Author Topic: Voltage resistance  (Read 4334 times)

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Offline Gasant

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Voltage resistance
« on: March 04, 2014, 03:21:19 pm »
Hi

I am trying to find a resistor to resist 234 v.
I only have volts. No amps etc. No other info to help you.
Is there perhaps a formula to do this.

This is what I am trying to do
Supply volts = 250v
need only 16v of that 250v

Thus 250v - 16v = 250v
How do I get rid of that 234v

Hope you understand what I am saying
Hope you can help

I thank you
Gasant
 

Offline rstoer

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Re: Voltage resistance
« Reply #1 on: March 04, 2014, 03:28:56 pm »
Not a good way to drop that much voltage... If this is AC (I assume so but you don't specify) then use a transformer.
 

Offline electronics man

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Re: Voltage resistance
« Reply #2 on: March 04, 2014, 03:39:34 pm »
the amount of voltage a resistor drops depends on the current being drawn through it, the way you drop that much voltage is by using a tranformer, try to find one that outputs 16V.
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Offline SoundTech-LG

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Re: Voltage resistance
« Reply #3 on: March 04, 2014, 06:01:09 pm »
Hi

I am trying to find a resistor to resist 234 v.
I only have volts. No amps etc. No other info to help you.
Is there perhaps a formula to do this.

This is what I am trying to do
Supply volts = 250v
need only 16v of that 250v

Thus 250v - 16v = 250v
How do I get rid of that 234v

Hope you understand what I am saying
Hope you can help

I thank you
Gasant


So no amps... curious. If you do not need amps (current). You will have no power at all to do anything useful.
Volts x Amps = Watts    16 volts x 0 amps = 0 power  What are you trying to do???
That's just an easy resistor divider ckt., but somehow I think you DO need amps... or maybe uAmps, or mAmps if you really want to do something with the 16 volts.
 

Offline electronics man

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Re: Voltage resistance
« Reply #4 on: March 04, 2014, 06:03:31 pm »
Hi

I am trying to find a resistor to resist 234 v.
I only have volts. No amps etc. No other info to help you.
Is there perhaps a formula to do this.

This is what I am trying to do
Supply volts = 250v
need only 16v of that 250v

Thus 250v - 16v = 250v
How do I get rid of that 234v

Hope you understand what I am saying
Hope you can help

I thank you
Gasant


So no amps... curious. If you do not need amps (current). You will have no power at all to do anything useful.
Volts x Amps = Watts    16 volts x 0 amps = 0 power  What are you trying to do???
That's just an easy resistor divider ckt., but somehow I think you DO need amps... or maybe uAmps, or mAmps if you really want to do something with the 16 volts.
+ no current through a resistor = no voltage drop.
follow me on twitter @get_your_byte
 

Offline SoundTech-LG

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Re: Voltage resistance
« Reply #5 on: March 04, 2014, 06:08:27 pm »
The OP never stated the current capabilty of the 250 volts either. But if he doesn't need current, I guess the voltage will be 0 no matter what he does... :)
 

Offline electronics man

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Re: Voltage resistance
« Reply #6 on: March 04, 2014, 06:16:59 pm »
no i think he meant he doesnt know what the curren the needs is.
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Offline Weez

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Re: Voltage resistance
« Reply #7 on: March 04, 2014, 07:16:59 pm »
Can you give some more info?  Where is this 250v supply coming from? 

You mentioned using a resistor to drop the voltage...if the circuit is series, this will work.  But you don't mention current, so as an example, if you need 100mA of current, you'll need a total of 2.5 k ohm resistance.  To tap 16v from that, you could use two resistors:  one 160 ohm, and one 2.34k ohm (btw, I did check Mouser, and both of these values are available).  Tap after the 160 ohm resistor to get your 16v.

If the circuit is parallel, it's not so easy.  Resistors in parallel all carry the same voltage, so alterations and /or additions to the circuit would be required.  A simple rule to remember:  resistors in series are voltage dividers, in parallel they are current dividers.

As has been mentioned, if your 250v is ac, your best bet would probably be a transformer.

I now defer to the smarter/more experienced members for advise beyond this  8)

 

Offline Dongulus

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Re: Voltage resistance
« Reply #8 on: March 04, 2014, 07:23:27 pm »
Please don't touch 250 V if you don't have any clue about what you are doing.
 

Offline Dave Turner

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Re: Voltage resistance
« Reply #9 on: March 04, 2014, 07:53:32 pm »
And define the load too.
 

Offline electronics man

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Re: Voltage resistance
« Reply #10 on: March 04, 2014, 08:03:06 pm »
all of this info wont help as this is a very bad idea anyway.
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Offline SoundTech-LG

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Re: Voltage resistance
« Reply #11 on: March 05, 2014, 03:54:26 am »
If you have a 10 meg ohm input on a DVM, you can read 16 volts on the meter if you put a 160 megohm resistor in series with your 250 volts.
You still have current (in amps)running through that series ckt. though. Roughly 1.5 micro amps I think...
This is a tough one, because current flows if you make a complete circuit with your resistor...  no matter what. That never changes.
I agree that you probably should not be using 250 volts for this experiment, until you figure this out. Try 25 volts, much safer.

Looking at this made me remember that, even though meters read out in VOLTs, it's really the CURRENT flowing through them (very, very tiny current) that gives the voltage indication.   :scared:   An "ideal" meter could still indicate volts with zero current flowing, hasn't happened yet...
and a meter measuring current, is usually, actually measuring voltage across a known resistance.

Oh, and maybe the most important thing to remember, the TOPIC was "voltage resistance" well, that is not what a resistor does. it can "drop" the voltage at a point in a series circuit, but it can only "resist" if there is CURRENT flowing. So really, it's voltage dropping...  and  CURRENT resistance.
« Last Edit: March 05, 2014, 05:04:54 am by SoundTech-LG »
 

Offline mariush

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Re: Voltage resistance
« Reply #12 on: March 05, 2014, 09:09:06 am »
Gasant, read this application note from Microchip titled Transformerless Power Supplies: Resistive and Capacitive:

http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf

This application note shows how to make small non-isolated power supplies using resistors and capacitors and diodes (and optionally zener diodes). However, keep in mind that such power supplies are only meant for low current (a few mA) and they should only be used where there's no chance of human person touching something exposed to electricity.

For safety, a transformer is always recommended.
 

Offline cybermaus

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Re: Voltage resistance
« Reply #13 on: March 05, 2014, 09:47:39 am »
Not sure if it is wise showing a diagram connecting diodes and stuff directly to main so someone so much at the beginning of the skill. Please be careful Gasant !

Anyway, for reference: These sort of resistor/diode/capacitor "transformers" are commonly used in cheap Chinese LED lightbulbs.
I took one open once. Its interesting: advertised as a 4W LED lightbulb, they forget to mention 3W of it is wasted on the resistor.
 


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