Power Isolation

Thread Starter

lindula

Joined Sep 23, 2016
76
Hello, I am designing a small board to provide power to a motor driver module. I need to provide 5V to the motor driver.

Here is my circuit:
Relay.jpg
The VCC5V in an input power rail to my PCB. The VCC5V also provides power to my Switching Regulators. I think I should provide some isolation between the PCB board and the external motor drive module. Should I provide some isolation on the VCC5V or is that not necessary?

Thank you
 

DickCappels

Joined Aug 21, 2008
6,776
Additional isolation is probably not needed. Check the solid state relay specs to make sure the control circuit is sufficiently electrically isolated from the switches. With only 1.8 volts will you be able to get enough current through the LED?
 

dendad

Joined Feb 20, 2016
3,657
When you say "The VCC5V in an input power rail to my PCB", is that to your FPGA board?
If you are using the same +5V for both sides of the isolator, you do not have isolation.
The 0V connection will need to be common too.
I would produce a second 5V supply from the motor power so you can completely isolate it.
 

Thread Starter

lindula

Joined Sep 23, 2016
76
When you say "The VCC5V in an input power rail to my PCB", is that to your FPGA board?
If you are using the same +5V for both sides of the isolator, you do not have isolation.
The 0V connection will need to be common too.
I would produce a second 5V supply from the motor power so you can completely isolate it.
Yes, VCC5V goes to all my switching power regulators and also 5V digital devices. I didn't realize this at the time of entering the schematic but I was thinking that if something stupid happens on the cable that carries the 5V it could ripple back, maybe blow up the SSR or maybe cause some kind of noise on the 5V. I don't know that is why I thought maybe I need to think about some isolation. I guess the simplest solution would be to use a separate supply.
Thank you very much,
Joe
 

Thread Starter

lindula

Joined Sep 23, 2016
76
Additional isolation is probably not needed. Check the solid state relay specs to make sure the control circuit is sufficiently electrically isolated from the switches. With only 1.8 volts will you be able to get enough current through the LED?
Hello, thank you for responding to my message. What spec should I look for in the datasheet? Isolation Voltage, Input to Output is 1500 Vrms.

Thank you
 

Thread Starter

lindula

Joined Sep 23, 2016
76
Thank you very much responding to my message.
Hello, quick question for you about Solid State Relays. I see in the datasheet the term:
Input Control Current to Activate which has a max value of 1mA. Is that the amount of current that is needed to close the relay? Or is the Forward Current (If) 5mA the amount of current to close the relay?
Thank you very much.
Joe
 

bertus

Joined Apr 5, 2008
21,024
Hello,

With 1 mA the relays just closes.
With 5 mA the relays is fully driven.
The max allowed current for the led is 50 mA.

Bertus
 

Thread Starter

lindula

Joined Sep 23, 2016
76
Hello,

With 1 mA the relays just closes.
With 5 mA the relays is fully driven.
The max allowed current for the led is 50 mA.

Bertus
Got it. I tell you with Electronics your always learning something new.

Thank you very much for all your responses.

Joe
 
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