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| Help me design a PSU |
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| nemail2:
sure, there are several caps for stabilization in the circuit. two on the opamps for voltage and current control, an input- and an output-cap. i have built the psu already, nothing oscillates so far. did test it with resistive, capacitive and inductive loads. |
| VEGETA:
--- Quote from: nemail2 on December 04, 2018, 07:14:15 am ---sure, there are several caps for stabilization in the circuit. two on the opamps for voltage and current control, an input- and an output-cap. i have built the psu already, nothing oscillates so far. did test it with resistive, capacitive and inductive loads. --- End quote --- How much max voltage and current does it output? Can it work with pre-regulator? down to 1v diff for example? |
| nemail2:
--- Quote from: VEGETA on December 04, 2018, 10:35:20 am ---How much max voltage and current does it output? Can it work with pre-regulator? down to 1v diff for example? --- End quote --- Dave did implement a pre-regulator, however he never finished the µSupply (until now, he says from time to time that it IS actually happening). My design does not have a pre-regulator but I might implement that later, maybe. The PSU does 0-12V (potentially 0-20V but you'll have to change the Opamp supply line and pay attention to the linear regulators power dissipation as well as maybe tune some other parts to the higher voltage) and 0-4A. if you change the opamp gain, you'll get out even more amps, however you'd also have to pay attention to heat dissipation, especially without the pre-regulator and at low output voltages. |
| Efe_114:
--- Quote from: VEGETA on November 14, 2018, 06:43:03 am ---To make it go down to 0v you would need a negative rail. To make this very easy for you, get ICL7660 charge pump IC. Feed it with say 5v and it will output -5v with low current of course. This will be the negative rail of your op-amp which sets the output voltage... it could be something like +25v positive rail and -5v negative rail which will ensure 0v output. It is better to put more filtering caps on 7660 output since it generates lots of noise, or better yes as I did in my dummy load design.. which is to give 9v to 7660 so it outputs -9v (or -8v with losses) then put TL431 after it which generates 2.5v in my case (you can make it anything via 2 resistors) with very low noise, now add another 100uF cap after it and you'll be done. I hope this helps you. --- End quote --- when i was re-reading the replies i re-noticed this comment and i came up with these questions: why do i need negative rail? if im going to use darlington pair and op-amp. i could use a lm358 which could go down to 0 volts with single supply? i didnt understand why do i need to use op amp. dont i need negative supply for lm317 adj pin ? I am confused. which configuration i can use 7660 lm358 tl431 and lm317 in the same time?(as you say in the next reply) |
| nemail2:
--- Quote from: Efe_114 on December 18, 2018, 08:52:35 pm ---when i was re-reading the replies i re-noticed this comment and i came up with these questions: why do i need negative rail? if im going to use darlington pair and op-amp. i could use a lm358 which could go down to 0 volts with single supply? i didnt understand why do i need to use op amp. dont i need negative supply for lm317 adj pin ? I am confused. which configuration i can use 7660 lm358 tl431 and lm317 in the same time?(as you say in the next reply) --- End quote --- if you are going to use a darlington and an opamp like LM358 oder OPA2197 (like I do in my design), you won't need a negative rail. if using a DAC to set the voltage, you'd be basically limited by the minimum voltage the DAC can output + opamp gain you'll be using (e.g. DAC can output 5mV * opamp gain of 3, would give you 15mV minimum output voltage). of course if your opamp can't output such a low voltage, you'll be limited by the opamp. but basically you can go pretty low without having a negative rail here (I do that, and it works perfectly). if you are going to use an LM317 you'll need a negative supply if you want to go all the way down to zero to negate the minimum output voltage of about 1,2V of the LM317. but I wouldn't do that, might make the LM317 under certain circumstances. you do need an opamp for the darlington because in this constellation the opamp would be the "controlling device" which in the end sets and maintains the voltage you wanna have. you can apply current to the base of a darlington but how'd you know the voltage it will allow to pass? with an opamp, you feed in the voltage you want into the non inverting terminal of the opamp, the opamp output goes to the darlington base and the darlington emitter goes to the inverting terminal of the opamp. the opamp will output whatever voltage it has to (limited by its supply voltage) to make the inverting and non-inverting inputs the same. so it will do whatever it can so the output voltage of the darlington matches the input voltage at it's non-inverting terminal. because you don't have DACs which can output tens of volts usually, you'll want to put a gain into the opamp circuit to increase the maximum output voltage (e.g. DAC output is 5V max, you want 20V so you have to put in a gain of 4). also, I think only opamps which are "unity gain stable" can be used without any gain resistors. the datasheet will tell you if your opamp is unity-gain stable. for other opamps, you'll need a gain to make them work stable. there is much more into making opamp circuits stable than I understand so here would some real PRO have to kick in but I hope you get an idea of what it is all about. |
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