Design new 0 to 30V power supply.

[PicMaster]

I changed the resitor values to 82K&10K this give me a max current of 4.36A

Not sure if it is me me, Put when I alter R20 the current does not seem stable and I get higher amps out put. I have got R20 set about half way current pot set near minimum voltge set to 15V then when I switxh on a .1R load the output current goes upto 16amps I turned if off quickly, I set R20 back to min and tired it again and I thought it was ok, But when I turned off the power supply then turned it back on and the same thing 16amps. It only does this once after that it does not seem to do it again until I turn off/on the Power.

I opened up the sim version on the PC and I noticed that is does the same in there but because it's a sim prog it's goes to 66amps but it does drop off down to it's normal current draw, I do not have the guts to let it run long enough in real life

Any ideas Why this should be happening

 

[Hero999]

R20 is not supposed to alter the current limit, that's R21's job. R20 controls the delay before the current limit kicks in so when it's set to the highest resistance setting there will be more of a surge before the current is limited.

There will be a surge anyway due to C7 rapidly discharging but the delay will make the surge last for longer.

I didn't experiment with changing the gain and sense resistor much with the simulator. It could also be possible that it's not so stable with the gain set lower but I doubt it, I imagine increasing the gain will make it worse and as you've reduced it, there's no need to worry.

 

[PicMaster]

R20 is not supposed to alter the current limit, that's R21's job. R20 controls the delay before the current limit kicks in so when it's set to the highest resistance setting there will be more of a surge before the current is limited.

This was before I changed the resistors to 82K&10K with a 0.1R shunt, The max current is 4.36Amps and altering tR20 does not effect the current readings now but I thought the output voltage was limited to 32V but I get 35V. Is this correct ?
I will investigate some more but can't understand why I get 16amps(to scared to leave it on for more than 2 seconds to see if it drops). Could this be-caused by me changing the shunt resistor I know you said it should have no effect, But really i can't go any further with all the other tests until I find out why or what is causing high amps

 

[PicMaster]

Hero
Can you explain a little how the current limit side of thigs work and about the role of R20 just has so I cna get a better understanding of how the circuit works

cheers

 

[Hero999]

R20 and R23 are in series so  for the sake of this discussion I mean R23 as well as R20 when I mention R20.

R20, C5 and U1C form an integrator or low pass filter. The cut-off frequency is equal to 1/(2πRC), in this case R20 and C5 are R and C respectively.

The interesting thing about an integrator is when the input is suddenly changed from one voltage to another, the output takes awhile to reach the same voltage as the input. An integrator is just an RC circuit, the time for the output voltage to reach within 38.7% of the input voltage is equal to one RC time constant, see the graph on Wikipedia.
http://en.wikipedia.org/wiki/File:Series_RC_resistor_voltage.svg

Note that the output voltage never, ever reaches the same voltage as the input but after five time constants it's within 0.7% so for practical purposes, it's equal to the input voltage.

The value of R20 makes no difference to the current or voltage setting of the power supply. It just alters how long it takes for the current limiting to kick-in. The integrator is connected to the current amplifier so there will be a delay from when the current limit is exceeded to when the output transistors are turned off.

 

[PicMaster]

I have done a bit more playing and something is not right, When I adjust R20 the amps do adjust I know you said it should not. The other thing I noticed instead of using a resistor has a load I used a 12V fan I set the voltage to 12.56V the fans runs OK then if I turn down the current pot to the Min the fans starts to slow down then stop on turning the current back up the fan spins mega fast, So I tried it again this time with a multimeter set up and I measured the voltage and it went up to 22volts ??? and that even without the voltage pot adjusted only the current
Now I'm not sure but it could be my fault I used IFRP3710 Mosfets has these are the only one I add at the time. could this be causing the problem ? to much gate capacitance ?

I shall have a look through my bigger junk box for different types  ;D

 

[roughrider1]

Hero,

Although the circuit will work the way it is, with a zener of 39volts, isn't the attached what you really want?

View attachment 40951

 

[Hero999]

PictMaster,
That does seem odd that the voltage shot up so high when the current limit was increased.

Did it go back down to 12V or did it stay at 22V?

roughrider,
Tha't's not what I intended, although it will work.

The zener was never intented to be a regulated voltage source, it's just there to ensure that the voltage to the op-amp never exceeds 39V.

It might be better that way because the supply to the op-amp will be more stable.

 

[PicMaster]

It stayed at 22V, what I shall do is move the zener diode like drawing above, and investigate more when I get some spare time

 

[Hero999]

You could try that, I don't know what could be going wrong, it sounds like one of the op-amps is latching up.

 

[PicMaster]

Hopefully with a bank holiday here I might get chance to have a look, I shall have a run over the PCB with a scope when the voltage shoot's up and see why the R20 is messing around with the current,
I know it should not but it does I can get upto 8amps by messing around with that.

I have checked my schematic against yoour and to me it's correct, Can you just double check it got me jsut incase it's wrong

thanks

mosffet_power_supply.pdf