Help with old low voltage switches

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150830-0027 EDT

junkhound:

How do you avoid burning out a relay coil without some explicit current limiting?

Using some round numbers assume that a 24 V 30 VA transformer can supply about 1 A. But for calculation purposes use 0.5 A in a 50 ohm resistor and the power dissipation is 1/4 of 50 = 12.5 W. This almost certainly would burn out a GE RR relay coil. On my system I limit steady-state current such that coil power is about 0.2 W in a single coil.

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How do you avoid burning out a relay coil without some explicit current limiting?

Good question, I should have provided more detail, my original reply was certainly misleading without all the details.


I have a single diode on the output of the 30VA xfmr, so 1/2 wave rectification also helps limit the short circuit current . Normal operation is from a solid 34 Vdc since there is a big capacitor on the line, am not trying to operate the relays on 1/2 wave rectified ac.


The old RR3 relays in the house are 86-90 ohms per side. If there is a constantly closed switch and activated relay, the dc voltage drops to just under 25 Vdc when the cap is loaded. Dissipation = 625/87 = almost 8 watts. I just threw a meter on a new RR8 and it is 100 ohms, so newer RR relays would dissipate less.

The RR3 apparently can handle 8W (mounted in steel box) as have never burnt one out.

Have never done a thermal analysis of an RR relay, so here is a quickie analysis.
RR3 here are all mounted in steel boxes, so say there is a 16 sq inch heat radiator (none of the boxes have insulation stuffed around IIRC). Rule of thumb is 125F rise per watt per sq inch is still air. That give a temp rise of (8W/16in sq) * 125F = 62F rise, a not unreasonable number to avoid burnout.

Probably the heat conducted to the steel boxes saves the relays from destruction - a bare relay or if any of the boxes were buried in insulation they would likely burn up as gar was observing, even at 8 W.
 
150831-1210 EDT

junkhound:

The new RR relays are 50 ohms per coil, and 100 ohms for the two coils in series. But only one coil is powered at a time, and they are not used in series.

Your 34 V DC says the transformer is 24 V AC. The time constant of 50 ohms and 20,000 ufd is 1 second. Thus, there should not be much drop in voltage from ripple, and most of your drop is from transformer internal impedance. The new RR relays are 50 ohms per coil and the old were about 43.5 ohms per coil.

On the thermal aspects of your installation there is going to be thermal resistance from the maximum hot spot in the coil to the relay case. and then thermal resistance from the RR case to the box, and resistance within the box. It would be interesting to do an average coil temperature rise measurement using the change of coil resistance with different power inputs in your typical installation.

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150902-2130 EDT

A correction to my previous resistance measurement of the RR7 relay. The previous measurement was with a Simpson 270 and I had not checked its calibration. That meter reads 44 ohms on a 49.9 ohm 1% resistor. My Fluke 27 reads 50.0, and a General Radio 1650A bridge reads 49.8.

With the 1650A the
blue-red leads read 55.0 ohms, and
blue-black leads read 55.3 ohms.

I have started a limited temperature rise measurement with no heatsink.
In about 30 minutes the resistance of blue-red has increased to 63.5 ohms at 138 ma. This is a power level of 1.21 W, and coil resistance may not have stablized yet. This calculates to an average temperature rise of about 40 C or 72 F. This is not maximum hot spot, which might be 10 C higher.

If we doubled the power input, then the rise might approximate double 40 or 80 C.

The aboved applied voltage is about 8.8 V DC.

The above measurement is close to a stablized point in time.

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150903-1716 EDT

Continuing the temperature rise experiment.

I made a flat plate aluminum heat sink 6" * 6" from 0.090" 3003 material. At the center is a 7/8" hole made by a Greenlee socket punch. The diameter made a very tight fit for the RR7 relay. This heat sink was horizontial for the test.

Same test as last night. The relay was allowed to stablize for about 1 hour. The same initial resistance of 55 ohms was measured. The resistance at the end of 1 hour was 62.0 instead of 63.5 ohms. Current was 140 mA. Power input = 62.0 * 0.14 * 0.14 = 1.22 W. Very close to the previous test of 1.21 W.

Estimated average coil temperature rise is 62.0/55.0 = 1.1273 - 1 = 0.1273/0.00393 = 32.4 C or 58 F. The heat sink reduced the rise from 40 C to 32.4 C.

Using a Fluke 62 Mini the IR temperature measurements were about 79 F on the RR body, on heat sink near the 7/8" hole 82 F, midway from the hole to an edge 74 F, and ambinet on surfaces nearby 74 F.

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