tom_itx just like my programmers.. it keeps me in the hobby and helps support it
21:48 rue_more cause its the hobbyists that form the new business
21:49 rue_more if eagle dosn't stay with the hobbyists, its dead
21:49 rue_more kicad is where its at now
21:49 rue_more eagle was never technically free
21:50 rue_more kicad will have 3 major forks in the next 5 years
21:50 Tom_itx what's the gui like?
21:50 rue_more one of them will be pretty slick
21:50 rue_more its in need of some fine tuning
21:50 rue_more and its all a bit too stickshift right now
21:51 Tom_itx i'm ok with a clutch
21:51 Tom_itx :)
21:51 rue_more me too, 5 and 4 stickshift takes some getting used to tho
21:52 rue_more esp if it dosn't have a synchonizer
21:52 Tom_itx i should update my partlist to see what i've used up
21:52 rue_more did you ever finish you database?
21:52 Tom_itx no, i'm still using a spreadsheet
21:52 rue_more ah
21:53 rue_more is the design of the spreadsheet stable?
21:53 Tom_itx but it works ok for now
21:53 Tom_itx so so
21:55 rue_more I should go confirm the next part of the overload baord for buddy
21:55 Tom_itx what's the kids doing tomorrow?
21:55 rue_more last night I worked out that running the amps off a 5V power supply for the current sense wont work
21:55 rue_more one is doing computer input via BASIC for a switch flipping robot
21:56 rue_more other is programming for robotic arm
21:56 rue_more working on his waypoint clode
21:56 rue_more code
21:58 rue_more complex brain error: that l was intentionally inserted
21:58 rue_more cl
21:58 rue_more its attached to the c
21:58 rue_more code
21:58 rue_more code
21:58 rue_more code
21:59 rue_more code code code code code code code code
21:59 rue_more I think I unlinked it
22:00 rue_more I should check on buddys battery pack thats charging
22:00 rue_more I have 2 fully charged so far
22:00 rue_more looks like about 3 cells need replace so far
22:00 rue_more I should make a spare battery pack too
22:01 rue_more then I'm gonna have to make the cell balancer, that bit is trickey
22:01 rue_more balancer for a 6series x 8 parallel li-ion grid
22:02 rue_more 8A 24V
22:02 rue_more 192W
22:02 rue_more should be about 36Ah
22:02 rue_more er no 18Ah
22:03 rue_more 18Ah 24V
22:04 rue_more with an inverter that can take about 18A (24V to -24V)
22:05 rue_more I'm using parallel ferrite cores for that...
22:06 jadew hey guys, I have a little dilema
22:06 jadew http://imagebin.org/199498
22:06 rue_more you still trying to make apower supply?
22:06 jadew in order to drive 1amp, I would have to feed on the non inverting input a VCC - 1V voltage
22:06 jadew rue_more, heh yeah :)
22:07 rue_more your feedback has to come off your load
22:07 rue_more or is that supposed to be voltage or current feedback?
22:07 jadew voltage to current
22:08 rue_more no, is that circuit supposed to control voltage or current
22:08 jadew basically 1v drop on R SENSE should be 1 amp across the load
22:08 rue_more or is that a transconductance amplifier?
22:08 jadew I think that's how it's called
22:08 jadew (I'm a noob, sorry)
22:08 Tom_itx rue will fix that
22:08 rue_more what do you want to happen, describe it as a black box
22:09 rue_more voltage goes in, current comes out?
22:09 jadew what I want to happen is feed 0-1v from my mcu and have 0-1 amp out
22:09 jadew yes
22:09 rue_more 1V in = 1A thru the load? regardless of voltage?
22:09 jadew yep
22:09 rue_more ok
22:09 rue_more next does the load have to be attached to ground?
22:09 jadew yes, that's the most important thing
22:09 rue_more ok
22:10 rue_more let me think for a sec
22:10 jadew with RSENSE between ground and load, it would work fine, however I would run the risk of shorting RSENSE
22:11 jadew when I'd attach another circuit
22:11 rue_more your right you have a reference problem
22:11 jadew the problem with my design is that in order to provide 0-1 amps, I would have to push VCC - 1V to VCC, in the noono inverting input, which I don't know how to do
22:11 rue_more let me see if I know how to reflect it
22:12 rue_more whats the max current output you want?
22:12 jadew 1 amp
22:15 rue_more Il = Vset+0.7/Rsense ...
22:16 rue_more 1R = 1V
22:16 rue_more whats the transistor gain?
22:16 rue_more Hfe
22:17 jadew about 200 I think, let me check
22:17 rue_more 5 or 200?
22:17 rue_more heh
22:18 rue_more 1/200 = 5mA
22:18 jadew scratch that, it's 750 or something
22:18 jadew it's a darlington
22:18 rue_more k
22:18 jadew I will use a resistor between the op amp and it's base in the real circuit
22:19 rue_more so Vd = 1.4V
22:19 jadew what is Vd?
22:19 Tom_itx something you don't wanna catch
22:20 jadew I was thinking I could somehow replace V2 in this schematic: http://imagebin.org/199500
22:20 Tom_itx voltage drop
22:21 rue_more 12V/5mA
22:21 jadew with something else and then be able to bring it to VCC with an outside voltage
22:21 rue_more 2.4k
22:21 jadew rue_more, the problem here is that I don't have a lot of control over V1
22:21 rue_more 1k will do
22:21 jadew it could be 12 it could be 15v
22:21 jadew the power source I'm using is rated 9v and it outputs 14
22:22 rue_more I did it
22:22 jadew how?
22:22 rue_more I reflected the control voltage
22:23 jadew how do you do that?
22:26 rue_more 1 min
22:27 rue_more wow, I can carry on a complex phone conversation while designing a circuit and come out of it with no idea what I said
22:27 Tom_itx average conversation is kinda boreing isn't it?
22:28 jadew I always forget what I talk to people
22:28 rue_more ok I think I can unload the camera now
22:29 rue_more ok image on its way..
22:29 jadew nice, thanks
22:29 rue_more jadew, you have to add to op-amp to this yourself
22:29 rue_more you will see what I did tho
22:30 rue_more !assist images/voltreflect.jpg
22:30 tobbor Possibly http://eds.dyndns.org/~ircjunk/images/voltreflect.jpg
22:31 rue_more now its all based off ground
22:33 rue_more see how it works?
22:33 jadew trying to understand it
22:33 rue_more the top 1K and the bottom 1K are in series
22:33 rue_more so they have the same voltage drop
22:34 rue_more the transistor is set up as a current driver, the emitter current is (the base votlage +- BE voltage)/Rsense
22:35 rue_more if the voltage on the bottom 1K is 1V the voltage on the top 1K will be 1V
22:35 jadew I see
22:35 rue_more (ideal transistor, no BE drop) so the emitter current is 1V/1Ohm = 1Amp
22:36 rue_more the collector current is the same as the emitter current
22:36 rue_more so the load gets 1A
22:36 jadew that's nice, thanks a lot
22:36 rue_more you will need to adust for the BE drop
22:37 rue_more its a addition offset, so you cant just change the resistors
22:37 rue_more you might be abel to add a diode in there to match the drop
22:38 jadew I'll try it out in the simulator
22:38 rue_more I think if you put a diode in series with the line comming off the bottom 1k it corrects it
22:38 rue_more not sure...
22:39 rue_more two diodes for a darlington
22:39 rue_more the diodes shoudl be thermally connected to the output transistor too
22:40 rue_more hmm yea I think the diodes either go in series with the bottom 1k or in series to the op-amp
22:45 rue_more yoru simulator should work it out quick
22:45 rue_more its not happening in my head
22:45 rue_more it looks like the diodes would go between ground and the bottom 1k
22:46 rue_more cause that would add the forward voltage thats lost in the BE
22:46 rue_more its giving me trouble cause altho the voltages goes up, the voltage difference goes down
22:47 rue_more jadew, well?
22:47 jadew I thought I get it, but I dont
22:48 jadew my real vref should go in the left transistor's base, right?
22:49 rue_more no
22:49 rue_more you have to put an opamp that outputs to the left transistor and uses the 1K as feedback
22:50 jadew ah
22:50 rue_more the voltage off the bottom 1k is almost the same as your load current
22:50 jadew now that makes sense
22:50 rue_more +-Vbe
22:50 rue_more of the driver transistor
22:51 jadew yeah, that works
22:51 rue_more it working?
22:51 jadew yep
22:51 rue_more now its slightly out right?
22:51 jadew yep, by 1mA
22:51 jadew 10
22:51 jadew sorry
22:52 jadew 10mA
22:52 rue_more to fix the error you need to add a diode
22:52 rue_more try putting a diode in series with the bottom 1k, between it and ground
22:53 jadew it's off by a lot more now
22:53 jadew hold on
22:53 rue_more ah , wrong way then
22:54 rue_more try putting it between the top of the 1k and the op-amp then
22:54 rue_more top of the bottom 1k
22:54 rue_more to drop the percieved feedback voltage
22:55 jadew still off by a lot
22:55 rue_more off by more of less?
22:56 jadew didn't check
22:56 rue_more I think those diode locations should cause opposite effects
22:56 rue_more one should make it worse, the other better
22:57 rue_more if its a darlington, you need to use 2 diodes to mimmic the Vbe drop
22:57 jadew I used a simple NPN for the one on the left
22:57 rue_more thats fine
22:58 rue_more the error is from the voltage drop across the right transistors BE
22:58 jadew so with the diode between bottom res and ground, I get 0.42A
22:59 rue_more !assist projects/heatplant/tempsens.jpg
22:59 tobbor Possibly http://eds.dyndns.org/~ircjunk/projects/heatplant/tempsens.jpg
22:59 rue_more new i2c temp sensors! :)
22:59 rue_more !assist robots/switchrobot.jpg
22:59 tobbor Possibly http://eds.dyndns.org/~ircjunk/robots/switchrobot.jpg
22:59 jadew what are you using them for?
23:00 rue_more heating system
23:00 rue_more switch robot is a sad excuse for a robot
23:00 rue_more but its good at educating the basics
23:01 jadew with the diode between the emitor and the bottom resistor + op amp junction, I get 0.99, so the same thing
23:01 rue_more jadew, what is the error with no diode(s) and with the diode in
23:01 rue_more .99A?
23:01 jadew yeah
23:01 rue_more opposed to .42A
23:01 rue_more so thats the right way
23:02 rue_more oh wait, whats it with no diode?
23:02 jadew 0.99
23:02 rue_more oh wrong place
23:02 rue_more hehe
23:02 rue_more ok put the diode in series with the op-amp input
23:03 rue_more I dont mean to dissapoint you but the real world is gonna make mincemeat of your simulated design
23:04 jadew heh, I'm sure :)
23:04 jadew I get 0.25 on the opamp input
23:04 e_more scratches
23:04 rue_more oh oh I know
23:04 rue_more put the diode between the top 1K and +12V
23:05 rue_more yea!
23:05 rue_more that makes it symmetric
23:05 rue_more why didn't I think of that earlier
23:05 Triffid_Hunter why not mirror the sense signal down to ground?
23:05 rue_more Triffid_Hunter, ....
23:05 rue_more haha
23:05 rue_more uh
23:06 rue_more cause the complex solution is best?
23:06 rue_more Triffid_Hunter, :/ thanks...
23:06 rue_more Triffid_Hunter, you gonna draw it for him or do I ahve to?
23:06 jadew 1.61 with the diode at the top
23:06 rue_more oh yea!
23:06 rue_more hmm
23:06 rue_more ok
23:06 rue_more jadew, in the simulator
23:07 rue_more use another of the exact same transistor as the output driver
23:07 rue_more but use the BE as a diode
23:07 rue_more leave the collector not attached
23:07 rue_more or if you have to connect the collector to the base
23:08 jadew 1.55
23:08 rue_more huh
23:08 rue_more touchy think
23:08 rue_more g
23:08 rue_more ok, Triffid_Hunter was right
23:08 rue_more we can so something else
23:08 e_more scratches
23:09 e_more looks at Triffid_Hun
23:09 Triffid_Hunter rue_more: I'm having a fiddle, feel free to race me
23:09 rue_more ugh
23:09 rue_more your thinking current mirror to a resistor off ground?
23:09 rue_more I can do it with an op-amp but thats got to be overkill
23:10 e_more tries to remmeber how transistor current mirrors
23:10 jadew I will have another opamp free on the same chip
23:10 jadew so if the opamp version is better, we can go with that
23:11 rue_more well, I dont know about better, but I know how to do it
23:13 rue_more its just a high side current sense amp
23:13 jadew I think I can come up with that one too
23:13 jadew let me see
23:16 rue_more ok
23:18 Triffid_Hunter rue_more: tell me what you think of http://triffid-hunter.no-ip.info/high-side-sense.png just as a first draft
23:18 Triffid_Hunter will that work or am I crazy?
23:18 rue_more hmm
23:18 rue_more I'm not sure
23:19 rue_more at a glance it looks plausable
23:19 rue_more its good of you to reduce the sense to .1
23:19 Triffid_Hunter yeah, I hate burning off power un-necessarily
23:19 Triffid_Hunter R3:R4 ratio should be gain if I'm thinking clearly
23:20 Triffid_Hunter although R2 is involved somehow too
23:20 jadew I can't come up with a working schematic
23:20 Triffid_Hunter jadew: plug in the one I just posted, let us know what it does :)
23:20 rue_more I have something simular to Triffid_Hunter but simpler
23:20 Aggrav8d hey guys.
23:20 Triffid_Hunter rue_more: do show :)
23:21 rue_more http://t2.gstatic.com/images?q=tbn:ANd9GcQyYxoDB8dbKctlzjnSNLOUE92DgwjWqLs5R28h8dKuwTZ3_2ZiOQ
23:21 rue_more its based on that
23:21 rue_more 1 min
23:22 rue_more !assist images/curreg2.jpg
23:22 tobbor Possibly http://eds.dyndns.org/~ircjunk/images/curreg2.jpg
23:22 rue_more no...
23:22 rue_more 1 min
23:23 Triffid_Hunter don't forget the Vbe changes with current
23:23 rue_more refresh
23:23 rue_more http://eds.dyndns.org/~ircjunk/images/curreg2.jpg
23:24 rue_more I know the current sense circuit works, I was just playing with it a few days ago
23:24 rue_more Aggrav8d, hi
23:24 jadew interresting
23:25 rue_more its a different version of the same thing
23:25 Triffid_Hunter yeah that should work
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23:26 rue_more oh god
23:26 Triffid_Hunter rue_more: heh 'copy image' not your friend ;)
23:26 rue_more http://i.cmpnet.com/planetanalog/2009/02/C0363-Figure4.gif <-- friendly url!
23:27 rue_more but instead of the current mirror you use a current to voltage converter
23:27 rue_more and skip the buffer
23:27 rue_more :)
23:27 Triffid_Hunter heh "current to voltage converter" seems a convoluted way to say 'resistor' :P
23:27 rue_more :)
23:28 Triffid_Hunter jadew: go with http://eds.dyndns.org/~ircjunk/images/curreg2.jpg
23:28 Triffid_Hunter although I do suggest using an 0.1R sense resistor
23:28 jadew I'm trying it now
23:28 jadew Triffid_Hunter, I only have 1R
23:29 Triffid_Hunter jadew: it'll dissipate 1 watt at full power, prepare for it to be fairly warm ;)
23:29 jadew actually, I might have some 0.1
23:29 rue_more yes, you can change the bottom resistor to 10k to compensate for only having 0.1R
23:29 jadew yeah, I bought 10w resistors on my last order :)
23:29 rue_more Triffid_Hunter, but a resistor is a voltage to current converter...
23:30 Triffid_Hunter rue_more: they swing both ways :P
23:30 jadew yeah, I do have 10w 0.1 ones
23:31 Triffid_Hunter jadew: 0.1R will only dissipate 0.1w with 1A going through, a standard 1/4 one would be fine ;)
23:31 Triffid_Hunter and 0.1v is still plenty of sense when op-amps are involved
23:31 jadew oh, that's nice
23:32 Triffid_Hunter jadew: so use your 10w if you like, and it'll be stone cold :)
23:32 rue_more Triffid_Hunter, WHATS THAT? TWO OPERATIONS FOR THE PRICE OF ONE!! __BUT WAIT__ YOU GET MORE!
23:34 rue_more I should go check my li-ion pack
23:34 jadew ok, tried that schematics, it outputs 0.23A no matter the vref
23:35 Triffid_Hunter jadew: did you get the input op-amp the right way around?
23:35 jadew screenshot comming
23:36 Triffid_Hunter jadew: I'd use an NPN for output with this, but a PNP should work just as well with the drive logic inverted
23:36 jadew http://imagebin.org/199504
23:37 Triffid_Hunter jadew: yeah you got it inverted, swap the + and - inputs on U1A
23:37 jadew I used a npn one (I have more of those anyway)
23:38 Triffid_Hunter jadew: remember, the voltage across R4 is equal to the voltage across R1
23:38 Triffid_Hunter jadew: ah well then switch Q1 for an NPN in your sim
23:38 Triffid_Hunter and leave the op-amp alone
23:38 jadew and I get 0.99V drop on the sense resistor but 1A on the load
23:38 jadew how does that work?
23:38 Triffid_Hunter jadew: should be right with an NPN.. with a PNP, you're measuring base current as well as load
23:39 jadew yeah, it works with NPN
23:39 jadew but why do I have 0.99v on R SENSE, yet I get corrent current on the load?
23:40 Triffid_Hunter jadew: base current again
23:40 jadew http://imagebin.org/199505
23:41 Triffid_Hunter jadew: yeah put a current sense in series with R1 and another current sense on Q1's base
23:41 Triffid_Hunter I bet R1 will see 0.99A and there'll be another 0.01A or so from the base
23:42 Triffid_Hunter ooh I know! swap R1 and Q1 around, so you're sensing the emitter current rather than the collector current
23:42 Triffid_Hunter only problem then is it'll supply 100% power if you short the output
23:42 Triffid_Hunter jadew: so, time to choose your compromise ;)
23:43 jadew I don't get a voltage drop if I put another sense res between Q1's base and the op amp
23:43 jadew at least nothing over 0.01v
23:43 Triffid_Hunter jadew: yeah, probably gets lost in the rounding
23:44 Triffid_Hunter that's a darlington right? it'll have pretty high gain
23:44 Triffid_Hunter may only need 10mA or less to drive 1A output
23:44 rue_shop2 .99 is prolly the difference caused byt eh few ma taken off fromt eh sense circuit
23:44 rue_shop2 expand it to 5 places and I bet you find its .999980
23:45 Triffid_Hunter rue_more: sense will only take whatever the op-amp input impedance is.. with a jfet op-amp it should be completely negligible. LM324 is pretty crappy all round, but they're cheap and ubiquitous
23:46 rue_shop2 ok then, .99999999980
23:46 jadew well, for 1V of input I get 995mA out
23:47 Triffid_Hunter jadew: 0.5% is pretty good.. if you want better I think you'll need a much more elaborate circuit
23:47 Aggrav8d http://www.newwoodworker.com/basic/kickback2012.html
23:47 jadew yeah, I guess I'll have to live with it
23:48 jadew so either this, or have R SENSE between load and ground
23:48 jadew which I don't really like
23:48 jadew the circuit will be fed from a separate transformer, nothing else will be connected to that transformer
23:49 jadew do you think I could still run into trouble if I connect the ground of another circuit with the output ground from this one?
23:50 Triffid_Hunter jadew: you could adjust R4 slightly
23:50 jadew not talking about this circuit, I'm just considering my options
23:50 jadew it's either this, which has proper current limiting on vcc, but it has a slight error
23:51 Triffid_Hunter jadew: with a regular transformer? naw hook it up however you like. gets tricky with switchers though
23:51 jadew or the simpler one, with no error, that has rsense between ground and load, let me show you
23:51 Triffid_Hunter jadew: I think you can adjust R4 to compensate the error. the amount to change it will depend on Q1's gain
23:52 jadew http://sub.allaboutcircuits.com/images/03039.png this would be the other option
23:52 Triffid_Hunter jadew: maybe a 100k potentiometer in parallel or something
23:52 jadew gonna give it a try
23:52 rue_shop2 whats the headroom on a lm324?
23:52 Triffid_Hunter jadew: ah, the old low side sense
23:52 Triffid_Hunter rue_more: pretty dreadful, I wouldn't use it on the +ve rail like that
23:53 rue_shop2 yea
23:53 jadew what's headroom?
23:53 rue_shop2 a typical op-amps output cant get to the + or - rail
23:54 rue_shop2 Triffid_Hunter, have a fav rail to rail amp?
23:54 Triffid_Hunter some won't sense near the rails either
23:54 Triffid_Hunter rue_shop2: not off the top of my head
23:54 rue_shop2 hmm
23:54 Triffid_Hunter most jfet input should sense rails
23:54 jadew ah, I know, I have some rail to rail opamps
23:54 rue_shop2 what part nums?
23:55 jadew tlc2272cp
23:56 Triffid_Hunter jadew: those look nice
23:56 Triffid_Hunter 16v max though, watch out for that
23:57 jadew thanks, didn't notice that
23:59 Triffid_Hunter jadew: also, datasheet says don't put inputs within 1.5v of VDD