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US3578987A Expired / Lapsed US ⏱️ 0.0 years remaining

Resistance-sensing system with fail-safe circuit

Assignee: Dover Corp
•
Filing: 1968-12-09
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Granted: 1971-05-18
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Expires: 1988-12-09
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Abstract

Abstract A control circuit suitable for use in a forced-draft gas-fired heating system permits a solenoid gas valve to be turned on only if a blower is operating. The solenoid of the gas valve is controlled by a silicon-controlled rectifier (SCR) which receives its gate signal from a thermistor circuit. The blower, when operating, blows air over the thermistor to limit the extent to which it heats up in response to current passed through it, thereby maintaining the thermistor resistance high enough to operate the SCR and the gas valve so long as the blower operates properly. If the blower does not operate properly, the thermistor warms up further, its resistance decreases markedly, and the signal which it develops and applies to the gate of the SCR becomes too weak to turn on the SCR and the gas valve. To prevent operation of the gas valve should the thermistor become opencircuited, there is employed a second silicon-controlled rectifier circuit comprising a second SCR having anode and cathode electrodes in shunt with the thermistor and having a gate electrode which derives its voltage from across the thermistor by way of a capacitor having a predetermined minimum value sufficient to cause the second SCR to turn on before the first SCR should the thermistor become open-circuited. When the second SCR is thus turned on, the first SCR cannot be turned on, and the solenoid gas valve is thereby prevented from being turned on if the thermistor becomes open-circuited.

Inventors

  • 👤 Douglas W De Werth
  • 👤 Earl J Weber

Bibliographic Details

Application Number —
Jurisdiction US
Data Source google_patents
Last Synchronized 2026-11-04 07:07
CPC Classifications
F; F23; F23N; F23N5/00; F23N5/24; F23N5/242; F23N1/00; F23N1/02; F23N1/022; H; H03; H03K; H03K17/00; H03K17/51; H03K17/56; H03K17/72; H03K17/725; F23N5/18; F23N2005/181; F23N2233/00; F23N2233/06

Claims Analysis

Total Claims: 6
Claim 1
Independent Claim 38 words
The apparatus of claim 1, in which said alternating voltage produced across said variable resistance means and said alternating voltage applied between said anodes and cathodes of said rectifier means are of substantially the same form and frequency.
Claim 2
Dependent Claim 63 words
Apparatus in accordance with claim 1, in which said first gate circuit comprises a first resistor in series with said first gate electrode, said second gate circuit comprises a capacitor and a second resistor in series with said second gate electrode, and the impedance of said second gate circuit of said varying alternating voltage is greater than that of said first gate circuit.
Claim 3
Dependent Claim 20 words
The apparatus of claim 1, wherein said second gate circuit comprises capacitive means for providing said advanced phase of current.
Claim 4
Dependent Claim 49 words
The apparatus of claim 1, wherein said means for producing an alternating voltage across said variable resistance means comprises a source of alternating voltage, impedance means in series with said resistance means, and means connecting said source across the series combination of said impedance means and said resistance means.
Claim 5
Dependent Claim 311 words
Fluid flow-sensing apparatus comprising: thermistor means having a negative temperature coefficient of resistance; means for passing a heating current through said thermistor means to heat it and reduce its resistance, whereby a flow of a cooling fluid adjacent said thermistor means will reduce the temperature and increase the resistance of said thermistor means and increase the voltage across said thermistor means; first semiconductor-controlled rectifier means having an actuated high-conduction state, a normal low-conduction state, and first anode, cathode and gate electrodes; first gate circuit means connecting said thermistor means to said first gate electrode to actuate said first rectifIer means to said high-conduction state thereof in response to said voltage when said resistance of said thermistor means is greater than a first predetermined value but less than a second predetermined value and to permit it to assume said low-conduction state thereof when said resistance falls below said first predetermined value; second semiconductor-controlled rectifier means having an actuated high-conduction state and a normal low-conduction state, and second anode, cathode and gate electrodes; means for applying an alternating voltage between said first anode and cathode electrodes, and between said second anode and cathode electrode; means connecting said first gate and cathode electrodes effectively in parallel with said second anode and cathode electrode; second gate circuit means connecting said thermistor means to said second gate electrode and having a phase shift characteristic different than that of said first gate circuit means so as to actuate said second semiconductor-controlled rectifier means to its high-conduction state in response to said voltage across said thermistor means and to actuate said first semiconductor-controlled rectifier means to its low-conduction state when said resistance increases beyond said second predetermined value greater than said first predetermined value, whereby operation of said first semiconductor-controlled rectifier means in said high-conduction state thereof in response to opening circuiting of said thermistor means is prevented.
Claim 6
Dependent Claim 66 words
Apparatus in accordance with claim 6, in which said first gate circuit means comprises a first resistor in series with said first gate electrode, said second gate circuit means comprises a capacitor and a second resistor in series with said second gate electrode, and the impedance of said second gate circuit means to said varying alternating voltage is greater than that of said first gate circuit.

References Cited (Backward Citations)

US3390275A Cited

USPTO Annuity & Maintenance Windows

Annuity Stage Window Opens Payment Deadline Surcharge Period Until Estimated Fee Status
3.5 Years (Year 4) 1974-05-17 1974-11-16 1975-05-18 $2000.00 Window Closed / Lapsed
7.5 Years (Year 8) 1978-05-17 1978-11-16 1979-05-18 $3760.00 Window Closed / Lapsed
11.5 Years (Year 12) 1982-05-17 1982-11-16 1983-05-18 $7700.00 Window Closed / Lapsed

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