Extremely hot resistor

[Kingston]

The regulator eats most of the conducted harmonics, yes. Radiated harmonics (transmitted by the transformer and the loop formed by transformer-bridge-cap) end up in insufficiently shielded loops and audio transformers (for the magnetic component) and on nearby conductors (for the electric field). The stronger HF components in the charging current of a cap connected to the bridge can also pollute ground if you're not careful, especially in a layout that didn't take this cap into account to begin with.

(You might say that a large fraction of linear supplies have a cap directly connected to the bridge rectifier, and thus suffer the same problem. Can't argue with that, but if you're adding a $1 cap to reduce ripple you might as well add a $0.01 resistor to reduce the cap's side effects.)

JDB.

Great lesson. I did not know that. Would be interesting to know in detail just how much it might make the PSU transformer radiate. I've certainly had my share of problems with PSU transformer magnetic hum (haven't we all really). Curious how much of it (if at all) could be cured by the resistor tweak. I'll have to test sometime soon.

 

[Samuel Groner]

From my experience you need rather high values of resistance to get a significant improvement. Here's the hum residual on a two-channel GreenPre with and without transformer secondary series resistors I've built some years back: hum_10ohm.pdf

Samuel

 

[syn]

Great lessons, definitely. I'm not sure, is it B+ after the bridge, to the resistor (47-120r) in series to the cap (recommended value?) to gnd  R33 in series to the B+ (as it is now)?

 

[Kingston]

syn, looks like your multimeter just isn't accurate enough for accurate PSU ripple measurements. Just making sure, are those VAC or mVAC values?

 

[syn]

VAC, it can not go below 0.001VAC.

 

[syn]

O.K. got it sorted. After trying various proposed things, I could not bring the ripple much down. Then I took 230:230VAC transformer that I was saving for another project, also replaced R33 with 47ohm (was 470ohm) 1W part (was 15W) (I know it is not doing much droping only a few volts). Regulator is getting ~ 290VDC and outputs ~252VDC, all in spec range(783). The ripple droped dramatically and this 1W resistor is now getting hot but not so unpleasantly. I might increase it's value to a bit more like 100ohm or so, to take it easier on to the regulator, but I'll do that tomorrow.
Thank you all so much
Best
Milos

 

[gyraf]

Don't make R33 too small - it protects the regulator against too large an input/output voltage difference at power-up..

Jakob E.

 

[syn]

I settled  for  2x2W270ohm in parallel for ~135ohm total resistance and 4W dissipation. This gives me ~280V before the regulator and ~250V after the regulator. Once more thank you all, especially to JDB who even explained how my DMM works.