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Old 9th August 2004, 02:15 PM   #1
Vivek is offline Vivek  India
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Default Relay acting funny

Hello all,
I built this http://users.swing.be/edwinpaij/modu...cc_pour_hp.htm circuit. It worked the first time I switched it on. But there is a small problem. Sometimes, the relays (Goodsky make, 12V coil) do not switch properly and there is the constant crr-rrr-rrrr sound. A small tap on the relays and they work okay. I think that the relays are not in a good shape though they are brand new.
The above circuit uses a power supply of +/- 12V. I am supplying about +/- 10V. Could this cause the relays not to switch properly?
Thanks for any help.

Cheers,
Vivek
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Old 9th August 2004, 03:48 PM   #2
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Well, using a too low voltage is probably not a good idea. After all, the force excerted by the solenoid is proportional to the current through it, which in turn depends on the voltage across it, right?

It's also possible that the realy is using enough current to cause the power supply to sag, with instability as a consequence.

Rune
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Old 9th August 2004, 03:58 PM   #3
Vivek is offline Vivek  India
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I was playing around with it and noticed one more thing. If I keep the PCBs uprght this switching problem occurs but when I put the board on its side the thing works like a dream.
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Old 9th August 2004, 04:35 PM   #4
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Turning the relay is likely to affect the required force to reliably close the contacts. With insufficient voltage, that could be enough.

Measure the voltage across the relay coil to make sure the voltage is close to the supply voltage. There's always the possibility that the transistor in series with the coil isn't fully conducting.

Rune
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Old 9th August 2004, 04:44 PM   #5
Vivek is offline Vivek  India
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I will measure the voltage across the coil.
Will a 12V coil burn up quickly if about 14 or 15V is fed to it?
Vivek
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Old 9th August 2004, 05:42 PM   #6
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You should try to get a datasheet for the relay. Normal relays must have 9-10 V if they are 12 V and 16-18 V for a 24 Volts. Some high efficiency relays can handle twice their nominal voltage without any problems.

A normal relay can handle at least 150% of nominal voltage. Just check the temperature of the coil. It should be less than 60-70 deg C.

If you happen to have a rather normal type, 8 V is too little to achieve max contact pressure.

I suggest that you first check the voltage across the T5. You should have less than 500 mV. Down to 100 mV is possible. Decrease R12 to 4k7 or 5k6. If you can increase your power supply voltage, this is the best.

(My company sells relays so I know some... not everything though )
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Old 10th August 2004, 06:08 AM   #7
Vivek is offline Vivek  India
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I measured the voltage across the coil and it is about 7.3V. I suppose that is too low. I had a 9-0-9V transformer but the rectified output was in the region of +/- 15V. I was not sure if 15V would be okay for a 12V coil and that is why I used a 6-0-6V transformer. Maybe I should use the 9-0-9V transformer.
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Old 10th August 2004, 07:36 AM   #8
Jennice is offline Jennice  Denmark
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Just a guess: You measured the transformer output voltage with no load?

A transformer always have a load dependent output - comparable to an ideal voltage source followed by a resistance, R_internal. A bigger transformer will notmally have a smaller R_internal, and hence less load regulation.
Always load a transformer when measuring the output voltage.

Jennice
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Old 10th August 2004, 12:38 PM   #9
Vivek is offline Vivek  India
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Yes, I was measuring the voltage with no load. I increased the supply voltage and the circuit is working better already. The under load voltage is +/- 22V and no load voltage is +/- 30V for a 9-0-9V transformer.
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Old 10th August 2004, 12:47 PM   #10
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Oh, oh!

If you get those voltages you're either not measuring right, or you've connected the rectifier wrong.

With a 2x9v transformer, you should have 9*1.414 - 0.5 = 12.2 volts on positive and negative after the rectfier and caps.

What you're measuring just doesn't make sense.

Rune
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