Simple EE help needed


Active Member
It's been too long... I need some assistance here (probably very easy)

I am connecting power plugs (wall warts) to the switched power outputs on my amplifiers so my HA system (HomeVision Pro) can tell when various amplifiers are on or off.

My digital inputs accept +/- 30V DC. All of my plugs work fine except one... when it is turned on, it gives my 8.4 VDC, but when I check my log - my HVPro is sensing very rapid switching from a signal to 0 (so it thinks it's turning on and off about 10 times per second) (HVPro detects anything less than 1.1V as a 0, and anything above 1.3 as a 1 (on)).

I have checked the plug straight off a wall outlet (to make sure the amp outlet wasn't bad) and it gave me the same result. I also checked it into another input on HVPro (to ensure the input wasn't bad). If I check the signal with a DC voltmeter - it's a nice steady 8.4 VDC.

Can I mount a capacitor in series to smooth this signal out? If so, any suggestions on the size of it? Any other ideas? I know... I could go buy another wall plug and be fine... but now I want to know why this one isn't working and how to fix it! ;)

So you are saying you are plugging in an 8.4 VDC wall wart, then switching it with a series switch (only switching one leg), then feeding it into a digital input correct?

Are you sure the wall wart is "DC" and not "AC". You say you are getting 8.4 VDC when you measure it with a voltmeter. I'm assuming then that the voltmeter is on the "DC" volts scale. What exactly is the rated output of the wallwart (what does the label say)?

Just for kicks, put the meter in "AC" volts and see what it says.

I reread your post and you don't have much of a swing between off and on. (1.1 vs. 1.3). could be the cheap wall wart is not filtered and noisy as you suggested. You could try a a cap, but can you change this swing (make a one something like five volts?)
The problem with putting a filter capacitor across the input is that it will delay OFF detections because it will have to discharge befor you can get down to 1.1 volts. I agree with BSR that you need to do some more research about what is really coming out of that wall wart. Are you sure it is a regular transformer and not a mini switching power supply of some kind? Is it the same model as the other ones you are using?
I just double checked, and it is a DC Wall Wart, but it's 9.3VDC it's reading, not 8.4 (must have been one of my others that gave me that reading). It is not the same type as the others, but when I got the nice stable reading - I assumed it would work fine.

I don't mind it taking a little longer to read that the Amp is Off (a few seconds won't cause any problems)...

I'm wondering if whatever it was plugged into (can't even remember) puts a load on it that keeps the voltage stable or something... I might try the capacitor route just for grins to see if it works... and if it doesn't, buy a cheap DC plug from RadioShack...
If it's a linear power supply (not a switching supply) but it's not regulated, then the output voltage will vary from 0 to peak. It's using a diode bridge to make the negative voltages positive, but it won't smooth them out. It will vary as if it were an AC supply but with half the peak-to-peak magnitude. You can try a load across it or adding a regulator to smooth it out.

If it's a cheap wall wart, my guess is that it's a non-regulated DC power supply - just a transformer and diode bridge. Whatever it was originally used for probably had an internal regulator or conditioned the output in some other way to get the required voltage.

In this case, the voltage will be changing from zero to peak 120 times a second. But this may be faster than your instrumentation can register - therefore giving you only about 10 transitions a second.

If it's a switching supply, it probably won't give you a good smooth voltage without a sufficient load. In addition, adding this to the back of an audio amplifier may introduce noise in the audio signal.

If you have access to an oscilloscope, take a look at the actual waveform. The type of supply it is should probably be obvious once you've seen it.
Thanks for the information Smee. So if it's not regulated, it won't work for my needs? I was considering purchasing this wallplug in replacement... but guess I won't now.

Buffalo 9VDC Power

thanks again... guess I was just lucky with my other powerplugs - they all worked fine! ;)
If it's not regulated, you can add a regulator (e.g., a 7805 5v regulator for $1 or so, plus a few capacitors and some wire). You may also just be able to add a capacitor to smooth it out.

You could probably use something like this inexpensive regulated supply:
Jameco 190512CH
Ahhh cool. I did a quick search on the 7805 regulator, and can pick them up cheap (Radio Shack carries them too which is convenient). I might experiment with this (might be a good learning experience). Thanks!
yes it would, I missed that when I posted. Good eye - and good to point out in case any one was following this thread.

And thanks for the link upstatemike - that is a great price!
Make sure you follow the specs on that regulator. You are going to be drawing some current so you may need to limit the current with a resistor. If you are not sure then make a drawing and post it before you lose an eye.
hehehe - excellent advice squintz, and I appreciate the comment because I might not have realized this issue until it was too late. However, I'm leaning towards buying a few of the plugs from the link in upstatemikes post. I will be adding a couple more amps and will need some more anyways... and at that price, I could barely buy the components to fix the other one ;)
Sometimes these wall warts have built-in filter caps so when power is removed there is a delay of a second or two before the voltage drops to zero. An easy fix for this is to get a small "bi-pin" or "grain of wheat" style 12V incandescent bulb from Radio Shack and connect it across the input at your controller. This will discharge the filter quickly once power is removed and will provide a status indicator at the controller for troubleshooting.