Can I parallel two Solid-State Relays ports to gain a 1 AMP maximum current in USB-6525?

[Benw327]

Hi,

I am plan to use USB-6525 to switch a 7VDC/1 AMP DC power supply.

Because USB-6525 have only 500mA maximum current for each channel, can I parallel two Solid-State Relays ports to gain a 1 AMP maximum current?

Does this safe to use this way?

If not, is there any other USB device can provide 1 AMP Relays channel?

Thanks,

Ben

[margasan · Accepted solution]

Hi Ben,

Yes you can do that, please refer to this KB, it contains the info you need

Can I Use Multiple Relays on One Device to Allow More Current To Pass?

Regards,

Margasan

[Peter_B]

Hi there,

I'd like evidence that I can parallel up the outputs to increase the load capacity without causing problems.  However when I click on Can I Use Multiple Relays on One Device to Allow More Current To Pass?   I get the following error

"Error

You are not authorized to view this document”

[crossrulz]

In theory, you can use two switches to double the current capacity.  In the real world, I would recommend against this.  What if one fails?  The other will soon fail spectacularly.  Plus you can't guarantee the load balance, so one may be going over the current limit while the other is just under (again, just asking for trouble).

Instead, you should be using one of the switches in the USB-6525 to control a relay of some sort that can handle the current.

[Peter_B]

Thanks crossrulz.  If the solid State Relays are MOSFETs then it could be OK to parallel them up as per

AN11599 Using power MOSFETs in parallel - NXP ...

"The Positive Temperature Coefficient (PTC) of RDS(on) is a stabilizing influence that promotes power sharing between the MOSFETs in the group."

The reason I posted above was because the claimed solution for the OP's question exists in an NI kb article  !!!  So there must have been some substance to the original answer above.

The approach you suggest of using the USB-6525 to power external relays is a waste of relays in the 6525 and if I needed to do that I'd simply purchase a USB-6501 which is less than 1/3rd the cost and with it use the following Solid State Relay.

[Peter_B]

I think I know how it can be done.  The USB-6525 incorporates "a depletion-mode MOSFET-based current-limiting circuit,"  

This means if I parallel up two of the SSRs and when one SSR switches on first, its current will be limited to 500mA, then the other SSR switches on and shares the load current (approx 700mA in total shared between them both).  They might not share them evenly, but that doesn't matter, I can place small (5ohm, 1 watt) resistors in series with each relay to load balance and promote more even sharing of current at the expense of a bit of addiitonal power wasted in the resistors .  Each SSR's  "On resistance = 550 mΩ max".