Single 5 volt supply op-to isolate 5 volt relay?

Community PCB Design
1

Hi all,

I am seeking some advise on a problem I have:

I want to build a circuit that controls some 230 volt equipment with a pic micro switching 5 volt relays the problem I have is that

I have a power supply with only one output voltage 5 volt 5 amp.

I have just designed a relay board with op-to isolation between the pic and relays…but have just realized there is no point op-to isolating the relays if they share a common supply with the pic controller circuit as any spikes will just bypass the op-to isolators.

The thing that I was wondering is can I separate my single 5 volt supply into two so I can isolate the relay coils from the pic with op-to couplers ? if not is the only way another to stop any spikes reaching my control circuit or is the only way two totally separate dc supply’s with two transformers one for the relays and one for controller?

I think it must be possible to isolate with a single supply because a SSR can do it? I just cant understand how

Any help would be much appreciated.

Thanks Brett

2

It wouldn’t be isolation if everything shared the same line. Yes you would have to have 2 supplies. Otherwise there wouldn’t be much isolation.

Oh, and +1 for PICs.

Hope your design goes well!

3

Hi

I use power and data (I2C) isolation for medical device (small thermostat)

if it’s critical and you welling to make more effort look at analog devices

digital isolation, I think you need the isoPower family

http://www.analog.com/en/interface/digi … index.html

4

Get an isolated DC/DC converter with a +5v input and a +5v output. Murata Power Solutions, RECOM Power,and TDK makes good ones.

5

Is switching the relays on resetting the microcontroller? Otherwise why do you need isolation at all? The relay provides isolation from your 5V to the 230V mains. I would rather suggest putting a 10 uF or larger cap near the device switching the relay and a 0.1 uF cap near the microcontroller, done deal.

6

I don’t think that is the point of the isolation he is trying to get.

7

The relay already gives you isolation from the 230V – why do you want more isolation? If the relay is causing spikes, it sounds like you may not have a diode across the relay coil. The diode is essential to prevent voltage spikes when the relay coil is de-energised. Also, you normally wouldn’t drive a relay coil directly from a microcontroller output (which can normally only drive 10 or 20mA) – a transistor should be used as a buffer.

8

With a single supply it’s important to keep your grounds in control (starred). The ground from the relays should travel directly back to the supply. It shouldn’t be tied into the controllers grounds until those reach the supply. The crappy diagram below should make this very clear. Note: this is for the grounds only. If you wire the relay ground through the uController first then you risk the negative effects of when that relay dumps current to the ground.

text diagram incoming:

GOOD

relay ------Power supply-------uC

BAD

relay ----uC------Power supply

Based on the parameters of the circuit you should be well isolated from the 5V perspective. Though if you are worried about drawing too much current from the uC output. You should get a cheap op-amp and make a voltage follower to protect those outputs. If for whatever reason too much current is requested from the output the cheap VF will burnout and not the uC.

I think that you should be ok overall. If you do burn something well there is a reason it is called “sparkfun”

9

Grimfox:
With a single supply it’s important to keep your grounds in control (starred). The ground from the relays should travel directly back to the supply.

A good solid ground plane would work too, and has other benefits (eg EMC). If you’re going to use tracks instead of a groundplane, then your advice should be followed.