How To turn Dc to Ac

Dodgydave

Joined Jun 22, 2012
9,923
You can use an ordinary 50/60hz transformer, as long as its a centre tapped type, choose one that will give you a ratio of 3:1, you can wind your own or get a ready made one and unwind it,
so for example an input of 120 V and output of 40V is a 3:1, or 230V in and 80Vout.
 

Thread Starter

szymonm2

Joined Jan 26, 2016
17
You can use an ordinary 50/60hz transformer, as long as its a centre tapped type, choose one that will give you a ratio of 3:1, you can wind your own or get a ready made one and unwind it,
so for example an input of 120 V and output of 40V is a 3:1, or 230V in and 80Vout.
I have a 2:1 power supply lying around. Would that work?
 

spinnaker

Joined Oct 29, 2009
7,835
I was hoping that you could make a variable dc-ac circuit?

My power supply already has a variable 0-30V Dc range? Would that help in any way?
So you want to go through the complexity of building an inverter or expense to buy rather than purchasing a transformer or variac?
 

tcmtech

Joined Nov 4, 2013
2,867
Scratching my head all the way around on this one. Seems like a way overly complicated and unnecessary process to just run a small AC motor.

If it was me I would just use a variac and if isolation was a concern add a isolation transformer in some place.
 

Kermit2

Joined Feb 5, 2010
4,162
To bad he didn't want run a tube radio in his car. I could have showed him how a vibrating relay can make 300 VDC out of a car battery 12 volt input. Nary a transistor or regulator chip in sight. One "could" build the entire device and never use an electrical power tool of any kind except lead acid cells.
Could is the word the truth pivots on. You could do it, but why would you? Unless, of course, someone double dared you? Then you have to do it. :)
 

CraigC

Joined Jul 23, 2012
10
If you don't want to use a transformer, variac, or inverter, and the voltage needs to stay the same, then you are scraping the bottom of the barrel in terms of options.

You can use a MOSFET H-Bridge and filter circuit which could be operated at 50/60Hz with only a minor drop in the voltage. This would be the simplest way of doing it yourself, but I think everyone elses suggestions would be way easier/faster to implement.
 

Tonyr1084

Joined Sep 24, 2015
5,830
Forgive my lack of knowledge about such things, but I have a variable fan speed controller that came out of a kitchen exhaust fan. This is a dial-up type of motor speed controller that I've plugged many different fans into and have successfully varied their speed from virtually zero to full speed. I mostly use it to vary the speed of my polishing equipment, as excessive speed can cause heat that can warp plastic.

Now: I don't know how that varies the power going to the motor. But if (and that's a BIG "IF"), but IF it's varying the voltage (scaling the sign wave down for the lower speeds) wouldn't that also work on a transformer? HEY! I'm GUESSING, but could that possibly solve your problem? Using a variable motor speed controller to power a step down transformer? I suppose I could go out in the garage and grab the thing and bench test it on the O-scope to see what it's doing to the voltage and wave from. But if I suspect this DOES work then - - - well - - - I believe in "Why build when you can buy?" OK, I know it's fun to build, and that's what I do a lot of. Even taking things that it is cheaper to replace than to repair, but I repair them anyway, just for the experience and satisfaction of figuring it out.

I see a lot of questions being asked and not getting clear answers either. And what about a MOSFET? Can't you use a device that can handle 75 amps to regulate the flow of AC from a step down transformer? This isn't anything I've given much thought to, but it makes me think it might work. NO?
 

apqo1

Joined Oct 5, 2008
52
... I have a variable fan speed controller that came out of a kitchen exhaust fan. This is a dial-up type of motor speed controller that I've plugged many different fans into and have successfully varied their speed from virtually zero to full speed.
That's almost certainly a phase angle controller using a zero-crossing detector to drive a TRIAC. It's a very common way to cheaply accomplish incandescent light dimming or control speed in small AC motor applications. These circuits can be electrically noisy and are load-limited by the current rating and heat sinking of the TRIAC.

Something similar would probably work fine for the OP's purposes, but should be approached with great care, as it provides no mains isolation.

See: https://en.m.wikipedia.org/wiki/Voltage_controller
 
Last edited:

avayan

Joined Oct 30, 2015
38
All you need is an H Bridge. Now the question is whether you want to use an square wave or actually synthesize a sine wave. Since the output frequency is about 60 Hz, pretty much any H Bridge with any microcontroller could do this.

With Square wave Output:
  1. Find a 60V or higher voltage H Bridge (80V would be optimal as there will be voltage swings). Ideally, the H Bridge is of the PHASE/ENABLE control topology.
  2. Put a 60 Hz TTL signal on the PHASE input.
  3. place the logic signal required to assert the ENABLE signal.
  4. You will not observe a 30V output AC square wave at the H Bridge outputs (60V Peak to peak).
With Synthesized Sine Wave Output (this will require a microcontroller):

  1. Pretty much everything as before, but now we will use a microcontroller with a PWM output to drive the ENABLE line and a GPIO to drive the PHASE input.
  2. You will need to program on the microcontroller code a lookup table with values which will make the PWM output's duty cycle oscillate such that half of a sine wave is generated. You can use excel to generate this lookup table information. It will have to go from duty cycle 0 to duty cycle 100%, but must have the shape of a sine wave.
  3. Run the table twice. One with PHASE HI and one with PHASE LO.
  4. Do this fast enough so you end up with a 60 Hz sine.
Hope the info helps!

JIQ <SNIP>

Moderators note: removed unwanted link
 
Last edited by a moderator:

Plamen

Joined Mar 29, 2015
101
I just happened to have a rather chunky stepdown xformer of 800 VA capacity. 480 to 120 single phase.
I used it with a variac to get a 0 to 30 volt AC out with 120 AC input.
It was convenient for testing the two universal motors I scavenged out of old vacuum cleaners. Full voltage on them without a significant load means possible runaway self destruction.
And they would really spin with 12 volts DC input from a lead acid battery
Petkan:
AC powered vacuums use universal motors that can sustain no load indefinitely.
Lowering the voltage (say by dimmer) may allow lowering the speed.
Battery powered vacuums use permanent magnet DC motors. Their speed tracks proportionally the applied voltage.
Bear in mind motors are rotating transformers and as such flux density in the core tracks the applied voltage minus the de-magnetizing effect of the load. Normally the applied voltage is partially compensated by back EMF leaving relatively small portion to drive the current. Upon application of voltage the back EMF is not established yet -- hence the inrush current and characteristic sound. Both motor (even unloaded ) and transformers have this inrush current (up to 5 times the rated).
 
Top