# Help with finding a resistor to drop 220v to 110v

Discussion in 'General Electronics Discussion' started by betacrash, Dec 3, 2014.

1. ### betacrash

3
0
Dec 3, 2014
I run a manufacturing facility that has no documentation whatsoever. We do have a guy that has been building our machines since day 1 and just does what he has learned but has no information beyond this. Here's the dilemma.....

We build a machine that uses a small AC lamp (find below)

http://www.alliedelec.com/search/productdetail.aspx?SKU=70081530

When we build a 220v version of our machines we solder a resistor to one leg of the lamp to allow it to run without burning out prematurely. The resistors are old (I know, I know) so the indicating marks on them are unreadable. I just need to know if anyone would know what size resistor we would need to use for this lamp.

Specs on the lamp:

Base Style : Cartridge
Bulb : Cylindrical
Color : Red
Color, Emitted : Red
Color, Lens : Red
Current Rating : 1.2 mA
Current, Rating : 1200 mA
Dimensions : 1.45 in. H x 0.28 in. Dia.
Lens Color : Red
Life : 25000 hrs.
Life Hours : 25000
Resistance, Series : 4.7 Kiloohms
Type : NEON
Voltage Rating : 118 VAC/VDC
Voltage, Breakdown, AC : 110 to 125 VAC
Voltage, Breakdown, DC : 110 to 125 VDC

2. ### Bluejets

5,251
1,101
Oct 5, 2014
Arouse1973 likes this.
3. ### KMoffett

723
75
Jan 21, 2009
Looking at the actual DIALITE spec sheet:
http://www.dialight.com/Assets/Brochures_And_Catalogs/Indication/507-xxxx-xxxx-640.pdf
The internal resistor is 47KΩ, not the 4.7KΩ as shown in the Allied link. That had seemed awfully low. I have no idea where the "Current, Rating : 1200 mA" came from. So the voltage drop across the internal resistor 1.2mA*47000Ω is 56V. Neons trigger at about 90V and then drop to ~70V. So 56V+70V=126V...the approximate supply voltage. So, to go from 220V to 126V a resistance of (220V-126V)/1.2mA=94V/1.2mA=~78000Ω. The old resistors were probably under rated for power. The power rating should be at least twice the calculated value of 95V*1.2mA=0.11W, so 1/4W, as Bluejets said. Also, 78K would be easily replaced with a more common 82KΩ 10% 1/4W metal film resistors:

Ken

Last edited: Dec 4, 2014
hevans1944 likes this.

1,114
159
Aug 13, 2011
Ken, you have a little unit typo (0.0012A or 1.2mA).

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
1/2W metal film resistors are about the same size as 1/4W carbon resistors.

I (being one to err on the side of caution) would go for the extra insurance of a 1/2W resistor over the 1/4W especially given the voltage. My limited experience suggests voltage ratings of between 200V and 500V for name brand 0.25W metal film resistors. This one will have a peak of around 140V across it which is probably well within the bounds of reasonable for a 1/4W metal film resistor. In any case, 1/2W resistors may be more easily available and are also suitable.

My guess is that 1200mA was a typo from 1200μA in the specs of the device.

Arouse1973 and hevans1944 like this.
6. ### betacrash

3
0
Dec 3, 2014
Thank you thank you for your help with this........

But please forgive my ignorance. Now I need to know what Ohm I would need. When I looked up 1/2W I saw a selection of 10 10K and 100K

723
75
Jan 21, 2009
8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
As much as Radio Shack is a supplier of last resort, and even considering that a single resistor should cost you single digit cents, if this is all that you're ordering them that's ok given it seems to include postage.

9. ### KMoffett

723
75
Jan 21, 2009
Some times you just spend a one-off price for a prototype part at a down the street Radio Shack, before you buy in bulk for production. Especially if you have little or no experience.

Ken

10. ### betacrash

3
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Dec 3, 2014
You guys are great! Thank you very much!