more questions on voltage dividers
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So I did some calculations for the voltage divder I need: input 0-300
volts and I need to see 0-5 volts. I came up with one resistor of 295k
ohms and one at 5k ohms. This gives me a current of .001 amps. I don't
have the technical info in front of me on the bs2 stamp and can't remember
how much current it can handle but think .001 is fine. Now my question is
where or how do I get/make these resistor values. I looked on the
internet and only found 1K,10k, 100K ohm resistors. Is this even
posssible? Thanks so far for all the help...
Jason
volts and I need to see 0-5 volts. I came up with one resistor of 295k
ohms and one at 5k ohms. This gives me a current of .001 amps. I don't
have the technical info in front of me on the bs2 stamp and can't remember
how much current it can handle but think .001 is fine. Now my question is
where or how do I get/make these resistor values. I looked on the
internet and only found 1K,10k, 100K ohm resistors. Is this even
posssible? Thanks so far for all the help...
Jason
Comments
>
> So I did some calculations for the voltage divder I need: input 0-300
> volts and I need to see 0-5 volts. I came up with one resistor of 295k
> ohms and one at 5k ohms. This gives me a current of .001 amps. I don't
> have the technical info in front of me on the bs2 stamp and can't remember
> how much current it can handle but think .001 is fine. Now my question is
> where or how do I get/make these resistor values. I looked on the
> internet and only found 1K,10k, 100K ohm resistors. Is this even
> posssible? Thanks so far for all the help...
>
> Jason
You can buy 5k registors (or something very close) and a 500k potentiometer (a
variable resistor). You can put the resistor and potentiometer in series and
adjust the potentiometer to exactly the value you need it to be. I would start
by looking for these parts from a site like Digikey.
Neil
>So I did some calculations for the voltage divder I need: input 0-300
>volts and I need to see 0-5 volts. I came up with one resistor of 295k
>ohms and one at 5k ohms. This gives me a current of .001 amps. I don't
>have the technical info in front of me on the bs2 stamp and can't remember
>how much current it can handle but think .001 is fine. Now my question is
>where or how do I get/make these resistor values. I looked on the
>internet and only found 1K,10k, 100K ohm resistors. Is this even
>posssible? Thanks so far for all the help...
How much current the Stamp can handle isn't relevant since you will be
measuring voltage across the smaller resistor. All the current will flow
thru the divider.
This divider will dissipate 0.3 watts at 300 volts, most of it in the
larger resistance. I'd use a standard 330K 1-watt resistor, which will be
asked to dissipate 0.33 watts, in series with about a 7.5K (use the nearest
standard value that's decently above 5.5K - read on and you'll see why)
potentiometer, which will only be asked to dissipate 0.0075 watts so heat
isn't a consideration for this component. So get a hefty 330K resistor and
an easily adjusted (multiturn) pot and adjust the pot to calibrate the
divider. If the big resistor is exactly 330K, then the value you want to
adjust the pot to would be 5,593 ohms. You can buy a precision 330K
resistor and then use a precision ohmeter to set the pot to 5,593 ohms, or
you can just get a standard 330K resistor (cheaper) and apply 300 volts to
the divider and adjust until you read 5 volts across the pot... or if you
wish, as measured by your Stamp. Then check your Stamp's accuracy at
several different applied voltages to assure yourself it's reasonable
linear. Since very accurate voltmeters are easier to find (and cheaper)
than precision ohmmeters, I'd calibrate by measuring voltage, not resistance.
Good luck.
Jim H
>--- In basicstamps@y..., miller <miller12@r...> wrote:
> >
> > So I did some calculations for the voltage divder I need: input 0-300
> > volts and I need to see 0-5 volts. I came up with one resistor of 295k
> > ohms and one at 5k ohms. This gives me a current of .001 amps. I don't
> > have the technical info in front of me on the bs2 stamp and can't remember
> > how much current it can handle but think .001 is fine. Now my question is
> > where or how do I get/make these resistor values. I looked on the
> > internet and only found 1K,10k, 100K ohm resistors. Is this even
> > posssible? Thanks so far for all the help...
> >
> > Jason
>
>You can buy 5k registors (or something very close) and a 500k
>potentiometer (a variable resistor). You can put the resistor and
>potentiometer in series and adjust the potentiometer to exactly the value
>you need it to be. I would start by looking for these parts from a site
>like Digikey.
Whoa, you probably want to make the pot the lower resistance. The larger
resistance dissipates most of the heat so make it the fixed higher wattage
resistor. The pot will dissipate so little that you don't even need to
consider it's wattage rating. We're talking a few milliwatts. No need to
pay for a 500K multiturn 1-watt or more pot if you can even find one.
Jim H