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 Post subject: Re: Variable frequency AC power supply
PostPosted: Dec Fri 07, 2018 2:28 pm 
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[quote]Third, do you need 60 watts continuous? If so, is your amplifier rated for that. I think there are a lot of 60-watt amps out there that would go into meltdown if they were delivering a 60 watt sine wave into a load. (Many here will be smarter on this question than I am)/quote]

Just had to share a little story ... there are amps that can do this. Back in the day I was at an audio show, looking at a Crown DC-300 being driven by an HP sine wave generator. Very impressive display... especially when you noticed that what the amp was driving was a TOASTER ..... yes, the kind that cooks toast.

So, get one of those. ;-)

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 Post subject: Re: Variable frequency AC power supply
PostPosted: Dec Fri 07, 2018 3:00 pm 
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Thankfully the Bogen can handle continuous 60 watts far as I know. At least I know it does great for powering a phono, 8 track and cooling fan.

Cooling fan blowing on the heatsink is likely not necessary, but I do like to keep transistors as cool as possible.

Amp does have a thermal cutout where if it reaches a certain temperature it opens up the secondary's center tap connection to ground.

Not sure if I would be able to get the full 60 watts of drive at 120Vac though. Possibly due to transformer losses which is why I want a single transformer instead of two to minimise said losses.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Dec Tue 11, 2018 3:06 pm 
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Thinking about it.

If I wanted to save some money on transformers I could find a couple microwave oven transformers if they are small enough to fit, remove the HV secondary then wind whatever secondary voltages I need.

I could wind one for the amp B+ and the function generator B+ and wind the second one for 14Vac to step up the amp output to 120Vac since from what I understand they are 1 turn per volt and can handle some amperage.

I looked at one transformer online and an 800 watt one is

L 3"
W 3.54"
H 3"

So one may fit.

EDIT:

The power transformer needs to have a secondary of either

1. 80 Vct with the full wave rectifiers
2. 40 V with bridge rectifier.

At several amps.

Plus a secondary of at least 8Vac for the function generator power supply.

I may look for the proper transformer and see what I can find.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Jul Fri 09, 2021 9:59 pm 
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An update.

A few months ago I was using the function generator for testing some other stuff and managed to damage its output chip.

So this power supply sits unused currently.

I'm seriously thinking about finding a 15 volt to 120Vac transformer and getting maybe a used pro audio amp that can do at least 100WPC to drive the transformer.

I could get a second transformer and another function generator then I'd have two variable frequency AC supplies.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Jul Sat 10, 2021 3:59 pm 
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I had a similar requirement, but it was for a 400Hz supply for testing Avionics instruments and supply perhaps 100W. And it had to be three phase, just to make things more complicated.

For this application the audio power amplifier was considered too, however there was one thing that concerned me, that was amplitude control. I had considered it should be operated with a feedback loop and with over-voltage detection protection in case the input signal went up in level or became erratic or noisy. And there were some size & efficiency issues to consider.

Then I considered the Motor VFD (as was suggested early in this thread), most can set the base frequency from very low to 400Hz, these produce a PWM output that is effectively integrated by the motor's inductance, but there was no published design for a supply that I required, so it was a trip down a little traveled road, but I started playing around with power filters and I found that this integrated beautifully into a sine wave with a power L-C filter:

https://www.worldphaco.com/uploads/3phase.pdf

The method would work just as well for single phase variable frequency supplies, but the max frequency might not suit your application.

For this Avionics instrument application at least, the 0.75kW rated VFD gave an excellent stable & safe result and it does not have a failure mode where its output voltage can jump up more than a few % due to the tight control. The output voltage can be varied (from about 20 to 100%) as well as the frequency, but possibly not as much range as you might require.Being a switching design they are highly efficient.


Attachments:
VFD.jpg
VFD.jpg [ 238.12 KiB | Viewed 550 times ]


Last edited by ACORNVALVE on Jul Sat 10, 2021 4:19 pm, edited 1 time in total.
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 Post subject: Re: Variable frequency AC power supply
PostPosted: Jul Sat 10, 2021 4:08 pm 
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The frequency needs to be adjusted in .01Hz steps.

One frequency is 59. something Hz I believe and the other is maybe 61. something Hz.

Also whatever I use as a power supply must produce absolutely no RFI.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Jul Sat 10, 2021 4:25 pm 
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Tube Radio wrote:
The frequency needs to be adjusted in .01Hz steps.

One frequency is 59. something Hz I believe and the other is maybe 61. something Hz.

Also whatever I use as a power supply must produce absolutely no RFI.


I don't think you can get such fine frequency steps from the VFD.

The sine wave filtered output is largely devoid of RFI, but the VFD itself would have to be isolated from RFI sensitive equipment.

It would be interesting to see the schematic of the unit you use at work that goes from 40Hz to 5kHz.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Jul Sat 10, 2021 4:30 pm 
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The VFD then won't work for me given it has to be isolated from RF sensitive equipment.

I don't really need anything to keep the voltage stable as the design I originally did pretty much held the voltage stable to where it only varied slightly based on the load.

The load in this case is a record player and 8 track player both with an AC motor.

Only one will be used at a time.

So i did some calculations for a 100 watt amp. Ideally I'd want a 200 watt amp. In this case one of those subwoofer plate amps might be my best bet.

8 ohms
100 watts
28.285 Vrms
3.535 amps

That means I can use a transformer with a higher secondary voltage of maybe 20-25V with a 100 watt amp. I'd lean more towards 20V though.

It would need to be a standard power transformer though as I at one point tried a toroidal power transformer on the 60 watt bogen amp and it didn't like it at all.

The supply at work is basically a variable frequency oscillator that drives a linear power amp which itself drives a step up transformer. The voltage output control is not part of the oscillator circuit but is like a volume control on an amplifier.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Jul Wed 14, 2021 1:54 am 
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Got bored and decided to take another look at the power supply.

Probed around the function generator board and found a point where there was a good clean sinewave. I knew it had to be before the output chip given I know the output chip is bad given it badly distorts one half of the sinewave.

So I removed the chip and that same point had a sinewave still.

I connected it to where the output of the function generator went to the amp and got a decent output from the amp.

A slight modification of the voltage divider from the function generator output to the amp input (removed the resistor to ground after the series resistor) and I now have full output. According to the meter the amp can do 130Vrms easy no load.

The filter cap I originally used for the amp was a 6800uF cap which when the amp was loaded with a soldering iron produced about 2-4Vpp ripple. I paralleled another 6800uF cap with that one and the ripple went down some.

I also paralleled the 3300uF cap that coupled the amp to the speaker with a 8200uF cap.

I would have used that for the power supply, but its an older cap I had laying around plus I already had the power supply cap wired in and secured with double sided tape.

Tested the amp with the Sansui record player and the JVC 8 track. Seems to work just as it did before.

The only issue is the auto eject mechanism of the 8 track. If I keep the eject button pushed in it will drag down the voltage a good bit. At first I thought there was a problem with the eject mechanism until I realized I needed to wait a few seconds before pushing the button as the cap has to fully charge before there's enough current to fire the solenoid enough to eject the tape.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Sep Fri 17, 2021 4:49 am 
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Another update.

I got tired of the cooling fan just sitting behind the amp blowing on the heatsink so I went digging in my stuff and found a heatsink that originally had two TO-3 transistors on it and is 5" wide and the 5" fan fit perfectly. So after a little cutting of the amp chassis so I could get at the output transistor terminals I got it mounted and wired up.

Bogen sure must have made their CHS series of amps very robust.

When I wired up the transistors I had reversed the collector and emitter connections of the transistor that is supposed to have its emitter grounded. The amp still put out full voltage but would trip its built in breaker maybe 10 seconds after it was turned on. So I swapped the connections and the amp works just fine.

With a 30 watt soldering iron (cheapie from Wal-Mart as my decent one failed during soldering) as a load and my scope and voltmeter connected I could get a clean sinewave at a max of about 127Vrms. Also the heatsink barely gets above room temperature.

So the amp is at least good for powering a 30 watt load at 120Vrms.

I think part of the lower wattage is the fact that I use two transformers to get 120Vrms.

Attachment:
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The 70 volt transformer is 200 watts and is connected as follows.

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Here's more pictures.

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Attachment:
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I am so far well pleased with the heatsink update as before with the stock heatsink, without a fan it would get hot enough to nearly burn, but is perfectly fine now.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Sep Sat 18, 2021 7:32 pm 
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An update. I've been running the supply off and on over the last day with the expected load and it has worked well.

Today I decided to load it a bit more to see what it could do.

My load as it was convenient is an RCA WP-25A isolation transformer powering a Hammond 261M6 115V to 215V @269mA with the secondary connected to the low isolated output and the isolation transformer's voltage select switch to 130. That in turn powers a LED light that requires about 65Vdc to provide a constant current of 105mA for the LED panel. I use that light when I'm streaming on youtube as it is over me and lights up my green screen quite nicely.

Frequency is 60Hz and the output of the supply is 120Vrms.

Attachment:
11.jpg
11.jpg [ 477.86 KiB | Viewed 321 times ]


Here's a video of the waveform. Was impossible to get a photo given the sweep speed and the fact that my phone camera cannot allow for shutter speed adjustment.

https://www.youtube.com/watch?v=354I3q65D2Q

Running the load I have, the heatsink gets warm after a bit, but not hot. Now with the stock heatsink it would have been hot.

Now given the heatsink has fins on both sides of it what I would like to do is take a thin piece of metal and bend it so that the air from one end of the top side of the heatsink will be directed to the bottom fins and given the heatsink is partially open to the inside of the amp that would allow some air to get inside the amp and out the amp's vents in the top cover.

Two things I want to do to possibly make the amp better.

1. Install a power transformer with a higher maximum secondary current.
2. Replace the two transformers used to step up the voltage with a single 14V to 120V (can't get 14V but 15V will work fine I think) transformer to minimize transformer losses.

For the step up transformer I tried a 15-0-15 volt toroidal transformer, but the amp did not like it at all. I suppose it's due to the higher inrush current those transformers have.

Another thing I could do to make the amp better is to find another identical heatsink and connect a second pair of transistors using double the value of resistor that is in the collector of one and emitter of the other output transistor. That would cut the current each output transistor handles in half and might make for a beefier supply, however I am well satisfied with the supply as is. Plus if I was gonna go that far I might as well go ahead and follow the schematic changes to make it the CHS-100A version to ensure it is as robust as it is now or better yet find a CHS-100A amp.

So I'll leave it as is.

I'll be testing the amp a lot more possibly leaving it running while I stream my game to youtube for a few hours tonight with the scope connected to the output to monitor the waveform and the light as a load. That way I can be sure it will be good long term.

Oddly enough yesterday I heard a frying sound and maybe slight smoke, but after shutting it down immediately and powering it up it seems just fine.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Sep Mon 20, 2021 9:03 pm 
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I have a GR 1308A, contact me after 7 PM , 814 643 0603


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Sep Mon 20, 2021 9:18 pm 
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Unfortunately I need a digital display for the frequency as it has to be precise given it is used to power a record player and an 8 track player.

EDIT:

Wasn't satisfied with the stock power transformer as I felt it was a limiting factor.

Looked around and remembered I have the power transformer from a beyond repair Carver CM-1090 integrated amplifier.

I measured the AC voltage on the secondary that feeds the power supply for the output stages. Think it is almost 5Vrms higher than stock, but I figured that since the amp is already very robust the increase in B+ wouldn't harm the amp.

B+ is now 62.3Vdc no output and 58.8Vdc with a 30 watt load.

Now I suppose the higher B+ has something to do with this, but the heatsink gets a good bit warmer driving the light, however the sinewave is almost a perfect sinewave now.

Also I can get maybe a few more volts AC under load which is good.

Maybe now I can easily use a 15V to 120V transformer.

I'm almost not comfortable running the amp on just two power transistors.

I may go ahead and find a suitable sub for the power transistors then order four and four of the same resistors. That is provided I can find the other identical heatsink like the one I used.

That is provided the driver transistors can handle the increased load.

The light is more of a let's try it and see how much of a load it provides. Turns out it is at least over 30 watts and maybe close to 40 watts.

However the amp output goes to one transformer then that goes to a second transformer and it goes to a third transformer and finally a fourth then the light. So take the typical transformer losses and multiply them by four.

If I want more cooling I could get a better fan, but I like the fan I used as it's real quiet while moving a decent amount of air.

Cooling is where a second pair of output transistors will help as the current each transistor sees is 1/2 what it would normally see, but I must be careful as that will also increase the available output wattage which could cause the driver transistors to fail, however looking at the schematic it doesn't show any changes to the driver circuitry for the CHS-100A version. Also it may mean a larger breaker is required, but I would leave the stock circuit breaker in as that would serve as a good limiting device.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Sep Fri 24, 2021 2:49 am 
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I got a question.

Is it possible to use a higher rated output transistor in place of the stock transistors?

The amp uses two 2N3055 transistors which I can still get.

https://www.mouser.com/ProductDetail/on ... KWusgQc%3D

Looks like Bogen did build this amp very robustly judging by the datasheet for those transistors.

https://www.onsemi.com/pdf/datasheet/2n3055-d.pdf

Knowing what I learned in a Facebook solid state amp group from someone who designs switch mode power supplies I would want transistors of a higher voltage rating.

So the plan is to use the 2SC5200 since I'm not limited to TO-3 devices.

https://www.mouser.com/datasheet/2/308/ ... 313658.pdf

I used four of those in an amp kit that person helped me modify for higher power which I built in an old car audio amp case since it is all aluminum and makes for a great heatsink.

That amp can easily do about 93 watts into four ohms all day on a +/- 35Vdc supply.

Now if I find the amp with the new output transistors can put out more voltage then I can use a power transformer with a lower voltage ratio.

Instead of being stuck to 15V to 120V I may be able to use 20V to 120V.

That would be better for the amp.

However I need to make sure the bias is right and that the amp plays nice with those transistors.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Sep Fri 24, 2021 11:43 pm 
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He wants 40HZ to 5000 HZ ,use an audio generator with a power amp and a 28 to 35 volt 4 or 5 amp transformer wired as auto transformer , it will keep the loading on the amp to a minimum .

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 Post subject: Re: Variable frequency AC power supply
PostPosted: Sep Fri 24, 2021 11:46 pm 
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That's basically what I did but I put the generator and transformer inside the amp.

I ordered two transistors and two .5 ohm resistors to replace the original .47 ohm resistors.

I wanted thick film resistors, but I couldn't justify the cost.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Sep Tue 28, 2021 1:21 am 
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Did some calculations and a 15 volt to 120 volt transformer capable of 60 watts would require the 15 volt secondary be rated at 4 amps which would provide a maximum current of 500mA from the primary.

I already know a toroidal transformer won't work as I already tried a 15 volt to 120 volt one and the amp did not like it at all.

I say a 15 volt transformer as that's the closest I could get to the ratio of the two transformers I use now.

However with the new output transistors I may be able to get a higher AC output before clipping.

As it is now unloaded I can get 170Vrms out of the power supply. That means I need a smaller voltage ratio.

I do have a power transformer from a defunct soldering iron which might have a better voltage ratio so I may try it.

EDIT:

The transformer is 120V primary 29V 1.92A secondary.

Calculations show a 2A secondary is needed for 60 watts at 29V. However I doubt the supply would be able to supply 60 watts no matter what transformer I use.

That also depends on if the amp can put out 29Vrms without clipping, but ideally it would need to be more.

Given the B+ is 62.5Vdc ideally I'd get a max of 31.15Vrms no load, but I know it will be less than that so the transformer isn't going to work.

Now if it was a 120 to 24 volt transformer it might work. That would require a secondary of 2.5 amps.

I can get this one.

https://www.mouser.com/ProductDetail/Tr ... xq3g%3D%3D

115V to 24V 4A

To get 120Vrms that would require an input of 25.052Vrms.

I'm pretty sure the amp can do that just fine.

However I can use an 8 ohm load resistor on the output of the amp without the transformer connected just to see how much RMS output voltage I get before clipping. That will determine if the transformer I linked to will work. It will also be better on the amp if it does work given the amp won't be supplying as much current for a particular load so the amp will run cooler and I may be able to drive a larger load.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Sat 02, 2021 9:18 pm 
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I added a second identical heatsibk and second pair of 2N3055 transistors.

For those I temporarily used .5 ohm resistors just to get it working.

The one pair with the .27 ohm resistors as expected gets warmer.

I ordered four .27 ohm thick film resistors.

The power supply now can supply enough current to where the stock circuit breaker trips before the sinewave starts to distort. Thought of using a larger circuit breaker but I like how the stock breaker works as it will protect the supply better. Plus the only load will be an 8 track player and record player but not on at the same time.

I'll post a picture later.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 05, 2021 3:12 pm 
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I noticed something interesting.

After installing the four .27 ohm resistors for the same output load (the light I mentioned earlier) the stock breaker trips when the output is at 110V, but is fine no load. Now with just the two transistors and the four with the .5 ohm resistors for two I could go up to 120V with the same load and a little higher without tripping the breaker.

I am assuming that's due to the increased B+ current drawn by the second transistor pair.

The breaker is 1.6A according to the schematic and for the 100A version the breaker should be 2.5A so I will need at least a 2.5A breaker but I could possibly use a 3A which may be easier to find.

I may get one of those toggle switch/circuit breakers that some equipment use as a combination power switch and breaker.

EDIT:

What I don't get is this.

While monitoring the waveform with my scope it is at one amplitude, but when I turn the light on the amplitude drops slightly, although it looks like it's just the negative alternation that is changing in amplitude.

I suppose that I should have gotten two more new 2N3055 transistors instead of using the originals for the second pair.

Also I do need to replace the carbon comp resistors. I learned just how much a carbon comp resistor that is out of tolerance can affect a solid state amplifier when I worked on a Magnavox console amplifier about a year ago.


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