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Author Topic: Dropper resistor...  (Read 335 times)

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Online emjaybee

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Dropper resistor...
« on: March 01, 2019, 10:36:54 AM »
I'm working on assembling various items in readiness for various little projects at the moment.

I'm in need for a few dropper resistors to enable a couple of Jap. micro chassis to work on my dc test loop. They'll be under two different Sentinel shunters, so slow running rather than 'stabbed rat' mode.

The general consensus seems to be 120 ohm, 1 watt is about right for this purpose, but I'm struggling to find somewhere to purchase these from. Squires only go up to 0.5 watt, and previous  users suggest  1 watt minimum to prevent overheating.

If anyone can suggest an online supplier that would be great, failing that I guess I just trawl FleaBay.

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Offline Dorsetmike

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Re: Dropper resistor...
« Reply #1 on: March 01, 2019, 10:56:43 AM »
I've used 1/4 watt resistors in mine not had any problems. As they are shunters they would normlly only run short distances between stops so little chance of overheting. I've run mine continuously for a few circuits on a 4'x2' test track, again no problem.

I would think you'd have problems finding room in a Sentinel for a larger resistor anyway. one alternative could be to have the reistor in the feed between controller and track, with a switch across it to short it out when running other locos, I have done that for use with some of the Union Mills and Dapol locos that have problems slow running.
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Offline Lawrence

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« Last Edit: March 01, 2019, 12:18:04 PM by Lawrence, Reason: New hyperlink »

Online chrism

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Re: Dropper resistor...
« Reply #3 on: March 01, 2019, 12:41:13 PM »

The general consensus seems to be 120 ohm, 1 watt is about right for this purpose, but I'm struggling to find somewhere to purchase these from. Squires only go up to 0.5 watt, and previous  users suggest  1 watt minimum to prevent overheating.

Would not 2 x 240 ohm ½ watt, or even 4 x 470 ohm ¼ watt, resistors wired in parallel achieve the same result and possibly be easier to tuck into a confined space?
Provided they aren't jammed up against each other or something else they should be able
to dissipate the heat adequately.

Online ntpntpntp

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Re: Dropper resistor...
« Reply #4 on: March 01, 2019, 01:27:50 PM »
Rather than use dropper resistors which waste power and generate heat, I use an inverse parallel string of SMD diodes. Each drops the voltage by around 0.8V - 1V ish.   IIRC 100 diodes cost me 99p from ebay (China).

Recently I needed to slow down a Tomix TM-23 for my automated shuttle line, I used a string of 4 diodes in each direction (actually as two strings of two wired either side of the motor simply for neatness within the chassis)



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Offline jthjth

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Re: Dropper resistor...
« Reply #5 on: March 01, 2019, 08:52:04 PM »
I’m afraid the diodes will also generate heat, you’ve just distributed it around more components.

Online emjaybee

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Re: Dropper resistor...
« Reply #6 on: March 01, 2019, 09:32:04 PM »
Thanks for all the replies fellas. I'd already placed an order with Squires this morning, so rang them lunchtime and added some 1/2 watt 120ohm to the order, nothing like pushing the boat out at 7p each.

I like the diode array, but I think this is probably beyond my needs.

I'll try the half-watt, and see if it heats up when I give it some laps of my loop.

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Offline Webbo

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Re: Dropper resistor...
« Reply #7 on: March 02, 2019, 02:31:51 AM »
Diodes do indeed generate heat as jthjth suggests. If the voltage drop across the diode is Δ and if the current through it is I, then the power consumption of the diode will be Δ x I. This power consumption will appear as heat generation except in the case of a light emitting diode where some will appear as light generation. The situation with the resistor is pretty much the same in that its power consumption (and heat generation) is equal to the voltage drop across it times the current through it. 

Webbo

Offline JanW

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Re: Dropper resistor...
« Reply #8 on: March 02, 2019, 08:16:38 AM »
Diodes are a MUCH better option than resistors.
Not because of a possible difference in generated heat but because the voltage drop is fixed.
The voltage drop across a resistor depends on the current through the circuit (U=I*R)
When starting the mechanism has to overcome some friction which means higher current
(more voltage drop) than when running.
So if the loco has started the motor draws less current, the voltage drop over the resistor decreases and the loco races away.
I have seen this happen on a OO9 layout with a very small loco, I'm sure the resistor didn't help there.

Jan

 

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