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Topic: Computer controlled PS/function gen/multimeter (Read 228637 times) previous topic - next topic

Re: Computer controlled PS/function gen/multimeter

Reply #225
Maxim MAX5137 are 2 chanel 12 bit high precision low power DAC with SPI interface and ready out for easy ddaisy chaning. the 12bit version is around 3$ and the 16 bit is around 6$  witch is not much, with the benefit that maxim is briliant at giving out samples world wide.
best regards FIlip.

Re: Re: Computer controlled PS/function gen/multimeter

Reply #226
Microchip MCP4728 is a quad 12-bit I2C that has eeprom so you can set default power-up status. It's a 0.5mm pitch MSOP-10 though.

Re: Computer controlled PS/function gen/multimeter

Reply #227
silabs have micros with 12bit d/a and 16 bit a/d not sure witch ones from memory

i'm currently using a c8051f121 has more than enough ram/program for this aplication, a 500ks 12 bit d/a, and ~1Ms 12bit a/d(internal mux)

Re: Re: Computer controlled PS/function gen/multimeter

Reply #228
just don't go with an analog device ADuC series micro.  the sigma delta converts wind up being extremely slow if you want decent resolution; even then there was errata that required the use of the lowest clock speed (otherwise even more noise was coupled into the readings).

Re: Re: Computer controlled PS/function gen/multimeter

Reply #229
Accuracy is completely at odds with the convenience of on-board uC A/D.  If you need anything more precise than reading a basic potentiometer, I would recommend a stand-alone A/D with careful layout to avoid noise.  On-board D/A may not pose the same issues, since a PS will probably only require DC output and not any kind of signal - thus the DAC output could be massively filtered to remove any uC noise.

Re: Re: Computer controlled PS/function gen/multimeter

Reply #230
Another reason to use external spi dac and adc is that electric isolation with cheep optocouplers becomes easier than isolating on the USB side.

Re: Computer controlled PS/function gen/multimeter

Reply #231
I just found some excelent IC-s from maxim, those guys seriosly know what they are doing.

MAX9611, MAX9612
they are high side current sensing amplifiers with integraded 12bit ADC, I2C interface, and opAMP/comparator.

by adding  only a resistor and a Pmos to it you get current limiting, and 12bit A/D over the i2c for output voltage, current, temp, even set reference. by driving the reference via a DAC you get full digital control over current limiting. and you dont have to use any MC A/D chanels since the IC does it all for you. a

nd by using a lm78s40 or lm723 by driving its reference with a DAC you get full voltage regulation, and if you implement the i2c DAC you get full control over only 2pins over the whole power supply.
best regards FIlip.



 

Re: Computer controlled PS/function gen/multimeter

Reply #234
sorry, but i am unable to acess that part of the forum, i just asumed when i saw dp.com post right after my post on the forum that they were corelated.

anyway I developed an idea for a digitaly controled PS, that i am gonna build just as soon as my exams are over, in about a month.

the idea is relativly simple, it uses one 555, 3 comparators and 2 curent sense amplifiers.
its a clasic buck topology.
the 555 controles the switch.
the 555 is setup in such a whay that when a low pulse comes on its triger it gives out a low pulse witch is >= to fixed RC time. it goes low as soon as the trigger goes low, and stays low either for a fixed time or if the triger is still low it stays low untill the triger goes high.

the low output turns the switch on, high turns it off.

conected to the triger are 3 comparators. one that compares output Voltage to dac set referance. the second compares the inductor current trough high side curent aplifier, to a pre set maximum inductor current reference.
and the third inductor compares the ouput current to a second dac set refference.
(here the max9611-12 could be used)
when either of them goes low, the triger goes low and the switch is off.

I simulated it and it works rather well. the frequency is variable and depends on many things but two things can be controled:
the charge rate of the inductor,
and the 555 minimum off time, witch is used to stop any comparator oscilations contoling the switch.

Inductor charge rate is  calculated trough its maximum current and inductance. and the 555 minimum off time trough the rc network. for this cicuit to work on higher Frequancies and thus give more stability and fewer cost of the inductor, it needs comparators with low propagation delay and a 555 with a low propagation delay, because what is the point if you register that the current has crossed a pre set 4 amps when it takes the circuit a further time to respond and thus your current jumps to 5A, witch fries your inductor.

Plz coment and critique the design if you find any faults with it, thanks.
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the output current comparator is mising but its a simply the max 9612 added to the triger.

the reason for doing this is that is easy to understand, simple to work with, and gives full control of the circuit to the designer. by seting up different reference voltages you could  control maximum Inductor current, maximum output current, output voltage. by lets say making the maximum inductor current reference a function of the output current you could have an addapting maximum current for incresed stability, and higher freq operation. and this is done by driving the maximum current comparator with the output of the output current amplifier.
best regards FIlip.

Re: Computer controlled PS/function gen/multimeter

Reply #235
[quote author="arakis"]I just found some excelent IC-s from maxim, those guys seriosly know what they are doing.

MAX9611, MAX9612
they are high side current sensing amplifiers with integraded 12bit ADC, I2C interface, and opAMP/comparator.
[/quote]

I was looking at the MAX9611/12 earlier, it seems pretty slick for an all-in-one solution. 

I am working on a version of the old plug-in ATX power supply thing, with built-in current and voltage measurement.  I thought about using the 9611, but was hoping to get enough resolution to measure very small (uA) currents.  Right now I'm looking at a combination of MAX9923 (current sense amp) + MAX9939 (programmable gain amp) paired with a PIC to produce an auto-ranging current meter. 

With a 0R1 current sense resistor and gains of 20-15700 (100 from the MAX9923H * 0.2-157 from the MAX9939), I ought to be able to get a range of from 1A to 1uA (using a 16-bit ADC).  I haven't quite figured out how much error will be a factor with these parts and with gains that large. 

I also haven't figured out an easy way to add a current limit circuit or a non-fuse-based overcurrent protector.  That seems to be the trade-off with the ATX-based things -- it's so easy to have the different voltages all at once (3V3, 5V, 12V, and even an adjustable), but adding other features is a pain.

Re: Computer controlled PS/function gen/multimeter

Reply #236
[quote author="arakis"]sorry, but i am unable to acess that part of the forum, i just asumed when i saw dp.com post right after my post on the forum that they were corelated.[/quote]
Sorry, I had a feeling that was a private area. Basically, I send lots of chip descriptions, press releases, and application notes to the core team at Dangerous Prototypes.  Then Ian selects the ones that he thinks might appeal to the larger community and posts them on the blog.  There is often a delay between my forum post and his blog post.  Plus, there are other people contributing to those chip and app note threads.  Personally, I like the fact that Ian filters the list, because I think that makes the blog more interesting.

Re: Computer controlled PS/function gen/multimeter

Reply #237
I have been aiming to do something like this myself a long time ago, but due to me having a few other projects I am working on, I ended up leaving my psu project in the dust and neglected :(

So I will post my progress here, for all of you who want to use it for your own ideas. Keep in mind this is un finished, and I have yet to make a prototype to see how it works, so I cannot guarantee that it is all correct.

My main idea was to have something that would give me voltage sensing, a few voltage sources, and a single adjustable voltage source capable of giving me at least 12v (for things like programming certain chips).

I use a CPLD on it because I wanted to gain some experience with them and it gives me a tremendous amount of IO for a super cheap price. It has an external 8 channel ADC for sensing voltages and giving a very low speed scope.

For the adjustable SMPS, it uses a digital potentiometer to give a feedback voltage to the FB pin of the smps. The problem with this is I do not know what speed it needs to adjust the FB for a desired voltage, and if this is even the right way to control a SMPS. I got my information from asking this question on chiphacker: http://electronics.stackexchange.com/qu ... table-smps

It also has a small display, it is meant to be able to be used without a computer powering it (Li poly) so you can control and see the voltages and other things on the LCD. When running from a battery, you can turn off the LCD to save battery, and instead have the LEDS display the power state of any of the voltage sources, and for what ever else the user would want them to indicate. :P When it is connected to a computer, the user would be able to control the voltages and sense voltages on a PC program.

There are a few other intended features, and I originally wanted to make this into a product and sell it online, hence the price quotes as a PDF in the file, but I feel that it can be used to help the open source community make a better bench power supply at a quicker pace. My only request is that if any of the ideas are made from my uploading of this design, someone somewhere would put my name on it :P

Good luck with this project, and I hope that I was of some assistance to it.

*uploading the files now*
http://8486.a.hostable.me/Breadboard%20 ... supply.zip <-- uploaded :D

Re: Re: Computer controlled PS/function gen/multimeter

Reply #238
I like that you included all the different files. It is cool to see the design evolve.

I think the feedback control will probably be ok. The ad7376 can take 30 volts. The wiper and R10 create a voltage divider that can be changed to control the feedback voltage.
Got a question? Please ask in the forum for the fastest answers.

Re: Re: Computer controlled PS/function gen/multimeter

Reply #239
I am not sure what the state of this project is as I did not read all 16 pages. But I skimmed through a few of the early design ideas and have some thoughts.

First of all I really want to produce things like PS and so on but I can't because of liability (at least not for sale). Even my 5W design for USB is a bit scary to sell without some sort of UL type approval.

Second, the early designs seemed to be switch mode and filtered digital signals for output. I would be very concerned about noise in such a design. There was a rule in my first lab for low noise reason: nothing digital. One way to perhaps design a high efficiency lower noise PS is to use switch mode DC-DC followed by linear regulation. IE have a switch mode PS supply a few volts more than the output, then use a regular linear stage. Many of the HP power supplies from the 80s have complete internal schematics online.

Third, integration. Sometimes integration is good and I get how you can with DAC/ADC do all three of these tasks but I am not sure they are the best way to do them. I would think about the problems separately and see if the designs really share components, if so you can integrate them at a savings. If not then it probably is not a wise choice to try to integrate the design.

The DSO board + function generator might be more appropriate combo for instance than having a function generator in a PS.

I think building a source meter for cheap would be nice though and that is almost what you guys are trying to construct. A Keithley 2400 runs about 2000 dollars.