dual power supply with two different temsions - Page 1
Forum section: electronics / beginners

author: arivel
author_rank: Regular Contributor
timestamp: August 14, 2020, 12:55:12 pm
content: Hi everyone. I would like to know if it is possible to derive a smaller dual voltage from a larger dual voltage by making use of capacitor voltage dividers (or resistors) and mosfet voltage followers. I attach a photo . the scheme is simplified, do not look at the values that are put at random because it only serves to render the idea

author: Zero999
author_rank: Super Contributor
timestamp: August 14, 2020, 01:19:30 pm
content: Not, at DC. Capacitive voltage dividers only work at AC.

author: arivel
author_rank: Regular Contributor
timestamp: August 14, 2020, 01:43:40 pm
content: the function of the capacitor dividers is only to power the gate of the two mosfets configured as voltage followers.

author: Ian.M
author_rank: Super Contributor
timestamp: August 14, 2020, 03:14:28 pm
content: Still no. MOSFETs make *LOUSY* voltage followers (too much Vgs_threshold uncertainty) and need a DC (i.e. via a resistor divider) gate connection to initially charge the gate to the desired voltage and maintain it there against leakage.Just use another pair of regulators for the desired output voltage. If you don't need much current you can probably get away with tapping off the outputs of the existing regulators, and wont need much if any heatsinking, but if they are going to be dissipating more than a few hundred mW, you'd do better to tap of the unregulated input to the existing regulators and heatsinking the new regulators properly.

author: Brumby
author_rank: Unknown
timestamp: August 15, 2020, 12:03:33 am
content: Change C12,13,14 & 15 to resistors and you have half a chance.But the simplest solution (and the way I would do it) is this:Quote from: Ian.M on August 14, 2020, 03:14:28 pm

author: arivel
author_rank: Regular Contributor
timestamp: August 16, 2020, 02:53:15 pm
content: I'm looking for the best way to get two dual power supplies with the least possible waste of energy.the smallest is + 2.5-2.5 and the other much larger + 12-12 but it could be bigger. The difference between them is high.no the current is not high, it must supply logic integratorsI think at most 500 but

author: Ian.M
author_rank: Super Contributor
timestamp: August 16, 2020, 03:08:39 pm
content: Quote from: arivel on August 16, 2020, 02:53:15 pmWell, if you need energy efficiency you are going about it completely wrong. Some sort of switched mode solution running off the raw unregulated DC input voltage, would be far more energy efficient. Linear regulators like the 7812 and 7912 on your schematic throw away as many Watts as heat as the voltage drop from input to output multiplied by the load current, (plus for 78xx/79xx regulators, 5mA multiplied by the input voltage for their own quiescent consumption).To get better advice, what's the max. and min. load current for each rail, the mean voltage tolerance range and how much peak to peak ripple and noise can each rail tolerate?Also 500 what? 500A @2.5V would be a major $EXPEN$IVE$ project. 500mA would be reasonably doable with common parts or off-the-shelf modules, and for 500uA you wouldn't even bother trying to save power - a resistor dropper running a 2.5V reference IC from the closest higher regulated rail would be the best/cheapest option.

author: arivel
author_rank: Regular Contributor
timestamp: August 17, 2020, 02:12:03 pm
content: Quote from: Ian.M on August 16, 2020, 03:08:39 pmI'm not good at English, I didn't understand this sentence well:"a resistor dropper running a 2.5V reference IC from the closest higher regulated rail would be the best / cheapest option."

author: Ian.M
author_rank: Super Contributor
timestamp: August 17, 2020, 03:06:05 pm
content: Quote from: arivel on August 17, 2020, 02:12:03 pmSee attached 500uA dual (+/-) shunt regulator schematic. Max. current is set by the resistors' value and the supply voltage - 2.5V, up to the LM385's abs. max. current of 20mA, but over a mA or so you'd be much better off using low quiescent current series regulators, which will only draw what they need for its output current + their quiescent current.LM385-2.5datasheet

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 17, 2020, 03:29:16 pm
content: NEINdivide DC with resistors like others already suggestedregards,pierre