METHOD AND APPARATUS FOR TESTING RESISTIVITY OF REED RELAY CONTACTS

• US 3,822,398 A
• Filed: 11/05/1973
• Issued: 07/02/1974
• Est. Priority Date: 11/05/1973
• Status: Expired due to Term

• Current Assignee: AG Communication Systems Corp. (Nokia Corporation)
• Original Assignee: Richard Michael Rovnyak
• Inventors: Rovnyak, Richard Michael
• Primary Examiner(s): Smith, Alfred E.
• Assistant Examiner(s): Hille, Rolf
• Application Number:
  US05/412,860
• Time in Patent Office: 239 Days
• Field of Search: 324/28RS,28R,28CR
• US Class Current: 324/419
• CPC Class Codes: G01R 31/3278 of relays, solenoids or reed switches measuring contact resistance G01R27/205; high voltage magnetic switches G01R31/3271, G01R31/333; testing electric windings G01R31/72; monitoring of fail safe circuits H01H47/002

Abstract

Signal generator circuits are connected to a reed switch under test to apply a linearly changing current superimposed on a bias current level simultaneously to an operating coil and to its reed blades. Voltage developed across the contacts is amplified and differentiated while the changing current twists the blades as a result of magnetostriction. When the contacts are made through rough or contaminated areas as they are being moved by the twisting of the blades, the differentiated voltage exceeds a reference voltage and causes an indication of rejection.

Claims

1. The method of testing reed switch contacts, comprising the steps of;

providing an ample flow of operating current through the operating winding of a reed switch under test for at least a first period sufficiently long to close its contacts;

applying a flow of current through said contacts during a second period following said first period, the flow of current through said contacts including a DC bias level component and a superimposed gradually changing component, the point through which current is conducted through the mating surfaces of said contacts being moved over said surfaces as a result of a slight twisting of the reeds caused by said changing flow of currents; and

detecting across said contacts voltage that is abnormally high at any instant during said second period during which the flow of current is being changed through said contacts.

2. The method of testing reed switch contacts according to claim 1 wherein the detection of said abnormally high votage across said high contacts is determined by differentiating said voltage developed across said contacts, said differentiated voltage being detected when above a maximum acceptable constant level.

3. The method of testing reed switch contacts according to claim 2 wherein said operating current changes from an initial ample flow to a minimum amount of flow still sufficient to maintain said contacts closed while said current flowing through said contacts during said second period is being decreased.

4. The method of testing reed switch contacts according to claim 3 wherein said test is repeated while applying said operating current in a reversed direction.

5. The method of testing reed switch contacts according to claim 1 including the further step of providing a flow of current through the operating winding of a magnetic latching reed switch under test to unlatch said switch during a third period of time following said second period, the polarity of said unlatched flow of current being opposite to said flow of current in said first period.

6. The method of testing reed switch contacts according to claim 5 wherein said unlatch flow of current is applied in the opposite polarity when said test is repeated.

7. A test circuit for testing reed switches, comprising:

a timing signal generator for marking successive timing periods;

a current generator having delay means connected to said timing signal generator and current generator means connected to said delay means responsive to signals from said timing signal generator for developing a maximum level of current starting a predetermined time after the start of each of said timing periods;

additional delay means connected to said timing signal generator and a ramp current forming means connected to said additional delay means and to said current generating means, said ramp current forming means being effective a predetermined interval after the start of said maximum level of current to decrease linearly the output of said current generating means to a minimum level of current before the end of each of said timing periods;

means connecting a test coil for reed switches under test to the output of said current generator;

contact current generator means having a pair of test terminals adapted to receive the reed terminals of a reed switch under test for providing current through the contacts of the reed switch, said contact current including a DC bias level and a superimposed gradually changing component; and

voltage measuring means connected across said pair of test terminals for determining any peaks of voltage above a predetermined maximum value across said pair of test terminals while the output of said current generator is being decreased linearly.

8. A test circuiT as recited in claim 7 wherein said voltage measuring means includes an amplifier, a differentiator and detecting means connected to said test terminal, said detecting means being biased to provide an output only when the voltage across said pair of test terminals exceeds said predetermined maximum value.

9. A test circuit as recited in claim 7 wherein said means for connecting said test coil includes a current reversing switch.

10. The method of testing a set of contacts, comprising the steps of;

applying a DC bias flow of current through said contacts;

superimposing a gradually changing flow of current on said DC bias flow of current through said contacts after a predetermined time delay measured from the initial flow of DC bias current, said changing flow of current being of smaller magnitude relative to said DC bias component of current, the point through which current is conducted through the mating surfaces of said contacts being moved over said surfaces as a result of a slight twisting of the contacts caused by said changing flow of current; and

detecting across said contacts voltage that is abnormally high at any instant during which the flow of current is being changed, said detection being determined by differentiating said voltage developed across said contacts, said differentiated voltage being detected when above a maximum acceptable constant level.