Download HCPL3700, An optocoupler with a difference

HCPL3700, An optocoupler with a difference.
Optocouplers, or optoisolators as they are sometimes referred to, allow us to transfer a signal
between two circuits while keeping them electrically isolated from one another. In general they consist of
an Infrared LED and a Phototransistor. All this is enclosed in a light tight case under a dome that reflects
the light from the LED onto the Phototransistor. Infrared is used because the spectrum more closely
matches the sensitivity of the Phototransistor.
Generally these are a simple LED and Phototransistor in a four or six lead package. They also
come in dual, triple or quad (two, three or four) to a package in an eight, twelve or sixteen pin package.
CTR – Current Transfer Ratio.
This is a ratio of input current to output current. Most part numbers have a suffix that gives this
information. The part number may have an “A”, “B” or “C”, or perhaps a “-25”, “-50” or such indicator, at
the end of the number to give specifics. This should be taken into consideration when buying them from
distributors instead of buying them from the original equipment manufacturer (OEM). OEMs usually do
Incoming Inspection when they buy a batch of such parts. Out of every hundred they may check a few to
see that they operate as expected. This is one parameter they would likely check. At a set input current they
should get a known output current at a predictable output voltage.
Speed
We have a limitation on speed of these devices. This is a question of how fast we can get the LED
to turn on and off. This may be just a few KHz for some devices and up into MHz for other devices. In this
industry we more often find the slower types.
DC to DC type
These have an LED and Phototransistor. Some may have a simple phototransistor as the output.
These may have a CTR of 25% to 50%. Others may have a drive transistor following the Phototransistor.
This usually brings the CTR up to 100% or even more. Some may have a Schmitt Trigger output that is
designed to interface to a TTL or CMOS logic world. In this case CTR is without meaning.
DC to AC type
Ah, the Solid State Relay (SSR). Our input section may be the same simple LED, but the output
stage is a Triac. This lets us control an AC signal from a DC circuit just as a relay does. We often find these
in games where we drive an AC load. Typically the load is a higher current than the SSR can handle and
we find the SSR drives a higher current Triac.
These can be broken down into two types. One with a plain Triac output, the other has a ZeroCrossing circuit built into it. Consult you data sheets if you are in doubt.
Other possibilities
Browsing through the distributor catalog of your choice you will find an assortment of these
devices we didn’t bother to cover here primarily because they are just not popular to the gaming world. We
find them with Photoresistor outputs, FET outputs, SCR outputs.
The HCPL3700
The HCPL3700 is an optocoupler with a few extra features added we don’t usually find. We find
these used by Williams, Bally, and perhaps others I just haven’t noted. It is designed for an AC input.
Looking at the schematic we find the input side to has a Bridge Rectifier and a Constant Current LED
driver set at around 3.7 mA. We have both the AC input to the bridge available and the direct connection to
the constant current LED. We can drive this guy from AC or DC. Driving it from the DC inputs (pin 2 and
3) we should have about 3.8 Volts at 4 mA. Driving it from the AC inputs (1 and 4) we should have about 5
Volts at 4 mA. Since the output is designed to interface to logic we have no meaningful CTR quoted. Speed
of the HCPL3700 is only about 4 KHz but that makes it fine for checking the AC line on a 50 or 60 Hz
system.
HCPL3700S – indicates a Surface Mount case. An 8-pin DIP with the leads deformed and cut short.
HCPL3700W – indicates a Wide pin base. An 8-pin DIP with the leads spread out to 0.4” wide.
Troubleshooting
The most we should find across the inputs, pin 1 to 4, is about 7 Volts; or about 6.3 Volts across 2
and 3. Maximum input current should be around 10 mA, typically around 4 mA. As we apply an input
voltage the output should go low. Low should be no more than 400 mV, typically 40 mV. Since the output
is Open Collector we need a pullup resistor to VCC to see an output voltage.
In the circuit shown with AC applied the output of the HCPL3700 should be constantly Low. This
keeps the capacitor constantly discharged. Q1 should be off and the output pulled high through a pullup
resistor. If we miss a cycle of the AC line the R-C time constant of R4 and C4 allows the capacitor to
charge up enough to turn the transistor on and the output of Q1 goes Low signaling an AC Failure
Warning.
Since the circuit shown has current limiting resistors we can short the inputs of the HCPL3700
together for a second and confirm that the Q1 output goes low to check and see if our circuit is operating.
In the normal circuit the operation of this circuit would only occur in the final tenths of a second following
power down so in order to check this on a test fixture we need to build a circuit to look for specifically this
signal.