ADC Multimeter- how to calculate current and current limit?

Hello.

I've got multimeter created from my project on atmega8.

It work's quite good when measuring voltage on output stage of power supply:
0-30 VDC STABILIZED POWER SUPPLY WITH CURRENT CONTROL 0.002-3 A

But I can't figure it out how to determine current on output and current limit.

First of all, here is schematic where I have probes:
**broken link removed**

Plus one probe on point 13. It's there because I based on similar multimeter. Unfortunately this other multimeter has closed source and no explanation is given.

Measuring current flow should be a piece of cake because it's basically measuring voltage drop on resistor R7. This is first problem because when I've got no receiver it shows full range of from 0.00-4.5A. But maybe it's a good symptom?

And finally, my nightmare. How to measure current limit? (Yes it's possible)
I get the idea. We need voltage from 13 point because it's related with current limiter (U3). Also we need output voltage (which is voltage reference) of U1 (in my case it's 10V).

I found some calculations but when I implemented it, multimeter behaves strangely.

We need voltage reference from U1 to further calculations. We can measure it after calibrations. The only value which does not change after connecting load its the value of voltage divider P2 with resistors. We need to calculate it:

Dz = P13 / (UR7 + U1ref)

P13 - probe on point P13
UR7 - Voltage drop on R7
U1ref - voltage reference on output of U1

Current limiter will work when there will be the same voltage on inputs of U3 so:

(UR7 + REF) * Dz = UR7
Knowing previously calculated divider we can state on what voltage drop UR7 with this divider current limiter will activate.

UXR7 = (REF * Dz) / (1 - Dz)

Click to expand...

Can anyone help me with this?

I can provide a source code but i don't think it's important.

How about a schematic diagram?

You mean PSU or my multimeter?
If PSU then all is here:
0-30 VDC STABILIZED POWER SUPPLY WITH CURRENT CONTROL 0.002-3 A
There is even "how it works" section but since my English is not so good I can't actually comprehend everything.

If you mean my multimeter then here (but I don't think it's necessary. I can also regulate voltage amplification because i soldered potentiometers to instead of resistors):
**broken link removed**

But that's not a problem. Measurement calculations is the problem.

Use the - output (tp4) for 1 probe. The current measured is the top of R7 -- .47 volts = 1 amp. To know the current limit setting measure from the output to TP13. This is also .47 volts per amp.

The current measured is the top of R7 -- .47 volts = 1 amp.

Click to expand...

Well not exactly because my R7 is 0.28Ohm and 10W which means its 0.28 volts per 1A

To know the current limit setting measure from the output to TP13. This is also .47 volts per amp.

Click to expand...

You mean I should just measure output voltage and voltage of P13 to the ground and subtract it?

Yes.
When the + and - at the op amp are the same it will begin current limit.

I managed to measure current but current limit is still a problem.

It works like that:
I set up voltage to 18V and current limit on maximum (no limit) voltage does not drop. But when I start to turn potentiometer of current limit, voltage drops (and current too)

So as soon as voltage drops the current limit drops too (on display).
Here is my calculations:
current_limit = ((voltage2-((ADCL) | ((ADCH)<<8))*0.0252113)*0.00391168);
where
((ADCL) | ((ADCH) is measurement of ADC (0-1023)

*0.0252113 is voltage multiplier (it's wrong, I know but I wanted to test how it would behave)
*0.00391168 is current multiplier (works on current measure but it's still wrong.)

I don't worry about multipliers because i will calculate good ones. But I think procedure is wrong.
Isn't there some other way to calculate this without output voltage? I shouldn't probably use output voltage but let say voltage reference?

If you have a load that is drawing a current equal to the current limit setting, then the voltage has to drop as you increase the current limit to further limit the current. You can't limit the current without also limiting the voltage to a value of V = IR where R is the load and I is the current limit.

Yes I know that, but my question is, how to calculate current limit? I have probes as shown above + P13.