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Solid State AVR on an ST-3 Head - Any need for it ?

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"The best ones have acceptable waveform and voltage regulation by the stock harmonic system, though I expect THD is still about 15%."

Thanks BruceM,
Just to be clear, do you suggest not adding and AVR until the unit is installed and the output is evaluated?
If I was lucky enough to win the lottery and get a "good one" then no mods may be required ?
(Other than the usual bearings, Junction Box, and tidying of the wiring)


Yes, you won't know what you've got until you check it out and also take a look at the waveform, and see if you have issues with your various intended loads. (Check your AC voltage using a true RMS voltage mulitmeter at a range of loads.)  Many ST's are being sold that are wildly high voltage, and thus will likely require an AVR or a large dropping resistor in the harmonic to bridge rectifier circuit.   If you intend on using it for non-electronically regulated lighting, you will have less Listerflicker by using an AVR using the AC ouput as excitation instead of the harmonic , and this will also help with "harmonic hump" distortions if present. 

Some LED lights are electronically regulated with a small AC to DC switch mode power supply. They will get rid of the Listerflicker unless it's extremely bad.  The stock harmonic regulation amplifies Listerflicker, and during compression stroke, the harmonic output will drop so low that the AVR won't help, since it only limits the excitation and can't amplify it.


--- Quote from: BruceM on May 09, 2022, 03:29:54 PM ---Ha, thanks Butch, my experience with the ST's is just one of practical necessity.   It was an educational "opportunity". 

MikeNash, good point on the need for removing the doghouse, I forgot to mention that abomination.  Adequate heat sinking of a modern, metal cased bridge rectifier is also essential. 

It really is a shame that the ST's aren't made in a more reliable and consistent manner with decent QC.  The best ones have acceptable waveform and voltage regulation by the stock harmonic system, though I expect THD is still about 15%.
For a smooth sine, you really need skewed rotor windings, it seems.

--- End quote ---

Hi Bruce.  Appreciate the input, thanks.  I know less about electronics than the Queen of England does about the poor . . . so I figure - see pic - the bridge rectifier is the alloy-bodied gizmo in the pic?  I might have a google and see what they do.  But I should replace it with a "good one"?  Cheers

That may be OK, it looks like a modern one.  A picture from the other side would help.
I'd order a new one to have on hand, in case it's crap.  When your output voltage suddenly goes way low, that's the usual culpret (with Chinesium bridge diode).

Here's an example of what you want:

A bridge rectifier is just four diodes potted in a metal or plastic box.  Two AC inputs, usually opposite corners, and two DC outputs, usually marked + (plus)  and - (minus).  It turns AC (from the harmonic winding on the stator, which is very spikey and non-sine, and turns it into pulsed (spikey) DC for the rotor.  The rotor windings have a huge inductance, so they smooth the input current to create a more constant magnetic field at the 4 rotor poles. 

For this application, the metal body is better for heat sinking to some aluminum. A die cast aluminum case to replace the doghouse comes to mind. Use some thermal paste or a dab of silicone caulk It should be as thin as it can be, to just increase the surface contact area.

Mike, My 15KW ST had a rectifier like yours and it failed quickly as they are know to do. Luckily I had ordered a few 5010s per the advice of good people here. They do need to be screwed to a heat sink of some sort. the metal junction box has worked well for me.


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