Sorry, that won't work. Assuming the other end of the relay coils return to ground, you'd only get about 2 volts across the coils.
--
John Larkin Highland Technology, Inc
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Nah, your average 12V relay will pull in around 9V. 11.3ish is fat city.
Cheers
Phil Hobbs
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Dr Philip C D Hobbs
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Oh, dear. That'll give you about a volt and a half on the relay coils.
Get rid of D1 and both caps, connect R67 to the base of Q2, move Q3 to where C152 is now, and invert the logic level (low=ON).
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
Why is it that nobody helps the guy, rather than just letting him know you're all smarter than him.
The more common method to drive a relay is;
+12v to one side of relay, other side of relay goes to the collector of an NPN transistor, emitter of transistor goes to ground. Put a 1k resistor in series with base of transistor and apply your 3.3v the 1K resistor. Install diode across relay. Is there a reason you don't want to do it that way?
Looks like he wants to switch the coils and regulate the coil voltage down to 12. 18V is a bit much, 2.2x the normal coil dissipation.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
I've obviously led a sheltered life, since I've never seen a 12V relay datasheet where pull-in wasn't guaranteed at well below 11.3V. Do you have an example?
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
You're probably right--the actual intention is sort of hard to gauge from the schematic. In that case, the OP could keep the zener.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
He could use this as a $2 one-chip solution: TL751M12QKVURQ1 (active low input).
It's possible to modify the given circuit a bit and make it work okay too.. eliminate the caps, connect R68 to the input voltage (and make it 10K), reduce R67 to 330 ohms, add Q1 same type as Q3, E to ground, C to base of Q3, base to input through 2K. It will have a higher dropout voltage than the regulator solution so examine carefully the high ambient temperature (at the relay)/low input voltage condition. (this is also active low input).
Best regards, Spehro Pefhany
--
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Thanks for taking time to draw them. Yes, as you said I am trying to regulate as well as switch the power to the coils. In the fig-1, why is that pnp needed instead of resisting to limit current?
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Pull in voltage is rated at 8.4V for the 12V nominal relay. That's specified "without pre-energization at ambient temperature 23°C" (fine print).
If you require the type F insulation (155°C) they offer, then the coil resistance could then be more than 50% higher than at 23°C, so the guaranteed minimum pull-in voltage would actually be more than 12VDC!
Somewhat less than nominal voltage is normally okay if the input voltage is regulated and/or the environment is fairly benign, but that's not always true. Probably sluggish turn-on doesn't help relay life either. I don't like unnecessarily to give away voltage in a relay driver (eg. by using darlingtons).
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Fair enough, if you're running that hot. Of course in your business, it would probably pull in at a volt or so. ;)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
The resistor value could be a problem. If it's too big, there will be a lot of voltage drop in the resistor, because of the base current. If it's too small, it will get hot when its low side is grounded by the little mosfet. There may be no value that works.
Your relays need 200 mA. If the NPN transistor has a guaranteed beta of 100, you could need 2 mA of base current. Assume that when you have 12 volt power, you want at least 11 volts on the relays; the transistor drops 0.7, so that leaves
0.3 for the resistor. The resistors has to be 150 ohms. Now let the supply go up to 18 and turn on the mosfet. The current through the resistor becomes 120 mA and it dissipates over 2 watts.
If you think the relays will operate with less voltage, use a bigger resistor, and it might work. But it's a messy compromise.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
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The other approach would be to use #7 with a small resistor, and use the processor to measure the supply and PWM the relays appropriately. A Schottky catch diode would help.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
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