I could have my transformer guy make the primary DCR a bit higher than normal. That would soften up the source a little, but I am simulating an alternator, which is highish impedance already.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
TCA0372 is an amazing amp at a shocking price. They seem to be pretty tough.
Silk and solder mask are often misaligned. Which is why I don't do "mask defined" pads.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
TI does spec 60 mV max DC offset at the output, which is pretty good, but even that could push a lot of DC into my transformer.
There is probably some way to tweak the input with a little DC. Feedback loop or even a trimpot.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
60 mV max.
My transformer primary will be maybe 20 milliohms DCR. At the max 60 mV, that's 3 amps DC.
Why does TI use such goofy file names?
-- John Larkin Highland Technology, Inc lunatic fringe electronics
You can drive the transformer bridged but using current-sense feedback instead of voltage feedback; put a small sense resistor in line with each amp output going to each end of the transformer and take off the DC feedback to the opposite amp from the junction. That way the amps should act as their own servo to keep DC out of the transformer.
Lower power example like this for driving audio isolation transformer for XLR cable:
In the case of the TPA3255 I think you would put a low-offset op amp in front of the inputs and AC couple into that and do something similar but the 3255 just acts as a power buffer, I don't immediately see anything in the datasheet that says you can't intentionally apply a small DC feedback generated offset to its single input per channel as an error signal.
On a sunny day (Fri, 31 May 2019 15:36:39 -0700) it happened John Larkin wrote in :
As somebody else already pointed out, maybe a DC control loop could zero the current. Not so easy, low pass, some opamps, but not impossible.
The idea of supercaps is nice, maybe I should order some to see how those perform in a similar application (driving cryo cooler here). The ebay class D unit did not like the step up transformer... and died. The normal class B audio amps with output coupling caps work fine for that.
I'd just capacitor-couple the output, with a medium-size resistor parallel to the capacitor. The digital amps I've seen want to self-bias, do NOT like injected DC.
If this is for a three-phase power drive, a small three-phase motor with no load is a great filter/flywheel/phase balance component. So, how much ripple current is the blocking capacitor going to pass? The voltage should be kept low by the parallel resistor, of course, and that means a 6V capacitor is overrated for the job. Without blocking, you'd run DC into your three-phase loads (not recommended).
Do your AC loads burn up when given DC? Or, just magnetize and hum loudly?
** Hmmm - so a 4 ohm impedance in the working range with 20mohms of resistance. That's a ratio of 200:1. The transformer will have a regulation factor of 1%, or better.
150VA trannies normally have regulation factors of 8 to 10% and there is little difference between E-core and toroidal types other than size and weight..... Phil
..... Phil
Is it necessary to use a toroid, why not ordinary EI transformer with possibly air gap ?
Or how about a high inductance but low resistance external solenoid across toroid primary ? Apparently the frequency is in the 400 Hz ballpark, so even less solenoid inductance should suffice, compared to
50/60 Hz.
Ah, so that's how flux capacitors arose. I've always wondered. ;-)
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I should jolly well hope not!
No need. Lots of work has already been done to handle this hazard.
"The flux density bias of the transformer in such topologies is solved by u sing a fully digital strategy without any auxiliary circuit, thus, core los ses are decreased and the total conversion efficiency is improved. Circuit complexity and cost are reduced, enhancing at the same time the reliability of the system. The possibility of magnetic saturation due to steady-state or transients is strongly reduced, being an important performance."
There are plenty of other papers on exactly this topic. And there's a small niche market of add-on protectors.
That's what I'd do. No output cap, much smaller, much cheaper.
It is quite easy if you use that same loop for everything, DC as well as drive signal. That's how I do it with class D stuff. Mine have also always driven weird loads, not speakers. Use another opamp up front that has very low offset.
It would be good to have that blessed by TI, to make sure the analog loop filter in the IC doesn't become upset.
Be careful not to get into a resonant mode at some frequency. That can hammer them, they see an output AC short, become instable like an opamp with a capacitive load and potentially croak.
Not always. You were probably lucky. I had the overcurrent trip come on a regular big old stereo amp when I drove a load capacitively for aerospace testing.
-- Regards, Joerg http://www.analogconsultants.com/
It's not stated how the normally AC coupled inputs affect the DC offset, but we have the eval board and I could have a scut bunny set it up and try it.
One trimpot and 30 seconds of tech time, to turn it, is sure appealing. If it turns out we don't need it, we can leave it off the board.
The input resistance of the TPA3255 is 10K, and they want 10 uF input coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to me that the input caps are also used as lowpass filters for the DC feedback loop, which then suggests we could push the input pins gently to change the output offset.
Datasheets tend to hide the good stuff.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
It would take a giant capacitor, easily the biggest thing on the board. This is a nominal 400 Hz system, but we may as well go down to
50 Hz to have more market. I can just poke in a 33,000 uF cap per amp, I guess.We need to be frequency/voltage/phase agile, and we want to be able to program complex output impedance. The point of this box is that the customer wants to simulate alternators without spinning actual alternators, which they certainly could.
Problem is, this customer likes us so they throw all the hard problems at us, and we have to hope they'll buy 300, and not 3. I guess I shouldn't complain about being furnished interesting problems.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
A toroid would be about half the footprint and half the weight of a regular transformer, and would have lower output impedance. I know a couple of guys who do nice toroids. One did this for us, lower power.
I'm guessing that the TI amp would fight to maintain its output offset voltage. A series cap would be a lot easier. I could quit bitching and use some big caps. Probably don't need the diodes.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
On a sunny day (Sat, 01 Jun 2019 07:23:04 -0700) it happened Joerg wrote in :
No worry, I am making 60 Hz (almost)
This one never failed me in all those years :
It is not so critical, I have the original switch mode board too, and it works, but is difficult to control.
The problem is that the DeLoreans are getting hard to find.
ebay has an '82 for $40K.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
We're selling a lab-grade voltage source, so we want the output to be stiff. I estimated the 20 mohms primary DCR by scaling from a smaller toroid that we have. The AC output impedance is a separate issue from the DC primary current hazard.
We will be sensing output current to control the complex output impedance, so we can in theory tune the box Zout to zero by canceling the transformer impedance. That approaches building an oscillator, but we might take out some of the native impedance.
But alternators are very inductive, and our main goal is to simulate alternators.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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