Output noise from SMC1000C smart UPS

[BillP]

Hi! I had purchased the SMC1000C smart ups around a year ago to keep my computer up during outages. I recently decided to connect my consumer level FM Transmitter to the unit. The problem I am experiencing is a high pitched electrical noise broadcasting through the fm transmitter. I can confirm it only broadcasts this noise when it is plugged into the UPS even with no audio cable connected to the transmitter. The noise is not present when plugged into the wall. It makes this noise constantly even when not running on battery power. It gets much worse when on battery power and when recharging the battery. Any ideas what may be causing this interference?

[Brad_C]

On 4/9/2020 11:48 AM, Aidan said:

The problem I am experiencing is a high pitched electrical noise broadcasting through the fm transmitter.

This isn't particularly surprising. Both charging and on-battery there is a relatively high frequency PWM signal applied to the switching transistors to allow the UPS to do what it needs to do. Even when it's not "charging", it really is just with a much lower output to float the batteries, so it'll be continuous and just vary in strength and characteristic. This is highly likely to leak through onto the mains passing through the unit.

The easiest fix would be to use a power supply for your transmitter that has better noise rejection.

[BillP · Accepted solution]

Thank you for your help, that makes sense. I had assumed the unit had a simply relay to disconnect the charging circuitry when finished, allowing the mains to pass through. Knowing that it is continuously floating the batteries would explain the constant noisy output. It's a relief to hear this is normal.

I will have to try out a better power supply, as you suggested, and perhaps look into implementing a low pass filter to reduce the noise. Thanks again.