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Old 1st July 2018, 06:27 AM   #2691
Yuriy64 is offline Yuriy64  Russian Federation
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44250. R.rel on a small board- as I understand it, this resistor is of the same nominal as the resistance of the relay winding. How to measure the resistance of the winding correctly, or should it be taken from a datasheet on a relay?
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Old 1st July 2018, 10:14 AM   #2692
44250 is offline 44250  Serbia
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use ohm-meter and connect it to a relay powering contacts. i have learned that resistor in series with diode, connected paralel with relay reuces time to shut down relay,and in that way saving relay contacts. ESP33 Loudspeaker Protection and Muting

"Figure 2A shows an alternative method you can use to damp the back-EMF from the relay, but to implement it properly, access to an oscilloscope is helpful (if not essential). If the resistors have approximately the same resistance as the relay coils, the back-EMF should (!) be limited to about the normal relay voltage, give or take 50% or so. In the tests I carried out (see Tests, below) using a 24V relay, the back-EMF was limited to about -30V, which would be fine in most cases.

This method is slightly cheaper than using zeners, but is less predictable. An additional alternative is to use a catch diode to the -ve power supply. A 1N4004 between the top of the relay string and the -ve amp supply will limit the back-EMF to the voltage of the -ve supply, so for the example case this would be -40V. I expect that this would be quite acceptable, but have not tried it. Make sure that the diode is connected the right way around - the cathode goes to the top of the relays, and the anode to the negative supply."
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Old 1st July 2018, 11:37 AM   #2693
thimios is online now thimios  Greece
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Quote:
Originally Posted by 44250 View Post
use ohm-meter and connect it to a relay powering contacts. i have learned that resistor in series with diode, connected paralel with relay reuces time to shut down relay,and in that way saving relay contacts. ESP33 Loudspeaker Protection and Muting

"Figure 2A shows an alternative method you can use to damp the back-EMF from the relay, but to implement it properly, access to an oscilloscope is helpful (if not essential). If the resistors have approximately the same resistance as the relay coils, the back-EMF should (!) be limited to about the normal relay voltage, give or take 50% or so. In the tests I carried out (see Tests, below) using a 24V relay, the back-EMF was limited to about -30V, which would be fine in most cases.

This method is slightly cheaper than using zeners, but is less predictable. An additional alternative is to use a catch diode to the -ve power supply. A 1N4004 between the top of the relay string and the -ve amp supply will limit the back-EMF to the voltage of the -ve supply, so for the example case this would be -40V. I expect that this would be quite acceptable, but have not tried it. Make sure that the diode is connected the right way around - the cathode goes to the top of the relays, and the anode to the negative supply."
44250,sorry but any diode or diode-zener combination connected parallel to the relay coil doesn't protect the coil but the driver transistor.

Last edited by thimios; 1st July 2018 at 11:45 AM.
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Old 1st July 2018, 02:25 PM   #2694
44250 is offline 44250  Serbia
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it is true,but there is one more point - to reduce back-EMF and not to slow down relay opening its contacts. relay is being opened fastest when there is no zener diode or diode+resistor, and slowest when it has ordinary diode (without resistor in series with it) in parallel. this is "middle solution" or a compromise to save transistor from back-EMF and to save fast enough relay de-activation. there is a link at ESP33 page that directs to that theme.
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Last edited by 44250; 1st July 2018 at 02:29 PM.
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