Control device with current protected solid state relays
A control device, such as a smart thermostat, employs solid state relays as switches to activate and deactivate systems controlled by the device. By measuring the current flow through the power buses to one or more of the solid state relays of the control device, potentially damaging over current conditions can be distinguished from permissible transient over-current conditions and the control device can deactivate any solid state relays which would be damaged while allowing solid state relays which are experiencing allowable transients to remain operating. In the case of a severe over current condition, a current monitoring device can issue a fault signal, triggering an interrupt condition which will cause a processor in the controller to shut down the affected solid state relays very quickly.
1. A method of protecting at least one solid state relay connected between a load and a power bus within a control device from potentially damaging over current conditions, comprising the steps of:
(a) determining a set of five safety thresholds, each threshold comprising a maximum current flow level and a maximum permitted time therefore, the thresholds being defined according to a safe power dissipation capacity of the at least one solid state relay, the five safety thresholds comprising:
(1) if a maximum measured bus current for a power bus exceeds 12.5 amps, then the respective power bus switches are deactivated;
(2) if a maximum measured bus current for a power bus is less than 12.5 amps but greater than 12 amps for one full AC cycle, then the respective power bus switches are deactivated;
(3) if maximum measured power bus current for a power bus is less than 12 amps but greater than 11 amps for more than two full AC cycles, then the respective power bus switches are deactivated;
(4) if maximum measured power bus current for a power bus is less than 11 amps but more than 10 amps for more than four full AC cycles, then the respective power bus switches are deactivated; and
(5) if maximum measured power bus current for a power bus is less than 10 amps but greater than 3.5 amps for more than seven complete AC cycles, then the respective power bus switches are deactivated;
Wherein the maximum measured power bus current for a power bus between 0 amps and 3.5 amps is considered a nominal operating condition;
(b) measuring a current flow through the power bus;
(c) comparing that measured current flow to each of the five safety thresholds;
(d) determining if the measured current flow exceeds the maximum current flow level of at least one of the five safety thresholds;
(e) if the measured current flow exceeds the maximum current flow level of the at least one of the five safety thresholds, determining if the current flow has occurred for a period exceeding the maximum permitted time defined for that threshold and shutting down at least one of a plurality of power bus switches if the maximum permitted time threshold has been exceeded;
(f) repeating step (e) for each of the five safety thresholds; and
(g) repeating steps (b) through (f).
2. The method of claim 1 , wherein each of the three safety thresholds is based on a maximum power level that a FET of an associated at least one solid state relay can dissipate in a given time without being damaged.
3. The method of claim 1 , wherein steps (b) through (f) are repeated about every 1 ms.
4. The method of claim 1 , wherein the maximum permitted time defined for at least one safety threshold of the five safety thresholds is defined in terms of a predefined number of AC cycles.
5. The method of claim 4 , wherein:
the five safety thresholds includes a lower safety threshold and a higher safety threshold;
the maximum permitted time defined for the lower safety threshold and the higher safety threshold are each defined in terms of respective predefined numbers of AC cycles; and
instances in which a given maximum measured power bus current exceeds the lower safety threshold are counted toward the higher safety threshold.
6. The method of claim 1 , wherein shutting down each of the at least one solid state relays comprises receiving a fault logic signal and treating the fault logic signal as an interrupt.
7. The method of claim 1 , wherein the at least one solid state relay is part of an electrical subsystem which is part of an HVAC subsystem.
8. The method of claim 1 , wherein the at least one solid state relay comprises a pair of FETs; a charge pump to deliver a control voltage to gates of the FETS from a processor; and a fast shutdown circuit, operable to connect an output of the charge pump to ground when the fast shutdown circuit detects that the control voltage is less than a predefined value.
9. A method of protecting at least one solid state relay connected between a load and a power bus within a control device from potentially damaging over current conditions, comprising the steps of:
(a) determining a set of five safety thresholds, each threshold comprising a maximum current flow level and a maximum permitted time therefore, the thresholds being defined according to a safe power dissipation capacity of the at least one solid state relay, the five safety thresholds comprising:
(1) if a maximum measured bus current for a power bus exceeds 12.5 amps, then the respective power bus switches are deactivated;
(2) if maximum measured power bus current for a power bus is less than 12.5 amps but greater than 12 amps for one full AC cycle, then the respective power bus switches are deactivated;
(3) if maximum measured power bus current for a power bus is less than 12 amps but greater than 11 amps for more than two full AC cycles, then the respective power bus switches are deactivated;
(4) if maximum measured power bus current for a power bus is less than 11 amps but more than 10 amps for more than four full AC cycles, then the respective power bus switches are deactivated; and
(5) if maximum measured power bus current for a power bus is less than 10 amps but greater than 3.5 amps for more than seven complete AC cycles, then the respective power bus switches are deactivated;
wherein the maximum measured power bus current for a power bus between 0 amps and 3.5 amps is considered a nominal operating condition;
(b) measuring a current flow through the power bus;
(c) comparing that measured current flow to each of the five safety thresholds;
(d) determining if the measured current flow exceeds the maximum current flow level of at least one of the five safety thresholds;
(e) if the measured current flow exceeds the maximum current flow level of the at least one of the five safety thresholds, determining if the current flow has occurred for a period exceeding the maximum permitted time defined for that threshold and shutting down at least one of a plurality of power bus switches if the maximum permitted time threshold has been exceeded;
(f) repeating step (e) for each of the five safety thresholds; and
(g) repeating steps (b) through (f).
10. The method of claim 9 , wherein each of the three safety thresholds is based on a maximum power level that a FET of an associated at least one solid state relay can dissipate in a given time without being damaged.
11. The method of claim 9 , wherein steps (b) through (f) are repeated about every 1 ms.
12. The method of claim 9 , wherein the maximum permitted time defined for at least one safety threshold of the five safety thresholds is defined in terms of a predefined number of AC cycles.
13. The method of claim 12 , wherein:
the five safety thresholds includes a lower safety threshold and a higher safety threshold;
the maximum permitted time defined for the lower safety threshold and the higher safety threshold are each defined in terms of respective predefined numbers of AC cycles; and
instances in which a given maximum measured power bus current exceeds the lower safety threshold are counted toward the higher safety threshold.
14. The method of claim 9 , wherein shutting down each of the at least one solid state relays comprises receiving a fault logic signal and treating the fault logic signal as an interrupt.
15. The method of claim 9 , wherein the at least one solid state relay is part of an electrical subsystem which is part of an HVAC subsystem.
16. The method of claim 9 , wherein the at least one solid state relay comprises a pair of FETs; a charge pump to deliver a control voltage to gates of the FETS from a processor; and a fast shutdown circuit, operable to connect an output of the charge pump to ground when the fast shutdown circuit detects that the control voltage is less than a predefined value.