Circuit board starts only when warm air passes over it.

[silentrunning]

I have an air con appliance that doesn't always start when power is applied. I have removed the small control board which is separate from and supplied by a 24v power supply (that works fine) and inspected it for any obvious failed components but can see none. I have discovered that the board will start if I wave a hair dryer over it for a few seconds. The board appears to have no temperature sensors or sensitive components on it. Can anyone suggest what the most likely candidate for a failure mode of this form.
Many thanks.
John

[tehmatix]

Try to change crystal oscillator if it is existing one on your board

[stj]

post a picture
electrolytic capacitors are temp sensitive if they are borderline

[CapLeaker]

Upload some some high resolution pictures using the attachment function so we can see what you mean…

[silentrunning]

Picture of PCB attached. Many thanks.

[stj]

resolder the blue thing that is on the back of the board - it looks like a temperature sensor

[silentrunning]

resolder the blue thing that is on the back of the board - it looks like a temperature sensor

Thanks. I'll give that a go. Any idea where I might find information about that little blue thing?

[redwire]

It's a thermistor, look for any markings or numbers on it. There can be two of them- one for air temperature the thermostat uses, and another to detect an iced up frozen evaporator, it's near that coil.

[silentrunning]

Yes indeedy I thought it might be so I've been reading up on them.

The unit is actually a positive air flow fan that simply blows air into the house so no evaporator or anything like that. Could the ntc be being used for some other purpose, it is connected to one of the pins of the MCU via a tiny capacitor or two with the other end I think going to ground. Having said that given it's location on the back of the board I guess it's ambient temp it's after.

Measuring it's resistance I can see that change as I heat it up slightly.

I have resoldered it but with no change in behaviour, the unit refuses to come on unless it's been off for less than a minute or warm air is passed over it. Additionally passing warm air only over the thyrmister does not turn it on!

[silentrunning]

Oo forgot to say it has no markings on it. It's resistance is about 8k at about 16c

[redwire]

This is some laboratory unit? A fan controller? What model?

It could be the MCU is having issues reading the temperature sensor due to noise on its power. I would check the 3 electrolytic capacitors on the board, they may have gone bad. You can also try just heat each and see if that affects things, instead of heating the entire board. Try heating just select parts to narrow it down. Check the MCU power it looks like a 5V system.

[silentrunning]

Here's a link to the unit

https://www.nuaire.co.uk/residential...lation-systems

I've tried warming just separate component's and areas of the board but with no consistency. I've tried a separate power supply with no change. The MCU is a PIC18F24K20

The caps look ok but I'm a newby to electronics.

[stj]

heat the capacitors and see if it effects the unit

[silentrunning]

Done that but to no effect. I can't pin down any one component that when warned brings the unit on.

[silentrunning]

And only warm air will do the trick. Air from an unheated fan does nothing

[CapLeaker]

Then do the opposite. Take a can of air upside down and freeze spray components one by one and see if it quits.

Did you heat that orange cap with the brown band too?

[silentrunning]

Yep tried em all but couldn't get any consistent behaviour. Do you think replacing the three big caps is worth a shot?

[CapLeaker]

Before I would do that, I would put an oscilloscope on them and measure the ripple. Or take them out and check the ESR

[silentrunning]

Cool. I've been looking at the pwm fan signal that it generates with an oscilloscope with the possibility of making my own control board if I can't get this one to behave. What should I look out for regarding oscillscope readings for the caps. If I remove them I'll probably just put new ones in?

[silentrunning]

Not sure if this means anything but when I measure the resistance of the thyrmister I get 7.5k one way but 9k when the leads are reversed ! What's that all about?