resistors

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RexHea

RAIL BENDER
Oops! forgot you were in Malta and you may not have a Radio Shack store nearby. However, any 6 volt supply would work. We can figure the current from the wattage, but usually the current in milliamps is listed either on the pack or can be looked up in a catalog. Do you know the manufacturer or have any other info?
 
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Maxitrains

Member
No I know nothing, but I think its something similar or is a T-1 Bulb ( miniture ), I took a look on the net and found the following, a 0.75 Watt, 5V and 0.15 Amp, but as I said, it is similar to what I have in the lamp posts.

The bulb in the lamps is mostly covered so I cannot see it very well, all I know is there are 2 really thin wires leading out of the pole, one of the wire is bare ( not coating ) the other has a thin isolation on it.
 

RexHea

RAIL BENDER
Max, there are a lot of experiments that I would do that I can't recommend to you...could cause smoke:eek: :D .
Reading back, I see that you had concern about wiring direct from your PSU because of more wires. Well, if you would look under my layout or anyone else's, you would think you were in a giant spider web.:D This would be the easiest and safest way to go in solving your problem. Otherwise, we would be guessing and could damage something. Looking in the Walthers catalog, current ranges for lamps go from 20ma all the way to 200ma. If you happened to have a higher rating than we figure, it really could cause smoke or even a fire. Back to you soon on the email.
 
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Maxitrains

Member
If I take one of these lamps and connect it to a 6.5V PSU and in between the positive wire of the lamp and the positive connection to the PSU, I connect an amp meter, could it be possible to work out something, or my mind is running too much in the future?

(Do not laugh at me but as I said I'm not much into electronics):D
 

RexHea

RAIL BENDER
Uh, yeah, but put the meter in series with the bulb and power supply, i.e. minus power supply to minus meter, positive meter to bulb to positive meter.
Keep in mind that it will be relatively low current...probably in the ranges we have discussed.
 

Maxitrains

Member
I'm sorry but I don['t seem to understand, I see too many METERS written

Negative of the PSU direct to the meter, that's OK, then the positive of the PSU goes to the positive of the lamp , then the negative of the lamp goes to the positive of the meter.

is that it? or I got more confused then before?
 

Maxitrains

Member
I didn't manage to see the ampers that the bulb was taking, but I switched my meter to Ohms, and it gave me a result of 10.6 Ohms on this bulb. does this make any sense to you, and can you calculate anything with that figure, to bring you to an adequate resistor?

Thanks
 

RexHea

RAIL BENDER
No the resistance of the bulb is tricky because of a neg temperature coefficient. Hope you didn't check Ohms with the power on:eek:. That will blow a meter fuse.
The incandescent lamp doesn't have a pos or neg. It doesn't care. Otherwise you are correct: For current, -neg of power supply to -neg of meter, +pos of meter to bulb, other bulb lead to +pos of power supply.
Unless your meter can read less than 1 amp, you won't see anything on it. Is it a digital meter? Did the bulb light up?
 
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Maxitrains

Member
Hehe, about the Ohms reading I connected the meter only to the bulb, no power supply was attached.

But when it came to read the Amps, I tried everyu combination, but still gave me no result, my digital meter (TES 2340) reads in milli amps and also (if I saw correctly ) in nano amps, but surprisingly it wasn't giving me any reading when I tried. Should the light of the bulb come on when connected, via the meter, correctly?

I will be trying again, to see if I missed a step.

Thanks again.
 

RexHea

RAIL BENDER
Yes, the bulb should light. Does the bulb light without the meter in?
Nano...I don't think so. Make sure the meter is in the current setting and at the ma range...two buttons. Even if you have reversed leads, the digital meter will still read it, but with a -minus sign.
 
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Maxitrains

Member
OK, finally I made it, I had the wrong configuration of leads set on the meter, because I have to switch leads on the meter so that I can read the Amps and mAmps. So when I set it on AMPS it worked, bulb lighted up and it marked 0.07 Amps on the meter. This reading was given on 7.75V, as before I did the Amp test I checked the voltage comming out of my variable PSU, actually I set it on the 4.5V marking and gave me out 7.75 volts.

But anyway, I got the above reading of 0.07 Amps on 7.75 Volts ( bulb marked 10.6 Ohms in earlier reading) and this might give a better clue on what resistor should be used.

Thanks guys.
 

RexHea

RAIL BENDER
Ok, the 70ma @ 7.5 volts concerns me with the resistor wattage. If we figure 80ma @ 6 volts then you would need a little over 100 (106)ohms for your resistor to drop 8.5 volts.
You should be able to get by with a 1 watt resistor at 100 ohms or the next higher size you can get. Now, this size is a guess sense you didn't put exactly 6 volts on the bulb in your test circuit.

Try this size outside of your building for test. Measure the voltage across the bulb and see if it is close to 6 volts. If the voltage is over a volt higher, the life of the bulb will be shortened accordingly. If the bulb is too bright, go to a larger resistance.

Also, check and make sure the resistor is not too warm that it would melt something. You may need to insulate it someway or keep it outside of the building under the layout.

REMEMBER: Test first!;) :)
 
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Maxitrains

Member
OK I will go purchase some different resistors 1Watt from 100 Ohms upwards and test them. What do you suggest best as insulation?
 

RexHea

RAIL BENDER
Well, first see how hot it gets. It may not get hot enough to damage plastic. Otherwise, you could support and shape it so that it doesn't touch the plastic or other materials. For insulating, you will have to decide on what would work best, possibly a metal or wood block. If metal, be sure and tape or use shrink tubing on it so that it doesn't short.
 

Maxitrains

Member
went to the nearest stores ( well everywhere is near here :) ), and I asked for the 1 WATT 100 OHMS resistor, they didn't have any in stock :S, so I got some 82 OHMS and some 150 OHMS, I will try them and see how it goes. Then I also got 2 WATT 120 OHMS. I hope I can work with these :S.
 

Maxitrains

Member
OK, now I can post some results.

I tried both, 2 Watt 120 Ohms and 1 Watt 150 Ohms resistors, for 35 mins. Both work fine, but the 1 Watt gets really warm, it can be held in hand, but if laid over a pice of plastic, on the long run it will slightly melt the plastic. The 2 Watt one lights up the bulb even better and it is slightly warm, and I think it won't even do any damage to plastic. I don't know if this is because of the different Ohms, or of the size, maybe of both together, but in any way I think the safest is the 2 WATT
 

RexHea

RAIL BENDER
That sounds great, Max. You still may want to check the voltage across the bulb and make sure its around the rating. It should be close or maybe even under, which is good. If it is over the rating, go to the next higher ohmic value you can find at 2 watts. Otherwise, your bulb won't last as long.
 




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