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# 12V resistor question...

Tue 1 Sep, 2015 12:07 pm
I'm re-wiring a lamp, and the input is 120V, then goes to a electronic transformer that drops it to 12V. The bulb is a MR-16 style LED.

A friend showed me how to wire all of it up, but there is one issue. The original switch for the lamp is a toggle switch, with three positions. Center is off, one way is high, other way is low. When it came time to wire the low position, he said I'd need a resistor in line between the 120V and the low position of the switch to drop the current. When I asked which resistor I'd need he said "I don't know, just try a bunch". Does anyone have any more specific answers to what type resistor I'd need?

Thanks-
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dalehileman

1
Tue 1 Sep, 2015 12:17 pm
@Electronicguy,
If you could indicate current consumption of the M-16 I could give you a couple pretty reliable guess and even wattage

Otherwise Guy your buddy is right on

0 Replies

Ragman

1
Tue 1 Sep, 2015 12:20 pm
@Electronicguy,
If it were me, I'd try a bunch as your friend suggested.

What was in the place before it was re-wired? Resistors are measures in Ohms..not Volts. They also have heat dissipation rating measured in watts.

Also, the value of your MR16 LED hasn't been stated. They can be anywhere between 3.5 wt, 5 or 6.5 watt. Why not try a dimmer?

If you're going to use a resistor, you need to use Ohm's Law and figure out the resistance (or why not use what had been there before?). You'll need to try a few: 2.5 wt, 5w, 6.5 wt and a resistor value in Ohms based on the wattage value of the LED and the current drawn using Ohm's Law.
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engineer

1
Tue 1 Sep, 2015 02:23 pm
@Electronicguy,
Put a volt meter on the outlet of your transformer and try the switch in each position. That should tell you how the system is going to operate. Is the LED dimmable? You might not need to do anything if the bulb can handle the voltage differences. If you do need to do something, find out how much current the bulb will use. If the voltage difference on the output of the transformer is 3VDC between the two switch positions, you can use R=V/I to find the resistance required.
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Electronicguy

1
Wed 2 Sep, 2015 11:05 am
I don't have the bulb in front of me at the moment, so I will have to post the current info. tomorrow.

"What was in the place before it was re-wired? Resistors are measures in Ohms..not Volts. They also have heat dissipation rating measured in watts."

It had an old style transformer (magnetic I think it was called). It was a lamp that was gutted, so no idea about wiring before beyond that.

Supposedly the LED bulb I bought IS dimmable, so that's why I wanted to use the hi / lo settings.

I will post bulb info ASAP. Thanks to all for the replies so far!
dalehileman

1
Wed 2 Sep, 2015 11:11 am
@Electronicguy,
And Guy how about a schematic of the MTR-16 appliance itself

Sure I went to Google but I was overwhelmed
0 Replies

Electronicguy

1
Thu 3 Sep, 2015 10:14 am
Ok, I was able to find out that the bulb is 9W.

I don't understand much about electronics, that is why I have no idea how to read or create a schematic. That is why I've come here for advice. Therefore I cannot post one. I can solder pretty well when someone shows me what to connect!

Thanks.

dalehileman

1
Thu 3 Sep, 2015 11:14 am
@Electronicguy,
Quote:
... bulb is 9W
Okay Guy but forgive an old fella who isn't up to the latest, but is 9W the power of the entire MR-16 appliance or just the bulb itself

Incidentally I looked up the MR-16 and find it's not an LED appliance but halogen. A rewiring was suggested so I'd assume it's indeed now LED

Thus it would be interesting to learn whether the appliance contains a transformer or a resistor stepping down to the 2V or 3V required by a typical LED. If it seems heavier than you'd think it should, it's probably a transformer. To just the LED that's P = 3V x 3A = 9W, quite a potent LED I must say but a thorough Googling on the subject would take me a week

Yes at 84 I'm kinda slow

That's years

With internal resistor on the other hand you might note appliance shell getting pretty hot. Now 120V at almost 3 amp P=EI gives a power waste of approaching 400 w, hot as hell, so I'd guess transformer instead to give you P = EI at 3V x 3 A = 9W

But returning to the title I note a reference to 12 v, again suggesting that internal transformer. However the link above indicates that some such replacements use a switch-mode supply instead of transformer, suggesting DC might be preferable over AC

Let us mull it over a bit longer, and in the meantime just try different resistors.

But if the entire appliance uses 9 watts for an effective current of roughly 0.1 A, then its effective resistance is (say) R = E/I = 120/ 0.1 = approx 1200 ohms so if you use a resistor of this value it should cut the current in half . Wattage? P = EI = about 120/2 x 0.1/2 = 3 W

.....maybe cutting brightness by two-thirds. We're getting into the relationship between voltage and wattage here, Guy, so let's put it off for the time being

If you feel I've been of no help, Guy, I can well understand. If anybody finds a typo or two or an outright miscalculation, or heaven forgive me, Guy, you attempt a reply to one of the earlier postings... it's because the a2k software had cut me off.....
engineer

1
Thu 3 Sep, 2015 11:29 am
@dalehileman,
MR 16 is a size of bulb. You can get them in LED.

dalehileman

1
Thu 3 Sep, 2015 12:53 pm
@engineer,
Quote:
MR 16 is a size of bulb. You can get them in LED
Thanks Eng but my link above cites a conversion, hence the confusion. Regarding wattage and stuff like that, however, without spending all day at Google I can only bet there many versions with several different means to achieve the necessary voltage drop

(Unless I'm 'way off base and the MR-16 LED junction itself doesn't require 3V x 3A but actually 117V at (about) 3/4A

....in which case, Guy, ignore my entire posting above)

If at additional 12,000 ohms the lamp might be entirely off but Guy don't get frustrated; the typical semi bulb will shut off with only slightly decreasing voltage. Just try lower value R
0 Replies

Electronicguy

1
Thu 3 Sep, 2015 01:59 pm
Ok, the MR-16 refers to a "style" of bulb. For the last 30 yrs or so they were halogen, usually 35-50W, very bright and ran very hot. Now a days you can get the same style bulb (in terms of size, socket) but it's LED instead of a super hot halogen.

The lamp was originally designed for halogen MR-16. I got the lamp and for some reason all of the guts had been removed. I do know it had a magnetic transformer originally.

Now when I wanted to re-furbish, my electrical friend said I could get one of these to use as a transformer:

http://www.hatchlighting.com/products/electronic-low-voltage-transformers/80-watt-electronic-transformer-567.html

If I understand correctly, it takes the 120V and converts to 12v. The specs list it as 80W max. So I assume I could run any MR-16 style bulb, halogen or LED up to 80W with it. Having opted for a LED, the wattage will be 9W. So I assume it's ok. I wired it up, and it seems to work fine. Everything's great until I wanted to use the "low" setting on the toggle switch.

I have NO idea what resitor to order, so I guess I'll order an assortment and wire then in place to see how it looks then?
dalehileman

1
Thu 3 Sep, 2015 04:11 pm
@Electronicguy,
Quote:
I have NO idea what resitor to order
Surely Guy at least one of your buddies is into such and can loan you a handful of resistors. Anyhow good luck

0 Replies

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