Power Resistor Considerations & Fused Resistors

When using power resistors- 5 watts and up it is important to leave air space around them for heat dissipation. Best not to put capacitors real close to them also, as the heat can cut down the life of the capacitor and especially electrolytics.

The power rating of the resistor is over the entire resistor. So if you have, say a 50 W resistor and you move the tap close to one end, you MUST derate the resistor wattage.  (  I am sure there is a formula for this, but I don't know it. )

Most tubular resistors of high wattage, say 20 W or more typically use a pair of mounts that bolt to the chassis. They allow air flow and better heat dissipation. (The mounting pieces can be hard to find these days ).

When you use the "rectangular" ceramic power resistors, they do make clips to mount them to the chassis for better heat dissipation. I have even seen some with holes in the ceramic to bolt them to the chassis or a heat sink. Often the 3W and up to 20W are just supported by their leads for heat dissipation.

 The old "Sand Ohm" types, most are just lead mounted. A few types of them, the Fused Resistors, looked like Sand Ohm types, but had metal pins that went into a specific type of terminal strip. Mostly used in the old tube TV sets. DO NOT replace them with the new style resistors. They were designed to handle high inrush currents and blow if the circuits pulled too much current for a certain time period.

Interestingly, I have not yet found any of the rectangular or "Square ohm" resistors bolted to the chassis using heat sink compound. I would think that would help the heat dissipation.

Speaking of power resistors and fuse resistors. if you are working on an old b&w tv or early color TV , you will find a pair of small gauge wires in the filament circuit. They are fuse wires to protect the power transformer winding. I remember seeing replacement wire kits being sold at the distributor, but never saw one sold. Personally, I never saw the filament fuse wires ever open in hundreds of sets from that time period.  In reality is was a great idea, as filaments do short out and can draw excessive current damaging the power transformer of other filaments in tubes (If it was a series filament set).