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DIY 4 Phase Sinewave Generator for Turntable Motor Drive
DIY 4 Phase Sinewave Generator for Turntable Motor Drive
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Old 5th June 2018, 12:25 AM   #631
Altanir is offline Altanir  Brazil
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Join Date: Aug 2017
Hello Bill, how are you?

So, I was facing some issues with the regular TDA7492 board sold on Ebay and I've found a better built (Sure Electronics' webstore 2 x 50Watt @6Ohm Class D Audio Amplifier Board Compact - TDA7492) as I posted here a few months ago. It was finally delivered and I made the change today but I'm not getting the usual 120 VAC output, at 128 setting on the SG4.

The SG4 was running just fine, except for the TDA's shutdown issue when leaving stand by mode, forcing me to reset the power in order to make it run again. That's why I decided to replace it with a better board. The new board's specification are on the attached picture. I think (actually, I hope) it's just a matter of adjusting the gain board, but I have no idea how to choose the new resistors for the voltage divisor.

I've got a new multimeter, a real RMS one, and this is what I've got:

1) SG4 with everything normal, outputting 1.783 VAC on each phase/gnd pair;

2) Voltage divider with 1k8 and 8k2 resistors, with 0.315 VAC RMS on the 90+GND output and 0.320 on the 0+GND output;

3) TDA board outputting 6.93 VAC and 7.01 VAC for 90 and 0 phases;

4) Transformers outputting 56.5 VAC on the 90+GND, 55.9 VAC on the 0+GND and 81.3 VAC on the 90+0, with the SG4 set on 60Hz with voltage attenuator set on 128. On 81 Hz, the measurements are pretty much the same.

Except for this small issue, the Sure TDA board is a great improvement. No more crashes when exiting stand by or changing between 33/45. At least with the multimeter, I hope that this does not change with the motor plugged in.

Please, what should I do to get the correct voltages in order to power the 120 VAC hurst motor?
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Old 5th June 2018, 12:45 AM   #632
Pyramid is offline Pyramid  United States
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The best method would be to replace the fixed voltage divider with a stereo pot so you can adjust the output.

Using the numbers you gave me, if you keep the 8K2 resistor, change the 1K8 resistor to 5K (2x 10K in parallel or 4K7 plus 270 in series). That should get you fairly close to 115VAC output.
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Old 5th June 2018, 01:48 AM   #633
Altanir is offline Altanir  Brazil
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Thanks for the reply, Bill!

Unfortunately, I do not have any stereo pots here, only mono ones. I do have a lot of resistors and voltage divider pcbs I had made.

Using a 5k6 and 8k2 combo I managed to get 115.3 and 114 VAC from 90+GND and 0+GND, I guess that's enough to get the motor started right?

(I will reduce the attenuator to get 98V)

Later this year I will build the MA3D board to use with the BLDC motor.

Thank you very much, Bill!
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Old 5th June 2018, 06:40 AM   #634
nevans is offline nevans  Australia
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Default Further Help

Following up on my tails of woe, I have now put a pot in to represent the voltage divider. Playing around with the pot control definitely triggers the runway amp oscillation as the voltage on the output of the amp rises. The output stage of the amp needs to be around 12-13V AC RMS, however oscillation triggering comes in around about the 8-9V AC mark. Still open to suggestions. Everything at the moment is pointing to some problem with the output transformer presenting a load that the amp cannot sustain ..... I am open to suggestions guys


Thanks
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Old 5th June 2018, 08:17 AM   #635
ralphfcooke is offline ralphfcooke  United Kingdom
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I'm pretty sure you're right. I had a similar problem when trying to power a Papst 3 phase motor (up to 20W per phase); in the end I added an NTC thermister NTC 5D-9 between each amp output and transformer.
You may well need a different value thermister owing to the amp/transformer characteristics, they are cheap and easy to obtain and this is the easiest way I found of eliminating startup problems.
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Old 5th June 2018, 11:56 PM   #636
nevans is offline nevans  Australia
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Ralph,
Thanks for your thoughts, didn't think to add a thermistor into the circuit, did however consider putting a high wattage 5 ohm resistor in series to up the overall impedance presented to the amp output from the transformer. Your idea is alot better. Will let the community know the results, once I have carried out. Have already source an appropriate thermistor from Ebay


Thanks
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Old 6th June 2018, 01:10 AM   #637
Pyramid is offline Pyramid  United States
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Quote:
Originally Posted by nevans View Post
Thanks for the response. A little more detail. Firstly, with or without load on the output transformer, initial triggering of the oscillation still can occur. I have included a couple of photos to give you an idea of component layout. The power supply amp module is rated at 24V DC @ 3.2 AMP. (Turned down to 20V due amp restrictions at higher DC voltages due to low impedances on the output of the amp connected to the transformer output). The output transformer is rated at 100VA (2 * 12V AC inputs feeding a 240V AC main out). So roughly speaking the transformer at max capacity at 240 V can supply a .4Amp load. It should be noted that I have connected a standard 70W light bulb to transformer out and let it run for hours without issue ~ .3 amp draw. Of course when the oscillation begins everything shuts down. Final important point is the that disconnecting the transformer from the amp does stop the oscillation completely which leeds me to think about input impedances of the transformer. Specs on the amp state very clearly that the amp can only sustain at the lower end a 4 ohm Impedance load. I believe the transformer is presenting a lower impedance load than this, hence the amp runs amuk!! Of course my thoughts might be barking up the wrong tree completely. Any thoughts would be greatly appreciated. Thanks
Sorry for the late response, I missed your post.

What size motor are you using? The 100VA transformer is most likely your problem; it is too large for the class D amp you are using. These chip amps have over current protection that trigger much faster than their class AB cousins. The core magnetization of the xfmr can cause excessive current at start up; the larger the xfmr (and core) the higher the surge current at start up.

Your options are:

Use a smaller xfmr closer to the power rating of the motor.

Use a larger class D amp that has a higher threshold for over current protection.

Add ballast resistors (or thermistors) in series with the amp output to limit the start up current. These will reduce available voltage, consume power and dissipate heat and are the least efficient solution.
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Old 6th June 2018, 05:12 AM   #638
nevans is offline nevans  Australia
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Pyramid,
Thank you for the reply, much appreciated. Yes I have essentially come to same conclusion.I will initially try a couple of thermistors (5 ohm / 3Amp) as an initial test, if this fails will swap out the transformer for a more efficient toroidal one. I will report back to the forum on results ... again thanks
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Old 7th June 2018, 05:23 AM   #639
coolmaster is offline coolmaster
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I installed a pair of 0.22ohm (0R22) 5W wirewound resistors in series with the amp output to transformer. Since I can get most values at my place as easy as buying candy, I think I tried a higher value but got some unexpected results, can't recall what. I settled on 0R22 and that's it. Since my output transformer is 2 x 115V (series for 230VAC), I grounded the center tap for a balanced mains output. I didn't leave the output floating. Its been months, flawless operation so far. My output transformer is 60VA toroidal. Same with the one powering the TDA8950 amp which I added on a slim PC fan onto the heatsink. This made the SC really run very very cool in my tropical weather.

Last edited by coolmaster; 7th June 2018 at 05:26 AM.
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