Power supply for a beginner - Questions  - Page 1
Forum section: electronics / beginners

author: Atom
author_rank: Regular Contributor
timestamp: February 16, 2018, 07:15:40 pm
content: so here i am. Searching for a suitable power supply for my lab....i stumbled upon the desing of peter oakes and i watched the videos that explain the working principles but there are some things that bothers me.. but before here it's the link and the schematics.https://www.element14.com/community/groups/test-and-measurement/blog/2014/09/15/the-modular-bench-power-system-the-essential-diy-build-for-every-ee-student-and-old-timer-alikeso from what i've understood we are sensing the current through the shunt resistor and the voltage at the outputs with 2 op amps, comparing them with a setpoint (voltage from a pot or something else)...and making the difference, then the integrators are going to ramp up or .....here it comes the tricky part....1)the op amps can only draw current from the gate of the mosfet (but the mosfet is a voltage controlled device )because of the diode....and then what is supposed to to the zener there??2)give a reference voltage to the gate and then the op amps drawing currrent are going to pull down also the voltage at the gate of the mosfet...??3)nope thats not it because if we are putting a load that requires lets say 3 amps the voltage on the gate or VGS has to be bigger than when whe have lets say a 100ma load... how are the op amps able to feed voltage with that diode in betweeen?so could someone explain this to me... anyways when putting the circuit on the breadboard ,... at the first attempt tyhe circuit didn't worked .. the voltage dropped around 4V even if it was set to 22V so i figured out that since the op amps can only draw current (am i right ?) the voltage should have benn provided by the zenerso i swapped out the 10K resistor of the zener with a 1K resistor and the circuit started working.4) why did peter used a 10K resistor?if there are some kind of calculations behind the zener and all the stuff that are going on onto the gate of the zener could someone explain them to me or give me some hints ,, surely i don't want the dish already served.5) why the output dosen't go all the way down to zero it is something related to the opamps?then there is all the part about the mosfet..i will open another post... currently using a IRFP240 as opposed to a IRFP064 ..and for the ones wondering the op amps that i'm using are NE5532for the reference a 7805(it will be replaced by something better) for the supply of the op amps 2 lead acid 12V batteries and for the main supply a 27V 11A meanwell psu (it will be replaced by a transformer diode bridge and caps)tested with the 22ohm resisto thet you see on the fotho and the voltage is rock solid , checked with the oscilloscope and it didnt't overshoot, 220uF cap on the out opposed to the 1000uFaiming for a lower capacitance around 50uF?so that's all if there are something that i didn't explained well fell free to ask and yes english isn't my main language

author: Jwillis
author_rank: Super Contributor
timestamp: February 18, 2018, 02:33:50 am
content: I may be wrong but it looks like he's trying to use a xenor shunt regulator for the op amps power supply.But it looks wrong.

author: Atom
author_rank: Regular Contributor
timestamp: February 18, 2018, 09:29:45 am
content: i'm pretty sure that's not it because the opamps are feeded by +-12V that i'm providing through 2 lead acid batteries with a common ground, the zener has something to do with the mosfet and it's gate, BTW thanks for replying it seems you ' re the only one.

author: tautech
author_rank: Super Contributor
timestamp: February 18, 2018, 09:38:06 am
content: Quote from: Atom on February 18, 2018, 09:29:45 amNot really.We look at the thread title and with beginner added I truly wonder why you'd want to do something that isn't just plain simple until skill level develops.What's wrong with the venerable LM317 or if you need more grunt LM338 ?

author: danadak
author_rank: Super Contributor
timestamp: February 18, 2018, 03:40:22 pm
content: Being able to put a power supply into current limiting more supervaluable. A way of powering up a new design, setting a limit, andkeeping it from frying if you have an error, one simple example.Not a guarantee in all situations, but useful for some.Regards, Dana.

author: paulca
author_rank: Super Contributor
timestamp: February 18, 2018, 03:46:39 pm
content: Current limiting has kept much magic smoke inside my componentsI would add that a current read out is nearly as important. When something should be using 10mA and is using 100 something is wrong.

author: Jwillis
author_rank: Super Contributor
timestamp: February 18, 2018, 04:31:25 pm
content: Oh i see how that works now. 12 volts to a 6 volt zener gives 6 volts across the 10K resistor .6volts across 10k using ohms law I=V/R or I=6 /10000.Current through resistor and diode is 0.6 millivolts . Is that right ?

author: Atom
author_rank: Regular Contributor
timestamp: February 18, 2018, 08:35:44 pm
content: well the thing is that i would say i'm pretty good and i've alrealdy developed a circuit with the lm317 and a preregulator for keeping the heat down but i really missed current limiting. so i stumbled upon this power supply and i watched all the videos that peter made, he explained the feedback loops very well but he didn't expained what that zener is supposed to do.. probably is a weak pullup that will be brought down by the opamps the actual value exceed the set value. but as i've already written i can't understand why that resistor is a 10K resistor.the first time i have realized the circuit with that 10k resistor it didn't worked so i swapped that out with a 1k and it' started to work.. the thing is that if iload the psu with a 5ohms resistor and cranck up the voltage i can only go up to 15Vsince the opamps can only pull , this means thatthe zener isn't capable of supplying the voltage to the gate of the mosfet.am i right? i'm only searching a solution to this problem. shoud i ask directly to peter oakes?since i'm new to the forum thingi dont't know is there any way to call him and see this thread?BTW for the ones intrested into the lm 317 + prereg here it ishttps://drive.google.com/drive/folders/1wjivQtn55H0sZZFarhMQlLaM8aowif6c?usp=sharing

author: Atom
author_rank: Regular Contributor
timestamp: February 19, 2018, 08:52:46 pm
content: So here there are some updates after banging the head onto the wall a few timesi still partially understand the zener thing but the psu now works for load of 3+ amps.(there was an issue with the shunt resistor) and with the current limiting opamp.now at around 22V and 4-5A the switching psu starts buzzing i dont know if it's the power supply or some kind of instability of the mosfet?i still have to check with the oscilloscopeany suggestions?(the circuit is still on a bredboard but it will be soon ported to a prototipying for further testings and maybe with 1% resistor for more accuracy)