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Mullard HSVT Safety Relay testing.

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frankmcvey
(@frankmcvey)
Posts: 15
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I've had a problem with the subject relay that involved modifying a spring to replace the original, which had been distorted by a previous owner, rendering the relay inoperative. The new spring seems to be working well, giving a good positive latching action in either direction.

My question concerns testing the relay to ensure that it still trips at the correct over-current. According to the manual for the CT80 version this occurs when approximately 200mA flows in the relay coil. The coil measures approx 22 ohms DC resistance.

I lashed up a simple test rig using a 12V motorcycle battery wired in series with a variable resistor and a DMM; by gradually lowering the resistance the relay trips at about 220mA, which would seem to be a good result.

However, in circuit this relay monitors the current flowing in the transformer centre-tap connection of a pretty standard FW double diode valve rectifier, so as far as I can see, it will be monitoring an AC waveform in real life, rather than DC.

Will this have any significant effect? In other words, is the DC test valid?

Cheers,

Frank

 
Posted : 03/12/2015 2:03 pm
Cathovisor
(@cathovisor)
Posts: 6467
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The test is perfectly valid, Frank.

If you think about it, there is actually just a DC current flowing into the centre tap of the transformer: that current comes from alternate halves of the secondary winding during a single cycle to provide a current through the rectifier.

 
Posted : 03/12/2015 10:36 pm
frankmcvey
(@frankmcvey)
Posts: 15
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Topic starter
 

Dohhh! Of course it will, pulsating, but still DC. Thanks for the reassurance.

Cheers,

Frank

 
Posted : 04/12/2015 2:41 am
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