Electronics advice needed

[chrismahon]

I am completely out of my depth really. We have a Spanish semi-auto incubator. Last year the heater stopped working and I could see a burned component on the board. Brinsea have been very helpful and a suitable replacement board is available at £45 plus delivery (to France). When I took the unit apart in preparation for the new temperature control board (which I haven't bought yet) I noticed that the surge protection fuse was intact and the burned component was just a resistor.

Doing some research it is an axial 'metal film' ½ watt item (based on published dimensions) which sits in the circuit feeding the temperature measuring 'thyristor'? I don't think it failed due to a power surge but rather by poor design, a 1 watt item was needed and it simply overheated and melted. The board is intact but the resistor has burned out at one end. My meter says the resistance of one half of the resistor is 50 ohms, so perhaps the original was 100 ohms. So I need to buy a 1W 100 ohm axial resistor and modify the board tracking slightly to cope with the larger size.

Does this make sense? If I have got the resistance wrong what will happen? Where should I buy the resistor from - Ebay only sells them in 20 plus quantities and although they are only a few pounds it does seem a waste throwing the rest away?

Advice from an expert would be gratefully received before I waste a load of time going up a blind alley.

 

[chrismahon]

Well I took the plunge and ordered 10 x 100 ohm 1 Watt metal film resistors which were about £3 delivered. When I removed the burned out resistor from the control board I noticed '150' printed underneath which I took to be the required value perhaps? Checked two of the other resistors on the circuit. One was labeled 22K and was, the other was labeled 47K and was actually 62K. So there has been some 'tweaking' since the board artwork was made. I took a gamble that the required value was the stated value and made up a 150 ohm resistor with three of the 100 ohm resistors I had bought (series parallel combination). After a lot of fiddling with the mouldings, because they were never designed to come apart, I managed to put the whole lot back together.

Would you believe it! Plugged the unit in and within an hour it had reached temperature and stabilised.

For the record. The alternative was scrap the original board and to fit a Brinsea control board. I selected the spare for the Octagon 20 MK111 which has mains outputs to both the fan and heater and a temperature sensor on a lead which was long enough to go into the original position. Unfortunately the temperature adjustment was in a different place, so the board had to be fitted upside down in my application. But the size 4.3cm x 9.8cm allowed me to fit it into the original enclosure to make a neat job. The down side was the cost -£45 I think plus delivery to France. A replacement incubator (Covatutto 24 eco) is readily available on the shelves of most Country Stores here at €170.

 

[Icemaiden]

Hi Chris.
Sorry I didn't see your original post in the pre-Christmas rush; congratulations on your success. 08

For future reference, assuming that it's the same as when I was at school (when bread boards were something you made up circuits on rather than sliced a wholemeal loaf on), you tell the rating of a resistor by the colours of its stripes. (Measuring the resistance of a burned out resistor may give you false results, as the cooked component may have had its chemistry changed in the singeing process.) I can't claim to be able to remember the colour codes offhand, but I'm sure they'll be lurking on the web. I haven't needed them for the last 27 years; your practical electronics is far more up to date than mine!

Happy hatching.

 

[chrismahon]

Unfortunately the surface paint had burned away completely so I couldn't read the stripes. My reading of 50 ohms for half of the resistor must have been wrong -it should have read 75 ohms so you are right about the change in chemistry Icemaiden. When I designed Printed Circuit boards for a living (years ago) we never put the resistor values on the artwork as it caused confusion if they were later changed during development. It would be far too expensive to scrap the boards just because the value shown was wrong. Whoever designed this board didn't follow UK standard practice -very fortunate for me!