Potentiometer question on PCB

[StevenJames125]

Hi

Just a quick question - most PCBs contain potentiometer's - eg CH or DHW pots - what exactly do these do? I know they can be adjusted but was just trying to work out what they are for?

When you turn the CH dial for eg on a combi - does this in effect increase/decrease the potentiometer to allow more of less power to the CH thermistor? Does anyone know?

 

[kevplumb]

variable resistor

 

[kirkgas]

i think it alters the electrickery to a wee red or green bit which eventually pulls on an elastic band which alters the gas pressure which ultimately controls the output of the appliance

 

[StevenJames125]

Thanks Kev - so basically by turning the control know ie CH, you allow more or less resistance to get through to the CH thermistor?

 

[kevplumb]

close

in basic terms
the gas valve then the valve modulates to the signal sent to the control board by the thermistor (now wait for glazier to jump in and tell me i'm wrong)

kirk still puts coal in the back of the tv of a morning (bleedin haggis bashers )

 

[StevenJames125]

Thanks for your help

Might sound stupid, but if you alter the potentioemeter from the inside (ie removing PCB cover to access them, not the knob side - will this make a difference, or will it do the same as turning the user control dial which also connects to it?

 

[kevplumb]

same thing

 

[StevenJames125]

close

in basic terms
the gas valve then the valve modulates to the signal sent to the control board by the thermistor (now wait for glazier to jump in and tell me i'm wrong)

kirk still puts coal in the back of the tv of a morning (bleedin haggis bashers )

thanks a lot Kev for your help I really appreciate it. Have you missed a part of this post out about the gas valve or is it just my eyes playing up tonight

 

[kevplumb]

ok once again

the pot regulates the output of the modulating gas valve in relation to the signal recived from the thermistor
to the call for heat

i'm no electronics whizz

but thats basically how it works

or are you pulling my chain

 

[croydoncorgi]

the pot regulates the output of the modulating gas valve in relation to the signal recived from the thermistor
to the call for heat

errr.....no, not exactly.
On older type boilers, the potentiometer used to be wired to form a 'resistance bridge' circuit with the Flow or Hot Water thermister (temperature sensors) inside the boiler. When the bridge was 'in balance' (ie. the setting of the pot and resistance of the sensor were the same), the burner would be turned off. As the resistance of the sensor increased (ie. the temperature falls - it was usually an NTC Negative Temperature Coefficient sensor), the burner was turned on.

These days, virtually all boilers have fully-electronic controllers. The boiler software (yup - mostly they've got micro-controllers on the PCB) reads the setting of the pots and converts them to digital values (usually in degrees celsius) that are then stored in the working memory of the controller as the current setpoint(s). The software then reads the thermisters and works out not only whether the burner needs to run but also at what level of modulation. In most cases, this translates to RPM of the fan, which directly controls the modulation level of the burner by increasing / decreasing the flow rate of the gas air mixture. The only control needed by the boiler gas valve is to open and close it. The amount of gas used is controlled by the fan drawing combustion air through a venturi. The pressure drop (proportional to flow rate) sucks more or less gas through the gas valve.

 

[kevplumb]

the lad asked for a simple explaination

he got one

 

[kirkgas]

close

in basic terms
the gas valve then the valve modulates to the signal sent to the control board by the thermistor (now wait for glazier to jump in and tell me i'm wrong)

kirk still puts coal in the back of the tv of a morning (bleedin haggis bashers )

man i dinnae put the telly on, that costs money, i tell the weans to go and look in next doors windae