Help With Circuit Information

[Randy Gross]

Please view in a fixed-width font such as Courier.

________________
| _____|_____
______ | | | R1 and R2 used as a
)|| __| | |
120vac )||(8vac R1 R2 test load to
)||(7 Turns 14 Ohms 21 Ohms
6A )||(__ | | determine output amps.
______)|| | | |
| |___________|
50 Turns |________________|


This schematic depicts a transformer under load for testing purposes. I
scoured the web in search of a high current dummy load, no luck! I came up
with the Idea of using 2 heating elements (R1 and R2) wired in parallel to
reduce the total load resistance.

The result was surprising. I expected a good deal of heat but, the elements
didn't even warm. The drawback is, how do I use the resulting data to
determine the actual circuit amperage. These are the figures:

OCV: 8.1v
Total secondary load resistance: 8.5 ohms
Secondary current (clamp-on meter): 1.02 Amps
Primary current under load: 8.3 Amps

SOS

 

[William Rees]

Hi Randy,
I might be missing something here...

With a step down xformer in ratio 50:7 I would expect a output voltage of
about 16Volts. Could you be taking it from the centre tap to get approx 8V?

Anyway if your O/P voltage is 8V and your load is about 8 Ohms you'll only
get 1Amp flowing (V= I*R) and the power is also about 8Watts (W = I * I *
R). Which in heating element isnt going to appear significant. I imaging
the current flowing through the primary is about 60mA?

If you want to get more current flowing through the secondary...use a
smaller resistance load!
Hope thats not complete gibberish
regards
Bill

 

[[email protected]]

Please view in a fixed-width font such as Courier.

________________
| _____|_____
______ | | | R1 and R2 used as a
)|| __| | |
120vac )||(8vac R1 R2 test load to
)||(7 Turns 14 Ohms 21 Ohms
6A )||(__ | | determine output amps.
______)|| | | |
| |___________|
50 Turns |________________|

This schematic depicts a transformer under load for testing purposes. I
scoured the web in search of a high current dummy load, no luck! I came up
with the Idea of using 2 heating elements (R1 and R2) wired in parallel to
reduce the total load resistance.

The result was surprising. I expected a good deal of heat but, the elements
didn't even warm. The drawback is, how do I use the resulting data to
determine the actual circuit amperage. These are the figures:

OCV: 8.1v
Total secondary load resistance: 8.5 ohms
Secondary current (clamp-on meter): 1.02 Amps
Primary current under load: 8.3 Amps

SOS

This is SERIOUSLY whacked!

The primary power is 120 * 8.3 or 996 watts.
The secondary power is 8.1 * 1.02 or 8.262 watts.

If the measurements you posted are correct, your transformer
was dissipating over 987 watts. Kiss it goodbye - it's toast.

Perhaps you reversed the currents and primary was 1.02 amps while
secondary was 8.3? That would give more reasonable numbers:
Primary power is 120 * 1.02 or 122.4 watts.
Secondary load power is 8.1 * 8.3 or 67.23 watts.
That's still too much dissipation loss in your transformer
122.4 - 67.23 or 55.17 watts, but far more realistic.

Your turns ratio should give you about 16.8 volts in the
secondary. Total power in the secondary side would be
16.8 * 8.3 or 139.44 watts. Here, the secondary power and
the primary power are roughly equal. This argues strongly
that your primary current really is about 1 amp, and your
secondary current is about 8 amps, as power in must equal
power out. Some of the power out is in the form of losses,
and the paragraph above shows about 55 watts lost (as heat)
in the transformer.

You asked about the actual secondary current. Your
meter tells you that, assuming the meter is accurate.
If what you really want to know is how much current an
unknown transformer can safely provide, there are three
methods you could use:
1) estimate based on how much the transformer weighs, or
2) measure temperature rise and current, to determine
the maximum current you can draw while the transformer
temperature does not rise above some particular figure
(120 degrees C is probably a safe number) or
3) measure the no-load voltage, then apply a variable
load and measure the current and voltage. Vary the load
until the secondary voltage drops by 10%. Measure the
current at that point.