POT (potentiometer) - it's purpose?

[staffordshireterrier]

Hello

Might sound a weird question, but have been searching everywhere.

What exactly does a POT (potentiometer) do in a gas boiler? I know roughyl it controls the amount the gas valve can open, via the thermistor, but how? Is it linked to the user control stat setting in any way?

Does anyone have an info please?

 

[kevplumb]

its a variable resistance afaik

 

[staffordshireterrier]

what's that ?!

 

[kevplumb]

bit like a valve on a water pipe

the further you turn it the more it lets through

if you want chapter and verse you would be better asking the sparks

mind they will probably get all technical

 

[OLLIE20]

The boiler thermostat knobs on the generally have a spindle which go through the centre on the potentiometer on the PCB generally situated behind

The higher it is turned the hotter water in boiler will get so to make it simple if you have it set to number 6, this means its set to 60 degrees so the boiler will run, untill water gets 2 around 60 when it does, the thermister will lower the burner pressure via the PCB and to the modureg on the gas valve. lowering the B.P. so as to keep it at set temp

When it gets to set temp boiler will shut down untill falls to a certain level and will then re-fire

This is a general over veiw some boilers work differantly hope it helps why do you ask??????

 

[doitall]

Does this help.

http://www.sengpielaudio.com/calculator-voltagedivider.htm

 

[ChrisR]

I know roughyl it controls the amount the gas valve can open

Er, no it doesn't.

They're a general purpose thing to connect on the back of a knob, so the electronics knows how far the knob's turned.
Yes they do it using a resistive strip, so if full volume is 100R, half is 50R, etc.
They also come as knobless little adjusters on pcbs.

If you want technical I can do technical - but don't ask a sparks, they don't really have to know about them.

 

[ChrisR]

Does this help.

http://www.sengpielaudio.com/calculator-voltagedivider.htm[/QUOTE]
I doubt it very much John, that's just one application of them in impedance matching of radio circuits...

 

[doitall]

Does this help.

http://www.sengpielaudio.com/calculator-voltagedivider.htm[/QUOTE]
I doubt it very much John, that's just one application of them in impedance matching of radio circuits...

I didn't think it would Chris, but it looked interesting.

You and Tony, can tell the OP without my interference.

 

[staffordshireterrier]

Thanks for all your help

The reason I asked is I'm trying to learn as much as possible about boilers and fault finding/repair. I am acs qualified and have been working with a heating company but they don't do breakdown's, which I am really interested in.

I have been learning abotu parts such as the APS, FAN, PUMP, DIVERTER VALVE, FLOW SITCH, THERMISTORS, GAS VALVE, PCB, IGNITION ELECTRODE, HT LEAD, FLAME RECTIFICATION PROBE, ETC - but just came across the poitentiometer in a book I've been reading and wasn't too sure what it was for or did.

I would like to learn more about boiler's but am not sure where to go with it. I have seen jobs advertised in my local; area for boiler breakdown enginners, service enginners, etc - I have doen CP12's/Service's before but no breakdown's - although I have replace a thermocouple before which the tip was burnt out.

 

[Paul Barker]

Please can I get technical it will be fun.

The temperature sensor (thurmister) inside or on the flow pipe of the primary water or the hot water pipe has a variable resistance dependent inversely on temperature. This is therefore negative temperature coefficient, a term to describe this is therefore NTC shortened. typically at 20 degrees celcius this gives a resistance reading of 10,000 ohms. Ohm was an old bloke and since he discovered some stuff about electrons flowing around a conductor his name was attributed to the laws of electricity just like there are laws of gravity.

Anyway the NTC therefore becomes a voltage divider which effect is amplified by a voltage regulator which ultimately sends a dc voltage to the gas valve that modulates the flow of gas via a dc solenoid which to a variable extent holds open the valve to allow more or less gas through. this is known amongst us as a modureg.

But there is a further need for purposes of luxury of a user control. So toy give you the user some control over the temperature of your water or your radiators, a second voltage devider is put in series with the NTC. So that the end signal applied to the regulator is made up of the 10kohm ntc and a 10kohm pot in series. Therefore, when you want cooler water you turn your knob and the pot wiper moves to a part on the resistive material (used to be carbon but various materials are available to the makers of pots) track which offers less resistance and vice versa.

So in short an electronic solid state component or components known as voltage regulator is controlled by a resistance which is composed of a pot in series with an NTC thurmister (variable temperature resister). the direct current voltage so determined opens to a lesser or great extent a plunger via an electromagnetic coil, which would spring close in the absence of voltage, to allow greater or less gas through to the burner determining temperature within combustion chamber thus affecting temperature of water passing through.

 

[staffordshireterrier]

thanks for that paul .... i get it, much helped

I know some thermistors can be PTC - the single pin ones...?

 

[Paul Barker]

Thanks for all your help

The reason I asked is I'm trying to learn as much as possible about boilers and fault finding/repair. I am acs qualified and have been working with a heating company but they don't do breakdown's, which I am really interested in.

I have been learning abotu parts such as the APS, FAN, PUMP, DIVERTER VALVE, FLOW SITCH, THERMISTORS, GAS VALVE, PCB, IGNITION ELECTRODE, HT LEAD, FLAME RECTIFICATION PROBE, ETC - but just came across the poitentiometer in a book I've been reading and wasn't too sure what it was for or did.

I would like to learn more about boiler's but am not sure where to go with it. I have seen jobs advertised in my local; area for boiler breakdown enginners, service enginners, etc - I have doen CP12's/Service's before but no breakdown's - although I have replace a thermocouple before which the tip was burnt out.

in that case, you will never ever have a failure with this pot. An ol;d style pcb may fail but not due to the pot.

the ntc's fail on a lot of boilers a good stock of ntc's is obligatory aswel as a complete range of fibre washers but the most common are 1/2in and 3/4 in.

The most common breakdown modes are failure to sense flow of water or combustion products and or failure to shift same, with combustion products being highest incidense.

 

[Paul Barker]

thanks for that paul .... i get it, much helped

I know some thermistors can be PTC - the single pin ones...?

On the whole it's all shifted to NTC, not shure why there was a good reason for it though.

Just as they shifted to flame rectification (which means the ac voltage applied across the flame will only satisfy the safety checks of the pcb if it is converted to DC, a flame apparently acts as a diode [1/2 wave rectifier]) Previous to this they just looked for a voltage, but it was found that if the sense electrode was shorted or leant over to or tracked to earth it would inform the pcb there was a flame when there wasn't. flame rectification will only take place in the presense of a flame.

As for thermocouples, less and less about. Things are ramping ahead in the complexity stakes..

 

[staffordshireterrier]

Paul, what do you mean in this case you will never have a failure with this pot?


when the ntc's fail, are the most common to fail wet pocket types due to sludge etc sticking to them?

with ntc's - do you need the specific one for the boiler or are there universal ones then?? always thought you needed the one for the boiler make/model as in the manual?!


what are the fibre washers for? wet pocket thermistors?

do you mean most common faults are flow switches'/diverter valves, aps's, blocked flues, faulty fans, etc??

sorry for the q's just really interested and want to learn