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Author Topic: resistor shielding  (Read 2665 times)

gerard

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resistor shielding
« on: August 22, 2012, 07:18:19 PM »

Can anyone give me suggestions on the best way to shield a model from the heat generated from a 470 ohm resistor. this is going to have to be very close to other wiring and paintwork.
Your help appreciated,
Gerard
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Tug-Kenny

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Re: resistor shielding
« Reply #1 on: August 22, 2012, 08:03:49 PM »


My first reaction is to ask  'Why does the resistor get that hot'.

If it's imperative that you use that value, then you could increase it's size  (wattage) then it would run cooler.

cheers

ken
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Subculture

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Re: resistor shielding
« Reply #2 on: August 22, 2012, 08:57:01 PM »

It'll still disperse the same amount of heat. A little shield of polished aluminium will work well at reflecting the heat back, and keeping it off of more sensitive areas.

john s 2

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Re: resistor shielding
« Reply #3 on: August 22, 2012, 09:11:58 PM »

Instead of shielding, why not heat sink? The heat has to go somewear so disburse it. John.
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NFMike

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Re: resistor shielding
« Reply #4 on: August 22, 2012, 09:27:47 PM »

Instead of shielding, why not heat sink? The heat has to go somewear so disburse it. John.
If really pushed you could water cool it.

Colin Bishop

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Re: resistor shielding
« Reply #5 on: August 22, 2012, 09:38:21 PM »

I would go along with Kenny. Why is so much heat being generated with a consequential affect on battery capacity?

Excess heat is generally an indication of inefficiency.

Colin
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malcolmfrary

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Re: resistor shielding
« Reply #6 on: August 23, 2012, 10:47:55 AM »

Running on 12v, and with nothing else in circuit, it will dissipate 1/3 watt.  A 1/4 watt resistor will get very hot.  A small vitreous enamel 2.5 watt resistor will get equally hot, but will live through it, although the surroundings will suffer.  One of the larger 1 watt (tubular) or white ceramic resistors of higher rating will have a much larger surface area, and will run cooler as long as the heat has somewhere to go.
Running on 6 volts, it will dissipate less than 1/10 watt, and be well within its capability of just losing any heat generated along its own leads.
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Tug-Kenny

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Re: resistor shielding
« Reply #7 on: August 23, 2012, 11:35:50 AM »


Where is this resistor please ?


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gerard

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Re: resistor shielding
« Reply #8 on: August 25, 2012, 07:56:05 PM »

All the lights run off 12v, this last light on the stern rail is lit as a serarate circuit therefore I have put this resistor in to reduce to 3.3v
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Tug-Kenny

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Re: resistor shielding
« Reply #9 on: August 25, 2012, 07:58:06 PM »


Ah !    8)

Now we can resolve the problem.  Over to you guys.

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john s 2

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Re: resistor shielding
« Reply #10 on: August 25, 2012, 09:39:52 PM »

Change light to an Led if possible.Will take far less power. John.
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philk

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Re: resistor shielding
« Reply #11 on: August 25, 2012, 10:51:18 PM »

use a voltage regulator much more efficient and regulated voltage

phil
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NFMike

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Re: resistor shielding
« Reply #12 on: August 26, 2012, 12:50:39 AM »

12V - 3.3V = 8.7V. W = Vsqd/R = 8.7 x 8.7 / 470 = 0.16W. So its about 1/6th of a Watt. I'd try a 1W resistor and see how warm it gets. Generally the higher wattage resistors have a larger surface area so run cooler provided they have a bit of space for air to circulate. These sorts of values are used in model trains where there is limited space and melted body shells are fairly rare - though not unheard of :)

malcolmfrary

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Re: resistor shielding
« Reply #13 on: August 26, 2012, 08:22:20 AM »

What Mike said, only I would look for a 6 volt bulb if possible.  Much less work for the resistor to do, therefore less heat generated there.  If going down the LED route, while the LED will have a longer life expectancy than a filament bulb, and will itself run cooler, the current will be very similar, and its current limiting resistor will have just the same amount of work to do.  Same applies to a voltage regulator - they work by dissipating unwanted energy as heat, whether being used as just a voltage regulator or as part of a constant current circuit.
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ACTion

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Re: resistor shielding
« Reply #14 on: August 26, 2012, 09:12:43 AM »

If it were mine I'd use a separate 3-cell battery pack, especially if it's switched separately from the other lights. I presume the lamp is one of those Graupner or Robbe units, enclosed in a nice metal casting with spare bulbs the price of a pint!
Dave M
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john s 2

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Re: resistor shielding
« Reply #15 on: August 26, 2012, 08:41:52 PM »

Can a member confirm that the posting about led draw current being similar to a bulb is correct? As a led has a far lower amp demand then surely the resistor is smaller? Leading to far less current draw.John.
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malcolmfrary

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Re: resistor shielding
« Reply #16 on: August 26, 2012, 09:43:47 PM »

Can a member confirm that the posting about led draw current being similar to a bulb is correct? As a led has a far lower amp demand then surely the resistor is smaller? Leading to far less current draw.John.
The figure for the resistor quoted by the OP would give a max current less than 20mA on 12v.  If this was actually tried and found to give an acceptable result by the OP (who is the important person in this thread), then the particular filament bulb would be pulling about the same as I and most others would expect an LED to require.  Normally, I too would expect an LED to require far less current for the same light output (or the bulb to require a lot more), but its always possible that light bulbs are not as they used to be.  Especially, as DM mentions, about the price.  A filament bulb of minute size with a very high efficiency might cost many times the price of a sub-min LED, but not be as readily available.  And the LED's resistor would be of a higher value, partly for lower current, partly to drop the extra volt or so.
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NFMike

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Re: resistor shielding
« Reply #17 on: August 27, 2012, 12:48:23 AM »

LEDs and miniature (grain of wheat) bulbs both typically draw 20...30mA current.
Obviously you need to check your specific item's spec, but the comment in the post above is not unreasonable.

Two advantages of an LED are long life and cooler running compared to a 12V bulb. For a small enclosure the latter point can be significant as you can put most of the heat (generated by the resistor) somewhere else. If you have a 1.5V or 3V bulb the difference is of course much less or nil.

The disadvantage of LEDs for 'white' lights used to be the colour, but these days you can get them in practically any shade of white you want - a bit like car paint O0
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