The relays only provide a limited degree of protection. Switching a mechanical relay is slow and with a high DC current a relay may not even reliably turn off. What is the purpose of the relays anyway.
the device (relay) is normally open, so mosfet is not connected to input voltage. mCU will sense the voltage through smaller relay SR1 at the beginning, if its over voltage (greater than 100V or whatever value it will be), SR1 will disconnect and indicates fault led, user need to reset (by pressing Reset button) or disconnect the Vdc to restart mCU. during device (mCU) switched off or fault condition, the mosfet (and mCU) is guaranteed to be isolated from outside voltage. SR2 and SR3 will only connected if (1) input voltage (Vp+) is within range at all time, (2) mCU will output 0A at iset, and (3) user press Run Button momentary switch. after conditions are met, mCU will connect the relays and start ramping up iset from 0 to user selected potentiometer/trimpot position or mCU calculated current limit based on the device specified power limit, whichever is lower.
the relay will disconnect on the following conditions. (1) user push again the Run (Stop) button. (2) Vp+ reach down to (equal or lesser than) the voltage cut level (vcut). (3) user disconnect Vp+. in both run or stop mode, ie Vp+ = 0 (4) mosfet damage is detected. in all conditions, before disconnecting the relays, mCU will ramp down iset back to 0, for the safe disconnect. only condition (4) and "Vds plate is shorted" will not be guaranteed 100% in which fault led will lit, run led will off, but tremendous amount of current will go through shunt resistors. in any case whenever fault led is lit, user should immediately disconnect the Vp+ to avoid further damage to other components such as the gate driver and upstream.
the design will be based on assumption that... mosfet are not damaged. if they are, the current flow will be unspecified and the only hope is the mechanical relay disconnect. we can do redundancy protection but i believe the hobbiest will not be interested at buying the price
If it is for protection, it might be worth considering two MOSFETs in a kind of kaskode configuration
i agree mechanical relays provide limited protection, but electronic switch such as you mentioned, is not 100% bulletproof either. there is even possibility that the protection mosfet will got damaged first during operation before the constant current load mosfets do, in which case there is no more protection, worst if user has no visible clue about this. tyring to do some checking on the protection mosfet will expand the circuit even further imho.
Would you, for example, not like to be able to use it to test a 3V supply, or discharge a Ni-mh cell down to .8V?
draining cell batteries below 0.8V is asking for trouble
anyway i appreciate your concern, if i want this device to work at low voltage i will not populate D6. but just in case i need it for high voltage purpose, i have pads to put it on.
No, but there are a huge number of other un-necessary components which you could include in your circuit; why choose this one?
because i anticipate the damage due to reversed gate voltage, say -1KV spike. and again i can simply unpopulate this component.
It's not about the 10V rail - it's because D6 clamps Vgs to a maximum of .5V above Vds. If the threshold voltage of both FETs is 4V (Vth spec is 2V min, 4V max), then Vds can't drop below 3.5V without the FETs turning off
ok now i'm getting you. thinking about it the matter you are trying to fight, it seems this is a strong reason to use zener clamp instead of double diodes. i will think about this. thanks alot.
Also with a larger mcu then you could easily add communcations to allow remote control and logging via serial port/USB/blue tooth module etc. for very little extra cost making it a considerbly better piece of test gear.
no no no please dont add insult to injury. i'm on a very time limited budget here. i have no time to program all that, and i still love analog potentiometer rather than push button and very deep menus
in version 2?, maybe yes. but i need this one working quickly as the project i should be working on right now (adjustable psu) is requiring this device. and 2nd point, i dont have large mcu other than atmegas that are stuck on the arduino boards. i think i have some atmega 44 or 30 pins laying around but, as i said, stock limited, large mcu is only for very serious projects (3 years development time budget) and no time for programming, it could take the whole year.
i'll read the rest of your post later as i'm at work now, thanks alot for your time and effort guys...