# Pilot light to detect break in resistive load

#### npomeroy

Dec 9, 2012
19
I'm working with 12 - 24v DC supply delivering 2 - 4 A through a small heating element. I want to be able to detect if the element goes open circuit. A parallel LED + resistor of course shows voltage is applied but not whether current is flowing in the other branch. I understand the voltage is too low to use a neon bulb that would light on open circuit. I'm thinking a parallel capacitor + resistor could be the starting point as the cap would not charge to supply voltage while parallel current was flowing. I need a very simple set up as I will be constructing over 70 such heating units.

Nelson

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,648
I'm thinking a parallel capacitor + resistor could be the starting point as the cap would not charge to supply voltage while parallel current was flowing.
Sorry, but you're wrong here. The parallel RC combination will "see" the full DC voltage across the heating element and the capacitor will charge.

You need to sense the current throught the heating element. As you probably don't want to drop many volts across the current sensor (you will need the power for the heater, not to heat the sensor ), you will need to resort to some kind of amplifier. Here is a collection of suitable circuits with different complexity. A single OpAmp, a handful of resistors and an LED are simple, as required.

#### npomeroy

Dec 9, 2012
19
Thanks Harald
I think the circuits are all beyond what I am prepared to construct for this application. My alternative is to use a piece of liquid crystal temperature sensitive sheet stuck to an exposed section of the element (element is silicone insulated resistance cable, about 12W/m). If the cable is warm there will be a colour difference on the sheet.

#### Colin Mitchell

Aug 31, 2014
1,416
Can you put a diode near the element to detect the temperature?

Can you afford to lose 0.7v across a resistor?

#### npomeroy

Dec 9, 2012
19
"Can you put a diode near the element to detect the temperature?" ... Can you elaborate please?

"Can you afford to lose 0.7v across a resistor?" Not really - I'd be adding 6% waste heat to the system wouldn't I?
I think the temperature sensitive tape would tell me all I need to know.

#### Colin Mitchell

Aug 31, 2014
1,416
Use a diode to detect when the temperature is low A diode will allow you to detect the temperature at a remote location and drive a LED or relay or buzzer

#### witsender

Dec 12, 2013
22
put a series transformer in the supply to the load; the other side of the transformer triggers a triac when the current stops flowing; the triac operates an alarm.

#### npomeroy

Dec 9, 2012
19
put a series transformer in the supply to the load; the other side of the transformer triggers a triac when the current stops flowing; the triac operates an alarm.

This would only work with AC or pulsing DC - actually with AC maybe a small coil around a supply wire might pick up enough inductance to light an LED. BUT my total system will be around 2000 watts, spread over 70 heaters, and the most economical power supply seems to be ATX computer power supplies that supply DC. Even single voltage switch mode power supplies (especially if imported via AliExpress) are cheaper than plain transformers for the wattage.

Guys, in general thanks for the input. But because 70+ replications of the sensing system are needed I was thinking of nothing more complicated than a couple of cheap components in parallel or series with the heating element. This does not seem possible so pieces of temperature sensing tape may be fine. I do not need remote warning: Naked sections of the elements will be plainly visible stacked on shelves.

#### witsender

Dec 12, 2013
22
This would only work with AC or pulsing DC - actually with AC maybe a small coil around a supply wire might pick up enough inductance to light an LED. BUT my total system will be around 2000 watts, spread over 70 heaters, and the most economical power supply seems to be ATX computer power supplies that supply DC. Even single voltage switch mode power supplies (especially if imported via AliExpress) are cheaper than plain transformers for the wattage.

Guys, in general thanks for the input. But because 70+ replications of the sensing system are needed I was thinking of nothing more complicated than a couple of cheap components in parallel or series with the heating element. This does not seem possible so pieces of temperature sensing tape may be fine. I do not need remote warning: Naked sections of the elements will be plainly visible stacked on shelves.

Ummm... I think the transformer+triac also works with DC... I better make a drawing if I can't explain in words... one more go:
The DC supply to the resistive load is interrupted due to a fault in the load. This causes a pulse of current in the secondary of a little pulse transformer in either the supply or the ground of the load.
Remember, a pulse is generated with every change in the load... not just when current starts but also when it stops.
The pulse is used to trigger a triac powering an alarm, in parallel with the load.
Clear as mud, yet?
I'll post a drawing when I've made one, shortly.

#### npomeroy

Dec 9, 2012
19
Ummm... I think the transformer+triac also works with DC... I better make a drawing if I can't explain in words... one more go:
The DC supply to the resistive load is interrupted due to a fault in the load. This causes a pulse of current in the secondary of a little pulse transformer in either the supply or the ground of the load.
Remember, a pulse is generated with every change in the load... not just when current starts but also when it stops.
The pulse is used to trigger a triac powering an alarm, in parallel with the load.
Clear as mud, yet?
I'll post a drawing when I've made one, shortly.
Yes I begin to see the idea - but the heaters are on a PWM control so are switching on and off every second.

#### witsender

Dec 12, 2013
22
Yes I begin to see the idea - but the heaters are on a PWM control so are switching on and off every second.
Ohhh.
I suppose you could use the same pulse transformer to supply little flicks of current, charge a cap, and detect the voltage on that... but that fails the KISS principle

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