[TriEmbed] Driving 32 5v relays

Thu Jul 16 14:46:11 CDT 2015

Thanks Pete,
So what you are saying is the 595 is wired to the ULN2003a.  And the
ULN2003a drive the relays?

On Thu, Jul 16, 2015 at 1:24 PM, pete at soper.us <triembed at triembed.org>
wrote:

> Figuring that you've already bought the relays and lights, I suggest five
> ULN2003a relay drivers:
>
> http://www.digikey.com/product-detail/en/ULN2003A/497-2344-5-ND/599603
>
> Here's how to connect it (sorry about the hi-res):
>
>
> https://electrosome.com/wp-content/uploads/2013/05/Interfacing-Relay-with-8051-using-ULN2003.jpg
>
> No way will the '595 drive the relays directly if they need 73ma and, as
> John said, if one gate does work, trying to get several on at once  is
> likely to exceed the limits for the whole chip, so the per-gate capability
> is only part of the story. Even if it could sink the current notice you
> don't get a full five volts out of the chip's gate and via Ohm's law the
> current through the coil will be lower.
>
> Relay drivers will add more complexity, but the combination will work.
>
> I agree with Robert that a triac would be the way to go from scratch for
> mains voltage lights, but if you already have all the relays and lights
> that was no doubt some significant cost.
>
> This driver chip has internal diodes so it can handle inductive loads. An
> ordinary 74HC logic chip, if it could somehow handle the coil current,
> would be sent straight to semiconductor heaven by back EMF without a diode
> or RC circuit or varistor to constrain the voltage as the relay's magnetic
> fueld collapses (when the relay is turned off).
>
> Oh, note this chip sinks current. So the '595 would drive an input and the
> corresponding output provides a ground path for the relay coil.
>
> Finally, two more important details: The internal transustor and (yet
> another) diode in the driver will drop significant voltage, so for nice,
> reliable operation with five volt relays I strongly recommend six volts to
> end up with something clise to five going through the relay coils.
>
> Here's a wall wart that looks like it would work well:
>
> http://www.digikey.com/product-detail/en/WSU060-3000/237-1626-ND/3105436
>
> Also, be sure to set up the relay supply with decoupling from whatever is
> powering the logic. The easy way is to use a separate wall wart for the
> driver commons.
>
> Pete
>
>
> ----- Reply message -----
> From: "Richard Prevette via TriEmbed" <triembed at triembed.org>
> To: "Adam Haile" <email at adamhaile.net>
> Cc: "triembed at triembed.org" <TriEmbed at triembed.org>
> Subject: [TriEmbed] Driving 32 5v relays
> Date: Thu, Jul 16, 2015 12:10 PM
>
> Thanks all for the comments.  Very informative.  Rodney, I wish I'd seen
> this board before I started but if had to buy more boards, I would look at
> solid state.  (I am using 4 8 channel boards.)
>
> I found a 5v 3a wall adapter on Amazon $12.00.  Looks like that will do
> the job.
>
> On Thu, Jul 16, 2015 at 10:33 AM, Richard Prevette <
> prevette.richard at gmail.com> wrote:
>
>> The answer the first comment is mostly money.  I am using c9 size
>> Christmas lights for a yard display.  I have 16 PVC pips painted like candy
>> canes with two lights per pole.
>>
>> The second comment looks like just the ticket but would it have to be
>> wired to car battery or a 12v wall adapter with at least
>> a 3 amp rating?
>>
>> On Thu, Jul 16, 2015 at 10:16 AM, Adam Haile <email at adamhaile.net> wrote:
>>
>>> Stupid question... why not just use digital LEDs?
>>> https://www.youtube.com/watch?v=_nbuSgYoCSQ
>>> Full disclaimer: I have a tiny business that does LED lighting stuff,
>>> but I am also just a huge fan of digital LED strips since they take away
>>> complications like those with relays and such.
>>>
>>> More on your original question... no, no... not a resistor to do 14v ->
>>> 5v! You would be dropping 9v and @ 3A be dissipating 27W!   I have bins
>>> full of buck converters like the one linked below that will do 8-23V input
>>> and output a nice steady 5V. They work great and are very efficient.
>>>
>>>
>>> http://www.amazon.com/Nextrox%C2%AE-Display-Supply-Converter-Module/dp/B00CBCGAL8/
>>>
>>> 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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>>>>
>>>
>>
>
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