Any electronics people on here?...

[Maz]

Hi

I'm no electronics whiz, so excuse my ignorance, but...

I have an electronic device in which there are some resistors. Occasionally, when switched on, some of the resistors burn out. Previous repairs have been carried out (not by me) where burned-out resistors have been 'bridged' with a new resistor (which appears to be identical).

I want to replace the existing resistors with same resistor value (2.2 ohm) but higher power rating resistors.

I know the resistors at the moment are 2.2 Ohm, but I don't know how to tell what their power rating is. Unless I know this, I don't know what the next power rating up is. The chap who made this device is not around to ask.

I'm guessing the resistors are operating at or close to their threshold, hence the burn-out.

Can you tell from the colour bands what the Wattage is? (probably not).
I'm talking about this style of resistor:

 

[MrJamie]

The bands just tell you the resistance value and IIRC tolerance %. I guess you could measure the volts and amps going across the ones that burn out and work out the typical wattage to guess what rating it *probably* is. I have a feeling you could probably take it to a shop like Maplins and just buy the next physical size bigger in the same resistance rating, but my electronics knowledge is pretty poor

 

[pablo666]

The bands just tell you the resistance value and IIRC tolerance %. I guess you could measure the volts and amps going across the ones that burn out and work out the typical wattage to guess what rating it *probably* is. I have a feeling you could probably take it to a shop like Maplins and just buy the next physical size bigger in the same resistance rating, but my electronics knowledge is pretty poor

Stick two in paralell and you half the current through each resistor so shouldn't burn out.

 

[Profpointy]

Stick two in paralell and you half the current through each resistor so shouldn't burn out.

Yebutt... you'll also halve the resistance won't you -( unless you use two with twice the resistance of course).

----------------------
Have a look on radio spares (or whoever's) site, and try and guess wattage from the size.
Solder in one a good few sizes up from the guessed wattage and job's a good 'un .

Let's say the original is 1/4 watt then a 2W resistor should give plenty of margin

----------------------------------

Another thought : when you say "bridged" - have the burnt out resistor been left in place, in which case they may still be allowing (some) current through, thus giving the wrong value overall. I'd remove them.

--------------------------


Still another thought - perhaps there's an underlying fault causing more current to go through than there's meant to be, thus burning them out. Perhaps less likely, given the item presumably works, but can't be ruled out.

Cheers

Hywel

 

[Markymark]

Stick two in paralell and you half the current through each resistor so shouldn't burn out.

Unfortunately that would half the resistance (http://www.sengpielaudio.com/calculator-paralresist.htm)

Just buy the highest power rating there is, having too high wouldn't compromise anything, just would cost a few pence more.

 

[Rohloff_Brompton_Rider]

Either the resistors are under sized from the onset or the fuse/MCB circuit protection is wrong....the weakest link in a circuit should be its fuse.

Power= voltage times current.....so unless you measure the current you'll be guessing.

As a general rule, the wattage of a resistor is equal to its size. So go to your electronics store and get on physically the same size and material or larger in the same material.....the ohmic value is what's important.

 

[slowmotion]

For carbon film resistors, typical sizes are as follows.....

Wattage (W), Body length (mm), Body Diameter(mm)

0.25, 3.4, 1.8
0.33, 6.2, 2.3
0.66, 8.5, 3.2
1.0, 12.8, 4.7
2.0, 16.8, 5.7

 

[Maz]

For carbon film resistors, typical sizes are as follows.....

Wattage (W), Body length (mm), Body Diameter(mm)

0.25, 3.4, 1.8
0.33, 6.2, 2.3
0.66, 8.5, 3.2
1.0, 12.8, 4.7
2.0, 16.8, 5.7

Interesting. Where did you find this table?
It's easy enough for me to measure the length of the resistors!

 

[Maz]

Yebutt... you'll also halve the resistance won't you -( unless you use two with twice the resistance of course).

----------------------
Have a look on radio spares (or whoever's) site, and try and guess wattage from the size.
Solder in one a good few sizes up from the guessed wattage and job's a good 'un .

Let's say the original is 1/4 watt then a 2W resistor should give plenty of margin

----------------------------------

Another thought : when you say "bridged" - have the burnt out resistor been left in place, in which case they may still be allowing (some) current through, thus giving the wrong value overall. I'd remove them.

--------------------------


Still another thought - perhaps there's an underlying fault causing more current to go through than there's meant to be, thus burning them out. Perhaps less likely, given the item presumably works, but can't be ruled out.

Cheers

Hywel

Thanks Hywel.
w.r.t. the question in bold - yes, the burnt resistor is still in there, and the new resistor bridges over the top of it.

 

[slowmotion]

Interesting. Where did you find this table?
It's easy enough for me to measure the length of the resistors!

I just had a quick flick through the resistor section of my RS catalogue and picked out the dimensions of Tyco Electronics CFR 16,25,50,100 and 200 Series resistors.
Bingo!

 

[Maz]

I just had a quick flick through the resistor section of my RS catalogue and picked out the dimensions of Tyco Electronics CFR 16,25,50,100 and 200 Series resistors.
Bingo!

Cool. I'll have a look through the RS or Farnell catalogue at work. Cheers

 

[Wobblers]

Thanks Hywel.
w.r.t. the question in bold - yes, the burnt resistor is still in there, and the new resistor bridges over the top of it.

You'll need to desolder and remove the old burnt out resistor. The resistor in your OP is a 0.25 W carbon resistor. The first three bands give the resistance value, the last band the tolerance. You'd be best to replace it with a resistor with a higher power rating - Maplin sell metal film resistors which are of the same size but have a 0.6 W power rating, IIRC.

There is one possible gotcha: those resistors may be fusible resistors. A fusible resistor has much the same role as a fuse - if you overload it, it burns out and goes open circuit. Quite a few of the little power supplies and adapters you plug into the socket are protected by fusible resistors, as there isn't space for a proper fuse. You need to be sure to replace a fusible resistor with a new one for continued protection. It might say on the circuit board or schematic (if you have that) whether the resistor is a fusible one. What sort of electronic device is this?

 

[slowmotion]

Cool. I'll have a look through the RS or Farnell catalogue at work. Cheers

The RS website is down at the moment, but I'm sure it will be back soon.
uk.rs-online.com

 

[Maz]

You'll need to desolder and remove the old burnt out resistor. The resistor in your OP is a 0.25 W carbon resistor. The first three bands give the resistance value, the last band the tolerance. You'd be best to replace it with a resistor with a higher power rating - Maplin sell metal film resistors which are of the same size but have a 0.6 W power rating, IIRC.

There is one possible gotcha: those resistors may be fusible resistors. A fusible resistor has much the same role as a fuse - if you overload it, it burns out and goes open circuit. Quite a few of the little power supplies and adapters you plug into the socket are protected by fusible resistors, as there isn't space for a proper fuse. You need to be sure to replace a fusible resistor with a new one for continued protection. It might say on the circuit board or schematic (if you have that) whether the resistor is a fusible one. What sort of electronic device is this?

Cheers. The picture in the first message was for illustration only (i.e. just to show you what type of fuse I was referring to...Do they call it a 'push-through' resistor?, i.e. because you can push it into an electronic 'bread-board?).
How do you know it is 0.25W just by looking at it?

I have a schematic describing the resistor as 2R2 NRF25.

 

[ColinJ]

Before replacing any resistors, I'd be trying to find out what caused the old ones to burn out in the first place!