Author Topic: Dave's power supply mods.  (Read 42548 times)

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Offline PeterG

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Re: Dave's power supply mods.
« Reply #25 on: January 09, 2012, 03:33:00 pm »
Is there a need for a 'Basic' version of this kit, with just the board and parts?
Even a board without the micro but with marked pins for a micro to be connected not even a case. This would make it more flexible for those who want to make there own custom psu's. In my case, a Keypad and maybe a 4x20 rgb lcd all running of an Arduino Mega1280 board.

Just an idea.

Regards
Testing one two three...
 

Online EEVblog

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Re: Dave's power supply mods.
« Reply #26 on: January 09, 2012, 03:47:07 pm »
Is there a need for a 'Basic' version of this kit, with just the board and parts?
Even a board without the micro but with marked pins for a micro to be connected not even a case. This would make it more flexible for those who want to make there own custom psu's. In my case, a Keypad and maybe a 4x20 rgb lcd all running of an Arduino Mega1280 board.

Yes, I expect some just to buy it for that.
Although the board is custom designed for my case, mounting holes mean it can be mounted in any bigger case on standoffs.

Dave.
 

Online firewalker

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Re: Dave's power supply mods.
« Reply #27 on: January 10, 2012, 10:47:21 am »
I made a quick lookup at DigiKey for another linear voltage regulator with down to zero capability and... Didn't found one!

I thought there would be more than LT308x.

Alexander.
Become a realist, stay a dreamer.

 

Offline JimmyM

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Re: Dave's power supply mods.
« Reply #28 on: January 11, 2012, 07:14:16 am »
I was thinking that if an additional opamp (call it Ux) were placed between the opamp U3A and the LT3080. The output of U3A could go into the non-inverting input of Ux and the inverting input of Ux was connected to a voltage divider at the very end of the output. The output of Ux would then be connected to the input of the LT3080 where U3A was connected. Any voltage drops caused by current sensing, etc would be compensated for. Am I wrong in how this would work? This could also give a kelvin sense capability (at least for the V+ wire)
Would this be worth looking into since the LT3080/3085 does not have an external voltage sense connection. Also, the LT3085 is capable of 3 Amps which would amplify voltage drops caused by any i resistance between the output of the LT3085 (or multiple LT3080s)
 

Online firewalker

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Re: Dave's power supply mods.
« Reply #29 on: January 11, 2012, 07:34:38 am »
I think you mean LT3083. LT3085 is 500 mA.

Alexander.
Become a realist, stay a dreamer.

 

Offline JimmyM

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Re: Dave's power supply mods.
« Reply #30 on: January 11, 2012, 10:01:09 am »
I think you mean LT3083. LT3085 is 500 mA.

Alexander.
yes. Right. Sorry.
 

Online EEVblog

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Re: Dave's power supply mods.
« Reply #31 on: January 11, 2012, 11:14:48 am »
I made a quick lookup at DigiKey for another linear voltage regulator with down to zero capability and... Didn't found one!

I thought there would be more than LT308x.

I think there are a couple of others from memory in some form, but that's what makes the LT308x special, unless you want to roll your own, or use negative supplies to compensate etc.

Dave.
 

alm

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Re: Dave's power supply mods.
« Reply #32 on: January 11, 2012, 05:17:36 pm »
I'm quite sure that Dave wouldn't have picked the LT3080 if it didn't have a significant advantage over the common parts. An experienced desigs engineer like Dave will avoid specialist, singe-sourced parts if possible. If the circuit would have worked just as well with an LM317, this would be the superior choice, due to the availability it's much easier and cheaper to source, so much less likely to cause supply issues. The LM317 won't go down to 0V however without adding extra complexity, hence the exotic LT3080.
 

Offline siliconmix

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Re: Dave's power supply mods.
« Reply #33 on: January 21, 2012, 08:17:31 am »
whay would you want to go down to zero ? ???
 

Offline IanB

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Re: Dave's power supply mods.
« Reply #34 on: January 21, 2012, 08:23:30 am »
whay would you want to go down to zero ? ???

For instance if you wanted to check the low battery function with a single AA cell you would want to go down to 0.6 or 0.7 V. Since an LM317 bottoms out at 1.25 V it would be unsuitable for this application.
I'm not an EE--what am I doing here?
 

Offline siliconmix

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Re: Dave's power supply mods.
« Reply #35 on: January 21, 2012, 09:01:09 am »
whay would you want to go down to zero ? ???

For instance if you wanted to check the low battery function with a single AA cell you would want to go down to 0.6 or 0.7 V. Since an LM317 bottoms out at 1.25 V it would be unsuitable for this application.
oops just watched the vid
I'm quite sure that Dave wouldn't have picked the LT3080 if it didn't have a significant advantage over the common parts. An experienced desigs engineer like Dave will avoid specialist, singe-sourced parts if possible. If the circuit would have worked just as well with an LM317, this would be the superior choice, due to the availability it's much easier and cheaper to source, so much less likely to cause supply issues. The LM317 won't go down to 0V however without adding extra complexity, hence the exotic LT3080.
 

Offline jpelczar

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Re: Dave's power supply mods.
« Reply #36 on: January 21, 2012, 11:16:39 pm »
Another idea after watching Makerbot episode: how about providing a connector for the ATX PSU ? There are lots for unused PSUs lying around. This would allow for cool feature: power on/off by tact switch ;)
 

Offline mobbarley

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Re: Dave's power supply mods.
« Reply #37 on: January 22, 2012, 12:00:18 am »
A bit bored today.. Do I win the title of the first mod?




PS: what have I missed?
 

Offline mobbarley

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Re: Dave's power supply mods.
« Reply #38 on: January 22, 2012, 03:25:06 pm »
eagerly awaiting part 8 with details of the pcb / case so that I know if this will fit.....  ;)
 

Offline senso

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Re: Dave's power supply mods.
« Reply #39 on: January 25, 2012, 06:39:39 am »
I have already created the necessary footprints for the max parts and for the lm3080 and copied the schematic to Eagle, but I'm using an atmega644p instead and the front panel with the rotary encoders, buttons and display will be included in a separated board.
 

Offline Rerouter

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Re: Dave's power supply mods.
« Reply #40 on: January 28, 2012, 03:34:13 pm »
Was just reviewing part 3 of the video and picked up on the fact that the constant current source only works down to 0.8V, but with daves external one, seemed to go down to 22mV,

it would seem with a fet with a 400mV threshold, and driven at the 2.048V reference would capable of handling down to the last 8 bits or 20mV, while it would sink 27mA under the highest voltage, i felt i would just raise the suggestion,

perhaps using another op amp, one could invert the gain of Vset and have it level out would be the solution to that,

also i realise there is little need to get that low, but at a guess atleast one of us must be interested in that capability,
 

Offline JimmyM

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Re: Dave's power supply mods.
« Reply #41 on: January 29, 2012, 12:28:40 am »
I have already created the necessary footprints for the max parts and for the lm3080 and copied the schematic to Eagle, but I'm using an atmega644p instead and the front panel with the rotary encoders, buttons and display will be included in a separated board.
Keeeeep talking. I like what I'm hearing.
 

Offline george graves

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Re: Dave's power supply mods.
« Reply #42 on: January 29, 2012, 01:31:49 am »
I hope there are some pre-set voltages.  Seems a waist to have a micro in there if it's not going to do the work for you.

Also - wouldn't mind a bigger screen, and big readable fonts.  Not that my eye sight is bad, it just makes it easier to read out of the corner of your eye, or at a quick glance.



Oh and a cool case - it's gotta have a cool looking case.  I wish there was a generic version of something like this available:



A keypad would be cool - but I'm not sure how useful it would be.


Or - I might make something out of wood and stain it - this guy used an old hard drive for the top and bottom.





« Last Edit: January 29, 2012, 06:23:15 am by george graves »
 

Offline Rerouter

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Re: Dave's power supply mods.
« Reply #43 on: January 29, 2012, 01:43:42 am »
the 4 buttons will be left to us to chose there purpose,

also are there any downsides, (oscillations, offsets, etc) from using mosfets to switch op amp gains,

as it would be nice, wanting higher voltage ranges, to switch down the gain as i fall below 10V and regain the original accuracy, 
 

Offline PA3BNX

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Re: Dave's power supply mods.
« Reply #44 on: January 29, 2012, 06:26:02 am »
Hello Every Body out there,


I have a question for Dave and his new designed powersupply.

Will it get a so called foldback current mechanism in software.

I mean if I short the power supply for longer as lets say three seconds

Will it switch the Current down to zero mA and the Voltage also to zero Volts?

So that I have to reset it before the Voltage comes up again ?
Greetings,

Lodewijk

Credo:

Home brew projects:
Build/Design  with minimum hardware
and maximal software.
 

Offline Rerouter

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Re: Dave's power supply mods.
« Reply #45 on: January 29, 2012, 07:50:42 am »
once again, i would think that would be something you might have to go enable, just because some people will want foldback and some will want it to hold, also the device itself will foldback at 1.2A, in thermal protection mode,
 

Online ejeffrey

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Re: Dave's power supply mods.
« Reply #46 on: January 29, 2012, 08:13:13 am »
Will it get a so called foldback current mechanism in software.

I mean if I short the power supply for longer as lets say three seconds

Will it switch the Current down to zero mA and the Voltage also to zero Volts?

So that I have to reset it before the Voltage comes up again ?

I am sure you could implement that, but foldback current limiting is not normally wanted in a lab supply and what is normally meant by it is not actually what you have described.

The normal purpose of foldback current limiting is to protect a fixed voltage linear power supply in case of a short circuit.  The problem is that when a load exceeds the current limit, the voltage will start to drop, but this increases the power dissipation in the pass transistor.  In the ultimate limit, when you have short circuited the output, the pass transistor(s) are dissipating more than 100% of the supply rating.  Bipolar transistors also have second breakdown which further limits their safe operating area.  Foldback current limiting uses a separate analog feedback loop to reduce the current limit and keep within the transistor SOA when the output voltage is below the setpoint.  Generally once the short is removed the supply will recover naturally -- it doesn't require a hard reset.  Of course, if the short circuit is caused by a crowbar circuit, it won't reset until the power is removed.

A software defined limit with a 3 second delay is not nearly fast enough to provide protection for the majority of circuits that need foldback current limiting.  That might help if your thermal limit is an undersized heatsink, but more often you have to deal with the package juction->case rating or second breakdown.  These time constants are quite fast, 3 seconds is much too long.

In any case, by design a bench supply must not be susceptible to such overloads.  The supply has to be able to survive being programmed to the lowest possible voltage and the highest possible current.  The transistor doesn't care whether the set point is 10 volts and drops to 1 volt due to the current limit, or whether the setpoint is 1 volt.

In any case, the LT3080 has built in thermal protection.  If you do manage to overload it, it will shut off to protect itself.  However, this would be a sign of a design error, or use of the supply outside its design parameters (i.e., too high ambient temperature).

If you want to protect the device under test rather than the supply, that would normally be considered overcurrent protection, and then you would generally design it to shut off the supply until reset.  There are not that many situations where you need overvoltage / overcurrent protection, and again, usually 3 seconds is way to long -- if you are powering a very delicate transducer or something, normally you would want to shut off the supply within microseconds.
 

Offline Rerouter

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Re: Dave's power supply mods.
« Reply #47 on: January 29, 2012, 08:19:41 am »
or in those cases, you would set the maximum current just above expected current of the device, so that it holds there, and will fall to near 0V if a bad short was made
 

Offline amspire

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Re: Dave's power supply mods.
« Reply #48 on: January 29, 2012, 09:48:48 am »
I don't think foldback current limiting has any place in a lab power supply.

If you set the current to 1A limit, you want it to limit to 1A, and not an unknown current that you haven't specified.

In using a power supply, I am using it regularly as a current source, and also in testing a new circuit or a faulty circuit, I want to be able to set the voltage and slowly wind up the current limit. Last thing I want is foldback decreasing the current as I am trying to increase it.

Foldback has a place in some fixed output voltage supplies where the regulators cannot maintain full current at 0V, but I would never build a variable lab supply that cannot maintain full current at 0V.

Richard.
 

Offline SeanB

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Re: Dave's power supply mods.
« Reply #49 on: January 30, 2012, 12:02:17 am »
I think it is pretty good as a low power PSU with a really accurate output voltage setting ability. If you wanted a higher rating then you would not need the resolution, and a higher current rating with lower resolution would be perfect. It is a precision supply, good enough to use as a 4 digit reference voltage, or a 3 digit current source. Would be good to build in a small case and have a companion ( same software and interfacing but coarser steps) lookalike that does 0-40V at 0 - 5A, possibly using a multitapped transformer to reduce dissipation in the regulator stage. You could even build them into a larger case if you made the precision one a floating unit, though them your interfaces would need to be opto isolated if you were going to have remote setting ability. a 2/4 port USB hub and 2 USB to RS 232 converters and a set of opto isolators would do that, then a single USB socket on the back will do. The extra USB ports could then be front panel mounted for convenience - extra points to connect a USB scope that is still isolated from the power supply rails.
 


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