How to isolate 5V and 12V circuit

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Hello everyone,

I have extend existing circuit design by adding a 12V relay to it. AT
the moment both 5V and 12V circuit are side by side. This doesn't work
well. Intermitent device failure happen.

I would appreciate opinion and instruction on how to isolate these two
circuit. I already have some idea to put the relay and it 12V voltage
supply on another board. But I have no idea whether this will work or
not.

Thanks in advance


Re: How to isolate 5V and 12V circuit
>
> Hello everyone,
>
> I have extend existing circuit design by adding a 12V relay to it. AT
> the moment both 5V and 12V circuit are side by side. This doesn't work
> well. Intermitent device failure happen.
>
> I would appreciate opinion and instruction on how to isolate these two
> circuit. I already have some idea to put the relay and it 12V voltage
> supply on another board. But I have no idea whether this will work or
> not.
>
> Thanks in advance

The most important points are:

The 12 volt supply and the 5 volt supply commons connect at one point,
only, at the relay driver

The relay supply be bypassed to this common point with a capacitor,
before the power heads for the relay coil.

The relay coils have reverse voltage suppression connected across
them.  This can be a diode, a diode in series with a zener or
resistor, or a series RC (or some other choices).  The point is that
if a driver turns off the relay coil current and there is no place for
the inductive current to go, the coil produces a very high voltage
pulse.

These 3 solve most relay next to logic problems.
--
John Popelish


Re: How to isolate 5V and 12V circuit
>Subject: Re: How to isolate 5V and 12V circuit
>From: John Popelish snipped-for-privacy@rica.net
>Date: 12/12/2004 10:37 PM Central Standard Time
>

>The most important points are:
>
>The 12 volt supply and the 5 volt supply commons connect at one point,
>only, at the relay driver
>
>The relay supply be bypassed to this common point with a capacitor,
>before the power heads for the relay coil.
>
>The relay coils have reverse voltage suppression connected across
>them.  This can be a diode, a diode in series with a zener or
>resistor, or a series RC (or some other choices).  The point is that
>if a driver turns off the relay coil current and there is no place for
>the inductive current to go, the coil produces a very high voltage
>pulse.
>
>These 3 solve most relay next to logic problems.
>--
>John Popelish

If I could add one more to Mr. Popelish's list:

*  Do what you can to minimize arcing at the relay contacts, especially if
you're switching an inductive load..  The EMI can cause upsets on logic level
lines.

One good way to do tis is to put an R-C snubber across the inductive load.
What this does is reduce the rate of rise in voltage across the load so the
relay contacts can get far enough apart to avoid arcing.

Good luck
Chris



Re: How to isolate 5V and 12V circuit
I have always totally isolated, using split planes and creepage distances,
any area of my boards that drive relay coils.  Yes, snubbers on the load and
diodes (TVS) on the coil help.  I would still supply an isolated 12V to the
relay-driving side of my circuit and communicate through optocouplers to it.

As an alternative, you might consider solid state relays.  They might be a
bit more expensive than the mechanical one you are using, but they won't
kill the rest of your circuit with +/- spikes.  They come in big and small
sizes (I use them to drive 6A motors!).

> Hello everyone,
>
> I have extend existing circuit design by adding a 12V relay to it. AT
> the moment both 5V and 12V circuit are side by side. This doesn't work
> well. Intermitent device failure happen.
>
> I would appreciate opinion and instruction on how to isolate these two
> circuit. I already have some idea to put the relay and it 12V voltage
> supply on another board. But I have no idea whether this will work or
> not.
>
> Thanks in advance




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