Electric radiator: suggest the source of the problem - Page 1
Forum section: electronics / projects

author: mekhall
author_rank: Newbie
timestamp: September 24, 2013, 07:39:25 pm
content: I have a 900W (400V) electric radiator that suddenly did not work. I opened it up and found a TDA1023 triac triggering circuit inside which looks pretty much like the reference design plus a couple of added features (such as overheat protection...)As can be seen on the attached photos there seems to have been a lot of heat. There is a 10W power resistor which obviously gets hot even during normal operation.Can you suggest / guess the cause of events that has caused the circuit to fail?(I don't know the answer. Yet.)

author: Unknown
author_rank: Unknown
timestamp: September 24, 2013, 07:57:14 pm
content: Quote from: mekhall on September 24, 2013, 07:39:25 pmPiss poor design. Seen pretty much the exact same fault in many many appliances.

author: mekhall
author_rank: Newbie
timestamp: September 24, 2013, 08:00:50 pm
content: Quote from: Lurch on September 24, 2013, 07:57:14 pmProbably a pretty poor design yes. Do you care to elaborate a bit on what you think is wrong with this design?

author: jahonen
author_rank: Super Contributor
timestamp: September 24, 2013, 08:01:21 pm
content: Heat source near electrolytic capacitor(s) forms an excellent warranty timer.Regards,Janne

author: mekhall
author_rank: Newbie
timestamp: September 24, 2013, 08:08:29 pm
content: That electrolytic capacitor is used as a smoothing cap in a half rectifier according to the reference design in the data sheet...What do you say about the scorch marks? Are they from normal <= 10W power dissipation from the power resistor, or has something exceptional happened?I've peeked at some of my other electric radiators (same model) and they are not blackened by soot...

author: ConnorGames
author_rank: Regular Contributor
timestamp: September 24, 2013, 08:11:49 pm
content: Check the solder joints. They sometimes become brittle when they get that hot, which they often do when a manufacturer decides that it is just fine to run devices way to hot in order to save a few cents on heatsinks or bigger devices. This may not be the cause of the problem, but if you decide to repair it, make sure to check for this before trying to fire it up again.I just repaired a '93(ish) Marantz stereo receiver (nothing special and probably not worth repairing, but I enjoy a challenge). In this case, brittle solder on overheating TO-92 devices basically meant that the bias circuit went open, promptly frying the output devices and blowing the fuse. The receiver needs a recap too, but it is just not worth the time and money.

author: mekhall
author_rank: Newbie
timestamp: September 24, 2013, 08:23:01 pm
content: Checking the solder joints sounds like a good idea, thanks!

author: Unknown
author_rank: Unknown
timestamp: September 24, 2013, 08:31:56 pm
content: Quote from: mekhall on September 24, 2013, 08:08:29 pmThe design would have called for something to built that was cheap and lasted for at least the warranty period. This would also mean that anything that wasn't 100% (dry joint, dodgy resistor) at build time would have shortened the life of an already flawed design. I don't think anything has gone wrong as such, it is just cheap electronics built to a price. Ideally you don;t want a power resistor that can do that sat on a PCB, but it is cheaper to do that than mount it off board or add heatsinks so there you go. You have 2 options, repair it back to factory conditions and get it working again or improve it and make it last longer.Quote from: mekhall on September 24, 2013, 08:08:29 pmI wouldn't be surprised if they all ended up the same way at some point.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 24, 2013, 08:40:48 pm
content: Looking at the blackening on the reverse side of the board, it seems like two resistors are both getting very hot. The big power resistor is obviously heating up, but also the smaller (1/4W? 1/2W?) resistor located between the electrolytics and the heat sink. Consider replacing it with a resistor rated for higher power, and consider elevating it a bit above the board to allow airflow around it.Slightly overheated resistors can keep on working, but heat will destroy those capacitors. If the blackened board is any indication, the capacitors are getting hot, too. "Ideal" capacitors don't get hot, but once the ESR goes up, the resistance can dissipate heat, which boils the electrolyte away in a vicious cycle of increasing resistance and heat output, leading to capacitor death. I'd check their ESR, or just go ahead and replace them if you don't have an easy way to check.The area of the board around that TDA1023 has been a bit warm, too.An interesting thing to note: The board area around the heat sink looks pristine. Plenty of other areas of the board are blackened. The heat sink isn't getting as hot as many of the other components.

author: mekhall
author_rank: Newbie
timestamp: September 24, 2013, 09:19:40 pm
content: It may not be visible on the photos, but the power resistor is actually elevated above the PCB a bit by means of two small metal pipes on its leads. It's not elevated much; approximately 5 mm or so.It seems like at least the power resistor is broken - I measured resistance on across the resistor on the board and got something like 1 Meg Ohm, and I would expect something < 7 kOhm if it was working. If the small resistor is broken as well, would that point to a single event causing the circuit board to fail rather than something going on for a longer period of time?If I decide to try and repair this, I would probably change the power resistor, the smaller resistor and the electrolytic capacitor and see what happens.There is a overheat protector circuit just above the power resistor. It has a ground lead consisting of a bare copper wire and a power wire. The power wire connector had some insulation on it, but it crumbled into dust when I touched it - apparently affected by the heat.

author: amyk
author_rank: Super Contributor
timestamp: September 25, 2013, 10:11:32 am
content: Given you have a few other units that (I'm assuming they've also been used the same amount of time) don't exhibit the same symptoms, another possibility is that some other part (the element?) is drawing too much current and burning out the resistors here.

author: nctnico
author_rank: Super Contributor
timestamp: September 25, 2013, 05:02:56 pm
content: Besides replacing the components I also recommend cleaning the board with alcohol. The black areas are most probably conductive.

author: mekhall
author_rank: Newbie
timestamp: September 25, 2013, 06:14:13 pm
content: Quote from: amyk on September 25, 2013, 10:11:32 amYes, that's a good point. The element itself might be involved...All things considered I'm leaning towards replacing the radiator with a new one. Or maybe replace all of that specific model.(After all it's illegal to even open a radiator up for inspection up here...)

author: Whuffo
author_rank: Regular Contributor
timestamp: September 26, 2013, 06:48:04 am
content: Also look for thermal fuses. It might be a 14mm or so disc device bolted to the radiating element, or maybe a cylindrical one about the size of a 1 watt resistor. If you find either one, check with an ohmmeter; if open, replace. Most heating devices include one and they're a frequent source of problems.

author: SeanB
author_rank: Super Contributor
timestamp: September 26, 2013, 07:38:18 pm
content: 10k resistor is open circuit, and really should be a 20W unit. the board around the TDA device got so hot the solder on the one side of the chip melted, so likely the chip itself is now very dead as well, along with the very well cooked capacitors. If it was me i would replace the resistors with a parallel pair of 22k 10w units ( larger surface to dissipate heat and thus run cooler) and mount them on the existing standoff spacers with 5mm between the 2 resistors. As well new caps all round and replace the other cooked resistors. I did this on a timer board, where the bastids used a 5W resistor instead of a 10W unit, and on top of it all used a glass fibre sleeve to cook the resistor nicely. i used a 10W resistor to replace it, and reduced the current through it by half, as it was driving CMOS, so did not really need to have 100ma of current through the resistive dropper, and at the same time upgraded the zeners on the board to 1W units as the original 400mw units were looking rather melted even though they still worked. New caps and it is still working 15 years later, though the contactor it drove did fail year before last from contact wear. Was expensive to order one airfreight over Christmas as usual.....

author: madsci
author_rank: Regular Contributor
timestamp: September 26, 2013, 08:45:49 pm
content: Quote from: Lurch on September 24, 2013, 07:57:14 pmI concur. This appears to be a case of chronically excessive dissipation in that resistor. I see this stuff *way* too often.