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Newbie needs help designing transistor circuit to drive relay

D

Dylan

Jan 1, 1970
0
Could someone please help a newbie design a small transistor circuit to
drive a relay (wiring, value of resistors, direction of diode across coil)


I have a 2N5089 transistor

I have 2 relays that I want to drive (separate projects):

- Relay #1 is a NTE R74-11D1-5 (5V, 178ohm coil, 140mw)
http://www.nteinc.com/relay_web/R74.html

- Relay #2 is an NTE R12-14D5-6 (6V, 160ohm coil, .9w)
http://www.nteinc.com/relay_web/R12.html


What do you suggest for power supplies? Could I use the PC power supply
5v/12v for the circuit or am I better off with an independent supply?

In both projects I want to tap into an indicator LED on a separate circuit
for on/off condition of relay.

If you could give me a rule of thumb for calculating resistor values for
different relays I would appreciate it.


TIA
 
A

Andrew Holme

Jan 1, 1970
0
Dylan said:
Could someone please help a newbie design a small transistor circuit
to drive a relay (wiring, value of resistors, direction of diode across
coil)

The diode is reverse biased when the relay is on i.e. with it's cathode
pointing towards the +ve supply.
I have a 2N5089 transistor

I have 2 relays that I want to drive (separate projects):

- Relay #1 is a NTE R74-11D1-5 (5V, 178ohm coil, 140mw)
http://www.nteinc.com/relay_web/R74.html

- Relay #2 is an NTE R12-14D5-6 (6V, 160ohm coil, .9w)
http://www.nteinc.com/relay_web/R12.html

What do you suggest for power supplies? Could I use the PC power
supply 5v/12v for the circuit or am I better off with an independent
supply?

Obviously, the 5V supply would be OK for a 5V relay.

The 6V relay coil is 40 ohm not 160 ohm according to that web page. To
operate it from 12V you would need a series dropper resistor which would
waste power. I see they do the same relay with a 12V coil. Why not use
that?
In both projects I want to tap into an indicator LED on a separate
circuit for on/off condition of relay.

The way you do this depends on the circuit you're tapping into. The LED
_might_ be connected to +5V through a series current limiting resistor. If
it is, the logic which drives it will pull down to GND to light it. The
2N5089 is an NPN transistor. If you drive the base of this transistor
(through a 10k resistor) with an "active low" logic output, the transistor
will be on when the LED is off. This may not be what you want. If it isn't
you would need to invert the logic or use a PNP transistor (with a 5V
relay).
 
N

N. Thornton

Jan 1, 1970
0
Andrew Holme said:
coil)

The diode is reverse biased when the relay is on i.e. with it's cathode
pointing towards the +ve supply.

line on diode body is same end as line on circuit diagram. Line points
to the positive end of the relay coil. Line is cathode.

Obviously, the 5V supply would be OK for a 5V relay.

Dylan has a 5v psu for a 6v relay, giving around 4.5v on the coil.
Below spec, but will almost certainly work ok.


youve got a resistor on your tr gate to limit base current to say 2mA.
4.4v 2mA = 2.2k, so its a 2.2k base resistor, one end to the tr base,
the other end is your input point. Just attach the input to a pin on
the LED and see if it responds: if it doesnt, attach to the LED's
other pin.


NT
 
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