Filtering an LED light's power supply

[jim hardy]

[offtopic]Just curious what they were used for.[/endofftopic]

For a reactor control rod position indicating system. We called it RPI .

Control rods on a PWR come out the top of the core. They are lifted by a drive shaft which extends into a pressure housing atop the reactor vessel.
Here's my rough functional sketch of one, we had 45 of them.
Dome is reactor head, rectangle atop it is a nonmagnetic stainless steel pipe that's the pressure housing.
Rectangle inside the pressure housing is the drive shaft that lifts the control rod out of the core twelve feet below.

Orange winding is secondary which carries effectively zero current. So it's a flux detector.
Its induced voltage gets rectified and filtered to indicate position of the drive shaft (and its attached rod.)That 120 VAC supply and the 500ohm resistor set the current through the primary winding.
Observe that since flux Φ is in proportion to i which is set mostly by R(because X_L was only around 50 ohms),
and secondary voltage is dΦ/dt , we have effectively differentiated (high passed if you prefer) it exactly like Tim9000 did except with inductance not capacitance .
So harmonic content is exaggerated in the secondary voltage signal.

Our RPI system was specified to operate with maximum of 1% total harmonic distortion in the supply . Our custom built inverters met that.
The Sola transformers were the backup supply to this system and we settled for best they could do, 2%, and that worked fine.

Someplace i have 'scope traces . But that's another story for another thread.
Wish i knew enough about something to write a PF Insights Article - these old power plant lessons were painful to learn but are fun to look back on.
Excuse an old man's boring anecdotes - they're fond reminiscences You are very kind.

Anyhow, in a nuke plant a portable FFT analyzer is really a lot of fun .old jim

 

[Asymptotic]

[offtopic]Just curious what they were used for.[/endofftopic]

Sola CVTs were heaven-sent when programmable logic controllers and other early industrial computers still used linear DC power supplies sensitive to AC line variation , and particularly when they shared an AC bus with high power SCR DC motor drives. Secondary waveform was practically unaffected even with fairly ugly commutation notches and spikes on the primary windings.

A CVT also makes a severe overload or short circuit on the secondary circuit painfully obvious. Rather than clearing the secondary fuse, what often happened was voltage collapsed, PLC outputs turned off, (with the short turned off) voltage was restored, PLC rebooted, and the sequence repeated. You could tell when a solenoid coil had shorted by watching cabinet pilot lamps flashing in unison every few seconds.

 

[jim hardy]

A CVT also makes a severe overload or short circuit on the secondary circuit painfully obvious. Rather than clearing the secondary fuse, what often happened was voltage collapsed, PLC outputs turned off, (with the short turned off) voltage was restored, PLC rebooted, and the sequence repeated. You could tell when a solenoid coil had shorted by watching cabinet pilot lamps flashing in unison every few seconds.

They indeed collapse so aren't much good for clearing faults.
One can however use that to advantage.
Consider a panel of branch circuits feeding instrumentation in a big factory or power plant.
Sometimes they're fed by battery powered inverters for ultimate reliability. Inverters are also weak when it comes to fault clearing.
A branch circuit that's fed by a CVT won't draw very much fault current from the upstream source. That means other branch circuits fed from same upstream source won't see a voltage perturbation from the fault on the CVT isolated branch .
That's useful when your upstream source also has limited fault clearing capability, such as inverters feeding instrument buses in a power plant. A fault on one branch circuit must not "glitch" the other branch circuits.
That's tripped many a nuke plant.
APRIL 1973 twice in two days...

https://cdnc.ucr.edu/cgi-bin/cdnc?a=d&d=DS19730403.2.171Inrush current to a SMPS equipped computer looks an awful lot like a fault. CVT is handy there too, to limit inrush to some value that doesn't glitch everything else on the bus..
CVT can be useful also for starting a motor that has high inrush. Makes the start a lot more gentle.

Like any other transformer CVT's are susceptible to first half cycle saturation should you have the bad luck to energize right at voltage zero crossing. So if you're using one to limit inrush, operate it at about 70% nominal input voltage else you'll get an occasional current spike on energize.

Personally I do like CVT power supplies where ultimate reliability is needed. They're just too simple to fail. No output regulator to latch up from a "spike" and trip the overvoltage crowbar.

my two cents, hope it helps somebody somewhere.

Wow really got off topic didnt i ? Sorry, guys.

old jim

 

[tim9000]

If CVTs weren't so inefficient they'd be perfect. So

I’m wondering if you were designing one for 50 hz or 60 hz, supply frequency for something small, like 1kW at 230V or 120V, how would you know how big to have your capacitor?

That 120 VAC supply and the 500ohm resistor set the current through the primary winding.
Observe that since flux Φ is in proportion to i which is set mostly by R(because XL was only around 50 ohms),
and secondary voltage is dΦ/dt , we have effectively differentiated (high passed if you prefer) it exactly like Tim9000 did except with inductance not capacitance .
So harmonic content is exaggerated in the secondary voltage signal.

Sorry, can you explain how the control rod is a high pass filter? So you're saying that the flux is in phase with the primary current, which is basically a 'real' impedance, but the secondary voltage is the differential of that, but I don't see the connection. Also, what did you mean, what I did expect with inductance, but not capacitance?

Thanks

 

[jim hardy]

but the secondary voltage is the differential of that, but I don't see the connection.

induced voltage e = L di/dt . Since primary and secondary link the same flux they see same induced voltage.

I = V/ Z = V / (R +jX) and since R >> jX,,
I ≈ V/R
so e = L d(V/R) dt = L/R dV/dt and that's differentiation. Each harmonic gets exaggerated by its order.

Also, what did you mean, what I did expect ?? with inductance, but not capacitance?

I don't remember asking that.

I drew a parallel - your example differentiated using capacitance, my example differentiated using inductance.

 

[dlgoff]

... these old power plant lessons were painful to learn but are fun to look back on.

And fun for us readers too. Keep 'em coming.

 

[jim hardy]

Thanks, Don

old jim

 

[tim9000]

so e = L d(V/R) dt = L/R dV/dt and that's differentiation. Each harmonic gets exaggerated by its order.

Thanks for the clarification as to how the harmonics exaggerates, I think I get it: So, the secondary voltage is the derivative of the primary voltage, and because the higher order harmonics have a greater rate of change than that of the fundamental, those higher order voltage components are larger in the secondary, compared to that of the fundamental component in the secondary voltage.

I drew a parallel - your example differentiated using capacitance, my example differentiated using inductance.

This may sound (and indeed be) a stupid request, but could you please re-state what your interpretation of my expectation of harmonics and capacitance was, in relation to the parallel example you outlined. Partly because I want to make sure we're both on the same page.

Thanks Jim

 

[jim hardy]

could you please re-state what your interpretation of my expectation of harmonics and capacitance was .

?? Expectations ?

Closest words to that i can remember is :

That's exactly what i hypothesized to explain your observed current wave.
What experiment could you propose to test that hypothesis?
What did you conclude from the waveform ?

I had told you what your traces looked like to me

and was soliciting your thoughts - what'd it look to you like was going on and what conclusions did you draw from your traces.

 

[jim hardy]

So, the secondary voltage is the derivative of the primary voltage, current, and because the higher order harmonics have a greater rate of change than that of the fundamental, those higher order voltage components are larger in the secondary, compared to that of the fundamental component in the secondary voltage.

 

[tim9000]

I understand that it's the current, not the voltage. But I thought you were drawing attention to the fact that e = L/R dV/dt, because you separated the 'rate of change of voltage' out.
But as long as I took the main point about how the higher order harmonics are exaggerated, than that's the main thing.

?? Expectations ?

Plainly speaking, I was trying to be reminded of what it was that I'd previously said, which spurred on your example, which you gave as a parallel.

Thanks

 

[tim9000]

How many simple ways (if any) are there to reduce the THD on the source caused by using a full-bridge rectifier load?

If you used a low pass filter and then an isolation TX (or the other way around), how much would that help (if at all)?

Would the low pass filter really do anything beneficial?

Thank you