Microclimate ministat300 repair help

[brett_dale]

Hey all hoping for some help on a repair for this one, Its been some time since i worked in electronics (12 years) and id appreciate some advice.

The unit is basically a thermostat for switching reptile heating, it uses a lm393 voltage comparitor and switches the output via a 24vdc relay.

The issue is that sometimes the relay will switch on for around 5 seconds and then softly switch off (led on light in the relay trigger circuit remains on). This fault seems to be temporary as sometimes the unit will run for days without fault, othertimes the fault is persistant and just seems to clear after been unplugged a while. Iv metered the drive to the relay coil and it would seem im only getting 6.5V there when working and when the fault is apparant the voltage seems to drop to around 6v, obviously something amiss for a 24v relay.

Already swapped the relay out for one with same specs as initially i believed this to be the problem.

Noticed some browning to the 470 ohm resistor (top of board) metered it out correct but not while live.

Hope someone can give me some advice on this one.

Cheers Brett

 

[OLIVE2222]

Hi Brett,

The top resistor in the middle of the board, part of the main supply, as suffer a bit as becoming brown. ==>EDIT: OK you find it.
The electrolytic caps (3 X black cylinder) are prone to fail, having them working for 12yeras is a good performance!

That's a first thought,

Olivier

 

[brett_dale]

Hi Brett,

The top resistor in the middle of the board, part of the main supply, as suffer a bit as becoming brown. ==>EDIT: OK you find it.
The electrolytic caps (3 X black cylinder) are prone to fail, having them working for 12yeras is a good performance!

That's a first thought,

Olivier

Thanks for the quick response Olivier, im not sure how much use this item has had since it came to me second hand a year back, its me that hasnt worked in electronics for 12 years so a bit rusty.
Would i be right in assuming the electrolytics, top resistor and the large capacitor above the relay are the key components in the 240V ac to 24V dc drop, my presumption is that something in this area is failing, dropping the voltage to levels that are not enough to keep the relay switched but enough to keep the rest of the circuit operating (considering the lm393 works down to 3v). Im more used to working with transformers when it comes to dropping the voltage so it kind of got me lost.

Cheers Brett

 

[OLIVE2222]

Hi Brett,

Here an app. notes on transformer less power supply.

http://ww1.microchip.com/downloads/en/appnotes/00954A.pdf
indeed the big cap is part of the power supply.

Olivier