Power resistors from appliances

Discussion in 'General Electronics Chat' started by kavkav, May 22, 2013.

1. kavkav Thread Starter Member

Jan 1, 2013
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I'm looking for power resistors between 100 to 1000 ohms from my appliances and preferably a minimum of 60 watts so I can use the to make high voltage, voltage dividers.

I opened up an old flat iron hair straightener and found the heating element to be a slim ceramic-like rectangle piece. I measured a resistance of 780 ohms which would be awesome to use but I have no idea what the wattage is. I'm afraid to test it in my circuit because what if it burns and shorts something.

Any estimates on the wattage? Any suggestions or ideas on what appliances I can take apart for power resistors which have a minimum of 60 watts?

Thanks.

2. crutschow Expert

Mar 14, 2008
13,475
3,361
The wattage is V^2 / R when V is the applied voltage (in this case the main's voltage).

3. kavkav Thread Starter Member

Jan 1, 2013
64
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Yes but I need to know the resistor will not burn at the wattage I use. The flat iron, I don't know if the rectangle piece is a resistor for sure, but I don't know what else it could be. Also it has circuitry before it so I don't know what the actual voltage being fed to it was. I don't remember seeing a transformer and if that were the case, would it be safe to assume that it was fed at least 120 volts?

4. Kermit2 AAC Fanatic!

Feb 5, 2010
3,845
963
That 'flat' piece which you refer to is the HEATER ELEMENT from the hair iron

It is almost certainly composed of a fire proof compound and a length of Ni-Chrome wire. It is designed to get hot and the resistance of the NiChrome will increase significantly the hotter it gets. I'm saying the resistance value will vary upwards from the 780 Ohms you measured depending on the amount of power you send through it.

Consult the original product from which you removed the element to find the maximum wattage for which it was designed. Probably somewhere in the 800 to 1500 watt range. At these power levels the element will most likely be 'red' hot and have a resistance of 1200-1500 Ohms.

Google the alloy Constantine(not sure about that spelling). It is/was used in ammeter shunts because it gives a very small change of resistance over a wide range of temperatures. This is the feature you need in a power resistor. A small, linear rate of resistance change with temperature.

Heating elements are made with alloys that have exactly the opposite property.

5. kavkav Thread Starter Member

Jan 1, 2013
64
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I made a mistake it is 78 ohms. So if I use two of them for a voltage divider, I'm assuming the resistance of both will equally rise as they both get hot. However if I add a load in parallel to one of them it will cause the resistance to change a bit right?

Okay so if I use NiChrome wire, ideally I would like if it was a high resistance per small amount so that I won't have a long wire for a resistor. Preferably minimum 100 ohm per foot. What gauge should I used, and also is it insulated? How hot until the insulation melts?

Thanks.

6. kubeek AAC Fanatic!

Sep 20, 2005
4,687
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Why do you want a voltage divider on the mians? You will just waste lots of power.

7. kavkav Thread Starter Member

Jan 1, 2013
64
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I built a simple high frequency amplifier and I want a high voltage output. I don't know where I can get a high voltage power supply for cheap. I want my input to be about 80 volts. So the best way I know how is to make a voltage divider on the mains. It's also dangerous but I'm open to better ideas. For now, this is what I'm working with.

8. kubeek AAC Fanatic!

Sep 20, 2005
4,687
805
First of all, the divider will not be regulated and will be swaying all over the place as you load the amplifier. Second thing is that creating something like that presents a seriously lethal danger to anyone who comes close to it. Any part of the circuit could become live all of the sudden, and I hope you don´t want any member of your family or pets or anyone alectrocuted just because you skimped on a proper transformer.
Seriously, just use a transformer.
Besides, discussion of un-isolated power supplies is forbidden on this forum and your thread wouldn´t get very far.

9. kavkav Thread Starter Member

Jan 1, 2013
64
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Well I take caution with everything and keep everything insulated. If I use a transformer, it would have to be 1:2 step down. Even then it will be unregulated because I don't have a regulator for 80 volts or higher. I have this: http://www.alldatasheet.com/datasheet-pdf/pdf/125951/IXYS/IXSK50N60AU1.html

But I don't have enough knowledge to configure it to act as a regulator. I use capacitors to keep it somewhat regulated but as you said if I add a load it will just un-regulate. Plus if I keep adding capacitors I believe my voltage will go too high (or do I have an incorrect understanding of that one?).

Like I said I'm open to suggestions because finishing this is extremely important to me.

10. kubeek AAC Fanatic!

Sep 20, 2005
4,687
805
A transformer still maintains much better regulation by itself than a resistive divider does. Capacitors on the output will increase the voltage from the average value like 120Vac to the peak value, which is Vac*1.414. You still didn't specify what amount of power you need from this 80V supply, which is the second most factor in designing it.

11. kavkav Thread Starter Member

Jan 1, 2013
64
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I want the minimum available power to be 10 watts. My signal amplifier is designed for high voltage and low current so it will probably end being around 2 or 3 watts. Are you hinting at building the transformer or buying one? I don't mind buying it because a transformer to the main can be tricky.

Thanks for your help.

12. kubeek AAC Fanatic!

Sep 20, 2005
4,687
805
Usually buying a transformer is cheaper than making your own, and you are right that it is safer too.

13. kavkav Thread Starter Member

Jan 1, 2013
64
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Okay I guess I should look around, honestly I don't know why that wasn't my first option lol.

14. kavkav Thread Starter Member

Jan 1, 2013
64
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Okay now I remember why it wasn't my first option. They're too darn expensive around \$100. I have lots of transformers lying around my home. What if I use one to step down the voltage, then use another to step up the voltage to a something around 80? Can I use step down transformers to step up? Will it burn?

15. kubeek AAC Fanatic!

Sep 20, 2005
4,687
805
No problem with that, you only need to stay below the max current of each transformer. You could also connect two secondary windings in series to get the voltage you need. To get 80Vdc after rectification and filtering, you will need around 60Vac from the transformer.