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Anonymous Poster

Voltage Step Down Using Resistors

11/04/2010 12:41 PM

I want to step down 110Vac to 78Vac. A transformer converts the mains supply to 110Vac. Now without using another transformer and any other complex circuitry I would like to step it down to 78Vac. ( max current is 1A)

Please suggest the design of such a circuit.

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#1

Re: Voltage step down using resistors

11/04/2010 12:52 PM

Unless the load is absolutely constant not really.

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#2

Re: Voltage Step Down Using Resistors

11/04/2010 2:13 PM

The simplest thing would probably be a 100va autotransformer with 70% tap, but I don't know if that is a commercial item. Alternatively, McMaster-Carr has a suitable variac for about $100, but that might be considered "other complex circuitry."

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#3
In reply to #2

Re: Voltage Step Down Using Resistors

11/04/2010 3:16 PM

I suspect "...without using another transformer and any other complex circuitry" is taken verbatim (or nearly so) from the homework sheet.

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#4
In reply to #2

Re: Voltage Step Down Using Resistors

11/04/2010 3:18 PM

Thanks for the suggestion.

Due to space constraint, using an auto transformer is difficult. Is there anything simple that can be used?

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Anonymous Poster
#5
In reply to #4

Re: Voltage Step Down Using Resistors

11/04/2010 3:23 PM

How about a resistor?

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Anonymous Poster
#6

Re: Voltage Step Down Using Resistors

11/04/2010 4:38 PM

A transformer converts the mains supply to 110Vac.

Rewind this transformer's secondary, or replace it with one with a secondary of 78V.

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#7

Re: Voltage Step Down Using Resistors

11/04/2010 8:25 PM

simplist thing to use would be a pot resistor. i knows this because i used a 110 source and stepped it down w/ said resistor and used it as a theft deturant. works awsome at 70ish volts

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#11
In reply to #7

Re: Voltage Step Down Using Resistors

11/04/2010 11:44 PM

I dread to think how your using this as a theft deterrent :D

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#8

Re: Voltage Step Down Using Resistors

11/04/2010 10:35 PM

Get hold of a simple ceiling fan or light switch dimmer - they can handle the 1 amp current. They simply cchop the wave & reduce the voltage to suit. I used one to set up a variable speed drill - simple compact & effective. Cost is in your budget too I suspect.

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#9

Re: Voltage Step Down Using Resistors

11/04/2010 10:51 PM

NO, you do not want to use a resistor, . . . YES, you definitely want a transformer, . . . To make it efficient, you want to use a small 110/24 volt transformer and hook it up so that 24 volts subtracts from 110 volts, . . providing you with about 85 volts. Oleh

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#10

Re: Voltage Step Down Using Resistors

11/04/2010 11:08 PM

Try this website to see how a dimmer switch works - It is what you need - don't over think the problem & don't get too complex with the solution.

http://home.howstuffworks.com/dimmer-switch1.htm

Simple & effective

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#12

Re: Voltage Step Down Using Resistors

11/05/2010 3:47 AM

If you want to use resistors, set up a resistor chain at 5 Amps, 22 Ohms total, 550 Watts.

Tap at 78 Volts, ie at 15.6 Ohms.

The regulation will not be tops, but it is a cheap solution (not including the cost of electricity) if you have lots of low value resistors in your junk box.

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#13

Re: Voltage Step Down Using Resistors

11/05/2010 4:46 AM

The easiest way is get a high power rheostat,such as surplus from a dancer roller from a web process, of a couple of hundred ohms, and use the wiper to pick off the desired voltage, according to load.If the load varies, you will have to adjust accordingly.

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#14

Re: Voltage Step Down Using Resistors

11/05/2010 6:31 AM

Boys boys boys. If you want to do it with a small load that is relativley constant the part that you use is a capacitor in series. THere is almost no dissipation and the power used determines the capacitor. Beware of stored charge when servicing the cap unless you have a bleeder resistor in shunt. For small fixed loads, this is not to be beat such as dimming a light, used it many times for small applications. Woody

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#15

Re: Voltage Step Down Using Resistors

11/05/2010 8:51 AM

What is your application?

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#16

Re: Voltage Step Down Using Resistors

11/05/2010 9:36 AM

There have been some responses I really like, but op asked to solve this problem using resistors. At the risk of oversimplifying, I suggest a 78 ohm resistor in series with a 32 ohm resistor. The two resistors go across the 110 volts and one takes the 78 volts across the 78 ohm resistor. Not efficient, but follows op's guidelines.

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#17

Re: Voltage Step Down Using Resistors

11/05/2010 12:12 PM

The best solutions might be to use an auto transformer, negating transformer or light dimmer as suggested earlier. But you requested a resistor circuit. So here is your resistor circuit.

The simplest circuit would be to use a single series resistor to drop the unwanted voltage.

SOURCE-------RESISTOR--------LOAD--------Neutral

110VAC ------ 32 OHM ------- 1A@78VAC ------ 0v

R=E/I = (110V-78V)/1A =32Ω

Power dissipated in the resistor is P=I x I x R = 1A x 1A x 32Ω = 32W

This approach has several problems.

  1. There is essentially no load regulation. Voltage supplied to the load is dependent on the load current.[If load drops to .5A, the voltage supplied to the load jumps to 94vac]
  2. Power dissipated by the resistor is directly dependent on the load current. If the current increases to 1.5A, the power wasted across the resistor becomes 72W and the voltage to the load drops to 62vac.
  3. The circuit is 70% efficient (not too bad)

A more complex resistor solution requires two resistors to make a voltage divider. The sizing of the resistors is highly dependent on the desired voltage regulation and load current variation. If the load current is fairly constant and the required regulation is not too tight, there may be a solution.

For example: Improved voltage regulation can be achieved by putting a resistor in parallel with the load to draw the same current as the load. (R=E/I : 78V/1A=78Ω) Because the resistor in series with the source now sees twice the current (2A) it has to be halved to continue to only drop 32V.

R(series) = 16Ω

Power dissipated the series resistor now becomes = 2A x 2A x 32V = 128W And the power dissipated in the parallel load = 1A x 1A x 78Ω = 78 W

Essentially, the doubling of the regulation costs 166W (120W+78W-32W)

Efficiency is also now only P(load)/P(source)*100 or 78W/220W x 100 = 35% (not good)

The bottom line: If the load is constant, a series resistor will work. If the load varies, consider another solution.

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Anonymous Poster
#18

Re: Voltage Step Down Using Resistors

11/05/2010 3:04 PM

Why not use a variac transformer?

Chhers

Joe

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#19
In reply to #18

Re: Voltage Step Down Using Resistors

11/05/2010 4:07 PM

Yes a VARIAC would do. A variac is an adjustable auto transformer.

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#20
In reply to #18

Re: Voltage Step Down Using Resistors

11/06/2010 1:23 AM

Cost of Variac doesnot justify the oversimpliied requirement.

A re-wound transformer may be a better solution as advised by one of the friends.

Actually the best solution can only be given when total information about the exact requirement is clearly stated by the poster.

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#21
In reply to #18

Re: Voltage Step Down Using Resistors

11/06/2010 6:16 PM

Because, as Doorman suggested, the homework called for resistors.

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