HomeRoast Digest


Topic: heater protection circuit?: the light finally comes on! (3 msgs / 83 lines)
1) From: Mike McGinness
Hey Deward, first thanks for your simple yet elegant suggested solution of
using a diode simply wired across the two trimmer leads. Second I apologize
for being slow on the up take. I only have a lowly AASEE and have worked in
the computer field for 2 decades. You know, ones & zeros, like dc. Theory,
especially ac, returns slowly!
It finally hit me this morning exactly why & how it works. The light comes
on in the mind as it where. Duh, pass 1/2 the ac wave straight across, the
other half controlled by trimmer. Sure my idea of using an ac triggered
relay would work but why bother? More connection points to wire & besides
relay must handle heater current. The diode solution simply ensures the fan
is always enabled, at the theoretical minimum 50%, but in reality will be
most likely be higher depending on the trimmer used. My trimmer doesn't dial
down to zero output... I'll do some voltage measurements (under fan load)
once diode inserted in circuit.
BTW Deward you said 250v 3A minimum diode. Still fuzzy on what to get. Would
3A 200A surge PIV 400 work? $1.49 Radio Shack pn 276-1144http://www.radioshack.com/product.asp?catalog%5FnameLG&category%5Fname
LG%5F010%5F005%5F003%5F016&product%5Fid'6%2D1144
or maybe 3A 200A surge PIV 200, $1.39 RS# 276-1143.
or is this the wrong type altogether?
Thanks in advance for the help!
MM;-) aka Kona Konnaisseur miKe mcKoffee
Dual Variable Transformer Rosto Roasting
Rocky grindin' - Miss Silvia brewin'
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2) From: Rick Farris
Mike wrote:
<Snip>http://www.radioshack.com/product.asp?catalog%5FnameLG&category%5Fname
<Snip>
(I'm not Deward, but I see he hasn't answered this yet, so I'll take a
crack.)
I'm not an EE, I'm a lowly programmer but one thing I *have* learned is Ohms
law. Ok, and the relationship between AC (RMS) and peak voltage.
During forward current flow, the 3A rating is plenty for your fan.
When the diode is blocking and the current is flowing through the dimmer,
then you'd see one half of peak AC line voltage, or (being conservative) 150
VAC.
VAC implies RMS, and the relationship between RMS and peak voltage in a sine
wave is sqrt(2), or 1.414.
That means the diode will see 1.414 * 150 = 212 Peak Inverse Voltage. (PIV)
So the first one you selected would be correct.  (PIV is like speed rating
on a tire: as long as the speed rating is higher than you plan on driving
you're ok.   Same with PIV.)
-- Rick
P.S.  I left out some details (like the fact that we're only looking at 1/2
the sine wave in our application, otherwise peak-to-peak (p-p) voltage would
be applicable -- 2.828 RMS...)
[RF]
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3) From: Ken Mary
<Snip>
You can use a diode from an old popper as I have done. It works as described
and the dimmer is more "controllable", a larger knob rotation is needed for
a corresponding change in fan speed. However, on my Lutron dimmer, the
maximum speed remains the same. There is still the same large increase in
fan speed when the dimmer bypass switch is closed. The Lutron will be
replaced soon.
--
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