Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Doctore, Aug 17, 2013.

1. ### DoctoreMember

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You mean by top on pot, the center pin of pot?
Or I have to set pot to max w/o load?

EDIT:
With out load, at POT set to max, I have on center pin .730V

Last edited: Dec 30, 2013
2. ### ronvWell-Known MemberMost Helpful Member

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Load shouldn't make any difference as the pot just has +12 and ground. So .73 is within tolerance.
The way the circuit works it will make the - input of the op amp the same as the + input. To do that the current across the .13 ohm must drop that voltage making a constant current. So . I = E/R or .73/.13 or 5.5 amps each fet. So you should have 55 amps maximum. You might want to measure each .13 ohm to make sure they all have the same voltage under some load - say .13 volts = 1 amp each or 10 amps total. It could be one fet is not on. That would make the total current low.

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3. ### DoctoreMember

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I measure all .13 resistors, and they have about .109-.120V, most .110V, at 10A load(and 13V supply)

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5. ### ronvWell-Known MemberMost Helpful Member

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Hmm. Well you can change the 68K resistor to 56K. Does your meter have a current shunt?

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6. ### DoctoreMember

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Ok.
My meter theorically can measure up to 10A, but wires are thin and voltage go down, but up to 2 amps is okay.

7. ### ronvWell-Known MemberMost Helpful Member

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It is probably just tolerances. I think it would be ok to change the 68K (R26, I think) to 56k. This will increase the maximum current.

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8. ### DoctoreMember

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I replace R26(68k) to 62k(I dont have 56k) and on center pin of POT I have .791V like in your calculations
But You calculate that is about 60AMPS, this is too high because FETS have max. 57A :/
Potentiometers required in next dummy load :|
I dont know if you remember, but we had problem with displaying amps and temperature on chinese panel voltmeters.
And when I have 50A load, on display I ses 5.0, but I find schematic of this voltmeter and ask my polish boys for increase 10x displaying value.
They tell that I have to replace R1(390k on SCH) to 10x smaller(39k) So I order this resistor(2\$ for 100pcs with shipping, from my country) and then replaced.
So for test I connect 5,6V and I see on voltmeter 39,6V, the problem is that the R1 is not 390k like in chinese auction, but 320k, so I have to put 32k, but I buy precision potentiometer(50k) and solder jumper in R1 place, and now I have correct 10x readings fitted with my multimeter!

9. ### ronvWell-Known MemberMost Helpful Member

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No, It is okay. That is 57 amps each when switched fully on. But in our case they are not fully on. They have 13 volts at 6 amps each. So our problem is power, not just current.
Ahh ha. So maybe the problem is the measurement. Here is my suggestion. The .13 ohm resistors are 5% tolerance and since there are 10 of them the average will be better than that statistically. So lets say you set the load until the average voltage drop across the .13 ohm resistors is .26 volts. Then set your meter to read 20 amps.
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10. ### DoctoreMember

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Wait, I tell you about voltmeter just for info, there are no incorrect readings.
Load set to 3,1A(displayed in chinesee voltmeter), current flow is 3,2A, voltage from PCB for AMPS is .327V, so I think readings are almost ok.
I have to test maximum load, my test station(kitchen ) is busy
What mosfet is the best ever you seen?

11. ### DoctoreMember

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Ok, after next test, I have some new info.
When I was turning load to max, 41,42,43,44,45,46,47...46,45.....42, pot turned to max.
So this is problem with potentiometer or bug with circuit, I try to use lover value of R26.
But when I want to read voltage on center pin of potentiometer, load amperage jumps to 60AMPS!!!
And I continue to test temperature with this 60amps(at 12V), this temp. go to 53,5C max.

12. ### ronvWell-Known MemberMost Helpful Member

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Sounds like a bad pot. The temperature measurements look good! .04C/watt. I think you are a victim of tolerances, so no big problems.

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13. ### DoctoreMember

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What is max watt, volt and amp for STP80NF12 used in DL1(by jocanon) with my good cooling?

14. ### ronvWell-Known MemberMost Helpful Member

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Your FET is a little better.

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15. ### DoctoreMember

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Who is the master?!

I get 90AMPS!
Mosfets are still working, but please don't suggest on temperature(first display) because water was from pail, not from tap as it should be.
Power supply was rated for 72AMPS, but OLP shut down PS on 93-95AMPS
And you can see case is not complete because I don't have time for this

16. ### ronvWell-Known MemberMost Helpful Member

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Looks good!

18. ### DoctoreMember

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My first soldering of mosfets to copper pipe was terrible, now I know how to do this correctly, even "proffesional" and I will post here how I will do this next time.
The biggest problem was not to heat too much mosfets and escaping solder from mosfets.
So I got idea with soldering paste and capton tape.

Example is on PCB but this is the same with copper pipe.

And here is what we need:

This is capton tape and soldering paste in syringe(http://www.dx.com/p/lodestar-soldering-paste-50g-4711#.U1zliVd5Kt8) this is heapest paste I find, but this paste is very good and do what it should do, but this paste don't like air and should be storaged in fridge.

So, frist clean copper pipe/PCB and put capton tape like here:

Soldering process can be done in kitchen oven, we don't need PRO oven with temperature controlling.
I use little oven.

Put your copper pipe/PCB on aluminum foil:

And in my dirty oven:

Then keep an eye on this, when solder paste will be melted turn off and open your oven
to cool off.

Then unstick capton tape and clean from flux using alcohole.

EFFECTS:

This solder looks like from fab!

This is simple method to hold solder in determined place and by surface tension of solder, mosfet is on place we want.
I not will be angry if anyone tell that this is not PRO

Last edited: Apr 27, 2014
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19. ### jocanonMember

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wow, that's a great idea. I didn't know you could cook the MOSFETs, it doesn't break them? How hot can you get the oven before the MOSFETs are toast?

20. ### DoctoreMember

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If you get them back quickly from oven they will be not burned, remember, in fabs electronics are soldered in ovens, but temperature is controlled and many other parameters.
I solder many circuits in this oven, eg. SMT resistors, capacitors, atmega uC etc. w/o any burned part.
Soldering point of this paste is about 200C and max temp possible to get in this oven, about 350(bimetalic thermostat), but when I have about 250C I turning off oven and open doors to coll of components.

EDIT:
Cooked in my dirty oven :

Last edited: May 1, 2014
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nice