S-70: Turn-On Shock Reduction

[Mooly]

I can just remember the Thorn B&W sets. We (lol, I say we... Focus/Multibroadcast/DER etc) had a few out on rental.

 

[DF96]

Looking at it from Ambers perspective, they were able to come up with a one component fix, for a problem they were having.

I'm not convinced it fixes the alleged problem. A genuine one component fix would be to include a suitable NTC thermistor in the circuit.

I'd take it as a given that they tested their solution in their R&D facility before dashing off a service bulletin to the masses.

"R&D facility" is a posh name for the place where the designer gets confused by circuits.

All this (the problem and the manufacturer solution) smacks of poor engineering to me.

I think you are being very polite.

 

[Bob Richards]

It would remove the shock hazard but the resistor would have to be specified for 24/7 use at the intended voltage.

All this (the problem and the manufacturer solution) smacks of poor engineering to me.

Also, this talk of the power indicator glowing all the time would drive me nutz 😉

A 1M ohm R across the 120VAC (at the switch when in off position) would draw 0.12mA and dissipate.0144 watts. I doubt if the power LED on the other side of the transformer would glow at all (?). I don't understand what's wrong with this idea (?).

 

[Mooly]

Didn't you mean having the 1Meg in parallel with the cap... so the cap is still present.

Although the power dissipation is tiny, the resistor must be a specially rated part. In 240vac locations it will see 340 volt peaks each cycle. Even conventional 0.5 or 1 watt parts often failed under those conditions. Resistors have a voltage rating as well as a power rating.

SMPS in TV's and so on often used a high value 'start' resistor to kick the PSU into life and their failure was common place. Based on wattage and they were fine though.

 

[Bob Richards]

Didn't you mean having the 1Meg in parallel with the cap... so the cap is still present.

Although the power dissipation is tiny, the resistor must be a specially rated part. In 240vac locations it will see 340 volt peaks each cycle. Even conventional 0.5 or 1 watt parts often failed under those conditions. Resistors have a voltage rating as well as a power rating.

SMPS in TV's and so on often used a high value 'start' resistor to kick the PSU into life and their failure was common place. Based on wattage and they were fine though.

Yes, the R would be in parallel with the cap. If such a resistor ever burnt out, it wasn't the concept that was wrong but the Engineer who didn't pick the right resistor. If getting a high voltage R was difficult, he could have put two R's in series.

 

[Mooly]

Ah !

All this (the problem and the manufacturer solution) smacks of poor engineering to me.

I might be wrong 🙂 but I think you might be taking that as being aimed at your solution... it wasn't.

It is the cap fix, the indicator glowing and the shock hazard is what smacks of poor engineering.

Adding a resistor is fine as long as its specified appropriately... as you say, that's the job of the one doing the fix.