UK Resistor for volt drop

[Dimspark]

Hi Guys
I was asked to look at a twin extract fan system that was showing a trip fault after checking that both fans ran by temporarily bypassing the fan controller after further investigation I found that a resistor was in serious with a bridge rectifier
The dc side of the rectifier is soldered on to a 48v dc relay.
Now to the question ? it would appear that the resistor is fractured and therefore open circuited it’s either a 2 or 3 watt resistor but here is my problem I can’t measure the resistance as I can’t get a reading there is no other unit to compare it against it has the colour bands so it’s either a 2.2k or 22k resistor I think it’s red red red but the multiplyer band could be orange which would make it 22k ohms
Just wondering if anyone has ever come across something similar and have an idea of the size of resistor.
I have found an old pcb with 2 x 1 kohm resistor and a bridge rectifier if the rectifier is rated above 230v I might build a wee test rig to measure the actual volt drop across the resistor’s but this will be a last resort.
Thanks
dimspark

 

[pc1966]

What is the 48V relay coil's resistance?

If it is 2-3W off 230V and had a '2' in it it then probably 22k as approximating to (230 - 48)^2 / 22k = 1.5W

 

[brianmoooore]

Be aware that some series connected resistors like this are special 'safety' types, and should only be replaced by the correct part. The fact that it has gone OC makes it more likely that it is one of these.

 

[Dimspark]

Be aware that some series connected resistors like this are special 'safety' types, and should only be replaced by the correct part. The fact that it has gone OC makes it more likely that it is one of these.

Thanks Pc1966 , brianmoooore
The resistor looks like a wire wound resistor and it’s got the gold band which if I remember correctly is +-10% of the resistor value.
Thanks
dimspark

 

[Lucien Nunes]

Agree with the above, if the source is 230V with no other resistance then it really has to be 22kΩ Making similar approximations to @PC966, (230-48)/22k = 8.3mA. 8 * 48=0.4W coil power which is realistic for a small relay and 1.5W dissipated in the resistor as he says. Had it been 2.2kΩ instead, the current would be 83mA giving a coil power of 4W and a resistor power of 15W which isn't sensible.

2W resistors with coloured bands and of that high a resistance are not usually wirewound, what makes you think it is? As per @brianmoooore it might be a fusible, or at least non-flammable, metal film type. Gold tolerance band is 5% although it won't be that critical here.

 

[Dimspark]

Agree with the above, if the source is 230V with no other resistance then it really has to be 22kΩ Making similar approximations to @PC966, (230-48)/22k = 8.3mA. 8 * 48=0.4W coil power which is realistic for a small relay and 1.5W dissipated in the resistor as he says. Had it been 2.2kΩ instead, the current would be 83mA giving a coil power of 4W and a resistor power of 15W which isn't sensible.

2W resistors with coloured bands and of that high a resistance are not usually wirewound, what makes you think it is? As per @brianmoooore it might be a fusible, or at least non-flammable, metal film type. Gold tolerance band is 5% although it won't be that critical here.

Thanks Lucien
All comments are much appreciated I wish I had taken a photograph I could actually see the windings as some of the enamel had split and exposed the winding
When I was looking at the resistor I could actually see where the crack in the resistor was as it like a semi circle
I am just after googling the relay it had a coil resistance of 5.67kohm
230/27670=0.0083A so both yourself and pc1966 were spot on with your maths as the volt drop across the 22k ohm resistor would be 182v leaving the 48v to power the relay
My intentions are is to purchase a panel mounted resistor of 22k ohm and new bridge rectifier and install them.
Again thanks for the replies
Kind Regards
dimspark

 

[Dimspark]

Thanks Lucien
All comments are much appreciated I wish I had taken a photograph I could actually see the windings as some of the enamel had split and exposed the winding
When I was looking at the resistor I could actually see where the crack in the resistor was as it like a semi circle
I am just after googling the relay it had a coil resistance of 5.67kohm
230/27670=0.0083A so both yourself and pc1966 were spot on with your maths as the volt drop across the 22k ohm resistor would be 182v leaving the 48v to power the relay
My intentions are is to purchase a panel mounted resistor of 22k ohm and new bridge rectifier and install them.
Again thanks for the replies
Kind Regards
dimspark

Guys
Again thanks just after checking the RS website and the resistor on the RS site is almost identical to the resistor installed on the fan motor controller
I feel like an --- now but I have learned a lot from guys that have taken time out of their day to give me some much appreciated guidance
kind regards
dimspark

 

[Lucien Nunes]

I suspect you were seeing the spiral element of a metal film resistor. The wire of a 22k wirewound even of 10W+ is very fine indeed and if a wirewound meeting your description exists at all it would be very expensive.

The numbers were necessarily approximate due to adding AC RMS and DC values, ignoring the flywheeling effect of the rectifer around the coil inductance etc,, but good enough to identify which order of magnitude the resistor should be.

 

[Sparky Pat]

Guys
Again thanks just after checking the RS website and the resistor on the RS site is almost identical to the resistor installed on the fan motor controller
I feel like an --- now but I have learned a lot from guys that have taken time out of their day to give me some much appreciated guidance
kind regards
dimspark

[automerge]1622316771[/automerge]

Guys
Again thanks just after checking the RS website and the resistor on the RS site is almost identical to the resistor installed on the fan motor controller
I feel like an --- now but I have learned a lot from guys that have taken time out of their day to give me some much appreciated guidance
kind regards
dimspark