300 VAC Voltage regulating issue - Page 1
Forum section: electronics / projects

author: MMDuino
author_rank: Regular Contributor
timestamp: October 11, 2020, 11:25:37 am
content: Hi all,I'm working on a small project and I need your help in the voltage regulation issue as following:I have; Vin = 300 Vac (100 mA) and I want to get 24 Vdc (45 mA) regulated from that AC source without using a transformer, CBB capacitors or SMPS...Your help is highly appreciated,Thanks...

author: Alti
author_rank: Frequent Contributor
timestamp: October 11, 2020, 11:48:31 am
content: Quote from: MMDuino on October 11, 2020, 11:25:37 amUse drop resistor or drop inductor followed by rectifier.

author: MMDuino
author_rank: Regular Contributor
timestamp: October 11, 2020, 11:53:32 am
content: Quote from: Alti on October 11, 2020, 11:48:31 ambro. (I need regulated)...

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 11, 2020, 01:01:12 pm
content: Add a shunt regulator....But the power dissipation of resistor(s) will be high, 15W or more !!!!!

author: madires
author_rank: Super Contributor
timestamp: October 11, 2020, 01:42:49 pm
content: Direct regulation would create about 18W heat and the heatsink would be larger than an SMPS.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 11, 2020, 01:47:00 pm
content: A piezoelectric "transformer" could work, but I'm not aware of where to get those off the shelf.I would say the most straightforward solution would be a capacitive dropper using something other than a "CBB" capacitor.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 11, 2020, 02:14:46 pm
content: Use a discrete shunt regulator to 100V-ish and then use high voltage buck converter.High power loss though, around 5 watts for the discrete regulator and whatever efficiency converter you can find. So, you'd need at least a heatsink you'd find on your typical southbridge.It's a lot easier to just get a ST viper chip with a wurth transformer.

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2020, 02:18:42 pm
content: A motor+dynamo+linear regulator.Seriously, why do you want to avoid a switched mode power supply and transformer?

author: madires
author_rank: Super Contributor
timestamp: October 11, 2020, 02:29:35 pm
content: The omniscient landfill, AKA Internet, says that CBB refers to a polypropylene film capacitor. But I haven't seen any class X rated CBB types. So a class X2 MKP should pass the OP's requirements.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 11, 2020, 02:31:14 pm
content: Light bulb + solar panel + linear regulator.Requirements are contradicting, though. The question is posed like it's a brain challenge and not an actual project, yet you are talking about it being a project.If it's an actual project, either use an off-the-shelf SMPS power supply brick or design your own.

author: TheMG
author_rank: Frequent Contributor
timestamp: October 11, 2020, 05:07:46 pm
content: We need some context here. What is this powering and why can you not use a transformer, SMPS, or capacitive dropper?Almost anything else is going to be very inefficient, and therefore large due to the high power dissipation. Not to mention that if you need electrical isolation, you pretty much need to have a transformer (either mains frequency or SMPS).Pretty hard for anyone to help you with this if you don't provide all of your requirements.

author: ConKbot
author_rank: Super Contributor
timestamp: October 13, 2020, 05:27:40 am
content: Rectifier (half wave is fine with enough capacitance ) and 5 microchip LR8K4-G paralleled with input and output resistors so they share current nicely. Thermals aren't impossible, 1.3-1.5 C/W heatsink to keep the heatsink under 50C, and the junction temps under 100C.Power dissipation could even be limited by sizing the input cap such that it sagged significantly while the half wave rectifier is off, but not enough to put the regulator into dropout.Regulated, no transformer, no SMPS, no poly film caps.

author: MMDuino
author_rank: Regular Contributor
timestamp: October 14, 2020, 06:34:53 am
content: Quote from: ConKbot on October 13, 2020, 05:27:40 amI would to say thank you very much , I think the best way is to use LR8 with low current to trigger a transistor base (MJE13005) then maybe I could regulate the output to the desired voltage, I will order LR8 and MJE13005 and I will see the results...

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 14, 2020, 10:37:37 am
content: All the solutions proposed (series or parallel regulation) all have the same drawbacks: excessive power dissipation.This is due to the high difference between the input voltage and the output voltage.It is therefore necessary to use a chopping of the input voltage to reduce its value.The solution that I propose: rectifier bridge without filtering, a PNP or Mosfet switching transistor in series with a current limiting resistor charging an electrolytic capacitor at 36V under 45 mA of consumption. A 38V 5W zener diode protects the 7824 regulator against surges and too high input voltage (max 40V).The 36V supplies a 7824 regulator which supplies the 24V 45mAThe switching is controlled by a 555 whose duty cycle is adjustable to obtain 36V under 45 mAIt's just an idea, I didn't build the circuit to test it.NB: to start up, the switching/chopping transistor must be fully on when the 24 or 36V rails are lower than 12V.

author: Zero999
author_rank: Super Contributor
timestamp: October 14, 2020, 12:20:32 pm
content: Quote from: akimpowerscr on October 14, 2020, 10:37:37 amUnfortunately, that won't reduce the power dissipation below the equivalent linear regulator. When the switch turns on, the peak current will be very high, as it charges up the capacitor, causing a high power dissipation in the current limiting resistor. The only way to reduce the power dissipation is to replace the current limiting resistor, with an inductor and freewheeling diode, but then you have a buck regulator, a type of switched mode power supply, which the origninal poster said they want to avoid.The only other way I can think of is to only allow the parts of the sinewave through, which not much greater than 12V. The simplist way of doing this would be to use a phase controller which fires, right at the end of the mains cycle, thus only allowing a small part of the sine wave through, but it's very noisy, which is probably why the OP wants to avoid a SMPS in the first place.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 14, 2020, 01:09:39 pm
content: Yes, you are absolutely right.We can then modify the principle of operation of the circuit that I have proposed: the switching transistor would only conduct as long as the rectified 300Vac voltage is less than 36V.The current limiting resistance could then be very low and his dissipation will be also low , its usefulness being just to protect the switching transistor against possible current peaks.The current limiting resistor could be replaced by a current limiting circuit of the transistor.The 555 is no longer needed, the rest of the circuit remains unchangedWhat do you think about ?NB: Schematic added. (not tested)

author: ConKbot
author_rank: Super Contributor
timestamp: October 15, 2020, 11:31:10 am
content: Quote from: MMDuino on October 14, 2020, 06:34:53 amIf going for the linear route, especially using input cap sag to lower power dissipation, be careful of capacitor aging (losing capacity, increasing ESR) which will increase how far it sags, making it potentially start going into dropout during the AC cycle after it has time to cook for a bit.Double check your thermals, I was assuming a 25 C ambient. 1C/W is going to be hard to hit in something passively cooled that isn't huge, so you're looking at a 300VAC fan also.

author: Pawelr98
author_rank: Regular Contributor
timestamp: October 15, 2020, 08:45:31 pm
content: Just use an SCR with a delay network.You may also get away with some off-the-shelf triac regulator (light dimmer) connected with a bridge rectifier.Make it fire at the end of the half-wave, where the peak voltage will be in the 40-50V range and from there use some kind of linear regulator to make it smooth.That is the most efficient option if you don't want to use a switching converter.Interference may be a problem, so please filter it well.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 16, 2020, 12:27:01 am
content: Triggering a thyristor with a rectified ac voltage of 300V (424V peak) to obtain a voltage which does not exceed 40V, even with a possible variation of 10% of the ac voltage, requires, not only a precision triggering, but also a regulation of the trigger point.A simple delay circuit is unable to do this.It is the certainty of damaging the 7824 in the short term.(Vin max = 40V)