[TriEmbed] Driving 32 5v relays

Thu Jul 16 15:22:52 CDT 2015

Carl, Pete, I think you both missed the post I made where the relay board
already has a transistor driver and opto-isolation, so he is only having to
sink a couple milliamps (for the LED) per relay - nothing more.

On Thu, Jul 16, 2015 at 4:10 PM, Carl Nobile via TriEmbed <
triembed at triembed.org> wrote:

> Using an ULN2003A in this case is very interesting. The only thing I'd be
> concerned about is opto isolation or the lack of it in this case. I'd put
> an opto isolator between the microcontroller and the ULN2003A so nothing
> can fry the circuit behind the ULN2003A. There is a built in clamping diode
> in the ULN2003A, so you shouldn't need to add an additional one across each
> relay coil.
>
> All in all not a bad idea.
>
> Carl
>
>
> On Thu, Jul 16, 2015 at 3:46 PM, Richard Prevette via TriEmbed <
> triembed at triembed.org> wrote:
>
>> 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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>>>>>> TriEmbed web site: http://TriEmbed.org
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
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>>
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>
>
> --
>
> -------------------------------------------------------------------------------
> Carl J. Nobile (Software Engineer)
> carl.nobile at gmail.com
>
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