how to drive 24V with 18V max darl. opto without extra supply???

Problem:

need to drive 24V 600ohm coil relay.

part on hand: 6n139t darlington output optoisolator, max V collector 18V.

I assume that one should not present the collector with more than 18V, even when
the opto is off. This means you need to have a voltage regulator, say 12V for
the opto and drive an external transistor or mosfet whose collector or drain CAN
float up to 24V.

Is there a way to do this without a second power supply? I was thinking a stiff
divider, but that seems silly.

Thanks,

Tom

 

You should have enough tolerence on the relay coil to pull in at less than
24 volts and maybe you can select darlingtons wich can handle a bit more
than 18v, depends if u wana risk it or not, make sure the emf is well
clamped.

Colin =^.^=

 

Tom,
It would be better to have an optocoupler with a simple 30V open collector
output, which can drive the relais or an external bipolar transistor
directly, since the saturation voltage of a darlington is at least 1V. But
you could do that also with your device and a couple of resistors. You do
not necessarily need an extra supply, you can either use a zener to keep the
voltage at safe levels or make a resistive divider. But be aware that a lot
of power is wasted even when the relais is off. if you have several outputs,
an extra +5...12V supply will make sense.

 

Sure. Use a 15V or so zener between the 24V supply and the collector
resistor. Then, drive another transistor with the collector off the
optoisolator. It'll invert the sense.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

Subject: Re: how to drive 24V with 18V max darl. opto without extra supply???

Something like this? (view in fixed font or M$ Notepad):

VCC VCC
+ +
1N4002| |
24V - C|
| ^ C|
.-. | C|
2.2K | | | |RY
| | | |
'-' | |
| | |
| '----o
| |
o---. ___ |/
| '------o-------. .--------|___|- o|2N3904
| | | | 10K | |>
1N4744(15V)| .-o---o---o---o--. .-. |
| | 8 7 6 5 | | | |
/-/ | | 10K| | |
^ | 6N139 | '-' |
| | | | |
| | 1 2 3 4 | | |
=== '-o---o---o---o--' === ===
GND | | GND GND
| |
+ | |
o--------------------' |
|
___ |
o----|___|---------------'
-
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


This has the added advantage of being non-inverting (relay on when opto on).

Good luck
Chris

 

Looks OK to me, except I would reduce the size of Rbe at the external
transistor to say 1k something
This for bypassing the leakage current of the opto device (upto 100uA if I
see datasheet)

Something like this? (view in fixed font or M$ Notepad):

VCC VCC
+ +
1N4002| |
24V - C|
| ^ C|
.-. | C|
2.2K | | | |RY
| | | |
'-' | |
| | |
| '----o
| |
o---. ___ |/
| '------o-------. .--------|___|- o|2N3904
| | | | 10K | |>
1N4744(15V)| .-o---o---o---o--. .-. |
| | 8 7 6 5 | | | |
/-/ | | 10K| | |
^ | 6N139 | '-' |
| | | | |
| | 1 2 3 4 | | |
=== '-o---o---o---o--' === ===
GND | | GND GND
| |
+ | |
o--------------------' |
|
___ |
o----|___|---------------'
-
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


This has the added advantage of being non-inverting (relay on when opto on).

Good luck
Chris

 

another alternative would be to use a comon base npn transistor, the base
biased to about 5v by a simple 2 resistor dividor, the darlington the drives
the emiter.

when the darlington turns on it pulls down the emiter of the 2nd transistor
driving it into saturation.

a comon gate n ch mosfet could easily be used instead, with the gat siting
at around 10 volts.