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Science Forum Index » Electronics - Design Forum » Amateur questions - how to reduce DC amps ? Auto shut...
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| buddy... |
 Posted: Sat Jul 19, 2008 5:16 pm |
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Guest
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1. Can someone explain, (in simple language, I basically know little
more than the names of various components) how to reduce DC amps?
I want to heat a container of water in an automotive use. I have one
of those cheap "heating coils" that you plug into the cigarette
lighter outlet, and drop the coil into a mug of water to make instant
coffee or tea. But two problems:
- I powered it from a battery charger and it drew 10 amps, way to much
- after only a few seconds, it gets way to hot to touch. I don't want
it hotter than, say, 100-120 F.
I think these two goals are compatible, but I don't know how to reduce
the amps.
2. I want the above to shut off automatically, both when power is
removed (i.e. car turned off) and after some amount of time, say 10
minutes - just like the rear window defroster. How to do this?
Thanks in advance for any ideas,
buddy |
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| MooseFET... |
| Posted: Sat Jul 19, 2008 5:59 pm |
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Guest
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On Jul 19, 8:16 pm, buddy <buddy... at (no spam) idcomm.com> wrote:
Quote: 1. Can someone explain, (in simple language, I basically know little
more than the names of various components) how to reduce DC amps?
This is really more of a "basics" question than a "design" question.
You also have a "design" question later.
First make sure you are reading this with the right font.
The "*"s should line up in the lines below.
*MMM*MMM*MMM*
*III*III*III*
*---*---*---*
Change the font until they do.
The term series means like this:
Wire More wire 3rd wire
------[Item1]------------[Item2]------
If you connect two identical resistive heater in series, the total
power and the current will be reduced in half. Each heater will end
up with 1/4th the power going to it.
I = V / R
I is current
V is voltage
R is resistance
Hooking stuff in series makes the resistances add.
P = I^2 * R
P is power
Quote:
I want to heat a container of water in an automotive use. I have one
of those cheap "heating coils" that you plug into the cigarette
lighter outlet, and drop the coil into a mug of water to make instant
coffee or tea. But two problems:
- I powered it from a battery charger and it drew 10 amps, way to much
- after only a few seconds, it gets way to hot to touch. I don't want
it hotter than, say, 100-120 F.
You can buy a thermostat that kicks off at 100F. If you wire this in
series with the heater (and it can take the current), you will have
something that turns on until the water heats above 100F and then off
until it cools below 100F. It will cycle on and off holding the
temperature in a narrow range.
If your thermostat can't take the current:
http://dkc1.digikey.com/US/EN/PDF/T082/Section.html
look around page 2063
Digikey may also have the thermostat you need.
Quote:
I think these two goals are compatible, but I don't know how to reduce
the amps.
2. I want the above to shut off automatically, both when power is
removed (i.e. car turned off) and after some amount of time, say 10
minutes - just like the rear window defroster. How to do this?
If you can live with very inaccurate timing, you can use a second
small heater and a second thermostat and a relay. A push button will
start the timing.
Hefty push
button
!
-----
---O O-------
! !
! !
! Contacts !
! 0f relay !
! / !
+12V -+---/ O--------+---+-------- -------------> To heater
!
[Therm]
[ostat]
!
+------------
! !
) [Small ]
Coil of ) [heater]
relay ) !
! GND
GND
When the button is pressed, the relay contacts close because its coil
gets power.
As long as the coil continues to get power, the contacts will remain
closed and the heater will remain on. If the +12V input goes away,
the relay is de-energized. If the small heater heats the thermostat
hot enough, the coil gets de-energized.
The thermostat and small heater should be inside an insulated
housing. Screwing them both down onto a block of aluminum would be
how I would hold them in place. The more aluminum in the block, the
longer the delay.
This is extremely crude but these sorts of things are hard to break.
Quote:
Thanks in advance for any ideas,
buddy |
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| RFI-EMI-GUY... |
| Posted: Sat Jul 19, 2008 10:34 pm |
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Guest
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buddy wrote:
Quote: 1. Can someone explain, (in simple language, I basically know little
more than the names of various components) how to reduce DC amps?
I want to heat a container of water in an automotive use. I have one
of those cheap "heating coils" that you plug into the cigarette
lighter outlet, and drop the coil into a mug of water to make instant
coffee or tea. But two problems:
- I powered it from a battery charger and it drew 10 amps, way to much
- after only a few seconds, it gets way to hot to touch. I don't want
it hotter than, say, 100-120 F.
I think these two goals are compatible, but I don't know how to reduce
the amps.
2. I want the above to shut off automatically, both when power is
removed (i.e. car turned off) and after some amount of time, say 10
minutes - just like the rear window defroster. How to do this?
Thanks in advance for any ideas,
buddy
A simple way:
http://europe.hbc.honeywell.com/products/pdf/en0r8445uk07r0402.pdf
It won't reduce the "amps" but you can set to shut off when the water
reached the desired temperature. It will of course come back on if the
water cools off. Also, you have to keep water in contact with the
heating element and the thermostat bulb or the element will continue to
heat up.
Since you don't explain what your end goal is, I can't advise you on the
safety of what you are doing. I would suggest that given the possibility
to over heat if the water runs out, you should have a float switch to
prevent the element from coming on without any water in the container.
--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©
"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."
"Follow The Money" ;-P |
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| John Popelish... |
| Posted: Sat Jul 19, 2008 10:38 pm |
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Guest
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buddy wrote:
Quote: 1. Can someone explain, (in simple language, I basically know little
more than the names of various components) how to reduce DC amps?
I want to heat a container of water in an automotive use. I have one
of those cheap "heating coils" that you plug into the cigarette
lighter outlet, and drop the coil into a mug of water to make instant
coffee or tea. But two problems:
- I powered it from a battery charger and it drew 10 amps, way to much
- after only a few seconds, it gets way to hot to touch. I don't want
it hotter than, say, 100-120 F.
In simple language, the resistance of the heater is very
low, about 1/10th of an ohm, so it sucks lots of current.
If you must use that heater and cannot modify it to
increase its resistance, you have to lower the voltage,
since voltage is what drives current through resistance. Or
you keep the voltage the same, but pulse it on and off
rapidly, so that the average voltage is lower. But when it
is on, the current will still be the same, unless you add a
filter to average those voltage pulses before they get to
the heater.
Quote: I think these two goals are compatible, but I don't know how to reduce
the amps.
2. I want the above to shut off automatically, both when power is
removed (i.e. car turned off) and after some amount of time, say 10
minutes - just like the rear window defroster. How to do this?
By the time you have gotten the power turned on and off many
times per second to lower the average current, it is pretty
simple to also add a timer to terminate the pulses after
some amount of time.
Is the point of this exercise a secret, or can you tell us
what you are trying to accomplish?
--
Regards,
John Popelish |
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