I have built a K6000 controller / timer and it has worked fine for about 3 years. Now the K6000 cuts out for a few hours every day: it appears to lose 230v power and switches over to internal battery backup and so sheds its switched loads (so my home heating goes off!!).
The problem appears to be that the 7806 6v voltage regulator is overheating and shuts down for a while to cool off.
The K6000 is in a metal case with a few air-holes. TRANSF01 is getting very hot and is close to the 7806. But the K6000 has worked OK like this for 3 years.
Is overheating a known problem with the K6000?
The K6000 is drawing about 40ma from the 230v supply = 9.2VA (display medium contrast, no sensors attached and output LEDs and relays all off/open). I suspect that this is too high, as it is nearly twice the combined maximum output rating of TRANSF01 (3.8VA) and TRANSF02 (1.2VA). So TRANSF01 may be faulty or C7 may be leaking. Does anyone know what the normal current into the K6000 is?
The secondary output voltage from TRANSF01 across diode bridge D16-19 is 16.8 VAC and the voltage across C7 is 19.2V. Are these values normal?
The solution may be more air holes or a bigger heat sink on 7806 but first I would like to know if there is a fault in a component - because something has changed.
The 40mA you measure is probably not correct, unless you measure this with a True RMS meter. Can you measure the DC current that is drawn from VR1 e.g. by cutting D20 and measuring across its connections. Furthermore, please measure the voltage over its two outer pins.
We have never had any heat problems with this item, so something must have changed or gone defective in your unit. Please measure the input voltage. We know the mains voltage in the UK is sometimes a lot higher than 230VAC. That could cause a too high drop over VR1, which can trip the thermal shutdown.
320ma for the first second or two after switch on, then drops back to a steady 104ma
13.4V
So the dissipation of VR1 is 1.4W which I think is 70% of its rating (? 2W) without additional heatsink.
238VAC on my old but accurate analogue meter
242VAC on my cheap digital meter
This is only 5% above 230VAC - surely within circuit tolerances? The secondary output voltage from TRANSF01 across diode bridge D16-19 is 16.8 VAC and the voltage across C7 is 19.2V. Are these values normal?
The current looks normal.
However, a TO220 regulator without heatsink cannot dissipate more than 1W. It it obvious that in your case it will trip the thermal shutdown.
The secundary of your transformer is rather high. Is it a 220 or 230VAC transformer? The easiest way to solve this problem is to put a 100 ohm / 1W resistor in series with D21.
Thanks for your suggestion. I will probably do this with a 2W resistor.
230VAC part #1120038M
However, I am still concerned the transformer is failing. Current into D21 is 104 ma (rechargeable backup battery is disconnected); voltage across R35 (display backlight resistor) is 15.7V so 71ma must be going in there; all LEDs etc off. So the load on the transformer is 19.4V x (104 + 71)ma = 3.4W. The power entering TRANSF01 is about 238VAC x 30ma = 7.2VA. So transformer efficiency may be as low as 47% or C7 is leaking. Most of the power going into the transformer would be coming out of it as heat, which (1) may well be causing the overheating and (2) may shorten its life.
Do you know what the transformer efficency should be?
Small transformers can suffer from core saturation, which causes the efficiency to drop, the temperature to rise and the sine wave will be more or less clipped. Hard to tell in this case. Transformers don’t go bad overnight. It cannot harm to replace C7
This is interesting - thanks for the info. The transformer may have been going bad for a while, so of course.
I have also “googled” some manufacturers and am surprised to learn that efficiences of small encapsulated transformers can be as low as 55% or even 50%. However, my transformer is a futher 5%-10% below this, its secondary voltage is also high, and its gets very hot. Taken together with your diagnosis, I think the transformer has gone out of spec and it is time to change it (and C7, just in case).
Thanks again for interest and quick help - it is a great service! One final question: Velleman components are only available to special order (and long lead times) from UK suppliers and the transformer pin out is unusual (single secondary, staggered pins): could you please tell me who is your largest retail dealer with Internet sales for components in Europe?
Sorry, but I’m sure that you will understand that we cannot recommend one of our distributors, that would be unfair towards our other distributors. Furthermore, this is an item that they won’t stock, so they will have to order it for you. Suggestion for a quick solution: Return the transformer to:
Velleman Kit Service Dept.
Legen Heirweg 33
9890 Gavere
Belgium
Once we have received it, we can treat it as a repair item and send out a spare transformer. Cost+shipping will be billed to your account, so you will receive an invoice before we send out the goods.