Modena wrote: > Frank, so my basic understanding is that the opto-isolator you speak > of is an SSR, if this is the case why do I need another relay in-line > after the SSR?
An SSR is functionally equivalent to an opto-isolator/relay circuit in
that it can be controlled by logic level signals and drive isolated
loads. The advantage of the SSR is that it doesn't have any mechanical
parts and it can be switched much faster than a mechanical relay
(although SSRs also have a switching speed limit that vary from one
model to another). The disadvantage of the SSR is that it costs more
(sometimes 10x or more) than an opto-isolator/relay solution.
When I need to switch loads on or off (no PWM), I use a 4N29
opto-isolator to drive an appropriately sized relay. The 4N29 can sink
about 150ma of current, which means it can handle all mini-relays
(1-15A) and most automotive relays (20-80 amps). Door-lock actuators,
however, draw much more than 150ma when the motor stalls (which is why
they can't be activated for too long as well), so you can't drive them
directly from the 4N29 and you'll need to drive them from a relay (a
little 10A mini-relay will suffice).
I've attached a schematic of how I use 4N29's to drive a relay coil. I
show a pushbutton on the schematic instead of the actual logic
connection for illustration purposes. (Clearly, if you wanted to
mechanically activate a relay, you would simply add the pushbutton to
the relay coil itself ... but that's another discussion.)
Basically, when switch SW1 is depressed (ie. your logic circuit goes
HIGH), 5 volts is seen by Pin 1 of the opto-isolator (OPT1) which causes
it's internal LED to light. That triggers the transistor between Pins 4
and 5 to allow current to flow to ground. When the transistor is
activated, current can flow from the 12v supply, through the coil,
through the 4N29 to ground, thereby activating the relay coil. When the
relay coil is activated, current can flow from the 24v supply to the
load on Pin 30 of the relay.
This is a very reliable circuit because of the following:
1) All three of the supply/ground circuits can be completely separate
from each other, allowing you to use different logic, relay coil and
load voltages. That keeps the electrical noise (and failures) generated
by each circuit isolated from the others.
2) Resistor R1 ensures that you don't (accidentally) pump too much
current through the opto-isolator LED.
3) Resistors R2 and R3 keep the opto-isolator inputs and relay coil
inputs shorted when SW1 is not pressed (floating input) and when
powering up. This prevents spurious activations of the relay.
4) Diode D1 suppresses the nasty little reverse voltage spike generated
by the relay coil when it turns off (simultaneously proving that both
Maxwell and Murphy were right).
Hope this helps get you activated.
Frank P.
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<<inline: Opto-Isolated Relay Circuit.png>>
