Author Topic: Lab Power Supply Design Part 5  (Read 45125 times)

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Offline shebu18

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Re: Lab Power Supply Design Part 5
« Reply #100 on: January 06, 2012, 12:45:36 pm »
Is it ok if we use for our own design SMD 0.1% and 0,1W resistors? I found some at tme and they are 3times cheaper then the thd ones.
« Last Edit: January 06, 2012, 12:47:22 pm by shebu18 »
 

Offline JimmyM

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Re: Lab Power Supply Design Part 5
« Reply #101 on: January 06, 2012, 03:38:02 pm »
I hear a lot of great ideas for functionality, but I think they exceed the scope of this project. I'm going to buy one of the kits however Dave develops it. Then with that experience I'm going to design my own with a much greater feature set based on Dave's analog design and control methods.
 

Offline Blue

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Re: Lab Power Supply Design Part 5
« Reply #102 on: January 06, 2012, 05:46:42 pm »
Yeah,

I also get one kit.  :)
Perhaps Dave can make a poll of who is wants to buy a kit. Then he knows in advance how many parts to order (and how many nights he has to worry  ;D). Oh, I can make one. Just hang on for a few minutes....
« Last Edit: January 06, 2012, 05:51:37 pm by Blue »
 

Offline Blue

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Re: Lab Power Supply Design Part 5
« Reply #103 on: January 06, 2012, 05:48:11 pm »
Oh,

Any idea how to increase the maximun output voltage? 5V is a bit on the low side.
 

Offline JimmyM

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Re: Lab Power Supply Design Part 5
« Reply #104 on: January 06, 2012, 06:45:55 pm »
The Max Output voltage is 10V based on the components Dave has chosen.
 

Offline Short Circuit

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Re: Lab Power Supply Design Part 5
« Reply #105 on: January 06, 2012, 07:29:16 pm »
The Max Output voltage is 10V based on the components Dave has chosen.
  • MAX4080 76V
  • LT3080 36V
  • LM334 40V
  • LM78M05 35V (actually, I don't think this is a good idea if this 5V is also provided on an USB connector, only 0.5amps)
  • NJM14558 14V but this one is easily changed to something else, like powered by an 78L12 (35Vin)
  • IRLU8721 gate 20V same 78L12
  • missed anything?[/il]
 

Offline shebu18

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Re: Lab Power Supply Design Part 5
« Reply #106 on: January 06, 2012, 07:41:15 pm »
Why would you want to program you PSU over an app when you have nice nobbs to rotate? Are you in one room, the PSU in another and having 2 long lead to your room and now program the PSU voltage?
 

Offline Nick Gammon

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Re: Lab Power Supply Design Part 5
« Reply #107 on: January 06, 2012, 08:03:23 pm »
We will need more PIN's, there is a need for a bigger micro, this means no arduino compatibility(maybe mega).

The Mega has an Arduino board (and therefore libraries), so you don't need to throw that compatibility out the window.

In any case, "more pins" can be obtained by connecting up your custom I2C devices to the existing I2C bus (I see currently used for the LCD device). Since I2C supports 119 individual device addresses (1 to 119), there is plenty of scope for adding in port expanders or similar.

I'm going to buy one of the kits however Dave develops it.

I agree with this, which is why I personally prefer something simple, including a processor I can physically plug and unplug if necessary. The more complexity, like Ethernet, Bluetooth, USB that gets added, the more it gets out of the range of hobbyists to understand and alter.
 

Offline Anders

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Re: Lab Power Supply Design Part 5
« Reply #108 on: January 06, 2012, 08:18:42 pm »
A fan  controller can be done off-board easily enough. To be honest, given the target client base for this project, i dont believe making a port available to a 8 pin header is a major change. My keypad idea was for my own use. It makes the kit more flexible for end users.

Code is easily adapted for users own setups according to there mods if the mcu is Arduino compatible.

I believe this design change would make this a very attractive kit. Given its ability to be customized via the available port and being able to select he screen back color. The back color can be set to change to indicate different conditions for example.

This is just an idea.

Regards

If you choose to measure the temperature and control a fan from the microcontroller it opens up for a number of options.
Among other options it will be possible to increase the fan speed on increasing current instead of waiting for the heat to spread from the wafer to the outside of the case. All the required variables for this and more are already in the “house” its just a few more lines of code.
Proper Planning Prevents Piss Poor Performance
 

Offline Rerouter

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Re: Lab Power Supply Design Part 5
« Reply #109 on: January 06, 2012, 09:46:22 pm »
now while i hate to make a redundant post

for short circuit, for this revision of schematic, (and this is only as a higher voltage modification 30.72V)
2 x http://search.digikey.com/us/en/products/MC33172N/497-7409-5-ND/1038900 op amps are pin compatible with u3 and u12
and a few gain resistor changes;

2K in parrellel with R29
2K in parrellel with R31 (for a gain of 15)

10K in parrellel with R35
82K to replace R34 (for a maximum input voltage reading of approx 35.6V in, yeah not a pretty number, still working)

and for the mosfet, like a 82K resistor tieing its gate to ground, to divide input voltage by 2 (about 18V max), so its safe from 0 to 30.72V out, (mosfets arent my area, tell me it thats wrong)

please note the modifications are only something i am pointing out for others looking to modify it to there end
 

Offline JimmyM

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Re: Lab Power Supply Design Part 5
« Reply #110 on: January 06, 2012, 09:52:39 pm »
The Max Output voltage is 10V based on the components Dave has chosen.
  • MAX4080 76V
  • LT3080 36V
  • LM334 40V
  • LM78M05 35V (actually, I don't think this is a good idea if this 5V is also provided on an USB connector, only 0.5amps)
  • NJM14558 14V but this one is easily changed to something else, like powered by an 78L12 (35Vin)
  • IRLU8721 gate 20V same 78L12
  • missed anything?[/il]
Sorry. I meant based on the gain resistors on the OpAmps, and voltage dividers, etc. His design is 10V max, you can change component values (gain resistors, etc) to get more.
 

Offline Short Circuit

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Re: Lab Power Supply Design Part 5
« Reply #111 on: January 06, 2012, 10:14:17 pm »
now while i hate to make a redundant post
...
Why redundant? And even if so, without 'm few forums would flourish  ;D

Oh and 18V is still dangerously high for a mosfet gate. Usually you'd want to stay below 12-15V (assuming 20 Vgs,max). The Rds does not lower significantly above 8-10V anyway.

Sorry. I meant based on the gain resistors on the OpAmps, and voltage dividers, etc. His design is 10V max, you can change component values (gain resistors, etc) to get more.
Ah, ok, well, resistors are easy to change.
Mind that the design isn't set in stone until the design is proven by prototype, and even then.
 

Offline amspire

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Re: Lab Power Supply Design Part 5
« Reply #112 on: January 06, 2012, 11:33:45 pm »
A limiting factor with the maximum voltage from Dave's supply is the power rating of the LT3080 IC.

If you want to be able to use it up to 40 degC ambient and you want to keep the junction temperature below a safe 90 degC, then even with an ideal heatsink, the maximum power that can be dissipated in the LT3080 is (90-40) / 3 deg/W = 17 watts.

If the power supply us designed with a maximum current of 1.0 A, that means a maximum output voltage of under 16V. With a real world heatsink, 10V output may be as good as can be done with a 1A output. The chip will thermally protect itself, but you really do not need a power supply that goes into thermal current limiting. If the output current was lowered to 500mA, then the output voltage could be increased.

If you have a switchable source voltage to the LT3080 (like many high end power supplies use) so that the voltage drop across the LT3080 is always less then 10V, then you could potentially go to over 30V out at a 1A current supply rating. This method usually requires custom power transformers, or a switching pre-regulator, plus a fair bit of design work.

You can parallel LT3080's but you have to add some series resistance that degrades regulation a little.

Richard
 

Offline Rerouter

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Re: Lab Power Supply Design Part 5
« Reply #113 on: January 06, 2012, 11:40:08 pm »
and just for those that have yet to see part 6's video, the lt3080 seems pretty much indestructable to reverse voltage, as long as you dont short the input,
 

Offline slateraptor

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Re: Lab Power Supply Design Part 5
« Reply #114 on: January 06, 2012, 11:53:06 pm »
and just for those that have yet to see part 6's video, the lt3080 seems pretty much indestructable to reverse voltage, as long as you dont short the input,

I suspect a few more volts beyond his bench supply's max voltage output would have toasted that zener and the low-end sense resistors.

Nevertheless, that linear reg is quite robust.

EDIT: ...or was that part of the schematic even incorporated into the testbed?
 

Offline Rerouter

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Re: Lab Power Supply Design Part 5
« Reply #115 on: January 07, 2012, 12:04:53 am »
i dont believe it was, but it was pretty much just a test to see what the reg could survive,

really if the rest of the circuit was left connected, the op amps and adc would have fried well before the  diode,

still i am now content and awaiting the next update, (think daves board is still being held hostage by aus post)
 

Offline slateraptor

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Re: Lab Power Supply Design Part 5
« Reply #116 on: January 07, 2012, 12:23:51 am »
i dont believe it was, but it was pretty much just a test to see what the reg could survive,

really if the rest of the circuit was left connected, the op amps and adc would have fried well before the  diode,

still i am now content and awaiting the next update, (think daves board is still being held hostage by aus post)

Agreed.
 

Offline EEVblog

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Re: Lab Power Supply Design Part 5
« Reply #117 on: January 07, 2012, 11:18:10 am »
For those who wanted an Arduino header footprint on the board, I've looking into this.
But there is of course a catch, to make it fully compatible, you have to have ALL the pins free on chip because you don't know what Shield may be used and what lines it will use. And if the micro in the PSU is using the lines, then they aren't available for use by the shield.
So the only lines you can get away with using on the micro are the I2C lines to drive all the circuitry using port expander chips, and that's getting crazy.
You could design it to make it compatible with a few key shields, but really, it's getting messy and lacks usefulness. Might as well just do a custom interface board like my original idea.

Not to mention there really isn't much room on the board for it all anyway (header + port expander chips)

Dave.
 

Offline metalphreak

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Re: Lab Power Supply Design Part 5
« Reply #118 on: January 07, 2012, 01:29:46 pm »
AFAIK all Arduino compatible Ethernet variants use the Wiznet W5100 chip which is fine pitch SMD.
Hardly suitable for a DIY kit.

Dave.

The microchip ENC28J60 is available in a 28pin DIP package. I'm sure there's an arduino library for it somewhere.

Offline JorgeCarbajal

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Re: Lab Power Supply Design Part 5
« Reply #119 on: January 07, 2012, 03:46:23 pm »
Hi Dave, about the DAC you are using, there is a variant from the same series, the MCP4822, it has internal reference of 2.048 V, just like the one you are using, and i think its good to have less components. You can even choose to put gain to use 4.096 V
 

Offline Nick Gammon

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Re: Lab Power Supply Design Part 5
« Reply #120 on: January 07, 2012, 10:19:33 pm »
For those who wanted an Arduino header footprint on the board, I've looking into this.

I'm not sure what that means, Dave. If you check out this page you will see that there are many variations on the Arduino hardware:

http://arduino.cc/en/Main/Hardware

For example, the Arduino Mini comes in a very small package, including crystal. Or you might choose the Pro Mini.

For me, the "Arduino compatibility" is simply using an Atmega168/328 or similar chip, and preferably being able to program it "in situ" which means you don't turn off the functionality of the reset pin. Although even with that dedicated to driving an LED, you can probably program it with careful timing (ie. using power-on reset).

If you are reworking the processor part, I would free up the reset pin for resetting, and do something along the lines of my earlier suggestion for interfacing with the switches. You can probably save yourself some pain by choosing I2C compatible chips (like the DAC and ADC) rather than mixing SPI and I2C. After all, I2C can address 119 chips. That frees up more pins again.

Indeed, for polling the front panel switches, a simple I2C port-expander could handle that, allowing something like 8 switches and 8 LEDs, all off one I2C part. (And port-expanders can generate interrupts, so you don't even have to poll them).
 

Offline EEVblog

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Re: Lab Power Supply Design Part 5
« Reply #121 on: January 07, 2012, 10:35:26 pm »
I'm not sure what that means, Dave. If you check out this page you will see that there are many variations on the Arduino hardware:

There are some people who have suggested that I add Arduino Shield headers connectors to the board.
That would be pointless without freeing up most of the pins.

Quote
http://arduino.cc/en/Main/Hardware

For example, the Arduino Mini comes in a very small package, including crystal. Or you might choose the Pro Mini.

For me, the "Arduino compatibility" is simply using an Atmega168/328 or similar chip, and preferably being able to program it "in situ" which means you don't turn off the functionality of the reset pin. Although even with that dedicated to driving an LED, you can probably program it with careful timing (ie. using power-on reset).

If you are reworking the processor part, I would free up the reset pin for resetting, and do something along the lines of my earlier suggestion for interfacing with the switches. You can probably save yourself some pain by choosing I2C compatible chips (like the DAC and ADC) rather than mixing SPI and I2C. After all, I2C can address 119 chips. That frees up more pins again.

Indeed, for polling the front panel switches, a simple I2C port-expander could handle that, allowing something like 8 switches and 8 LEDs, all off one I2C part. (And port-expanders can generate interrupts, so you don't even have to poll them).

That's what I have done. I had to fix that reset pin and free up the crystal pins.

Dave.
 

Offline LaurenceW

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Re: Lab Power Supply Design Part 5
« Reply #122 on: January 07, 2012, 10:36:13 pm »
Jorge, that MCP4822 dual DAC and built in ref is a nice chip for sure, but look closer at the spec sheet for the "precision" voltage reference. It's 2.048 V, but +/- 40mV  - that's +/- 2%.  Hmm... "precision"?? It may well be better than that, but you can't know it.

Look at Dave's design philosophy. So far he has yet to introduce ANY hardware fine tuning in the form of variable trimmers, and so far has stated his abhorrence of software "tweaks" (we'll see how long that lasts!!) to calibrate the unit. Instead, he has adopted the approach of precision parts throughout (where it's critical), and is looking to produce an unadjusted  close tolerance performance from the get-go.

Because it's a non-commercial project, and more about learning than making big bucks, Dave is quite OK to do that, just because he wants to. Were this a purely commercial design, the MCP4822 would have saved us the cost/space of a separate Vref chip, and the finished product would probably be calibrated as a quick operation on the production line with a pot or software tweak.

Having said that -  a non-exhaustive check suggests that the MCP4822 is MORE expensive that the design's MC4922 AND Vref chip... :(

As ever in electronics, there are several ways to skin the cat!

Lots of great input on these blogs  on how to tweak the project, and clearly there is no please-all-the-people-all-the-time combination of features, but this comes close.

WHAT I WANT is a loud Piezo sounder BLEEP that wakes the BLEEP dog up next BLEEP door, every time i change the BLEEP BLEEP voltage from 5.1 BLEEP to 5.0 volts...  and where are the flashing lights, Dave? One little LED just doesn't cut it... ::)

"Bang"
« Last Edit: January 07, 2012, 10:39:17 pm by LaurenceW »
If you don't measure, you don't get.
 

Offline LaurenceW

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Re: Lab Power Supply Design Part 5
« Reply #123 on: January 07, 2012, 10:49:30 pm »
Dave, I have another question about protecting the LT3080, which you may have shown now is not necessary, but I was interested to understand it.

Rather than trying to introduce various fat diodes to shunt away any voltage applied back into the output from a charging battery or something similar, could you not introduce a forward-biased diode into the output of the LT3080,  before the VSense signal? That way, the diode drop is inside the feedback loop. Would the feedback loop performance suffer? Yes, it loses us 3/4 volt, but that would not bother me...

It might even do away with the need for the reverse bias protection diode across the output?

(Learning the hard way -  a while back I sent a 7812 to Silicon Heaven in a fairly loud and spectacular way, by forcing -20V into the output stage, from a HUGE charged capacitor bank. Kids - don't try this at home :-[).


ONE more question, if I may. The LT3080 spec sheet mentions doubling up devices (more device sales!) for double the output current, OR using a series pass transistor. What are the pros and cons of each approach?
If you don't measure, you don't get.
 

Offline Nick Gammon

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Re: Lab Power Supply Design Part 5
« Reply #124 on: January 07, 2012, 10:55:54 pm »
There are some people who have suggested that I add Arduino Shield headers connectors to the board.
That would be pointless without freeing up most of the pins.

Yes, I can't see the point to that. The boards-with-shields are basically a development/experimental environment. And many shields assume that (at least) some of the pins are there for them.

My personal view is that, if you make the I2C pins easily accessible (eg. SDA, SCL, VCC, GND) then anyone that wants to can connect up a device of some sort (eg. display, comms, switches, temperature sensor) and then modify the code on the main chip to address such device(s).

You might want to take a closer look at your I2C pull-ups (currently 10K). Screenshots here:

http://www.gammon.com.au/i2c

I found that 4.7K was closer to the required value for a nice clean square wave. However that was at 5V, you might need a different value for 3.3V.

More screenshots here:

http://dsscircuits.com/articles/effects-of-varying-i2c-pull-up-resistors.html

Judging by a bit of a Google search, 4.7K could still be OK for 3.3V.
 


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