What Watt Resistors for Beebs?

[BeebMaster]

Generally I re-order the same parts when I am running low (metal film 0.25W, "MR25"), but recently when looking to bulk buy through-hole resistors, I noticed lower wattage resistors are now available. Normally Beebs use 1/4W resistors in most cases but I see you can now get 1/6W or even 1/8W resistors. Can these be used in place of 1/4W, assuming they are the same tolerance?

Sometimes I can't get the correct values in metal film, in particular the 1M5 used in the Econet interface, and for a long time I have been using carbon film instead (CR25). Are metal film and carbon film interchangeable in the Beeb world?

[1024MAK]

Can you replace a component with one having a lower power rating?

This should only be done if you know the actual amount of power that said component has to handle (and I mean it's worse case situation). Otherwise you risk the lower rated replacement component letting out the magic smoke.

Note also, that the physical size and type affect the voltage rating.

Having said that, for resistors running off a 5V supply, if the resistance is 220Ω or greater, then (assuming no external source of power can be present) it's not possible to overload a ⅛W (125mW) resistor.

Metal film resistors are considered to be superior to carbon film types. However, unless actually specified, most resistors in 1980s computers are carbon film types.

The advantages of metal film compared to carbon film:

• Can withstand a higher temperature,
• Lower noise,
• Smaller size compared to carbon types for the same power rating,
• Better voltage coefficient and temperature coefficient,
• Long term stability,
• Tighter tolerances available.

However, in digital applications, most of the advantages don't make any significant difference.

Mark

[wiggy]

It's also worth noting that most 0.5W (or some manufacturers give you 0.6W) metal film resistors are physically the same size as the common (and older) 0.25W carbon parts.

[KenLowe]

Sometimes I can't get the correct values in metal film, in particular the 1M5 used in the Econet interface

https://www.aliexpress.com/item/1005002670881002.html

[1024MAK]

1.5M 0.4W 1% https://www.rapidonline.com/1m5-1-0-4w- ... pe-62-3701

1.5M 0.4W 5% https://uk.farnell.com/vishay/sfr250000 ... dp/9476512

1.5M 0.25W 2% https://uk.farnell.com/multicomp-pro/mp ... dp/3650155

1.5M 0.25W 1% https://uk.farnell.com/koa-speer-electr ... dp/3541172

1.5M 0.6W 1% https://uk.farnell.com/vishay/mrs25000c ... dp/9465537

For example...


Mark

[BeebMaster]

Thanks for the replies, I should have replied sooner myself.

I was on the wrong tack to start with, I'd always assumed that the wattage of the resistors was how much power they consumed, not how much power could be safely put through them! So I had best stick to 0.25W I think, not that I am planning to buy any more, I already have about a trillion of them round here.

Might get some metal film 0.25W 1M5 though, now I know where to get them.