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Power supply with over-current protection of prescribed
current-time characteristic
Very good news:
The sane-backend for the EPSON PERFECTION 1260 PHOTO now works fine.
Thank you, Gerhard Jäger!
Power supply with over-current protection of prescribed
current-time characteristic
The power supply is designed for an output voltage of 15.2volt and a
duration current of 1.25amps as required by the Epson Perfection 1260
scanner. The switch-off-time-current characteristic of the
over-current protection is similar to that one of a super fast chip
fuse used in the Epson Perfection 1260.
If the load draws an over current longer than the corresponding
switch-off time then the power supply switches off. It can be switched on
again by removing the load and unpluggin the main-supply cable for
ca. 30 seconds.
Circuit diagram
The operational amplifiers employed with
the power supply are all open collector types. Therefore, they can
work on a common load. Thereby, the common load is the current source
built up with the transistor Q1 (SF117) and the base configuration of
the power Darlington transistor.
The TAA861 serves as the voltage
regulator. The resistor R10 serves for the fine adjustment of the
output voltage.
The twin-opamp TAA2761 realizes the over current
protection. The opamp IC2b works as an integrator the opamp IC2a as a
Schmidt-trigger (see below for
some more words about that).
The duration current can be adjusted
by the 470Ohms potentiometer R21 and the switch-off-fime
characteristic can be adjusted by the 100kOhms potentiometer
R23.
The output impedance of the power supply is
approximately 0.005 Ohms at 1 Ampere output current.
The output current is measured indirectly by the voltage over the
resistor R20.
If the output current exceeds approximately 3 Ampere (worst case),
then the voltage over R20 reaches the threshold voltage of the
base-emitter diode of the transistor Q3 and Q3 delimits the output
current at approx. 3A. If the output load is too high (e.g. short circuit
at the output) the output voltage Vo drops fast. Immediately, the
output voltage of opamp IC2b follows triggering the Schmidt-Trigger
IC2a and switching off the power supply permanently.
Now we assume that the output current is low enough such that the
voltage drop over R20 does not exceed the threshold voltage of the
base-emitter diode of Q3. Here is where the integrator IC2b comes into
play. In the initial state the capacitor C20 is discharged (the error
through the bias voltage treated in the following is marginal and can
be neglegted). The resistor R21 provides a negativ bias voltage with
respect to the potential at pin (3) of the power-Darlington
transitor. To admit a duration-output current Idur := 1.25A this bias
voltage should be adjusted to 1.25 A * 0.2 Ohms = 250 mV. If the
output current Io is bigger than Idur then the integrator (IC2b, C20,
R23, R4) will integrate the voltage R20*(Io-Idur) over time. More
exactly, for an constant output current Io the output voltage of IC2b
will be approximately equal to
Vo-(Io-Idur)*R20*t/((R23+R24)*C20) where t is the duration of the
over current. The resistors R26, R27 and R28 serve as the positive
feed-back network for the Schmidt trigger IC2a. But this network also
lowers the threshold voltage of the trigger. Therefore, a voltage
swing of approximately Uswing = 10V at the output of IC2b is necessary
to trigger IC2a. This determines the time-current-characteristic of
the over current protection: Uswing =
(Io-Idur)*R20*t/((R23+R24)*C20). Or, written with
the admissible current-time product Qover :=
Uswing*(R23+R24)*C20/R20
we obtain the equation
t = Qover / (Io-Idur).
for the time-current characteristic.
If the over-current protection has fired then one has to switch off
the power supply for a recovery time of approximately 30 seconds and
switch it on again to return to the normal operation mode (voltage
source). To prevent an activation of the Schmidt-Trigger at start-up
time of the power supply the capacitor C21 has been inserted.
Note, that for ordinary melting fuses not the current-time product is
a constant but the power-time product (also known as the
I²t-product).
But that doesnot harm too much for my application. I've adjusted the
current-time product to 4As. For that setting the scanner "Epson Perfection
1260 Photo"
behaves very much like when it was equipped with an ordinary melting
fuse.
The following lines are history from now on, since the new sane-backend
for the "Epson Perfection 1260 Photo" works fine!!!!
If one uses the scanner with the plustek-0.45-TEST4.tar.gz backend for
Linux-sane and a resolution of 1200dpi then it fires the fuse or the
over current protection, resp., after about 20 seconds. That is not
the case with the firmware driver for windows.