EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Romanizer on September 12, 2020, 04:49:18 pm
-
I am trying to rebuild the infamous Objective 2 Headphone Amplifier as a Balanced unit. The old Power supply consisted of a positive and negative 12V regulator, a on switch, and a power conditioner. the conditioner waits for the switch to stabalize the rails and then pass clean +/-12v to the op amps. My design is going to replace the last two components with a mosfet switch for both rails (see picture). my question is if this is going to work. I want a mcu to switch on the opto isolator to turn on both mosfets at the same time, exactly. the 10k resistors should create a crude virtual ground (doesnt need to be precise at all) and the mosfets should turn on at the same time. Are there any mistakes?
[attachimg=1]
The rails +12V and -12V are coming right off the regulators.
The rails V+ and V- are going to the Op Amps.
-
That should work.
You'll get roughly ~0V at each gate (+/- VCE of the opto) which should be enough to turn it on. Make sure those gates are safe with ~12V between G-S.
Bear in mind that there will be some leakage through the opto which could turn one/both of the FETs on when it's supposed to be OFF. With 10k/1k resistors it can probably be negligible. Same issue exists if it's not exactly +12 & -12V. eg if one is 13V and the other is -11V the opto will find the middle around ~1V (+/- VCE of opto). Could be some funny business when 12V/-12V are turned on or loaded down.
Also bear in mind that insufficient drive on the opto-input (and thus insufficient current sink thru opto C-E) will result in a half-on FET.
Finally, the actual value voltage at each gate won't exactly be zero--leakage through the gates will draw one side or the other up/down. Again, 10k/1k are nice and low values that it should work fine.
I'd personally use the opto to drive a NPN/NMOS + PNP/PMOS to pull the gates of the PFET+NFET respectively. No funny business with resistor values because you'd tie the (eg) NPN collector directly to GND so you know exactly what voltage it will pull to.
-
Usually switching the regulator itself is better because the output can be more closely controlled, but that should work.
-
Use 2 low-power solidstate relay if the current is small and a resistance of about 1 Ohm is acceptable. Or use 2 MOSFETs that are controlled by an cpecial optocouplers with a group of LEDs generating a control voltage for the MOSFET of about 9V Toshiba TLP3905/3906.