Diode across coil?

[iwire]

160122-0927 EST

iwire:

My goal is to try to help you understand how the circuit works.

A relay coil has both inductance and resistance that can be represented by a schematic series circuit of a pure (ideal) inductor and resistor. These two components in series are what you see looking at the relay coil terminals. In the real world these two elements are not separable. Thus, I can not put a scope across the resistive component of the coil, but I can measure the input current to the coil and from that deduce the voltage drop across the internal resistance vs time.

I can determine the value of the coil's internal resistance by a DC resistance measurement by waiting long enough for any transient affect to die out. For a typical small relay that is probably less than one second. Using appropriate instrumentation I can measure the coil's inductance independent of the coil's DC or AC resistance. An LRC bridge is that type of instrument. The LRC bridge with AC excitation can also be used to measure the inductance.

For a DC relay there are pull-in and drop-out voltages and currents. For one sample of a P&B KUP 11D15 24 VDC relay some measured values are:

1. 480 ohms DC coil resistance using a Fluke 27.
2. 476 ohms DC coil resistance using a General Radio 1650-A bridge.
3. 0.950 Henrys Q = 1.6 armature open at 1 kHz inductance using GR 1650.
4. 1.050 Henrys Q = 1.4 armature hand held closed with same instrument.

More later I have to leave.

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Gar, I know how the circuit works enough to wire and troubleshoot it.

I have no clue at all what a Henry is and no need to know the resistance of the coil.

 

[Besoeker]

May it be a ZNR, zener transient surge surpressor diode??

We routinely used a 1N4007.
Cheap, effective, and readily available.

 

[templdl]

We routinely used a 1N4007.
Cheap, effective, and readily available.

Thanks for that. I'm going to make a note of that in a place where hopefully it won't get lost.

 

[kwired]

Gar, I know how the circuit works enough to wire and troubleshoot it.

I have no clue at all what a Henry is and no need to know the resistance of the coil.

Henry? That was my grandfather's name

It is also a unit of measure for inductance, similar to microfarads for capacitors. Maybe named after whoever figured it out?

Can't tell you much more as I don't use these units of measure on a daily basis - or even an annual basis for what I typically do, but still in back of mind from basic electrical theory classes nearly 30 years ago.

 

[gar]

160124-0900 EST

iwire:

I won't address any more comments on this subject to you.

.

 

[gar]

160124-0902 EST

For those that are interes6ted I am working on plots and informattion on some snubbing methods and how these affect the relay operation, and transients generated.

It will be a while before I complete the experiments.

.

 

[gar]

160124-1612 EST

In my experiments I am using a P&B KUP 11D15-24 as the relay under test. In series with the coil is a 22 ohm resistor for current measurement. This is added to the coil resistance resulting in a coil resistance of 500 ohms. The series combination of the 22 ohm resistor and the coil resistance become what I call the relay terminal points. The 22 ohm terminal point is also what I call common. This is where I reference the scope common and the negative terminal of the regulated DC power supply. The DC supply is connected thru a mercury wetted relay contact to the top end of the KUP relay coil.

In effect this circuit allows me to measure coil current independent of what shunt devices I place across the above defined coil terminals.

Current to the relay when energized is 48 mA (24/500 = 0.048). If I have no shunt device across the relay terminals. then voltage breakdown appears to occur around 1200 to 1500 V. Most likely this breakdown is across the mercury wetted contacts. A shunt of 22k prevents breakdown with a peak voltage across the 22k resistor of about 1050 V.

Theory is working well with this circuit, except my measured value of inductance at 1 kHz seems to be very much in error. For the present I am putting off a determination of why.

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[GoldDigger]

Is if the inductance, L, or the inductive reactance, ZsubL, that you are concerned about (or both)?

 

[gar]

160124-1856 EST

GoldDigger:

The inductance.

The inductance of around 1 Henry measured at very low flux level and hand holding the armature closed at 1 kHz in combination with the series resistance does not predict the L-R discharge curve I measure. One time constant for 1 H and 500 ohms is 1/500 = 2 mS. Experimentally determined time constant is more in the range of 5 mS.

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[GoldDigger]

Just put of curiosity, how are you measuring the time constant experimentally? On application of voltage or when shorting the terminals of the charged coil?

 

[gar]

160124-2039 EST

GoldDigger:

Shunt diode across coil terminals. Apply 24 V DC to coil. Upon removal of excitation (voltage) trigger scope, and measure time to current drop of 63%. 50 mA to 18 mA. This is before the armature moves where there is a bump in the curve because of the change in inductance at armature opening.

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[gar]

160124-2114 EST

GoldDigger:

I was trying to avoid studying the measurement of L for the moment. But your question needs some present answer.

I thought I would try applying DC bias to the coil and measure the resonant frequency. Quickly I can not do that. So without DC bias I used a 1 mfd capacitor to form a series resonant circuit. Now I get about:

Armature open --- 100 Hz, so L = 2.5 H
Hand close ------- 80 Hz, so L = 3.9 H

These are more in the ballpark, but possibly high.

I really need DC bias. That won't happen for the present.

.

 

[gar]

160124-2141 EST

GoldDigger:

Attached is a plot:



.

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[gar]

160124-2218 EST

As I was playing around trying to run my quick experiments I was having a problem switching the voltage to the relay. So I got out an old test box I had my employees make for me around 1960. This had a lever switch controlled mercury wetted contact relay. The relay was a P&B JM1 112 11. I went on-line looking for a relay datsheet, could not find one.

I did find different locations having these for sale, but I don't believe this model is made today. What was astonishing was price --- $ 300 to $ 800 each. This is a 1"+ diameter 3" long octal plugin relay. I believe I have a second one somewhere. They are the equivalent of gold.