Need 555 timer @ 60Hz to FETs to 1:1 ration transformer 6VAC out

Thread Starter

DonFring

Joined Jul 23, 2020
7
I have seen several 555 timer projects here that run at around 60Hz but they are mostly using an output transformer to change the voltage ratio, such as in a 12VDC to 120VAC inverter. I need a pulsing AC signal of any shape that is isolated from the 12VDC power source that supplies it which a 1:1 transformer should provide.

I know people suggest for many to wind their own transformers for unique applications such as thee but these get bogged down in theory too quickly for me and I guess I am looking for something that tells me how many wraps of what gauge wire around which coil.

I had this level of training as my backgroun in the 1960's but let's just say it's been just a little too long since I left the classroom.

If anyone could help me out, it would be appreciated. I am looking to emulate in a very sloppy fashion the output of a 6VAC "wall puck" power transformer. Its only for charging a portable phone (they all require AC for some reason) from a 12VDC power supply.

I've also discovered that since VOIP adapters ground the phone line to the power supply, I have to have the 6VAC that keeps the phone charged isolated from the master 12VDC ground which is also my "sole" power supply (there's no AC outlet available in this isolated spot I'm setting it up and only 12V to run everything and running an energy wasting DC to 120VAC power inverter is not an option either, as I need something efficient)
 

AnalogKid

Joined Aug 1, 2013
9,043
I had this level of training as my backgroun in the 1960's
Hey, geezer - welcome to the forum.

High volume devices that long ago often used AC wall warts (1970's modems did this a lot) to keep cost down. Putting even a simple rectifier and capacitor circuit inside the wart was an additional manual labor process, a significant cost delta. OTOH, the device already had a power input connector and a pc board with a bunch of installed components, so adding two more parts was a very small delta.

As for your question -

What is the required output current?
How much current is available to power the inverter?

Depending on the answers, it might be easier/cheaper/faster to use two small power transformers back-to-back:

555 or other power oscillator circuit > 12 V to 120 v > 120 V to 12 V > device

If the output current requirement is low enough, two 555's make a passable H-bridge driver.

Where are you located?

ak
 

Thread Starter

DonFring

Joined Jul 23, 2020
7
Dallas, TX. Thanks! The output would be 2A at most. Need 6VAC from a 12VDC source with an efficient conversion and better if possible than using 12VDC to 120VAC inverter and a 120VAC to 6VAC wall wart combo.
 

BobTPH

Joined Jun 5, 2013
3,002
Before going ti all that trouble, you might want to see what the power jack goes to in the phone. If it only feeds a rectifier, then you can input DC. If it is a bridge, you don’t even need to worry about the polarity.

Bob
 

Thread Starter

DonFring

Joined Jul 23, 2020
7
Thanks for the reply...Actually some function can be gained with the method you are describing, however, in my case that same 12VDC power source is supplied via a 12VDC to 5VDC buck converter to a LinkSys SPA-2102-N ATA phone adapter and one side of the phone line is attached internally with the power supply which seems to contend with the shared ground when using that same 12VDC to a buck converter and supply the phone with 6VDC with the polarity that works with a random portable phone. Now I've used the subject ATA with a wall phone now and all is OK, but the moment I attach the phoneline from the LInkSys ATA to the phoneline going into the portable phone, if they are powered by the same source via respective buck converters (one is receiving 5V and the other 6V) then as soon as I attach the phone line to the two either I lose power or a horrific buzz comes across the audio. This is because the phone line is grounded inside the ATA, this shorts out the phone line if I try to share power supplies, so for this reason I still need to isolate the power supply and the portable phone would make the most sense since it is by design using a 6VAC wall-wart. Running a test using the phone's supplied 120VAC to 6VAC wall-wart, this problem doesn't exist because the portable phone is using an isolated power supply via the wall-wart.

I've tried other model VOIP ATA's and portable phone combos but appearently they all are based on a single design and the mfg either just alters the circuit enough to escape the existing pattents but more likely, all portable phones are basically the same and like most modern electronic equipment, competitor's equipment rolls off the same line only with a new company name when they switch production runs. Therefore as long as I want to use commercially available equipment including a VOIP ATA and a portable phone, attaching them together without some type of power supply isolation is not going to work.
 

Orson_Cart

Joined Jan 1, 2020
43
hello there, please see attached sketch for a full and simple solution, for more power you may need to up the 47uF caps, and change the xtors to bigger or use darlingtons, you can also put 40V 1A/2A schottkies across the xtors to maximise efficiency.
 

Attachments

Ian0

Joined Aug 7, 2020
1,581
If you need electrical isolation, but not safety isolation, use a mains transformer with two secondaries and ignore the primary. Or don't ignore it completely, put some insulation over the terminals!
 

Orson_Cart

Joined Jan 1, 2020
43
To design an isolating transformer, 6Vac to 6Vac at 60Hz, the governing formula is, dB/dT = V / ( N. Ae )

for a ferrite E55 core the Ae, or effective centre pole area = 355mm2 ( 355x 10-6 metres^2 )

thus N = V. dT /Ae. deltaB, where V = 6V, dt = 60Hz half cycle = 8.333mS & deltaB = -280 to +280mTesla for ferrite (0.56T total)

thus Npri = Nsec = 251 Turns - a bit too high it would seem

For a laminated steel core - the flux density, B, can run -1.2 to + 1.2 Tesla ( 2.4 total swing ) so the Ae.N product = 0.0208 ( m^2.Turns)

So for an EE electrical steel core of 1" x 1" centre pole area ( 645mm^2, x 10-6 = m^2 ) turns are 32.2, say 33T each.

For a steel core of 1" x 0.5" the turns would be 66, and so on.
 

Thread Starter

DonFring

Joined Jul 23, 2020
7
THank you! Hoping I can gather the materials and successfully perform the winding ....

I found some info on winding transformers:
https://ludens.cl/Electron/trafos/trafos.html
And found what may or may not be a source for laminates:
https://www.cwsbytemark.com/index.php?main_page=index&cPath=206_284

Since I will be "attempting" to wind the transformer myself, is there any advantage to wiring the primary of the transformer for 12V and center tapped and use a push-pull output from the 555 and a couple of MosFETs since the supply voltage is 12-14VDC (varrying with alternator charging or not)? Thanks again!
 

Orson_Cart

Joined Jan 1, 2020
43
... you can use a 2nd 555 as an inverter [ feed pins 2 & 6 ] to provide the other opposite o/p for a push-pull primary side arrangement, use 100 ohm turn on gate resistors for the mosfets, and a back diode across each for faster turn off - this will give you the required dead time, further, 33v zeners ascross the mosfets will add a measure of protection, 1W each.
 
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