Velleman K7200 or simular

Hi gaga2410
You should try 0.5 ohm resistor 5 or 10 W; just use 3 resistors ; it should be fine
bye

I built this power supply. for current resistors I used four 0.22Ohm- 11Watt. ıt works great thanks wizpic for PCB

generator***** said:

I built this power supply. for current resistors I used four 0.22Ohm- 11Watt. ıt works great thanks wizpic for PCB

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I would be supprised if you got the full 8amp max peak current and continous 5amps with .22R unles you have used more fo them.
Your welcome for the PCB.

I'm working on a new version of a power supply that uses mosfets and digitally controlled

I have a nice 200VA 12V toroid transformer that I would like to use but unlike the transformer used here, it has a single secondary winding. Is there an easy way to use that instead? Or could someone maybe tell me how to modify the given schematic to use with my transformer?

Also, how do you display the current limit?

KoZu said:

I have a nice 200VA 12V toroid transformer that I would like to use but unlike the transformer used here, it has a single secondary winding. Is there an easy way to use that instead? Or could someone maybe tell me how to modify the given schematic to use with my transformer?

Also, how do you display the current limit?

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Sorry to say the answer is no, The 12V would not be enough the max you would get out is about 12-15V max. You could use it but you would be limited to low voltage, then you would not need the relay and a few other bits,

The tansformer needs to be 0-15V 0-15V to get the full 30Volts out. I used an op-amp tp boost the voltage then displayed it on LCD using a PIC chip.

I actually also have a 12V-0-12V 38VA transformer. How many TIP3055 would I need for a PSU with such a transformer? The output would then probably be 1A max. Any other changes to be made?

you should get away with 1 or 2 TIP3055 and the shunt resistor you will get away with using a 5W 0.1R resistor but your supply voltage will be limited to just a little over 25V but above 23V you may get more ripple on it, Try it so how you get on

Hi wizpic,
with your PCB there's a lettel problem the Fluctuations of the current !!! the current display is not stable ?

Q80 said:

Hi wizpic,
with your PCB there's a lettel problem the Fluctuations of the current !!! the current display is not stable ?

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If I remember I did not do the meter side of things, The PCB of the power supply works peefectly I've not has not problems with mine over the years.

If you built the meter that come with the PSU then yes it does seem to be a little unstable

The problem with your PCB there’s no connection between the groundplane and the earth, but you can fix this problem with the shunt between the pin of current resistor and the body of your PSU.
I did the PCB & the meter just like the original one ; a 100% like the original one ; and its works like a magic

Q80 said:

The problem with your PCB there’s no connection between the groundplane and the earth, but you can fix this problem with the shunt between the pin of current resistor and the body of your PSU.
I did the PCB & the meter just like the original one ; a 100% like the original one ; and its works like a magic

Click to expand...

My PCB is an extact copy of the original version. I copied the schematic and the orginal PCB which I brought has a kit from maplins years and years ago. The metering I used a pic and a MAX7219 and this gives stable reading on voltage and current. I know the original meter was unstable and was hard to cure.

But at least if any one else has the same problem they now know what they can to cure it

I'm also thinking of using a PIC to display the voltage and current with transistor temp and fan speed on an alphanumeric lcd display. What I don't really understand is why on the original schematic is the current measured between the negative output and ground? Can someone explain to me in detail, how and why can I measure the current using the PIC's ADC? Also.. is there a way to show the current limit or is the LED the only way to know that the limit has been reached?

~K

KoZu said:

I'm also thinking of using a PIC to display the voltage and current with transistor temp and fan speed on an alphanumeric lcd display. What I don't really understand is why on the original schematic is the current measured between the negative output and ground? Can someone explain to me in detail, how and why can I measure the current using the PIC's ADC? Also.. is there a way to show the current limit or is the LED the only way to know that the limit has been reached?~K

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The current sense resistor is a 0.1R resistor and when you draw a current it produces a small volatge acrross it in Milli-volts, This can be between 23-400MV depending on how much current is been drawn(this is only an example). The PIC'S A/D converter cannot really really this and struggles to cope with such low volatge. The way I did it was to use an op-amp set up has again amplifier so the voltage then would be come say 0-5V. If I remember when I carrying out some testing without the current LED connected the PSU played up. I may be wrong but it has been some years since I built it. What you can do is leave the LED in place and use a pin on the PIC then when it goes high show it up on the LCD but I think the LED just does the job fine.

What I actually meant was if there's a way to show the actual numeral value of the set current limit without having to reach it.
Thanks for explaining how to read the current, I guess the tricky part will be playing with the amplifier gain. Although is still remains unclear exactly why is the sense resistor between negative terminal and ground. Wouldn't it be more logical to put the resistor between the circuit board output and front panel plug to be in the path of the current? Maybe I'm just misunderstanding the way the circuit works..

Here is a drawing how to set your op-amp up(This is what I used).
The sunt resistor (knwon has R37-R40 in original velleman kit) always work best in the negative line. There are reasons for this hs it is a common factor.

Now I usnderstand what your trying to do is to show the actual numeral value of the set current limit without having to reach it. Yes you can do this very easy. This is the way I would do it. is place your meter + at the center of the current limit pot(RV3) measrue the voltage with the pot turned to Min take a note then turn the pot to the Max and take a note on the second reading,(note you do not have to place a load on it). If I remember it goes from 0.364V to 1.02V(don't quote me on that) Then you can use one of the A/D port on the pic to read the voltage and convert that reading into amps, You will have to do a bit of maths on otherwise your reading will be higher than it is set for. EG. if the pot turned to Min and the reading is 0.364V you need to subtract that so that the meter displays 0.00A then turn the pot to max and make sure your reading display 0 - 5A. you would have to work out the maths to complete it.
If you can measure the voltage and let me know I will have a play with it or if I get time I will check my end.
Do you have a copy proteus ISIS ?
Hope this makes it a bit clearer

Ok, thanks. This makes things a bit more clear. Alright, I'm off to playing with the PIC's ADC and interfacing it to the display. I'll be sure to run into more questions soon so look out.

Because this is the best power supply that I have built and had it for many of years and it has had some use and abuse and thought I try to use a PIC to digitally control the voltage/ current setting via push buttons, The voltage side was perfect with brilliant accuracy I had a coarse and fine tune for it. But the current side was slightly harder for some unknown reason I could not get the current to regulate correctly. I had no real control over the current limit if I remember correctly.

May be I can re-look into it, I will make a drawing up of how I connect the digital part and someone could have a look to make sure that I have done it right or someone may have better ideas or other better ways.

May be if it can be done then we could use 1 rotary encoder to set voltage/current limit etc.etc.

What other PSU you could recommend that would be able to be modified to digital
?

Any idea where I can get the original transformer or a suitable replacement? I just killed mine a few minutes ago. I hadnt noticed the fan had stopped working and I was running at max voltage and max current for almost 4 hours. Both secondary sides of the transformer have a 0 ohm resistance. This thing was HOT!

I am located in the USA.

Thanks.

Any 0-15V 0-15V 300/500Va transformer will do the job, I'm supprised the output transistors did not go bang must be on a big heatsink

Wizpic

One of them did. They are mounted on the stock Velleman heatsink. I found this transformer: **broken link removed**