my first project :) - need a little help

[SnakeBite]

thanks for the clearification but i know what i meant.
there are ADC's that convert the voltage (analog signal) to digital Binary signal and they also have a seven segment driver in one package what i need is a way to convert the analog voltage in a resolusion of 4.5 digit but with a little more than 20000 counts and then drive the five seven segment that i have .

i almost sure that i can do it with PIC or AVR microcontroller but i'm in a learnning process of how i program and use microcontrollers .... so if the ONLY way to make it happent is with a micro i will have to speed up my learnning speed...

Ido 

[SnakeBite]

i found a way to make voltmeter with 40000 counts using MAX1499 ADC and MAXQ610 MCU but i don't have real experience with MCU yet and also the package of the two chips it's defiantly not for beginners (32 pins TQFP)

you can see all the info here
http://www.eetasia.com/STATIC/PDF/201101/EEOL_2011JAN31_EMS_POW_AN_01.pdf?SOURCES=DOWNLOAD

do you think i need to buy the starter kit and other accessories of this chip and get familiar with it OR there is more easy way let's say with pic micro+MAX1499 ? which pic do i need if the MAXQ610 is 16 bit micro?

what is better the pickit2 or the pickit3 ? do they fix the bugs of pickit3?

thanks
Idoaricha

[IanB]

I think the answer to your question depends on your objectives. If your intention is to learn about electronics, then you can go ahead and make it as complicated as you like.

But if your intention is to construct a practical power supply, then I do not understand why you are getting so hung up over the 40000 counts? What good will it do you to display 20.700 volts instead of 20.70 volts? Unless you are planning to make a super accurate, highly regulated, highly stable laboratory grade power supply, then the extra digit will be a waste of time.

[Bored@Work]

Before you think about stuff like which microcontroller to use, do the sensible thing, run the numbers.

If I got my math right then 1 count (aka the last digit) in 40000 is 25 ppm. Can you construct a resistive divider that good?

For example, lets assume one of your resistors is perfect, and the other is just 0.05% off, and the meter is perfect, too. Then you end up with a display of 39.990 V for 40.000 V input (or 40.010 V - i.e. overflow).

Now you might be lucky. You might get two resistors from the same lot which happen to have a similar error in the same direction, canceling each other's error out in the resistive divider. But do you feel lucky?

And if you aren't lucky, do you have the instruments to adjust the divider, or do a correction in software?

[Zero999]

The problem with some cheap transformers is they don't have enough iron and copper in so run hot and will catch fire if they don't have a thermal fuse and there may not be enough insulation between the primary and secondary which can be a shock hazard.

Automatic tap selection can be done with a relay or transistors, attached is a basic tap changer which uses transistors.

[SnakeBite]

The problem with some cheap transformers is they don't have enough iron and copper in so run hot and will catch fire if they don't have a thermal fuse and there may not be enough insulation between the primary and secondary which can be a shock hazard.

Automatic tap selection can be done with a relay or transistors, attached is a basic tap changer which uses transistors.

i can't find match transformer in sane price. and  even at anyprice i can't find something with the same rating output voltage is it really that critical ?

BoredAtWork and IanB i thought about it this Saturday and i came to your same conclusion -  that i don't need that much resolution and i'll lower it to 3.75 digit (4000 counts) 20.70 is enough for me.

thanks
Ido

[Zero999]

Oh sorry, I forgot to post the schematic of the tap changer.

[Zero999]

i can't find match transformer in sane price. and  even at anyprice i can't find something with the same rating output voltage is it really that critical ?

You won't find an off the shelf transformer with all of the specified taps. To get the exact specification you desire, you'll either need to wind it yourself or get it custom made. You could use two 6-0-6V transformers in series to get 0-6-12-24V but it'll be more expensive and have poorer characteristics (efficiency & regulation) than a 12-0-12V transformer with the deisred current rating.

[SnakeBite]

i do found one but you said that we can't trust it because it's chinese transformer.

my question is  : Is it really that critical to buy made in USA transformer after all it's just a wires and iron core.

 i started to read about transformer theory but it seems a big hassle to wind it yourself to many variables to consider and a lot of formulas of the Power loss , magnetic flux also types of cores etc...

Ido

[Zero999]

Of course it doesn't matter where the transformer is made, as long as it's good quality. I'm surer there are plenty of good Chinese made transformers but buying a non-name part is often a gamble.

[SnakeBite]

thanks . i contact the seller and he was honest enough to tell me that this transformer is NOT high quality one.
so I'll for more...

i have another question :

what is the difference in protection between EMI suppression capacitor and ferrite bead  ? do they make the same thing or one is different then the other ? i did a little research and the ferrite bead is generally a low pass filter that blocks RF and EMI from going through a cable and the EMI capacitor is do the same .

if they are the same which one is better to put in a power supply near the input transformer?

thanks
Ido
 

[Zero999]

A ferrite bead acts like a lossy inductor which is a high impedance to RF and is used to block it. A capacitor has a low impedance to RF and is used to short circuit it to earth.

EDIT:
A mains transformer will be very good at blocking RF because the eddy losses will be high at high frequencies. It's possible for some RF to get through the transformer to the secondary, via the interwinding capacitance. This can be minimised by using a screened transformer or by connecting both sides of the secondary to earth via Y capacitors.

[SnakeBite]

which one you think will be better to use at the input of PS ?