LEDs are dimly lit!

Thread Starter

Timjr66

Joined Mar 3, 2023
3
Hello all! This is probably simple to all of you, but is confusing me as a novice! I am an HVAC technician and I built a training board using multiple thermostats. They are powered by a 24VAC transformer. When the thermostats respective switch closes, it is wired so that each respective lamp lights. Very simple circuitry! My issue is when the circuits are not closed. The LED lamps are all dimly lit. I want them to be non-powered however, they are receiving power somehow. I suspect a low voltage trickling through the solid-state thermostat boards? I’ve tried All value of resistors between the switch and the LED lamp without success, is there a different type of LED I should be using? I used this from Amazon:
DMWD 12V-24V 20mA Energy Saving LED Indicator Light Mounting Hole 6mm.
Thanks in advance!!
 
Last edited by a moderator:

sghioto

Joined Dec 31, 2017
5,380
Try a resistor in parallel with the LED
Start with a 10K and see if that helps.
Those LEDs are designed to operate on DC voltage.
 
Last edited:

LowQCab

Joined Nov 6, 2012
4,026
Thermostats can sometimes use some pretty funky work-arounds to
make a working unit with as few, and as cheap as possible, parts.
They are seldom "simple switches".

The Thermostats need a Load similar to the type
that they would normally be attached to in the field.

A 5-Watt Power-Resistor,
with approximately the same Resistance as the usual generic Relay-Coil they
would be actuating in the field, would be the first half of the solution.

The other half of the solution would be a Diode in-line with the LED.
LEDs, even "Universal-Multi-Voltage-Indicators" ( like You are using ),
generally don't like being fed an Alternating-Current,
some will pop when fed AC.

The Diode turns the AC into "Pulsating-DC",
which the LED will like much more than AC.

The Resistor will suppress the normal leakage Current from the Thermostat.
The LED takes only a very tiny Current to produce visible Light.
.
.
.
 

Thread Starter

Timjr66

Joined Mar 3, 2023
3
Thermostats can sometimes use some pretty funky work-arounds to
make a working unit with as few, and as cheap as possible, parts.
They are seldom "simple switches".

The Thermostats need a Load similar to the type
that they would normally be attached to in the field.

A 5-Watt Power-Resistor,
with approximately the same Resistance as the usual generic Relay-Coil they
would be actuating in the field, would be the first half of the solution.

The other half of the solution would be a Diode in-line with the LED.
LEDs, even "Universal-Multi-Voltage-Indicators" ( like You are using ),
generally don't like being fed an Alternating-Current,
some will pop when fed AC.

The Diode turns the AC into "Pulsating-DC",
which the LED will like much more than AC.

The Resistor will suppress the normal leakage Current from the Thermostat.
The LED takes only a very tiny Current to produce visible Light.
.
.
.
Thank you. So if I’m understanding you correctly, if I continue to use the same LEDs, I should try a relay coil parallel to the LED?
 

LowQCab

Joined Nov 6, 2012
4,026
A Resistor, or an actual Relay, will do the trick,
plus a Diode inline with the LED if You are working with AC-Voltages,
which You probably are.

The Relay will add an audible "click", and maybe a slight "buzz",
both of which might "add to the experience".
.
.
.
 

Alec_t

Joined Sep 17, 2013
14,280
using multiple thermostats. They are powered by a 24VAC transformer.
Please post the technical spec of the thermostats.
Also, note that 24V AC has a peak voltage of nearly 40V, which may cause damage to those LED indicators if they are designed for DC use only.
 

LowQCab

Joined Nov 6, 2012
4,026
Thermostats don't generally provide Schematics, just Wire-Colors, or 2-letter-Acronyms.

"" Thermostats can sometimes use some pretty funky work-arounds to
make a working unit with as few, and as cheap as possible, parts.
They are seldom "simple switches". ""
.
.
.
 

MisterBill2

Joined Jan 23, 2018
18,176
Consider that an electronic thermostat MUST have a bit of power to operate, and that power will come either from a battery or the circuit it is controlling, where else is there?? Thermostats with a battery use it to keep working while they are switched on, so that whatever needs the 24 volts will get the full power. Thermostats without batteries usually have a series voltage drop of just a bit to keep them alive while the controlled device is energized.
That very small current, probably not even ten milliamps, is not enough to affect the typical 24 thermostat load, which is usually over 100 milliamps. So nothing else complains or is bothered.
To avoid the LEDs slight glow, shunt them with a resistor, probably180 ohms, 1/4 watt. If it is still that important.
I am thinking this only happens with electronic thermostats, not the mechanical switch devices.
 

Thread Starter

Timjr66

Joined Mar 3, 2023
3
A Resistor, or an actual Relay, will do the trick,
plus a Diode inline with the LED if You are working with AC-Voltages,
which You probably are.

The Relay will add an audible "click", and maybe a slight "buzz",
both of which might "add to the experience".
.
.
.
Thank you!
 

LowQCab

Joined Nov 6, 2012
4,026
Some of the "Old-School" Thermostats that were "just Switches" had a Heating-Element
that was powered only by having the Switch "Open" and drawing a small amount of
Current through the Relay Coil being controlled.
( this was to reduce the Temperature-hysteresis of the Thermostat ).

So, even back in the Day,
they were playing games with something that "appeared to be" really simple.

Even the newest Electronic-Thermostats try to be "backwards-compatible"
with methods that were used ~60-years ago.
This creates a tremendous amount of confusion.

I think the AC-unit manufacturers should get together
and establish some modern, updated Standards that make more sense,
and actually supply their own Thermostat-design that is guaranteed to be compatible
with any "updated", or "standardized" system of control.
.
.
.
 

crutschow

Joined Mar 14, 2008
34,285
Start with a 10kΩ resistor in parallel with the LEDs as suggested.
If needed, reduce the resistor value until you see no visible light from the LED when OFF.
 

MisterBill2

Joined Jan 23, 2018
18,176
Some of the "Old-School" Thermostats that were "just Switches" had a Heating-Element
that was powered only by having the Switch "Open" and drawing a small amount of
Current through the Relay Coil being controlled.
( this was to reduce the Temperature-hysteresis of the Thermostat ).

So, even back in the Day,
they were playing games with something that "appeared to be" really simple.

Even the newest Electronic-Thermostats try to be "backwards-compatible"
with methods that were used ~60-years ago.
This creates a tremendous amount of confusion.

I think the AC-unit manufacturers should get together
and establish some modern, updated Standards that make more sense,
and actually supply their own Thermostat-design that is guaranteed to be compatible
with any "updated", or "standardized" system of control.
.
.
.
Actually, those internal heater/resistors were also called "anticipators", with a purpose to heat the thermostat so that it would not bring about an over-temperature condition caused by not responding to the increased temperature of the surroundings. They were often calibrated on the basis of the gas valve current, or the current drawn by whatever the controlled device was.
Oil burners and especially coal furnaces, and also wood burners, have a fair lag time between the initial call for heat and the actual room temperature rise. Thus the thermostat was heated just a bit to "anticipate" the arrival of the heat.
 

MisterBill2

Joined Jan 23, 2018
18,176
There does exist a uniform connection and control scheme used by most, but not all, producers of heating and cooling systems. The exceptions being one rather disreputable company that utilizes an entirely different scheme so that they are the only ones who can provide alternative controls for their heating and cooling products. And interestingly enough, their controls cost twice as much as any others.
 

jeffkrol

Joined Dec 8, 2015
42
Hello all! This is probably simple to all of you, but is confusing me as a novice! I am an HVAC technician and I built a training board using multiple thermostats. They are powered by a 24VAC transformer. When the thermostats respective switch closes, it is wired so that each respective lamp lights. Very simple circuitry! My issue is when the circuits are not closed. The LED lamps are all dimly lit. I want them to be non-powered however, they are receiving power somehow. I suspect a low voltage trickling through the solid-state thermostat boards? I’ve tried All value of resistors between the switch and the LED lamp without success, is there a different type of LED I should be using? I used this from Amazon:
DMWD 12V-24V 20mA Energy Saving LED Indicator Light Mounting Hole 6mm.
Thanks in advance!!
Well in home LED lights some "bad" brands stay dimly lit due to (I was told) capacitance in the line.
Solution is to put an incandescent bulb in the circuit. Sops up the leakage?
Worked for me.. YMMV
No the incandescent doesn't light.

Doesn't take much current to light an led.
 

MisterBill2

Joined Jan 23, 2018
18,176
Are these thermostats digital display types? Or just mechanical switches like the round Honeywell devices with the mrtcury switch elements? Really, WE have no description of what kind they are.
One other suggestion is to use a digital meter and read the voltage across the thermostat when the LEDs are glowing. And one more question is why do you want the OFF?? Is your tester battery powered??
 
Top