Looking for a pure sine variable output inverter

Thread Starter

jimmiegin

Joined Apr 4, 2014
49
Ok guys thank you for your input. I did not expect so many replies so thanks again. Now, the reason I wanted to do it this long winded way around is simply because I have a load of circuits laying around that I have built and played with during my exploration of power supplies thus far. So have all sections of original idea already built apart from the inverter stage. So from what I can gather, I would not be able to get an output "sympathetic" to input voltage with an inverter?. The sine wave to power amp to step up has be put to me before (suggested @ 400hz although I can not remember why this was said) and I do plan to experiment with this also. But as I can find nothing of this idea, or my own on line I can only assume they are not the best ways to go. As for budget I do not have one, in that it is as close to nil as one could get but i do have a big junk pile and maybe up to £30. Dose anyone know of a circuit that has been built that even slightly resembles what I propose?. The 317 circuits i have use two 2n3055 pass transistors for extra current and i have a few quite large transformers in my junk pile. The reason i am thinking pure sine is that i am under the impression that anything other than sine will be hard to get a good dc from and cause noise in audio circuits??. Would it maybe be a viable option to run maybe a 1.5 volt transformer into a power amp and step that up providing the input signal to the power amp does not clip?. And what is a triac dimmer? I have a number of dimmers in my pile but do these not work by chopping the positive peek and so cause noise?. Thanks for reading my post, i wait in hope that someone can answer my questions. I will have more questions for individual replies when it is not so late at night.
 

studiot

Joined Nov 9, 2007
4,998
Ok guys thank you for your input. I did not expect so many replies so thanks again.
Did you read my posts #8 & #15 explaining why your proposal, as stated, will not work.

How about providing a proper block diagram for discussion.
You could include blocks already built with specs and point out those where you need help.

:)
 

Thread Starter

jimmiegin

Joined Apr 4, 2014
49
Did you read my posts #8 & #15 explaining why your proposal, as stated, will not work.

How about providing a proper block diagram for discussion.
You could include blocks already built with specs and point out those where you need help.

:)
Ah I see, Sorry I am not sure why I initially said 12V as it will not be that at all. It is actually a 26V transformer so will be 36.7v minus rectifier drop. In any case if biasing is an issue that limits operating input voltage then it matters not. But would there be some way so shift the bias along with input to give a fixed ratio of in/out as both bais and input are shifted? I will go through the net and post the schematics I copied from. But basicly ignoring the step down we have standard single rail positive lm317 variable from 0-30ish volts with two 2n3055 pass transistors and a resistor on the emitters to even the two. The missing link I am asking about, and ignoring the step up transformer I have a bridge rectifier with a with a couple of caps to smooth the output and that's it. What bout the feeding a lower voltage like 1.5VAC into a transistor power amp and stepping up the out? That is something I can do just from junk in the pile but is there actually a working, in service supply anywhere from this idea cos I can find nothing.
 
Last edited:

studiot

Joined Nov 9, 2007
4,998
Your 26 volt transfomer plus rectifier/filter and LM317 will give you a reasonable 2 - 30 volt direct supply.

Do you understand how an inverter works?
You keep saying "the input to the invertor".

An inverter is not really like an amplifier which has an input and its own separate supply.

The direct voltage you are providing forms the supply for the inverter.
The inverter comprises one (or more) controlled switches that draw current from this supply through the primary windings of a transformer.

The desired alternating voltage then appears at the secondary.

Let us say that you are driving the primary with X volt pulses in the range 2< X < 30.

Then since the transfomer has a fixed ratio, n, the output will depend upon x and be given by nX.

This is OK if you are using SCR switches and rectangular or square pulses, since you are simply switching between zero and the supply rail.

But you have specified that you want a sine wave.

This is not achievable with SCR (or any other) switching.

To achieve a sine wave you require tuned circuits and transistors (BJT or MOSFET) in the active or linear region.

Since these are working in the linear region they are acting as amplifiers and require suitable biasing as such.

This biasing will depend upon the supply volts. Therein lies your problem.

What happened to the block diagram?
 

Thread Starter

jimmiegin

Joined Apr 4, 2014
49
Thank you for the explanation, I had absolutely no knowledge of inverters other than dc to ac. I shall be drawing a block diagram and scanning after work. posting pics has been a problem on other thread but will scan a block drawing later. Thank you for your time and input :) Do you have a schematic so I may visualise your explanation please
 

Thread Starter

jimmiegin

Joined Apr 4, 2014
49
Having no luck uploading pdf of block diagram please will someone tell me how as i am doing the browse and selecting the file and uploading and it seems to do that ok but is not showing up in my reply. please help.
 

umphrey

Joined Dec 1, 2012
39
This is a really common thing to do in high power systems but their are obviously lots of problems with a proposal like this.

1. You want your DC voltage to be as high as possible. Otherwise you have to push large currents through the switches and it makes things harder.
2. You can build a variable inverter if you want but the problem is that THD is a function of load current (higher THD at light load) so you would have to settle for a less than perfect sine wave at full load and it would just get worse from there. Even getting a high quality sine wave at full load is going to take a lot of filtering.

http://i.imgur.com/InwcBHT.jpg
 

Thread Starter

jimmiegin

Joined Apr 4, 2014
49
I have tried to upload the documents as pdf but it simply will not work! They are wel within the max size specified. If someone would be kind enough to send me a message with a step by step that would be really cool. Thanks for your help guys :)
 

Alec_t

Joined Sep 17, 2013
14,379
1) Click 'Post reply'.
2) Scroll down below the reply box to Additional options.
3) Click 'Manage attachments'.
4) Click 'Browse'.
5) Select/open file.
6) Click 'Upload' and wait for indication that file has uploaded.
7) Click 'Close this window'.
 
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