Floating 5V 10mA power? - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: April 29, 2014, 04:55:47 pm
content: Hello.I need to provide isolated power to a piece of circuitry - 5V, 10mA max.What i'm thinking about is - capacitors let AC through, yet they provide isolation.So, if i pass 180* out-of-phased square wave over a couple of capacitors and rectify it, then i'll get my floating power supply.The schematic is attached.I tried a quick test with two attiny13 pins driving them at 200kHz, and it seem to work nicely, giving 3V and 8mA off a 4.5V supply.Now, attiny13 is over $1.5 apiece, can only give 40mA per pin, and can't work over 5V.So, two questions:1. Is that a sane solution for a floating supply? Any way to do better (minimising for component count, cost and exoticness)? A 1:1 transformer in place of capacitors is a commonly found solution, but winding a custom transformer is a couple orders of magnitude more complex than putting down a couple of capacitors.2. Is there a complementary square wave generator IC capable of sourcing&sinking a few 100mA (i need to float several pieces, makes sense to drive them off one source), that is under $1 a piece? Some 555 shenanigans perhaps?

author: cellularmitosis
author_rank: Unknown
timestamp: April 29, 2014, 05:19:04 pm
content: Interesting I've been contemplating similar ideas. I recently ordered a few of the smallest 1:1 transformers I could find, to see if I could make a 10mA floating rail:http://www.farnell.com/datasheets/1577973.pdfI figured I'd drive it with a relaxation oscillator. I'm hoping that will be the lowest noise solution possible.

author: mariush
author_rank: Super Contributor
timestamp: April 29, 2014, 05:32:13 pm
content: If money is not a problem, ADUM5000 is great chip, it's designed for usb voltage isolation, it does 5v to 5v @ 0.1 A :http://uk.farnell.com/analog-devices/adum5000arwz/ic-isolated-dc-dc-conv-180mhz-soic/dp/2102522

author: Artlav
author_rank: Frequent Contributor
timestamp: April 29, 2014, 05:42:11 pm
content: Quote from: mariush on April 29, 2014, 05:32:13 pmNo, that one is over-expensive overkill for my task.

author: sleemanj
author_rank: Super Contributor
timestamp: April 29, 2014, 09:20:03 pm
content: Search ebay for 5v isolated dc dc converter (or a 9v and follow with 5v linear), come in a small package, buck topology.

author: sleemanj
author_rank: Super Contributor
timestamp: April 29, 2014, 09:51:44 pm
content: Quote from: sleemanj on April 29, 2014, 09:20:03 pmAnd replying to myself, the DPS05R09 is boost 5 to 9 isolated, there is an ebay listing 5 for ~$10

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 29, 2014, 11:52:23 pm
content: I can't think of any complementary square wave IC but if you have something that outputs a square wave signal (such as a 555 or a MCU) then you might use something like a TC428 (datasheet) which is a dual MOSFET driver with an inverting and non-inverting output. Just feed the square wave into the inputs and connect the capacitors to the outputs.The IC has much beefier outputs than a MCU and supports higher voltages (up to 18V.) The inputs are TTL compatible so you can drive it with a MCU running at a lower voltage.I've used the IC along with a 555 to create a negative power rail before and it worked great.

author: Harvs
author_rank: Super Contributor
timestamp: April 30, 2014, 12:37:41 am
content: Look at MOSFET driver ICs. I've made some isolated low power supplies using them in the past, sorry I can't give you more since I don't own the IP.Just be aware what you're building gives you common-mode DC isolation only. Isolation will fall off with frequency.

author: ludzinc
author_rank: Unknown
timestamp: April 30, 2014, 02:26:49 am
content: Quote from: Artlav on April 29, 2014, 04:55:47 pmNo problem!http://www.paulinthelab.com/search?q=meter

author: Artlav
author_rank: Frequent Contributor
timestamp: April 30, 2014, 10:55:13 am
content: Quote from: Harvs on April 30, 2014, 12:37:41 amYeah, these seem to work, and there are dual channel ones available.555 plus one of these would be fine.I'd preferred a single-chip solution, however.Kind of like IRS2153D, only one that works down to 5V.Never quite figured out how to search for a chip by specific function - parametric searches like Digikey are of no help there.Any advice?Quote from: sleemanj on April 29, 2014, 09:51:44 pmNice, but these will be $10 EACH, while a MOSFET driver and capacitors approach can power dozens, for less price and space.

author: fcb
author_rank: Super Contributor
timestamp: April 30, 2014, 11:25:30 am
content: I've done it with little digital audio transformers and a 555 before. They are sub $1.Try the Murata DA101C, available in SMT or THT.http://uk.farnell.com/murata-power-solutions/da101c/transformer-dadt-1-1/dp/1362398

author: Joule Thief
author_rank: Regular Contributor
timestamp: April 30, 2014, 11:33:13 am
content: How abouthttp://www.ebay.com/itm/360860072660?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649I tried one on an LED meter drawing over 30ma. They worked fine with just 4 pins to connect and very small footprint on the pcb. Five volt variants are available alsoGood topic. Very timely for me as I have been trying to investigate a cheaper approach with discreet components. It functions but only to about 7 ma. You may be able to fine tune the circuit below to fit your needs.(scroll to the end of the web page).http://www.techlib.com/electronics/regulators.html

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 30, 2014, 11:52:31 am
content: Quote from: Joule Thief on April 30, 2014, 11:33:13 amThat's a good one-chip (and a few passives) solution.You could use a 74AC04 instead of the 74C04 to get more current. You might actually have trouble finding a 74C04 these days.

author: ajb
author_rank: Super Contributor
timestamp: April 30, 2014, 01:55:13 pm
content: Quote from: Joule Thief on April 30, 2014, 11:33:13 amMurata and Recom also make modules like this, beware though that some are regulated and some are not.

author: Zero999
author_rank: Super Contributor
timestamp: April 30, 2014, 05:58:47 pm
content: Rather than using a bridge and to outputs, try a Schottky doubler and one output.

author: zapta
author_rank: Super Contributor
timestamp: April 30, 2014, 06:02:38 pm
content: Quote from: Artlav on April 29, 2014, 04:55:47 pmThey provide isolation for DC only. Be careful if your goal is to isolate from mains power.

author: Zero999
author_rank: Super Contributor
timestamp: May 01, 2014, 10:15:40 pm
content: Quote from: zapta on April 30, 2014, 06:02:38 pmThat can be done, as long as you use Y2 rated capacitors and ensure the capacitance is low enough to limit the leakage current to a safe level.

author: David Hess
author_rank: Super Contributor
timestamp: May 01, 2014, 11:53:50 pm
content: Quote from: Artlav on April 29, 2014, 04:55:47 pmIt will work. Whether it is sane depends on the details. Remember to use capacitors with a rated voltage higher than your maximum isolated voltage difference. I would voltage derate them as well. I would not rely on this circuit for safety in an off-line application.CRT DC restorer circuits do something similar to cross a DC isolation barrier to the roughly -2000 volt DC cathode potential and drive the CRT grids.QuoteThe usual solution is to add a level shifting power stage operating at a slightly higher than needed voltage to compensate for the voltage drop across the diodes. A boost switching regulator works for this.

author: Artlav
author_rank: Frequent Contributor
timestamp: May 02, 2014, 03:26:40 pm
content: In the end, a 555 oscillator driving a TC4428 inv/non-inv fet driver does the job.Two outputs driving two capacitors, with two diodes and capacitors making a voltage doubler to account for the diode drop.Thanks all.Quote from: Joule Thief on April 30, 2014, 11:33:13 amNice, but these are modules that are A. $10 apiece, B. unobtanium (require ordering over the internet).I want a discrete component solution, something i can make at any time without having to wait a month for parts to arrive.

author: mariush
author_rank: Super Contributor
timestamp: May 02, 2014, 04:24:38 pm
content: That Adum5000 i mentioned above is 5$ when you get 100 pieces. One chip, 11mm by 8 mm , soic16... no other parts required (well, maybe some 0.1uF decoupling caps, optional and cheap, pennies).Your solution is maybe $0.25-0.40 for 555, 1$ for tc4428, 1$ for resistors and capacitors for 555, diodes, capacitors for voltage doubler .. so about 2-2.5$ ... but add the pcb space wasted (if you pay for square inches at a pcb factory) and you're up to about 3-4$.Is it really worth dealing with so many parts just to save 1-2$? It's up to you.

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2014, 12:34:38 pm
content: Are you sure the 555 timer is really necessary?It should be possible to get the inverting MOSFET driver to oscillate.The outputs of a 74HC14 can be paralleled and should be able to drive a 10mA load with very little voltage drop.