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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Feb 2020, 1:32 am   #1
Michael Maurice
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Default When ohms law doesn't seem to work.

I have been rebuilding tow old radiograms and both needed dropper resistors.

The first, a Murphy A242R needed the rectifier replacing, so a silicon one was fitted under the chassis with a 150R resistor in series between the rectifier and the smoothing capacitor.

Now the voltage drop across the resistor is around 17 volts, so that according to ohms law, V²/R = W the resistor should be dissipating approx 2W. But a 5W one gets very very hot, so I fitted a 17W resistor which got warm but not hot.

The second was a Philips F5G96A. This has an AC/DC chassis with droppers for the heaters.

One was open circuit it was 1.2K ohms and the voltage to be dropped is approx 122V. So according to ohms law, a resistor of 1.2K should dissipate 12.4W. But a 50W resistor bolted to the chassis gets very very hot.

I'm going to fit a 100W resistor, but have I missed something here? These resistors are rated above what ohms law says should be a safe dissipation, but clearly is not.

Can anyone explain where I'm going wrong, if indeed I am?
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Old 16th Feb 2020, 3:49 am   #2
G8UWM-MildMartin
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Default Re: When ohms law doesn't seem to work.

Yes!
But only in the first case, where the current is neither plain DC nor sinusoidal AC.
It's more sort of unidirectional current plus spiky pulses charging the reservoir capacitor, and horrendous and well beyond my mathematical capability to calculate.
The second case of a simple heater dropper should behave in accordance with your calculation. They originals normally ran hot enough to burn dust off, contributing to the usual "valve radio" smell!
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Old 16th Feb 2020, 5:59 am   #3
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Default Re: When ohms law doesn't seem to work.

Also, modern resistors, of a given power rating, have become a lot smaller than they used to be. 'Ye cannae change the laws of physics, captain' so this has to be at the expense of the smaller part having to run a lot hotter to dissipate the same amount of heat from its smaller surface area. Scale a component down by a factor of two in length and diameter and the surface area goes down by a factor of four.

The modern parts are run much harder than the old. The only justification of the modern ratings is that better materials survive these temperatures and still deliver good reliability figures.

As a consequence, modern resistors run with much higher surface temperatures than we're ued to, so we need to be careful about wires coming into contact with them, and our estimates of dissipation will be a long way off.

David
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Old 16th Feb 2020, 9:20 am   #4
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Default Re: When ohms law doesn't seem to work.

Yes, a simple calculation based on ohms law will be very misleading when dealing with peaky waveforms rather than with flat DC or sine wave AC.

As has been said, when pulsating DC is charging a capacitor, the dissipation can be higher than expected.

Similar effects can be observed in battery charging circuits. The heating of the transformer winding and of the connecting cables will be proportional to the RMS current, but the degree of charge put into the battery will be determined by the average current.
As a simplified example, consider a charging current that is 10 amps for half the time, and zero for the other half.
The RMS current will be just over 7 amps and the transformer winding and cables must be designed for at least this current.
The average current, will be 5 amps. This figure is relevant when estimating how long it will take to charge the battery.

Similar perplexing results can occur in discharge lamp circuits.
The light produced by a fluorescent tube is roughly proportional to the AVERAGE current through the lamp.
The heating effect in the supply wiring is determined by the RMS current.
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Old 16th Feb 2020, 9:29 am   #5
Craig Sawyers
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Default Re: When ohms law doesn't seem to work.

I use this http://www.duncanamps.com/psud2/ when checking the design of power supplies.

The other key reference is a paper published in 1943 by Schade, but is still copyright IEEE.

But one published by General Radio is here https://www.ietlabs.com/pdf/GR_Appno...r%20Design.pdf

But it seems that a good design rule is that the rms current through the resistor in your configuration is about twice the DC current that the supply supplies. So the power dissipation in the resistor is four times that of a more straightforward calculation.

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Old 16th Feb 2020, 11:52 am   #6
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Default Re: When ohms law doesn't seem to work.

I believe that some (most?) modern DVMs which claim a “true RMS response” actually only give the RMS value of the AC part of a voltage or current which includes both AC and DC components, such as is the case in your first example.

Back in the day, folks used a trick with small light bulbs to find True RMS values of currents. Get a small low voltage bulb which glows at about half brightness when connected in series with your 150R resistor. Measure the bulb brightness with a photo sensitive device such as the photo cell from a cheap garden solar light. Then find the DC current in the bulb which gives the same brightness as when it was in the radio, that DC current will be the same as the True RMS current in the bulb when it was in the radio. There will be an error because the bulb will reduce the current in the radio, but for the circuit you’re describing, the error should be small, the voltage across a bulb at half brightness will be less than it’s rated voltage.

For your second example, a 1k2 resistor will only dissipate 48W with full 240V mains across it, so a 50W resistor should be fine as heater dropper.

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Old 16th Feb 2020, 12:17 pm   #7
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Default Re: When ohms law doesn't seem to work.

Quote:
Originally Posted by Michael Maurice View Post
Can anyone explain where I'm going wrong, if indeed I am?
Second page, second paragraph:

http://www.r-type.org/pdfs/uy41.pdf

Lawrence.
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Old 16th Feb 2020, 12:48 pm   #8
mark2collection
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Default Re: When ohms law doesn't seem to work.

This all explains my Philips Philetta (220V set) on UK 240V.

I used 100 ohms from the power switch output to the sets original dropper. The Ohms law calculated wattage resulted in a very hot resistor.
An aluminium (gold in colour) resistor bolted to the chassis at ample wattage soon cured that.

The resistor sees 240V mains, the sets dropper sees 220V, a 20 volt drop, happy days

Great question and posts, this helps explain a lot.

The modification made to the set is completely reversible, should it be desirable in the future.

Mark
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Last edited by mark2collection; 16th Feb 2020 at 12:55 pm.
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