[TriEmbed] Driving 32 5v relays

Carl Nobile carl.nobile at gmail.com
Thu Jul 16 10:30:18 CDT 2015


Richard,

72mA is .072A, so your math is correct if a little round about.

2.3A = .072 * 32

I wouldn't use less than a supply that could deliver 3 Amps continuously.

Using a resistor to lower the voltage at this current is not a good idea.
The current won't always be the same since not all relays will be on at all
times making the value of the resistor you need to be different in each
case, which would not even be possible. You will need a voltage regulator
that can keep the voltage constant, a pure resistor cannot do that. Also
the power dissipation of any resistor you need will be quite high and the
resistor very big physically.

Look for circuits that use an LM7805, you will need a good size heat sink
on the device also. You could also look for a 5V PWM regulator, these don't
dissipate as much heat.

Carl


On Thu, Jul 16, 2015 at 11:10 AM, Richard Prevette via TriEmbed <
triembed at triembed.org> wrote:

> I am making a Christmas light display using 32 relays.  Using arduino and
> four 74HC595n shift registers.
> The relays are SRD-5vDC-SL-C.  Here is the question: I may be
> misunderstanding but the datasheet states each
> relay takes 72ma.  I put  a meter across the power and that seems right.
> At one point all relays will be on at the same time.
> That looks like I need a power source that will supply about 2.3 amps.  32
> x 72 x 1000 = 2.3a (Correct ?)
>
> I have a Dell AC adapter that states output at 14v dc and 3 amps.  All I
> need is a resister to lower 14 to 5 volts and
> I good to go?
>
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