Relay Driver Circuit
John W Bottoms
a 70ma relay (12volts) using a TTL signal. I don't believe it can
be done with 1 transistor without using an exotic transistor or device.
-jb (Concord, MA)
LStewart
Here (attached) is a schematic of a relay driver circuit. Hope it
helps. If not, write me back and I'll explain it further. Its quite
simple so I would imagine you'll have no problems implementing it.
jim_clay
> I'm looking for a simple 2 or 3 transistor circuit which will drive
> a 70ma relay (12volts) using a TTL signal. I don't believe it can
> be done with 1 transistor without using an exotic transistor or device.
>
>
>>>>
Hi John,
A single transistor can easily do the job in conjunction with 3 resistors
and a diode (to suppress inductive kickback). 70 ma. at 12 volts is a very
modest requirement that a large number of singal transistors can handle
easily. You should also consider the very inexpensive SN75475 relay driver
chip by TI or the even better ULN2803A by Allegro; both of these are available
for under a buck.
A 2N2222A is a commonly available NPN transistor with an HFE of about
100, the ability to handle up to 500 ma and block up to 40Volts. The emitter
should be grounded. The collector should be tied to one end of the relay field
coil. The other end of the relay field coil is tied to +12Volts. A diode
in parallel with the field coil is used to suppress inductive kickback when
current is turned of to the field coil. The base will need a 330 ohm pull up
resistor to +5 volts. The base then may be driven directly by the output
of a 74LS06 device. Other TTL drivers can also be used as long as Vbe break
down voltage of the chosen transistor is greater than 5 volts. In general,
74HCT00, 74LS00, 74ALS00, 74AS00, 74F00 type parts will all work in this type
of interface.
The SN75474 and ULN2803A minimize the wiring to almost nothing and are
very robust. For most applications using small field coils these would
be better choices
Regards,
Jim Clay
j...@lightside.com
To use a transistor,
Thomas Schaerer
: I'm looking for a simple 2 or 3 transistor circuit which will drive
: a 70ma relay (12volts) using a TTL signal. I don't believe it can
: be done with 1 transistor without using an exotic transistor or device.
You can, if you take a simple lowpower-MOS-device like the BS170 is.
That's no an exotic transistor :-))))
Bye
Thomas
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* Thomas Schaerer scha...@isi.ee.ethz.ch +41 01/632'2768 *
* Swiss Federal Institute of Technology Zurich, ETHZ (fax:01/632'1208) *
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Richard Steven Walz
John W Bottoms <jbot...@world.std.com> wrote:
>I'm looking for a simple 2 or 3 transistor circuit which will drive
>a 70ma relay (12volts) using a TTL signal. I don't believe it can
>be done with 1 transistor without using an exotic transistor or device.
----------------------------------
I guess you're not kidding, John, but I wish you were offering a prize for
all takers, you'd be broke! Connect as so:
+12VDC to relay coil wire A. (pick on for A, other is B)
Coil wire B to collector of any signal transistor, NPN, (2N3904, 2N2222A)
Transistor emitter to ground of 12V supply and TTL ground.
TTL output to lead A (pick one) of resistor, approximately 470 ohms 1/4W.
Side B of resistor to NPN transistor base.
Remember to use a 1N4001 diode across the relay coil wires A and B, the
band toward wire A. (freewheeling diode for inductive spike at off)
Vary value of resistor to bring NPN transistor collector voltage just below
the voltage at the base when the TTL level is HI. Don't fan out this TTL to
anything else. Watch the relay click on and off when you bring the TTL HI
and LO, respectively. You're welcome. Just make sure the +12VDC supply is
clean and not noisy.
-Steve Walz rst...@armory.com
Claude Frantz
>I'm looking for a simple 2 or 3 transistor circuit which will drive
>a 70ma relay (12volts) using a TTL signal. I don't believe it can
>be done with 1 transistor without using an exotic transistor or device.
A paper about a sophisticated circuit can be found on the host
bauv106.bauv.unibw-muenchen.de, in the directory /pub/claude/p3896367.
--
Claude F. (cla...@bauv106.bauv.unibw-muenchen.de)
This message may contain opinions which are not shared by my employer.
The facts can speak for themselves.
Alan Triggs
>>a 70ma relay (12volts) using a TTL signal. I don't believe it can
>>be done with 1 transistor without using an exotic transistor or device.
I did this recently with a simple 7407 chip. This is a buffer driver (non-
inverting) with open collector outputs. Simply hook one end of the relay to
your 12v and the other end to one of the 7407 outputs. When the input is
low, the output is open, so the relay is not powered. However, when you
apply a 5v signal to the 7407 input, it 'closes a switch' for the output to
be tied to ground. I know you can hook-up anything up to 30v to the
output. I'm not sure of the current rating, but I would guess that it's
good enough for your application.
7407
0/5v _____|\_______________ relay ________ 12v
|/
input result
0 no path for 12v's current
5v 7407 grounded - therefore path for current
Hope this helps
Alan Triggs
David Keith
One enhancement: connect a diode - anode to the 7407 output
and cathode to +12V. When the 7407 shuts off, the inductance
of the relay coil is unwilling to let the current decay quickly
and dumps its magnetic field energy into the circuit. If it
finds no other path it will generate sufficient voltage to
break down the output transistor of the 7407. The diode will
provide a safe path for discharge of the inductor and prevent
the collector output of the 7407 from being driven higher than
a diode drop above +12V.
Spehro Pefhany
: atr...@gab.unt.edu (Alan Triggs ) wrote:
[snip]
: One enhancement: connect a diode - anode to the 7407 output
: and cathode to +12V. When the 7407 shuts off, the inductance
I would consider it more obligatory than an enhancement. Another way is
to use a chip of the ULN200X (Eg. ULN2003) family which has the diode built
in. Remember to connect the common of the diodes to the + supply, or it
won't do it's thing. It is possible to get into SOA problems with the
transistor with large inductive loads and slow turn offs and small
xistors, but 70mA shouldn't be a problem. Just don't try to use a
transistor rated at 75mA to switch a 70mA inductive load unless you have
really looked at the data sheet...
Bon Chance!
--
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Spehro Pefhany "The Journey is the reward"
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Fax:(905) 332-4270 (small micro system devt hw/sw + mfg)
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Ralph R. Boudreaux
Connect TTL to the gate of a FET (ZVN2106A $0.60 Digi-Key), ground the
FET source, connect one end of the relay coil to 12V and the other end
to the FET drain. Also, connect a zener diode across the drain (cathode)
to source (anode). The zener will avalanche (like the FET) and conduct
the flyback energy. Choose a zener near the FET's rating for quick
relay turn off (43V 1N5260BCT-ND $0.22 digi-key).
May need a pull-up resistor, but probably not.
--Rusty Boudreaux
Martin T. Vinson
useing "real" TTL drive (such as the output of a 7404) connect the gate's
output through a 1k resistor to the base of transistor #1. The collector
of transistor #1 goes to 12 (or 5) volts through a 3.3k resistor. The
emitter of transistor #1 connects to the base of transistor #2. The
emitter of transistor #2 connects to ground and the collector to one end
of the relay coil. The other end of the relay coil connects to 12 volts
(or 5volts depending on the type relay you use). Connect a dampening
diode across the relay coil with the cathode (the end with the band)
connected to the end of the coil connected to the power supply. A two
transistor darlington is v e r y sensitive and will work easily. You can
use 2N2222's, 2N3904's (or is it 3906's, I can't remeber) Just make sure
they are general purpose NPN transistors. You can use fancy FET's etc.
but I design control stuff all the time that must survive lightening,
power surges etc. and have a hard time making FET's and MOSFET's work in
hostile environments.
>
John Lundgren
Ralph R. Boudreaux (ral...@eng.auburn.edu) wrote:
: Easy:
: Connect TTL to the gate of a FET (ZVN2106A $0.60 Digi-Key), ground the
: FET source, connect one end of the relay coil to 12V and the other end
: to the FET drain. Also, connect a zener diode across the drain (cathode)
: to source (anode). The zener will avalanche (like the FET) and conduct
: the flyback energy. Choose a zener near the FET's rating for quick
: relay turn off (43V 1N5260BCT-ND $0.22 digi-key).
: May need a pull-up resistor, but probably not.
: --Rusty Boudreaux
I thought that a regular depletion mode FET would conduct with zero bias
on its gate, just like a tube. Of you ground the source, and then the
TTL pulls the gate to ground, the FET will still be conducting. Hmmm.
: > >>I'm looking for a simple 2 or 3 transistor circuit which will drive
: > >>a 70ma relay (12volts) using a TTL signal. I don't believe it can
: > >>be done with 1 transistor without using an exotic transistor or device.
Just use a 2N3053 (Rat Shack) for the driver transistor, with the relay
connected between 12V and the collector, and emitter grounded. Also use a
1N4001 across the relay coil, cathode to 12V. Then divide the voltage
from the TTL down a bit, since TTL low is .8V and the transistor will turn
on at .6 V. The transistor should have a gain of 70, so the current
needed at the base will be 1 mA or less. If that ain't enough, then a
second transistor in darlington connection will take care of it. Or you
can just use a single darlington NPN transistor, I think it's a TIP120, or
something like that, also from Rat Shack.
--
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John Lundgren
Martin T. Vinson (mar...@ionet.net) wrote:
: In article <D8LyM...@mail.auburn.edu>, ral...@eng.auburn.edu says...
: >Easy:
: >Connect TTL to the gate of a FET (ZVN2106A $0.60 Digi-Key), ground the
: >FET source, connect one end of the relay coil to 12V and the other end
: >to the FET drain. Also, connect a zener diode across the drain (cathode)
: >to source (anode). The zener will avalanche (like the FET) and conduct
: >the flyback energy. Choose a zener near the FET's rating for quick
: >relay turn off (43V 1N5260BCT-ND $0.22 digi-key).
: >May need a pull-up resistor, but probably not.
: >--Rusty Boudreaux
: >> >>I'm looking for a simple 2 or 3 transistor circuit which will drive
: >> >>a 70ma relay (12volts) using a TTL signal. I don't believe it can
: >> >>be done with 1 transistor without using an exotic transistor or
: device.
: >Two plain-old 2n2222's will work in a Darlington arrangement. If you are
: useing "real" TTL drive (such as the output of a 7404) connect the gate's
: output through a 1k resistor to the base of transistor #1. The collector
: of transistor #1 goes to 12 (or 5) volts through a 3.3k resistor. The
: emitter of transistor #1 connects to the base of transistor #2. The
: emitter of transistor #2 connects to ground and the collector to one end
: of the relay coil. The other end of the relay coil connects to 12 volts
: (or 5volts depending on the type relay you use). Connect a dampening
: diode across the relay coil with the cathode (the end with the band)
: connected to the end of the coil connected to the power supply. A two
: transistor darlington is v e r y sensitive and will work easily. You can
: use 2N2222's, 2N3904's (or is it 3906's, I can't remeber) Just make sure
: they are general purpose NPN transistors.
2N2222's are no problem. But the 2N3904 will poop out way before it gets
to 70 mA, so don't use them. The 2N3906 is a PNP.
You can use fancy FET's etc.
: but I design control stuff all the time that must survive lightening,
: power surges etc. and have a hard time making FET's and MOSFET's work in
: hostile environments.
: >
Jerry E. Stone
several times. Place a 1K resistor between the TTL Output pin
and the base of an 2N2222A. Tie the emitter of the transistor to
ground. The collector to one side of the relay, and the other
side of the relay to +12V. Place a 1N4001 or whatever across the
relay with the cathode toward the 12V side.
Spehro Pefhany
: I have a much easier solution to your problem that I have used
This will probably work most of the time, but, with a LS gate, the
output is guaranteed to be only 2.4V min at 0.4ma out (over the temp
range). Now, consider Vbe of the transistor- allow say 0.7 volt, and we
have Ib= 1.7mA (which is _more_ than than 0.4mA, so you really have to
allow an even lower output voltage, worst-case, but let's go with the
optimistic value.. ). From the saturation characteristics of the
transistor, at room temp, and Ic=70mA, the min current to get well into
saturation at room temp is about 3 mA, and at -55C (well I _am_ in
Canada) is about 6ma. Of course, in practice the transistor will warm
up rather quickly if it isn't in saturation. :)
These are worst-case values, and _at 25C_, the suggested values are
probably ok with _most_ 2N222A's. Not intending to be critical, but if you
want it to work under all conditions, a fair bit more margin is required
than this.