Circuit to indicate failure of a heating element - help pls.

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I have a water tank with with 2 x 5kW heating elements (on separate
supplies).

Problem is: one is probably just about enough to supply all the hot
water I usually need, so how can I tell when one has failed?

I'd like to have an LED connected to each circuit, to warn me when
the
element has failed.

Grateful if you would tell me how. I have a bit of electrical
experience, but next-to-no electronic.

(Power supply is UK mains - 230V, 50Hz AC)

 

[TheM]

I have a water tank with with 2 x 5kW heating elements (on separate
supplies).

Problem is: one is probably just about enough to supply all the hot
water I usually need, so how can I tell when one has failed?

I'd like to have an LED connected to each circuit, to warn me when
the
element has failed.

Grateful if you would tell me how. I have a bit of electrical
experience, but next-to-no electronic.

(Power supply is UK mains - 230V, 50Hz AC)

Split mains line and make a current transformer on one of the mains leads.
Secondary can drive LED directly.

Maybe put a diode across LED for protection against too high voltage in
reverse.

M

 

[MooseFET]

I have a water tank with with 2 x 5kW heating elements (on separate
supplies).

Problem is:  one is probably just about enough to supply all the hot
water I usually need, so how can I tell when one has failed?

I'd like to have an LED connected to each circuit, to warn me when
the
element has failed.

Grateful if you would tell me how.  I have a bit of electrical
experience, but next-to-no electronic.

(Power supply is UK mains - 230V, 50Hz AC)

Does current flow in both heaters at the same time? You need to know
a little about how they are wired before you begin.

If they took their power from the same phase and through the same
contacts, the detection would be quite easy.

If they draw power at the same time, the detection is fairly easy.

If one heater is only used if the temperature drops below some value
because the other can't cope, the detection is harder again.

Detection that current is flowing into the heater can be done with a
current transformer. This signal can then be used to tell that the
heater is there when the contacts close.

If you need to detect that the heater is there with the contacts open
it is a lot trickier. I would suggest doing the detection at a
frequency way above the 50Hz mains. This would allow you to couple to
the heater through a capacitor.

I assume you know Ohms law. I assume that you know about capacitors
such as Xc=1/(2*pi*F*C). Since F is in the denominator, a small
capacitor that passes very little 50Hz, would pass quite a bit
of50KHz. This trick would need to be used if we can't wait for the
contacts to close to detect that the heater is there

 

[ojc]

Does current flow in both heaters at the same time?  You need to know
a little about how they are wired before you begin.

If they took their power from the same phase and through the same
contacts, the detection would be quite easy.

If they draw power at the same time, the detection is fairly easy.

If one heater is only used if the temperature drops below some value
because the other can't cope, the detection is harder again.

Detection that current is flowing into the heater can be done with a
current transformer.  This signal can then be used to tell that the
heater is there when the contacts close.

If you need to detect that the heater is there with the contacts open
it is a lot trickier.  I would suggest doing the detection at a
frequency way above the 50Hz mains.  This would allow you to couple to
the heater through a capacitor.

I assume you know Ohms law.  I assume that you know about capacitors
such as Xc=1/(2*pi*F*C).  Since F is in the denominator, a small
capacitor that passes very little 50Hz, would pass quite a bit
of50KHz.  This trick would need to be used if we can't wait for the
contacts to close to detect that the heater is there

Many thanks for your help. Ohms law I can handle, and I can just
about follow your capacitor equation.

The elements are in a water tank - one near the bottom (with
thermostat set to 35deg C) & one near the top (thermostat set to 55deg
C).
They're switched on & off by a timer which operates two relays.
Everything's on the same phase.
The cold water comes into the bottom of the tank & gets heated to an
intermediate temperature by the lower element.
As hot water is drawn from the top, the warm water which in the tank
rises & gets heated to a higher temperature by the upper element.

A current transformer sounds like the way to go - connected to an LED
which lights up when the current is flowing?
Could you tell me what spec CT I'd need to drive the LED directly?
Thanks.

 

[MooseFET]

Many thanks for your help.  Ohms law I can handle, and I can just
about follow your capacitor equation.

The elements are in a water tank - one near the bottom (with
thermostat set to 35deg C) & one near the top (thermostat set to 55deg
C).
They're switched on & off by a timer which operates two relays.
Everything's on the same phase.
The cold water comes into the bottom of the tank & gets heated to an
intermediate temperature by the lower element.
As hot water is drawn from the top, the warm water which in the tank
rises & gets heated to a higher temperature by the upper element.

A current transformer sounds like the way to go - connected to an LED
which lights up when the current is flowing?
Could you tell me what spec CT I'd need to drive the LED directly?
Thanks.

The current transformer has one turn on its primary. This is just the
wire feeding the heater. The many turn secondary has a load resistor
placed on it. The current transformer with its associated resistor
will give you several volts AC to work with.

Warning: Never leave the secondary side unloaded.

ASCII art:

T1 R2
------+-----/\/\------
!! ( ! !
--- !! ( \ ----- D1
) !! ( /R1 ^ V
--- !! ( \ -----
( ! !
------+---------------


T1 is the transformer
R1 is the resistor that is across its terminals
R2 needs some math
D1 is a two leaded two color LED I will explain


LEDs light up when you put about 10mA (0.01A) through them in the
forwards direction. They can be destroyed by about 5V being applied
backwards. With AC, there could be some risk of the backwards
voltage. Lucky for us they make LEDs with nose to tail pairs in them

http://media.digikey.com/pdf/Data Sheets/Lite-On PDFs/LTL-293SJW.pdf


About R2
The forward drop of the LED is just about 1.5V.

The secondary and R1 makes about V volts

R2 = (V - 1.5) / 0.01A

You will only know once you know what transformer you are using.