Need suggestion for freeze protection circuit

Thread Starter

BuzzTronic

Joined May 5, 2018
3
hi all,

this is my first post.

I am changing an air conditioner from digital to analog (the circuit board failed). I can easily power the compressor and blower with relays, and the compressor relay with a thermostat.

But there is a freeze sensor I would like to use to interrupt the compressor. It is about 1k ohms at room temp, and maybe 3k at freezing temp. I need a circuit that will interrupt the primary on the compressor relay when this goes above a certain ohms. Needs to be adjustable so I can get it where it needs to be (35deg farenheight?)

The simpler the better.

Thanks
 

Hymie

Joined Mar 30, 2018
883
hi all,

this is my first post.

I am changing an air conditioner from digital to analog (the circuit board failed). I can easily power the compressor and blower with relays, and the compressor relay with a thermostat.

But there is a freeze sensor I would like to use to interrupt the compressor. It is about 1k ohms at room temp, and maybe 3k at freezing temp. I need a circuit that will interrupt the primary on the compressor relay when this goes above a certain ohms. Needs to be adjustable so I can get it where it needs to be (35deg farenheight?)

The simpler the better.

Thanks
These days you can buy something that will do what you want, far cheaper than you can buy the components to make the circuit.

https://www.ebay.co.uk/itm/W1209-Di...hash=item212fb74398:m:mfFYkgFEdfzceM-pBIX040w
 

Thread Starter

BuzzTronic

Joined May 5, 2018
3
thats cool I will put that as option.

But I am looking for an extremely simple circuit that when the sensor ohms reaches a threshold, it either turns on or off, so I can control a relay. I'd like some adjustability (trimmer pot?).

I'm thinking maybe could use a transistor?
 

Hymie

Joined Mar 30, 2018
883
thats cool I will put that as option.

But I am looking for an extremely simple circuit that when the sensor ohms reaches a threshold, it either turns on or off, so I can control a relay. I'd like some adjustability (trimmer pot?).

I'm thinking maybe could use a transistor?
OK here is a very simple circuit that will do what you want using an op-amp driving a transistor & relay.

The set temperature is controlled by the 10k pot connected to the inverting input of the op-amp. The temperature sensor tc is simple voltage divider along with the 2k7 resistor.

In operation, when the voltage at the non-inverting input of the op-amp is greater that that set by the 10k pot – the relay will be energised; conversely when the non-inverting input is lower than that set by the 10k pot – the relay will be de-energised.

The capacitor C is provided to minimise relay chatter that might occur when the voltages Va and that at the inverting input to the op-amp are near equal.
 

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Hymie

Joined Mar 30, 2018
883
Here is a single transistor control circuit that might work.

Q1 is a darlington transistor (type TIP121) given the gain needed.

The 15k resistor value might need to be increased for the circuit to operate as you want.
You can have a play with the resistor values until it performs as you want.

One other thought, if the compressor ntc is embedded within the motor, the separation between it and the mains may be unknown – so my advice is to test the circuit with a variable resistor in place of the ntc, and treat the circuit with care once connected in final application.
 

Attachments

Marc Sugrue

Joined Jan 19, 2018
167
OK here is a very simple circuit that will do what you want using an op-amp driving a transistor & relay.

The set temperature is controlled by the 10k pot connected to the inverting input of the op-amp. The temperature sensor tc is simple voltage divider along with the 2k7 resistor.

In operation, when the voltage at the non-inverting input of the op-amp is greater that that set by the 10k pot – the relay will be energised; conversely when the non-inverting input is lower than that set by the 10k pot – the relay will be de-energised.

The capacitor C is provided to minimise relay chatter that might occur when the voltages Va and that at the inverting input to the op-amp are near equal.
For that circuit to work off that opamp you'll need a negative psu rail for it or change to a single rail amplifier.
 

DickCappels

Joined Aug 21, 2008
6,660
So...use an Lm358 or LM324 or such, or even better a single supply comparator like the LM393, which would require the base resistor be used as a pullup resistor.
 

Marc Sugrue

Joined Jan 19, 2018
167
Many circuits using an op-amp do not require a dual rail (+/-V) supply, including the one given above.

From my recollection the 741 needs a dual supply which was what the circuit diagram suggested. If not i stand corrected. I’ve no issue with the amps offered by Dick which are all designed to operate from a single rail.
 

Hymie

Joined Mar 30, 2018
883
From my recollection the 741 needs a dual supply which was what the circuit diagram suggested. If not i stand corrected. I’ve no issue with the amps offered by Dick which are all designed to operate from a single rail.
741 op-amps are perfectly happy working from a single supply rail – but as with other op-amps operating on a single supply rail, their outputs cannot drive negative, below the 0V.
 

Thread Starter

BuzzTronic

Joined May 5, 2018
3
One other thought, if the compressor ntc is embedded within the motor, the separation between it and the mains may be unknown –.
If NTC is a thermister, then I think we are talking about the sensor with two wires that is attached to the cooling evaporator coil. Thats what I need to reference so when it ices up, the circuit interrupts the power to the compressor relay, so the ice can melt.

The original computer board took care of this but that's dead so I am just replacing the basics here so as to use the air conditioner. Already have an old school ac thermostat coming from ebay.

Thanks for all the advice,

I will get the parts and make one or more of these circuits. :)
 

ebeowulf17

Joined Aug 12, 2014
3,274
OK here is a very simple circuit that will do what you want using an op-amp driving a transistor & relay.

The set temperature is controlled by the 10k pot connected to the inverting input of the op-amp. The temperature sensor tc is simple voltage divider along with the 2k7 resistor.

In operation, when the voltage at the non-inverting input of the op-amp is greater that that set by the 10k pot – the relay will be energised; conversely when the non-inverting input is lower than that set by the 10k pot – the relay will be de-energised.

The capacitor C is provided to minimise relay chatter that might occur when the voltages Va and that at the inverting input to the op-amp are near equal.
It might be worth adding a feedback resistor for some external hysteresis, which would provide greater protection against chatter near the transition point.
 

crutschow

Joined Mar 14, 2008
25,690
741 op-amps are perfectly happy working from a single supply rail
Depends by what you mean as happy. :rolleyes:
The 741 output will only go to a minimum of about 2V output with a single supply.
Must better to use a single supply op amp or comparator whose output can go to zero as Dick suggested.
 
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Hymie

Joined Mar 30, 2018
883
If NTC is a thermister, then I think we are talking about the sensor with two wires that is attached to the cooling evaporator coil. Thats what I need to reference so when it ices up, the circuit interrupts the power to the compressor relay, so the ice can melt.

The original computer board took care of this but that's dead so I am just replacing the basics here so as to use the air conditioner. Already have an old school ac thermostat coming from ebay.

Thanks for all the advice,

I will get the parts and make one or more of these circuits. :)
If you make the single transistor circuit, increase the 15k resistor to 33k; with the thermistor at 3k this will allow a transistor base voltage range of 0.9V - 2.3V depending on the pot position.
 
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