Silent Knight SK-5208 NAC Current

Hello everyone. Some years ago I purchased a Silent Knight 5208 FACP, and since then I’ve been confused on whether the NACs output filtered DC or FWR (Full Wave Rectified) current. I’ve received conflicting information from multiple sources: the manual says the NACs output FWR and so did a representative from the company, but all the mechanical horns I’ve hooked up to it sound smooth rather than raspy as they would on FWR. I’m asking this because I don’t want to damage any of my Simplex devices by powering them off this panel. I already had to replace the strobe in one of my Simplex alarms thanks to unknowingly powering it with FWR, and I don’t wanna have to replace more stuff by making the same mistake again. If you could give me a clear answer that would be great. Thanks.

I think that Silent Knight knows best how the NACs are powered. They designed and manufactured the 5208 panel. Experience has shows you by damaging a device that SK is correct.

The device I damaged on FWR was BEFORE I got the 5208, when I was using a power supply that unbeknownst to me output FWR current. And like I said, any time I have a mechanical horn hooked up, it sounds “smooth” rather than raspy, if it sounded raspy (as I’ve heard in numerous videos from other collectors) that would mean it’s being powered with FWR, but it doesn’t, meaning the NACs must output filtered DC.

I don’t go by what horns sound like. I have never noted whether one sounds “raspy” or not. I do use and believe scientific examination. Most digital multimeters can measure the AC component in a DC power source. If you have a digital meter try this.



Connect a load to a NAC on your panel.

Connect the meter to the NAC in alarm polarity.

Put the panel in alarm and read the DC voltage.

Switch the meter to read AC.

If the NAC power is filtered the AC should be 2 volts or less.

If the AC is greater than 2 volts the power is not filtered or poorly filtered.



If you have an analog meter the test can be done by placing a capacitor in series with one test lead. The capacitor will block the DC so the meter can read only the AC.



The only other test is to connect an oscilloscope to the NAC to observe the waveform.