Color code for fusible resistors?

[metertech58761]

I don't know if anyone has any time to answer my question, but here goes -

There are some fuses with a bright green body that have a color code like a resistor; how do you read the rating from its markings?

The one I have at hand has red / red / black bands. Is it 2.2A or 220mA?

[Xena E]

Usually a fusable resistor has its resistance value and tolerance marked the same as a normal resistor. The last band is usually white solely to identify it as fusible.

The power rating of the resistor vs the actual value is what is used to calculate when it is designed to open.

Yours is 22Ω. More than that I can't help you with, you need to know the rating from the manufacturers data.

E

[MathWizard]

On computer PSU's, I've seen pink bodied fuse-able resistors, the big ~1inch through-hole kind, I think they have an extra color band on them, but the normal bands still code the resistance, like 15mOhm was something with black, brown and green.

[metertech58761]

Quarter-watt, so I guess 100 milliamperes.

[metertech58761]

The thing is, 100 milliamps doesn't make sense.

The main fuse is 10 amps fast blow.

This fusible resistor lies in the main transmit B+ line, so 100 milliamps is absurdly low.

[TimNJ]

“B+ transmit line” of what? Show a photo? Is it an inductor?

[metertech58761]

Here's the schematic and picture of the board set.

The part in question is F2 on Panel B, and in the picture, you can see most of it by the positive end of the large silver electrolytic at top.

[metertech58761]

Found another picture - the fuse is on the corner, and a second one with the same markings is on the daughterboard.

[bobxyz]

I don't think the color code for the extra band is standardized, but here's an example from https://www.vishay.com/docs/28737/nfr25.pdf
"MARKING
The nominal resistance and tolerance are marked on the
resistor using four colored bands in accordance with
IEC 60062, marking codes for resistors and capacitors.
For ease of recognition a fifth ring is added, which is violet
for type NFR25 and white for type NFR25H."

[Kim Christensen]

This fusible resistor lies in the main transmit B+ line, so 100 milliamps is absurdly low.

100mA @ 170V is 17W input power which doesn't seem unreasonable for an amplifier with such limited heatsinking:

[metertech58761]

I guess that does make sense.

There is a later version that uses a lower voltage for that transmit function (calling it +48V for now), and the single pair of output transistors uses strips of 1.5" x 1/2" x 1/8" sheet aluminum as heatsinks; that circuit has no fuses at all.

So I'll consider this little mystery solved. Much appreciated.