# Driving multiple relays

Discussion in 'Electronic Design' started by Dh, May 17, 2007.

1. ### DhGuest

Hi guys,
I've got to drive some eight relays (12v, 200 ohm coil, 300mW
sensitivity). I'm planning to use the old 2n2222A to drive all of
them(in parallel). is it disastorous? would u suggest a better
transistor for this application? is it ok to use single transistor or
i should go with separate transistor for each relay??
Thanks!!

2. ### Fred BloggsGuest

There is such a thing as applying arithmetic. 12V and 200 ohms means
12/0.2=60mA per relay, and then 8x this would 480mA. But then 300mW at
12V means a current of 300/12=25mA per relay for 8x or 200mA total. So
and consistent.

3. ### D from BCGuest

This might be kinda silly but..How about using one more relay to drive
a bunch a relays?
D from BC

4. ### ehsjrGuest

Should be ok if you use 2 2N2222A transistors, each
driving 4 relays. Your circuit would need to be designed
to provide approximately 50 mA to the base of the
transistors whether you use one or two. If you want to
cut down on the base drive current and use a single
transistor, a power darlington like a TIP120 would work.

Ed

5. ### DhGuest

There is such a thing as applying arithmetic. 12V and 200 ohms means
Hi Fred,
ur arithmetic is correct, but the data i've given is correct as i
suppose sensitivity means minimum power required to turn the relay ON.
so, 25mA current would be sufficient, the 60mA is upper limit for the
current which is useful when we want to connect a resistance in series
with the coil for short circuit protection.
please correct me if anything is wrong..

6. ### DhGuest

why 50mA?? the maximum current i need will be (12/200 * 8) = 480mA as
pointed out by Fred, in that case my Ic of the transistor required is
480mA and to drive this transistor (hfe=75) into saturation i need
minimum base current of (480/75) = 6.4 mA or as a thumb rule atleast
double of it which makes base current = 12.8 mA.
Please tell me how u arrived at 50mA..

7. ### DhGuest

Hi guys,
one more addition, i'm going to use AVR microcontroller to drive the
transister, the pin current capacity is 40mA max.

8. ### WinfieldGuest

Lose the transistor and use a single MOSFET for all eight relays.
For example, Fairchild's RFP3055LE is a sixty-cent TO-220 through-
hole logic-level mosfet that will switch 480mA with only 50mW of
dissipation across its 0.1 ohm Ron, with 5-volt logic gate drive.
And, it sports the long-famous '3055 workhorse part number.

If you must use a TO-92 transistor, use one of Zetex' high-current
high-gain Super E-line types, like a ztx851, which will dissipate
under 50mW at 480mA with 10mA (beta=50) base drive.

9. ### MooseFETGuest

I'd suggest the supertex TN0604N3 if he had to use a TO-92. It would
be about as much current as I'd trust one for. The on resistance is
about 1 Ohm so you have about 1/4W to get rid of.

10. ### Winfield HillGuest

It's a shame theere aren't more powerful TO-92 mosfets. It's
not for a lack of small enough dies, because we see many good
new candidates in coming out small surface-mount packages.
It's a lack of interest in bringing out any new TO-92 parts.
We do still get new TO-220 through-hole parts, but there have
almost no new TO-92 parts introduced for quite a few years now.

11. ### JasenGuest

you want saturation or else you'll cook the transistor.
for saturation treat Hfe as approx 10

Bye.
Jasen

12. ### ehsjrGuest

Better rule of thumb: base current = Ic/10 for
guaranteed saturation. BTW, 50 mA Ib for saturation
with 500 mA IC is on the datasheet.

But in this circuit, what do you care? If you have
plenty of current available, that's fine, if you're
limited, use a darlington.

Ed

13. ### MooseFETGuest

Today, I mostly use surface mount for the small signal stuff. There
have been very few times that I wanted a power device in the 0.25 Watt
range. I can come up with a long list of things I'd like to see
before I see a new transistor in a TO-92.

A reference with less than 50nV/sqrt(Hz) noise

An LT1247 that really does work at 1MHz without getting red hot.

A PA7540 in low power CMOS.

A fast low noise op-amp that will work up to 35V

A line of analog filters you can program like a PROM to be the filter
you need.

A fast comparitor that can take large input swings.

Linear voltage regulatators with sense pins.

LDOs that are stable for all capacitive loads.

HC4046 VCOs that remain linear at the extremes.

Strong alloy "brass" parts that aren't magnetic.

1N5314 in a SMD package.

While we are at it: diamond heat sinks.

I think you will agree that many things are more important than new
TO-92 parts.