H28TR Digital Thermostat -55℃ to 120℃ DC12V/24V Mount Size 46×27mm Temperature Controller Module

The H28TR is a low-cost digital display electronic thermostat with relay switching output, powered by DC12V/DC24V, rated for 10A load and -55℃ ~ 120℃ temperature sensing range, with mount Size 46×27mm. It automatically cuts off or connects connected heating/cooling loads per preset temperature thresholds to maintain target ambient/object temperature, resolving manual temperature regulation inaccuracy and constant human monitoring demands for small-scale thermal control systems.

Product Information of H28TR Digital Thermostat

Item Name	Mini Embedded Digital Thermostat	Output Type	Relay Output (Switch Output)
Model No.	H28TR	Output Capacity	Max 10A
Temperature Measuring Range	-55℃ ~ 120℃	Display Color	Red+Red / Red+Blue, Optional
Display Precision	0.1℃	Operating Temperature	-10 ~ +65℃
Temperature Control Precision	0.1℃	Operating Humidity	10 ~ 80% (Non-condensing)
Allowable Deviation	1% (±1 digit)	Operating Air Pressure	80 ~ 106kPa
Operating Current	＜15mA	Sunlight Exposure	Avoid direct sunlight
Display Mode	Dual 3-digit 0.28-inch LED Digital Tube	Overall Dimension	48×29×38 mm (L × W × T)
Measurement Input	NTC10K Waterproof Temperature Probe	Mount Size	Recommended: 46×27 mm (L × W)
Probe Cable	Standard probe diameter: 5mm, cable length: 1m	Net Weight	40g
Power Input	DC12V / 24V Optional	Manufacturer	Smartclima

Product Features

1. Wide temperature sensing coverage from -55°C up to 120°C, adapts to both ultra-low temperature cold environments and high-temperature heating scenarios.
2. Two standardized DC power input modes (12V / 24V) to match most small DC power supply modules in DIY and industrial miniature equipment.
3. 10A relay contact load capacity, compatible with most low-power heating sheets, cooling fans, semiconductor refrigerators and small heating tubes.
4. Digital real-time display design, real-time ambient temperature visual feedback without extra temperature measuring instruments.
5. Fully adjustable temperature switch threshold; users freely set the temperature trigger point for automatic startup/shutdown of loads.
6. Compact integrated structure with plug-and-play wiring design, convenient for DIY assembly and equipment internal embedded installation.
7. Stable operation performance with low failure rate, verified by high buyer satisfaction and high on-time delivery rate (99%).
8. Low procurement cost with complete after-sales guarantee including 7-day no-reason return and free delivery service.

Specific Usage Scenarios

1. DIY small constant temperature incubator makers: Hobbyists build egg incubators, seed germination boxes and pet reptile breeding boxes; H28TR automatically maintains stable internal box temperature to improve hatching and survival rate.
2. Cabinet electronic heat dissipation control: Electrical cabinet, battery compartment users use this thermostat to control cooling fans; fans auto-start when internal temperature rises above the set value to prevent circuit overheating damage.
3. Small semiconductor refrigeration module temperature control: Electronic experimenters drive semiconductor cooling plates with H28TR to make mini cold storage boxes, cooling liquid containers with fixed low-temperature requirements.
4. Small heating equipment constant temperature maintenance: Users install the controller on small heating rods, heat tracing belts for antifreeze protection of water pipes in cold outdoor environments.
5. Aquarium & small water tank temperature auxiliary management: Aquarists use the thermostat to control small water heating equipment to stabilize water temperature for tropical fish breeding and aquatic organism cultivation.

Operation Instructions

Set Startup Temperature

On the normal measurement display screen, short-press the UP key to enter startup temperature setting mode. Only the upper screen number will be displayed. Adjust the target temperature value by short pressing the UP / DOWN keys. The value will flash to save automatically and revert to the normal measurement interface if no operation is performed within 3 seconds.

Set Stop Temperature

On the normal measurement display screen, short-press the DOWN key to enter stop temperature setting mode. Only the lower screen number will be displayed. Adjust the target temperature value by short pressing the UP / DOWN keys. The value will flash to save automatically and revert to the normal measurement interface if no operation is performed within 3 seconds.

Quick Check Preset Startup & Stop Temperatures

On the normal measurement display screen, long press the DOWN key for 3 seconds until the screen flashes. The upper screen shows the preset startup temperature while the lower screen shows the preset stop temperature. The system will auto return to the normal measurement screen after 5 seconds of inactivity.

Heating Mode Setup Instructions

Setup Rule:

Startup Temperature ＜ Stop Temperature (The system will automatically identify the working mode as heating mode)

Setup Steps:

1. Short press the UP button to access the startup temperature setting. Only the upper display value will appear. Set your target temperature by short pressing the UP/DOWN buttons. The setting will flash to save automatically and return to the normal measurement interface if no operation is made within 3 seconds.
2. Short press the DOWN button to access the stop temperature setting. Only the lower display value will appear. Set your target temperature by short pressing the UP/DOWN buttons. The setting will flash to save automatically and return to the normal measurement interface if no operation is made within 3 seconds.

Application Example:

How to configure the thermostat for a water heater: stop heating at 65°C and restart heating when temperature drops back to 50°C?

1. Confirm heating mode is activated by keeping startup temperature lower than stop temperature.
2. Short press the UP button to enter startup temperature setting, set the startup temperature to 50°C using the UP / DOWN keys.
3. Short press the DOWN button to enter stop temperature setting, set the stop temperature to 65°C. Setup completed!

Cooling Mode Setup Instructions

Setup Rule:

Startup Temperature ＞ Stop Temperature (The program will automatically identify the operating mode as cooling mode)

Setup Procedures:

1. Short press the UP key to enter the startup temperature setting page. Only the number on the upper screen will be displayed. Set the target temperature value with short presses of the UP and DOWN keys. If no operation is performed within 3 seconds after adjustment, the value will flash for automatic saving and the system will return to the normal measurement interface.
2. Short press the DOWN key to enter the stop temperature setting page. Only the number on the lower screen will be displayed. Set the target temperature value with short presses of the UP and DOWN keys. If no operation is performed within 3 seconds after adjustment, the value will flash for automatic saving and the system will return to the normal measurement interface.

Application Example:

For farm temperature control: stop cooling at 26°C and restart cooling when the temperature rises back to 28°C. How to complete the configuration?

Step 1: Confirm the cooling mode by setting startup temperature higher than stop temperature.

Step 2: Short press the UP key to access startup temperature setting, set the startup temperature to 28°C using the UP / DOWN keys.

Step 3: Short press the DOWN key to access stop temperature setting, set the stop temperature to 26°C. Configuration finished!

Code meaning

Main Menu	Description	Setting Range	Factory Default
P-1	High Temperature Fault Alarm Value	-55℃ ~ 120℃	120
P-2	Low Temperature Fault Alarm Value	-55℃ ~ 120℃	-55
P-3	High & Low Temperature Alarm Delay Time	0s ~ 999s	5
P-4	Minimum Interval for Consecutive Relay Actions	0s ~ 999s	10
P-5	Lower Display Content Setting	SA (Startup Temp) / SP (Stop Temp)	SP
P-6	Temperature Fine Calibration	+10.0℃ ~ -10.0℃	0
P-7	Parameter Lock	ON / OFF	OFF
P-8	Automatic Display Shutdown	ON / OFF	OFF
P-9	Restore Factory Defaults	YES / NO	NO

Notes: The above parameter settings are only for customers with special requirements. This parameter table can be ignored under normal usage scenarios.

1. How to access internal menu settings

1. Two key operation modes: long press & short press. Long press: hold the key for about 3 seconds then release; Short press: press and release the key within 1 second.
2. Long press the UP key to enter setting mode; P1 will be shown on the upper screen. Toggle among P1–P9 by short pressing the UP or DOWN key.
3. When P1~P9 is displayed, long press the UP key to enter the parameter value setting interface.
4. After entering the setting interface, adjust values with long press UP/DOWN key; switch ON/OFF status with short press UP/DOWN key.
5. After finishing adjustments, the screen will flash to save automatically and return to normal measurement interface if no operation within 5 seconds.

2. How to quickly check preset startup temperature & stop temperature

On the normal measurement display page, long press the DOWN key for 3 seconds until the screen flashes. The upper screen shows the preset startup temperature while the lower screen shows the preset stop temperature. The system will auto revert to normal measurement view after 5 seconds of inactivity.

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P-1: High Temperature Fault Alarm

Set the threshold value for high-temperature fault alarm. When actual temperature exceeds this value, the screen flashes for alarm reminder, and the relay cuts off output to protect circuit safety.

P-2: Low Temperature Fault Alarm

Set the threshold value for low-temperature fault alarm. When actual temperature drops below this value, the screen flashes for alarm reminder, and the relay cuts off output to protect circuit safety.

P-3: High & Low Temperature Alarm Delay Time

When the measured temperature exceeds the high alarm threshold or falls below the low alarm threshold, the relay will cut off output after this set delay duration.

P-4: Minimum Interval Time for Consecutive Relay Actions

This function is mostly applied to compressor refrigeration systems. If this thermostat is used for refrigerators or freezers, this parameter must be configured. Normally set to 3–6 minutes according to compressor backpressure. Skip this item if you are not controlling a compressor or do not require action interval delay.

P-5: Lower Screen Display Configuration

The lower screen displays stop temperature by default; users may switch it to show startup temperature based on actual demands.

P-6: Temperature Calibration

Use this function to calibrate readings when there is deviation between measured temperature and actual ambient temperature.

P-7: Parameter Lock

Turn this item ON after finishing all configurations to lock parameters and prevent unauthorized modification. All temperature and internal parameters will be locked unless P7 is switched back to OFF.

P-8: Automatic Display Off

ON: The screen will turn off automatically after 15 seconds of key inactivity under normal working status. Wake up the screen by pressing any key or when an alarm triggers.

OFF: Display keeps on permanently.

P-9: Restore Factory Defaults

Switch P9 to YES to restore factory default settings when parameters are messed up or device malfunctions occur.

FAQ