# RE: M.O.T. CURRENT plus calculation education (needed)

```I'd like to know the same thing.  Like what is a good max RMS current to
pull through the sec. Winding on these (easy to find) HV transformers?

Bench tests and a little brainwork revealed the following:

M.O.T.-----
Most secondary voltage (out of 10-15 transformers) measure 2500VAC no load.
Measurement taken with 10:1 probe and tripplett VOM.  Occasionally an MOT
will measure in at 4500VAC.
Figure the largest u-wave oven at anyone's house is rated for about 1800
Watts or so.  At 1800 Watts with 120 VAC line-in [1800/120 = 15] the line
must provide 15 Amperes.  So figure 2000/120 = 16.7 then line current
divided by 16.7 = sec current, about 898mA.

Consider that the line input to the microwave oven chassis is equipped with
a fusible link rated at 10, 12, or 15 Ampere breaking current.  Considering
that the output power of various microwaves will sometimes be greater or
less depending on the design of the original appliance.  Some ovens are
rated for 500W (small) while others are close to 2kW (large, like the ones
at 7-11 or in the cafeterias).  So the size/weight of transformer is a vague
indication to the current available in the sec.

For a more exact guide I think it is possible (and maybe someone could guide
me through the calculations) or may be possible to figure out the max RMS
current rating for the original application.  This would only work if you'd
obtained the fuse along with the transformer, i.e. either removed the unit
yourselves or have the actual data on the transformer (in which case you'd
Okay, so say you have the fuse and the transformer.  You've already
measured the voltage in the HV winding.  Next you've got to do some
research, find out about the characteristics of the OEM's fuse.  Most OEM
fuse of the u-Ovens are the ceramic envelope type.  This is where my
questions begin, I have no calculus or trig.  So, we know the breaking
current of the fuse and the voltages in the transformer.  My guess is that
there is a coefficient type of a thing for the amperage rise-time vs. link
material thermal characteristics which tells the engineers how fast/high the
current is allowed to surge before fusing occurs thus breaking the line EMF
and automatically turning the power "off".  Somewhere between zero * E*I and
the point where the fuse breaks is your RMS value.
How is the RMS calculated with little or no actual data?

-----Original Message-----
From:	Tesla List [mailto:tesla-at-pupman-dot-com]
Sent:	Friday, November 12, 1999 5:14 AM
To:	tesla-at-pupman-dot-com
Subject:	M.O.T. CURRENT

Original Poster: "J. B. Weazle McCreath" <weazle-at-hurontel.on.ca>

Hello Coilers:

What is the typical secondary current I can expect to get from a
microwave oven transformer?  Can the current draw given on the
oven's label be used to calculate a rough estimate of the current
available from the secondary?  Would two identical transformers
with properly phased, paralleled primaries, and secondaries in
series with the center tap grounded, work for a low power T.C.?

73, Weazle, VE3EAR/VE3WZL

Listening: 147.030+ and 442.075+
E-mail:    weazle-at-hurontel.on.ca
or ve3ear-at-rac.ca
Web site:  www.hurontel.on.ca/~weazle

```