# Elemental question

Discussion in 'Electronic Basics' started by Daniel Morrow, Jun 5, 2004.

1. ### Daniel MorrowGuest

What is the simplest way to reduce the heat coming
from the heating element on a cheap 110v, 8a, (650 deg
F) heat gun? My math says it uses about 880 watts, so
I looked and a 1000 watt dimmer switch would cost
about \$35, which is nearly twice the cost of the heat
gun. Its a lazy way to do it and slows down the fan
A \$100+ 1000 watt resistor needs a heat sink and
only makes sense if youre going to treat AC like DC.
(which I am familiar with) Im trying to teach myself
AC circuitry, but Im not looking in the right place
or just not comprehending it yet. Any tips on doing it
right?

2. ### Henry KolesnikGuest

Put a diode in series and cut the power in half.

4. ### happyhobitGuest

A diode will cut the voltage by half and the current by half. The power will
be cut by ¾ (75%).

A 1000 Watt heat gun will put out 250 Watts.

5. ### Tim AutonGuest

You're cutting the time in half.

Tim

6. ### happyhobitGuest

I don’t know where I came up with that.

I suppose it makes sense on some other plane of reality but not this.

A diode will block 1 / 2 of the sine wave, which produces a 50 % duty-cycle
for 1 / 2 of the power.

Ignore my last post.

7. ### Rich GriseGuest

I had it explained to me, and finally went and worked it out on a
spreadsheet, but half a sine wave gives .707 times the power.

It's RMS, so it's a different equation.

Cheers!
Rich

8. ### happyhobitGuest

Hi Rich,

I think we’re talking about two different things here.

Thing 1.
..707 of the Peak voltage (or current) Is the RMS or Effective voltage (or
current). This is what your voltmeter displays.

RMS current produces the same heating effect as a continuous (DC) current of
the same amount. An AC current of 10 amps (RMS) has the same heating effect
as 10 amps DC.

Thing 2,
If you have two half sine waves (one positive and one negative) and block one
of these, you end up with 50% of what you started with. (Voltage, current and
power)

My problem was that I initially viewed it with an analog mind when I should
have used a digital mind. ;-)

If you drop the voltage to 50% the current drops to 50% and the power drops
to 25%. (50% voltage X 50% current X 100 % time = 25% power)

If you PWM the output to 50% the current drops to 50%, the voltage drops to
50% and the power drops to 50%. (100% voltage X 100 % current X 50 % time =
50% power)

9. ### Daniel MorrowGuest

The heat gun I am using is adjustable - but the switch isn't variable
- just switchable, and I need to get the heat output to be variable or
at least at one particular temperature.