Resistor identification help.

[stuartb3502]

Hi,

Can anyone help me check my identification of these resistors? They're on a garage opener. I'm hoping to try replacing them to see if there's a simple/cheap fix. If not, it'll be at least a new board and possibly a new opener as I don't know if these were the root cause.

I think they are 33k 0.05% or 1% tolerance, but I'm not that used to identifying these things and some of the colours are not that obvious to me. On the top one R2 I can't tell whether the first band is actually brown or just perhaps discoloured. And I'm not sure whether the last band is brown or grey.

I'd also welcome thoughts on power rating for these. I'll try to find something similar size or bigger - possibly 2 or 3W?

Thanks
Stuart

 

[sparkymarka]

Very much doubt those resistors are faulty ! possibly whatever U1 is ...or have you checked the rear of the pcb to see if they have become dry jointed ?

 

[winston1]

R2 could be 13KΩ, R1 could be 33kΩ. Why don't you try measuring them with your multimeter?

 

[stuartb3502]

Very much doubt those resistors are faulty ! possibly whatever U1 is ...or have you checked the rear of the pcb to see if they have become dry jointed ?

Joints on the board all look good. I was just going by appearance (there are soot marks up the board under that penny where the resistors seem to have been smoking). U1 is apparently an optocoupler

I started looking at this thinking I'd find a dead relay, but they "look" OK (as in not heavily arc-ed/burnt like ones I've seen before (on a microwave)). Checked resistors with a meter in-situ. One is open circuit. I believe it's usually the other logic board not this power relay board which normally goes on these openers. Manufacturer's told me logic board no longer available. This board is but at £85+ on an 18 year old opener is not worth it vs a new opener.

R2 could be 13KΩ, R1 could be 33kΩ. Why don't you try measuring them with your multimeter?

Thank-you. That's what I thought they may be. I'll de-solder and measure at least one of them (other is dead), but I was conscious they may not measure correctly if breaking down anyway.

 

[sparkymarka]

What make is the operator ?

 

[stuartb3502]

What make is the operator ?

It's a Chamberlain Liftmaster.

Still confused by the resistors because looking closely it looks like R2 could possibly be Brown/Yellow/Orange/Grey (although 0.05% tolerance on this size of resistor seems very unusual judging by what's for sale). All the colours have faded and are fragile.

R2 measures as 12.2 kohm (de-soldered). R1 is dead.

 

[stuartb3502]

Taking another tack, I found these pictures of new boards. Still not completely clear since photo colours are not always accurate, but looks like 12kohms (5 or 10% tolerance)???

 

[endecotp]

The ones in the new photos are certainly 12k.

Regarding the power rating, it’s impossible to be certain as manufacturers differ but measure its size and compare with e.g. this datasheet:
https://www.seielect.com/catalog/sei-cf_cfm.pdf

My guess it could be 8.5 or 11mm long, which correspond to 0.5 and 1W respectively.
It would do no harm to use a higher-rated one of course!

 

[stuartb3502]

My guess it could be 8.5 or 11mm long, which correspond to 0.5 and 1W respectively.
It would do no harm to use a higher-rated one of course!

Good guess. The existing ones are 11mm. I'd ordered some 3W ones, but will try to find some 1 and 2W ones in case space proves to be an issue. Thanks very much.

 

[endecotp]

U1 is apparently an optocoupler

Just noticed that.
Presumably the other ends of the resistors have mains on them.
Total resistance = 24k, current = 240/24000 = 1/100 = 10 mA, which is vaguely right for a regular optocoupler.
But 10 mA through 12k dissipates I^2 R = 0.01 x 0.01 x 12000 = 1.2 W.
So i’m not surprised that they have got hot.

Is there any evidence of a diode in series with the resistors and optocoupler? When using mains voltage on an optocoupler (or other LEDs), the max reverse voltage becomes an issue. It’s possible that the optocoupler is fried. Try checking it with the diode range on your multimeter.

 

[stuartb3502]

Good call - I "think" the diode at least is dried as I have resistance in both directions . So I'll order one of these as well.

 

[bernardgreen]

as I have resistance in both directions

The 620 series of opto couplers have back to back emitting diodes

https://uk.rs-online.com/web/p/optocoupler-ics/6998253/

 

[endecotp]

Well done bernard for reading the part number!
Yes so in that case it should be OK unless the heat from the hot resistors has killed it.
I’m not sure what you mean by “resistance in both directions” - what exactly have you measured? Does your multimeter have a diode setting?

 

[stuartb3502]

Thank you both for your further thoughts on this. Yes, I had checked the optocoupler on the diode setting of the multimeter. I fitted the resistors today. 3W and substantially more beefy than the originals. I decided to try it before replacing the optocoupler (on the basis that I could damage something else whilst doing that). And it worked!

The resistors (+ 2 spare and 2 2W versions in case I couldn't fit the 3W ones) came to £3.10 including postage with the optocoupler being £4.60 (they are about 80p in bulk and wait). So definitely a worthwhile saving.

Thanks for all the help