Power-on Reset Circuits
By Robin Minnear of the Game Doctors
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A power-on reset circuit is a circuit
that is designed to cause a reset pulse
for a certain period of time , after the
power is first applied to the system .
When a solid -state system is first
powered up , all the logic handling
devices appear in a completely
random fashion . This is when a free
game or other undesirable things
may happen .
To combat this , the power-on
reset circuit in Figure Three is of
some help . It ill simple and versatile .
The circuit shown is for interfacing to
a TTL five-volt system. However, by
changing the resistors and capacitor ,
you may use this basic design in
almost any DC system .
When power is first applied, the
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PLAY METER, January, 1978
F\t:t. 3
by the 220 OHM resistor . This is also
the point you tie into your TTL
system .
Very qUickly the power on the
SCR's anode (A) reaches five volts .
Since the SCR is still turn ed off, the
anode is at five volts and the cathode
is at grou nd .
At this time the 10 mfd capaCitor
starts to charge through the 470K
OHM resistor . The diode (lN914) is
reversed biased so he doesn 't
conduct.
As the capacitor charges, the gate
(G) of the SCR soon reaches it's
trigger point . When the gate reaches
its trigger point , thE' SCR is turned
on .
This causes the cathode to be
pulled to five volts .
The reset pulse comes off of the
SCR 's cathode . It is a 10 pulse that
returns hi a couple of seconds after
the five volts is applied .
Should the power be interrupted ,
the diode becomes forward biased
and quickly discharges the capacitor.
At the same time the SCR turns off.
The circuit is now ready to generate
another reset pulse when power is
restored .
I usually tie this reset circuit to the
clear inputs of flip -flops in credit
circuits . This way the credit circuits
are reset when you power up .
However , this reset circuit will pulse
any TTL device . If the system you 're
working on needs a HI going reset
pulse , simply add an inverter to the
circuit. This reset circuit is especially
helpful on Wheels .
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