Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently May Thu 24, 2018 8:23 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 13 posts ] 
Author Message
 Post subject: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Thu 12, 2017 10:11 pm 
Member

Joined: Apr Thu 12, 2007 3:24 am
Posts: 1347
Location: Milwaukee,WI
I have a cheap inverter that I wanted to use to run some old tube radios when selling at a swap meet.Problem is that hash coming from it is so loud and annoying. Google searches on the subject of output filtering are heavy into formulas and light on actual values of parts for this application. So much of the discussion mention value changes for different types of loads. The radios I would be powering would be of the transformer type drawing between 40 and 70 watts.

Does anyone have a circuit or link with part values for this type of application?


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Thu 12, 2017 10:31 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 847
Location: Wayside, NJ Monmouth
Send you a PM


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Thu 12, 2017 11:37 pm 
Member

Joined: Apr Thu 12, 2007 3:24 am
Posts: 1347
Location: Milwaukee,WI
Thanks. Just to specify:

What I am looking for is a brute force 60HZ low pass filter with a large inductor and capacitor which will help clean up the 60hz square wave out of the inverter into something that kind of resembles a sine wave. That should help clean up the hash.


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Fri 13, 2017 5:57 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3427
I built this years ago and the hash filter does make the wave trapezoidal. It also reduces the power, in this circuit anyway. YMMV.

http://www.americanradiohistory.com/Arc ... -Lytel.pdf


Attachments:
Image1.jpg
Image1.jpg [ 148.63 KiB | Viewed 2385 times ]
Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Sat 14, 2017 1:06 am 
Member

Joined: Apr Thu 12, 2007 3:24 am
Posts: 1347
Location: Milwaukee,WI
Thanks. I'll give it a try. Were you able to wind your own smaller choke or does it need to be a good chunk of iron to avoid saturation and have room for the turns of thicker wire?


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Sat 14, 2017 3:07 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3427
forumuser wrote:
Thanks. I'll give it a try. Were you able to wind your own smaller choke or does it need to be a good chunk of iron to avoid saturation and have room for the turns of thicker wire?

I used a choke from Radio Shack that looked like this: https://www.radioshack.com/products/rad ... h-rf-choke

But mine has only a single winding that covers more of the core. It might measure anywhere from 10-50 uH. My recollection is that RS used to have two chokes available. Certainly you can wind your own. Even without a ferrite core 10uH wouldn't take a lot of wire.


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Sun 15, 2017 7:37 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4406
Location: Perrysburg, OH, U.S.A.
forumuser wrote:
What I am looking for is a brute force 60HZ low pass filter with a large inductor and capacitor which will help clean up the 60hz square wave out of the inverter into something that kind of resembles a sine wave.


I'm not sure your idea would give you the result you're looking for. In a square wave output inverter the RMS output is equal to the peak of the square wave, or in your case 120v. A square wave is made up of all odd-numbered harmonics starting with the fundamental for a 60Hz square wave these are 60Hz, 180Hz, 300Hz, 420Hz., etc. The relative amplitude of each of these to the peak value of the square wave is An = 1/(n * Π). In this case this would be, for the first through 9th harmonic:

Attachment:
Harmonic Amplitudes of 120v pk Sq Wave.jpg
Harmonic Amplitudes of 120v pk Sq Wave.jpg [ 57.75 KiB | Viewed 2317 times ]


If you were able to eliminate all of the harmonics above the fundamental you'd be left with a 54 VRMS, 60Hz output voltage.

John

_________________
“Never attribute to malice that which can be adequately explained by stupidity.”
― R. A. Heinlein


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Mon 16, 2017 12:59 am 
Member
User avatar

Joined: Jun Fri 20, 2014 7:10 pm
Posts: 828
Low cost modified sine inverters have an output waveform that looks like this:

Image

The peak voltage of the modified sine waveform is set so that it matches that of normal sinusoidal AC power (about 170 Volts in the US). The duty cycle is set so that the modified sine wave has the appropriate RMS voltage.

One thing worthy of note is that many AC inverters have a "bridged" or "balanced" output topology, where the hot and neutral prongs are both driven by push-pull transistor pairs. This is why these inverters carry warnings about not shorting the neutral to ground (say by passing the output through a breaker panel).

If you have some RFI filters, perhaps like this one Image where you know that the neutral won't be shorted to ground, you could try putting them on the output. The filter shown in this illustration has a balanced construction, which should be more effective than a simple pi filter because it will filter both the hot and neutral lines.


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Mon 16, 2017 2:24 am 
Member

Joined: Apr Thu 12, 2007 3:24 am
Posts: 1347
Location: Milwaukee,WI
My inverter will be in my truck simply for plugging in 2 pronged radio plugs for radio testing at swap meets. But my plans may all be for nothing.
After testing several radios with the inverter it seems this cheap one has an overactive current sensor and at least half the radios I tried with it had too much of a surge demand at startup and the inverter went into protection mode. I know the inverter and filter combo are already giving a voltage drop but I'm wondering if I could get away with a low value power resistor ( I don't have any inrush current limiters laying around ) inline with the inverter output to limit the current so the inverter doesn't kick off.


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Mon 16, 2017 7:07 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 7689
Location: Beautiful Downtown Burbank CA
Alfredo_T wrote:
Low cost modified sine inverters have an output waveform that looks like this:

Image

What happens to the waveform when an isolation transformer is plugged in?


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Tue 17, 2017 2:54 am 
Member

Joined: Apr Thu 12, 2007 3:24 am
Posts: 1347
Location: Milwaukee,WI
I didn't try an iso trans with a radio. However I attached a 6.3v stepdown transformer with a resistor load on the seconday and the scope wave form was still a sharp square wave.

I can easily whip together the filter and will give it a go if for nothing else than experimentation.


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Tue 17, 2017 4:34 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 7689
Location: Beautiful Downtown Burbank CA
Try a low tube count transformer set.


Top
 Profile  
 
 Post subject: Re: LC filter for 12VDC-120VAC inverter output
PostPosted: Jan Tue 17, 2017 2:51 pm 
Member

Joined: Apr Thu 12, 2007 3:24 am
Posts: 1347
Location: Milwaukee,WI
The ones I tested with were 5 and 6 tubers with power transformer.


Top
 Profile  
 
Post New Topic Post Reply  [ 13 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 8 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB