Author Topic: Lab Power Supply Design Part 5  (Read 44542 times)

0 Members and 1 Guest are viewing this topic.

Offline Anders

  • Contributor
  • Posts: 34
  • Country: se
Re: Lab Power Supply Design Part 5
« Reply #50 on: January 05, 2012, 07:44:13 am »
Why use so many peripheral chips when you can save space and money by using for example a reliable PIC18F46J50 or similar which got most of it onboard?
Just drop the “full of nasty surprises” Atmel chip.

Why don't you just do your own design and kit, if you know how to do it right?

Freeloading and a general lack of appreciation for the contributions that Dave has graced the community with, obviously. ::)

No, that’s not at all what it is! Maybe you like it to be but it’s not.
If we can’t discuss technical things and issues on this forum without getting personal about it what’s the point?
Proper Planning Prevents Piss Poor Performance
 

Offline Anders

  • Contributor
  • Posts: 34
  • Country: se
Re: Lab Power Supply Design Part 5
« Reply #51 on: January 05, 2012, 08:16:10 am »
Just wondering;

I’m not an expert, far from it but why reinvent the wheel?
Why use so many peripheral chips when you can save space and money by using for example a reliable PIC18F46J50 or similar which got most of it onboard?
Just drop the “full of nasty surprises” Atmel chip.

PIC18F46J50 only has 10bit ADCs. Atmel does the ATxMega series with 12bit ADC and DACs, but 1) they aren't available in a DIP package (Dave's making kits that customers assemble themselves) 2) Have you ever tried to reliably get stock of some of the Atmel chips.... its a bit of a joke sometimes. Not good for an open source design if nobody can get parts!

OK, I made a mistake picked the 46J50 from memory when I should have picked the PIC18F46K80 part as an example. Or if USB is preferred the 18F4553 or 18F4458 however these two only runs at 48MHz.

I got very disappointed with Atmel, their products and the availability and had since moved to Microchip and I’m very pleased with the decision. I believe I get more bang for the bucks and I always find and get what I want and in time.
Proper Planning Prevents Piss Poor Performance
 

Offline hans

  • Super Contributor
  • ***
  • Posts: 1035
  • Country: nl
Re: Lab Power Supply Design Part 5
« Reply #52 on: January 05, 2012, 08:52:24 am »
Just wondering;

I’m not an expert, far from it but why reinvent the wheel?
Why use so many peripheral chips when you can save space and money by using for example a reliable PIC18F46J50 or similar which got most of it onboard?
Just drop the “full of nasty surprises” Atmel chip.

Don't say that! ALL chips have nasty surprises. I am a fond PIC user myself, but I have had projects where 5 patch wires were required because several GPIO was an I-only  (or USB communication). Kinda annoying when you wire up the symbol saying 'D+/RD4' (or something). This also happened for the oscillator pins, apparently not capable of doing output kinda stuff.
Yeah, should have read the datasheet, but it should be clear from the pin-out and I/O table diagrams what I can use as an I/O and what not.
This also applies to some Altera FPGA's, where it said something like 148I/O pins on the datasheet. But as 10 of them were clock, it's more like '138 I/O, 10 optional I pins'. Unfortunately this project required 140 O's. Too bad the chips were already ordered..


Also, as Dave mentioned in the video, an internal ADC/DAC is nearly not as good as an external one. The Microchip parts are pretty bad at specificying noise, but the ADC used has an SNR of about 72dB there abouts. Considering THD is not an issue, you get about 11.6 effective number of bits. This means oversampling by 2-3x gets you 'noise free' readings (not free, but atleast not much left of it). I don't think an internal ADC can improve on that, in fact.. I've seen internal ADC's dancing around on 10-bits.

Also note that a direct USB connection is not nice, because you don't have an isolated supply any more.

I plan on building my own PC accessible dual  power supply, but with some tweaked specs. I think 10.24V out is too low, rather see 18V maximum. I will probably be using a higher specified ADC for the readouts so I can get even more accuracy on current and voltage. Especially the current consumption functionally is neat, as I was designing a device that I approached the other way around (good current measuring with internal supply options).
I don't think the uA range of 10mA may be wide enough, I'd rather get up to 20 - 25mA with a resolution of <1uA. Yes, that's about >20k counts, but a 16-bit ADC + oversampling should cope. 
Even more nicely would be to add an 'oscilloscope'-like feature to the PC (so USB communication actually has a good use!) so I can see the power consumption real time. It will require fast ADC's and a lot of post proccessing.

Will have to see how it goes. Especially streaming data to PC, isolated and fast is kinda hard to do. FTDI chips can only run so fast (like 2M-3Mbaud), so may have to resort to a FT232H chip (USB2.0 ,  up to 12MBaud).
Auto ranging the uCurrent range is only useful when you can also lock it to on/off. If you have a device with a modem drawing 1Amp peaks during transmit, you don't want the PSU to be smart and think "hey I can switch to low current measuring range", but isn't responsive enough on the transients (how fast is checked whether it's not clipping? once every 10ms?)

But I guess this goes far beyond the average user this kit is meant for, as the uA measurement is a nice extra, and the rest of the supply is more than sufficient for someone playing with microcontrollers..
 

Offline EEVblog

  • Administrator
  • *****
  • Posts: 29222
  • Country: au
    • EEVblog
Re: Lab Power Supply Design Part 5
« Reply #53 on: January 05, 2012, 08:58:07 am »
Using the external ADC and DAC gives builders not only the choice in their ADC (Microchip make pin compatible 10 and 12 bit versions), but also decouples the design from any particular processor or even brand, and lets people play with and maybe learn about some SPI devices.
It's not a bad choice.

Dave.
 

Offline Ajahn Lambda

  • Regular Contributor
  • *
  • Posts: 98
  • Country: us
  • quecksilberdampfgleichrichter
Re: Lab Power Supply Design Part 5
« Reply #54 on: January 05, 2012, 09:35:07 am »

I just have to say it's a bit of a moot point to argue with a designer after he's already chosen the micro with which he's going to control a system.  Dave, I'm not gonna question your choice; after all, it's your baby, you're the one that's gotta make it work, and you know what you're doing, so I'll just sit back and wait for the final product.   :)

I got very disappointed with Atmel, their products and the availability and had since moved to Microchip and I’m very pleased with the decision. I believe I get more bang for the bucks and I always find and get what I want and in time.


It's almost as varied as products in the auto industry, regarding µCs: each brand and line has its ups and downs, and there is no "one solves all problems" chip.  The combination of hardware qualities, development environment, support, tool availability/openness, part availability, etc., i.e. the 'biosphere' surrounding a chip, all play into the strengths and weaknesses of each.


Having said that, I'm still using AVRs because that's what I started with, there are many free and easy-to-use tools for the platform, and it's what I know best due to familiarity.  NOT the f**king Arduino platform.  Yes, there are many 'gotchas', but they're not show-stoppers if you're creative and RTFD.  Microchip has rapidly learned why AVRs have such a rabid fan base, and their lineup reflects that knowledge, so I may give them another go when I have the opportunity.  I've had the inkling to try out an ARM, a ColdFire, as well as a few oddball FPGAs, in addition to the PIC line.  Just for kicks, I've tried my hand at some assembly on a VERY old 8051 recently.  Wow, THAT was painful, but very enlightening!


My first experience with Microchip was in 2007; it was expensive and not much fun, involving MPLab and a PIC24F controlling a 3? BLDC CNC motor.  Yes, my first endeavor in embedded system design was a trial by fire...or at least that's how I remember it.  Nevertheless, I'm an intelligent guy (ha!), so I can accept that my point of reference may have been skewed by inexperience, stress, and 36-hour days.
I am not a smatr poney.
 

Offline slateraptor

  • Frequent Contributor
  • **
  • Posts: 833
  • Country: us
Re: Lab Power Supply Design Part 5
« Reply #55 on: January 05, 2012, 10:45:46 am »
No, that’s not at all what it is! Maybe you like it to be but it’s not.
If we can’t discuss technical things and issues on this forum without getting personal about it what’s the point?

I was being facetious, hence ::) Besides, the fact that you mentioned a chip down to its exact part number was, well, rather suggestive. ::)
 

Offline Rerouter

  • Super Contributor
  • ***
  • Posts: 4219
  • Country: au
  • Question Everything... Except This Statement
Re: Lab Power Supply Design Part 5
« Reply #56 on: January 05, 2012, 11:15:45 am »
just felt i would point out that this schematic is limited to a maximum input voltage of 14V due to IC's u3 and u12, apart from that it seems that all anyone would need to do is change the op amps and gains and they could run this supply upto ~32V (better to be safe than sorry)

dave any chance of a higher rated op amp after your proto board for the kit, being alot of the people that buy this kit will probably not catch on to that limitation, and hook it upto some 15V plug supply and blow them
 

Offline IanB

  • Super Contributor
  • ***
  • Posts: 9485
  • Country: us
Re: Lab Power Supply Design Part 5
« Reply #57 on: January 05, 2012, 11:20:28 am »
Assuming this power supply is for testing electronic circuits and not power devices, for what kinds of circuit might a 10 V supply be inadequate?
I'm not an EE--what am I doing here?
 

Offline IanB

  • Super Contributor
  • ***
  • Posts: 9485
  • Country: us
Re: Lab Power Supply Design Part 5
« Reply #58 on: January 05, 2012, 11:26:16 am »
I think this might have been covered in the videos, I can't remember, but what happens if you should connect a battery to the output, for example attempting to charge it? Is there a danger of damaging anything if the power supply is switched off or has a lower set point voltage than the battery?
I'm not an EE--what am I doing here?
 

Offline Rerouter

  • Super Contributor
  • ***
  • Posts: 4219
  • Country: au
  • Question Everything... Except This Statement
Re: Lab Power Supply Design Part 5
« Reply #59 on: January 05, 2012, 11:28:05 am »
ian, i test automotive based control systems, they need those higher voltages (yes they have a regulator stage but its good to test them aswell) also a number of users will try and repurpose this themselves,

so i am just laying out what i can see as the devices maximum capability

as for the op amps, they appear to be linked to input voltage, so it is a valid point about someone feeding in more than 14V
 

Offline Rerouter

  • Super Contributor
  • ***
  • Posts: 4219
  • Country: au
  • Question Everything... Except This Statement
Re: Lab Power Supply Design Part 5
« Reply #60 on: January 05, 2012, 11:46:28 am »
ian, from memory, aslong as the voltage being shove back into the supply doesnt exceed the input voltage of the supply you are good (please check this, i may be wrong)
 

Offline IanB

  • Super Contributor
  • ***
  • Posts: 9485
  • Country: us
Re: Lab Power Supply Design Part 5
« Reply #61 on: January 05, 2012, 11:53:47 am »
ian, from memory, aslong as the voltage being shove back into the supply doesnt exceed the input voltage of the supply you are good (please check this, i may be wrong)
I'm wondering what might happen if the supply was powered down with the battery connected. I recall that voltage regulators may not like having the voltage on their output pin raised above the input voltage.
I'm not an EE--what am I doing here?
 

Offline Rerouter

  • Super Contributor
  • ***
  • Posts: 4219
  • Country: au
  • Question Everything... Except This Statement
Re: Lab Power Supply Design Part 5
« Reply #62 on: January 05, 2012, 11:59:40 am »
in those cases there is usually a protection diode across the output and input supply pins, so that it never goes more than 0.7V above the input pin (regs can generally survive this, but for low droput may require a schottkey) and this protection generally goes the whole way up the chain until you hit your transformer (be it the plugpacks or your own)

and while i find it odd that it was left out while the negative protection was added, i feel dave may have had his reasones due to layout limitations or other potential issues we arent aware of at this time, (e.g. leakage current of the diode while testing down to half milli-amps)
 

Offline slateraptor

  • Frequent Contributor
  • **
  • Posts: 833
  • Country: us
Re: Lab Power Supply Design Part 5
« Reply #63 on: January 05, 2012, 12:50:57 pm »
ian, from memory, aslong as the voltage being shove back into the supply doesnt exceed the input voltage of the supply you are good (please check this, i may be wrong)
I'm wondering what might happen if the supply was powered down with the battery connected. I recall that voltage regulators may not like having the voltage on their output pin raised above the input voltage.

If the supply was powered on, the regulator feedback will compensate by adjusting the base voltage of the internal pass FET.

With the supply powered off, I don't see anything that would be an issue. The Schottky at the regulator's output is rated at 40V reverse breakdown, and reverse leakage is spec'd at 400uA@40V, so any leakage current will pass through the low side sense resistors. Assuming you're not charging 40V batteries, it'll eventually drain if you leave it hooked up, but that looks to be the extent of it.
 

Offline Rerouter

  • Super Contributor
  • ***
  • Posts: 4219
  • Country: au
  • Question Everything... Except This Statement
Re: Lab Power Supply Design Part 5
« Reply #64 on: January 05, 2012, 02:36:26 pm »
mate, he is getting at a positive voltage within the devices normal operating area being connected before powering it up, that 40V diode is only good for negative voltages or ones exceeding 40V,

the point still falls back, what protection is in place when a voltage higher than the input is present on the output, be it the 10uF cap when you switch it off, or an external source of power e.g. his 9V battery,
 

Offline Rufus

  • Super Contributor
  • ***
  • Posts: 2094
Re: Lab Power Supply Design Part 5
« Reply #65 on: January 05, 2012, 03:28:31 pm »
the point still falls back, what protection is in place when a voltage higher than the input is present on the output, be it the 10uF cap when you switch it off, or an external source of power e.g. his 9V battery,

It will reverse bias the emitter base junction of the LT3080 pass transistor and possibly kill it. I don't see any relevant specification in the datasheet or mention of protection against reverse bias. So who knows?
 

Offline slateraptor

  • Frequent Contributor
  • **
  • Posts: 833
  • Country: us
Re: Lab Power Supply Design Part 5
« Reply #66 on: January 05, 2012, 03:46:57 pm »
mate, he is getting at a positive voltage within the devices normal operating area being connected before powering it up, that 40V diode is only good for negative voltages or ones exceeding 40V,

the point still falls back, what protection is in place when a voltage higher than the input is present on the output, be it the 10uF cap when you switch it off, or an external source of power e.g. his 9V battery,

I'm wondering what might happen if the supply was powered down with the battery connected.

It will reverse bias the emitter base junction of the LT3080 pass transistor and possibly kill it.

???
 

Offline Rerouter

  • Super Contributor
  • ***
  • Posts: 4219
  • Country: au
  • Question Everything... Except This Statement
Re: Lab Power Supply Design Part 5
« Reply #67 on: January 05, 2012, 04:01:21 pm »
both are valid conditions,

before powering up, there is no power on the input stage,
when powering down there is only what is held in the capacitance of the input stage,

while the output capacitance is less of a concern due to the constant current source, the micro, lcd adc and dac are all powered off the input side, likely to drain it faster than the constant current on the outside if no load is connected,

in short, i am saying unless there is some higher level reasoning i am missing, an extra protection diode between output and input of that reg would solve those potential issues, making it more robust, and as dave has been sporting on about, guild the lilly one more step with a small fix,

 

Offline Nick Gammon

  • Contributor
  • Posts: 41
  • Country: au
    • Gammon Software Solutions
Re: Lab Power Supply Design Part 5
« Reply #68 on: January 05, 2012, 04:10:15 pm »
There's one more AVR catch I've noticed in the schematic, AREF pin tied to Vcc. You're not supposed to do that, you can select internal reference to be AVcc in software (REFS bits in ADMUX register) and then you should put a decoupling cap from Aref to ground or just leave it disconnected if you're not using micro's ADC.

If you (or someone plaing with the kit) accidentally switch reference to internal 1.1V it'll appear on the AREF pin and get shorted with Vcc.

Good point. AREF is usually left floating, but with the 100nF capacitor on it. So instead of the 2 x 100nF capacitors as shown, you really should have one on pin 7 (Vcc), one on pin 20 (AVcc) and one on pin 21 (ARef).

Using the external ADC and DAC gives builders not only the choice in their ADC (Microchip make pin compatible 10 and 12 bit versions), but also decouples the design from any particular processor or even brand, and lets people play with and maybe learn about some SPI devices.

Your project is very educational, and the ensuing discussion also. I am learning a lot about why you might choose to do this, rather than that, when designing electronics circuits.
 

Offline EEVblog

  • Administrator
  • *****
  • Posts: 29222
  • Country: au
    • EEVblog
Re: Lab Power Supply Design Part 5
« Reply #69 on: January 05, 2012, 05:59:24 pm »
None of the LT3080 apps show that traditional protection diode, even their lab power supply design, so I presumed it was somehow fairly well protected in that regard.
Might be worthwhile testing that, or adding the diode perhaps...

Dave.
 

Offline RTC

  • Newbie
  • Posts: 4
  • Country: fi
Re: Lab Power Supply Design Part 5
« Reply #70 on: January 05, 2012, 08:52:33 pm »
How about this?
 

Offline slateraptor

  • Frequent Contributor
  • **
  • Posts: 833
  • Country: us
Re: Lab Power Supply Design Part 5
« Reply #71 on: January 05, 2012, 09:19:50 pm »
How about this?

That component alone will increase BOM costs by ~$3, assuming the additional protection is needed at all.
 

Offline Rerouter

  • Super Contributor
  • ***
  • Posts: 4219
  • Country: au
  • Question Everything... Except This Statement
Re: Lab Power Supply Design Part 5
« Reply #72 on: January 05, 2012, 09:44:17 pm »
agreed, if needed, a diode still remains the better solution, the other protection diode he chose would make a decent part, which would only add 29c to the BOM
 

Offline Psi

  • Super Contributor
  • ***
  • Posts: 7119
  • Country: nz
Re: Lab Power Supply Design Part 5
« Reply #73 on: January 05, 2012, 09:50:51 pm »
It really depends what the goal is.

Sure you can design an awesome product that's built like a tank and can survive pretty much anything.
Building something like that can be extremely satisfying but it's not going to be cheap.

Or you can build something to sell and keep the costs as low as possible without sacrificing safety or the intended specs.

Both approaches have their uses :)
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline Short Circuit

  • Frequent Contributor
  • **
  • Posts: 439
  • Country: nl
Re: Lab Power Supply Design Part 5
« Reply #74 on: January 05, 2012, 10:42:17 pm »
Assuming this power supply is for testing electronic circuits and not power devices, for what kinds of circuit might a 10 V supply be inadequate?
In that line of thinking, just build a 3.3V/5V power supply, and perhaps a luxe-edition with 9V & 12V. That will be fine for 99.5% of all non-power circuits.

Most circuits that will be operated from batterypacks will need more than 10V; 3C lipo 11.1v, lead-acid 13.8v, and charging voltages even higher.
Any circuit with a 7812 or similar inside needs some 14-15V to work properly.

I'd say a lab power supply needs to provide at least up to 15~20V to be reasonably universal. (Also matches higher power mains adaptors nicely, think 19V laptop power supplies).
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf