Problems with P3 24 VAC Power Supplies

[eztech 0]

I am exhaustively troubleshooting one of our past installations that is constantly suffering video losses on multiple channels. I would be interested in hearing if any of you folks have any experience or advice regarding these power supplies. The PSU's used are 8 channel 24VAC 7.25 Amp units. These are advertised as 24VAC output but we've had 4 cams DOA at different site visits. At first we were baffled, but upon closer inspection and testing we realized that these PSUs are outputting between 28.5-29 VAC at the PSU. The cams in use are CNB 600TVL box cams, according to CNB tech support there cams have an over voltage protection circuit which attempts to save the camera in the event of over voltage. The tech suggests that the cams should receive no more than 26.5 volts, we have measured 28.3-28.5 VAC consistently at each camera. Is this high voltage output to be expected from other 24V PSU manufacturers?

[SEANHAWG 1]

Yes that is common to have the PSU's with a couple points higher than 24 Volts to account for voltage loss over the cable run but for them to be 28 or 29 volts seems kind of higher than normal. I normally see about 26 volts on most 24 VAC PSU's right at the PSU and at the camera end its about the same if the cable run is not very long but normally drops down to about 24 volts under load. We also had problems running the CNB box cams but we were using 28 Volt power supplies and they would not come on at all, once we hooked them up to a regular 24 VAC transformer, all problems were solved. But every PSU that we have worked with that was rated at 24 Volts were able to power the CNB cameras no problem. We have worked with Altronix, Ultrak, and Wren PSU's

[Ken Layton 0]

Is it possible that the wall outlet AC voltage is high?

[eztech 0]

The AC input voltage is high, we've been monitoring that for about 2 months and the voltage ranges between 117 - 123 volts.

[survtech 0]

Many Altronix 24VAC supplies put out 28-29 volts, depending on load. AC supplies are NOT regulated so they typically put out a nominal voltage. In 20+ years, I've never experienced a problem with AC-operated cameras that was shown to be related to those AC supply voltages.

 

DC supplies are another thing altogether. I've seen a couple of DC cameras that were quite sensitive to even 10% overvoltage. Dallmeier cameras, for one, were rated at +/- 5% (a stupid idea).

[Soundy 1]

Like the others have said... AC power supplies are not typically regulated as they simply use a step-down transformer (sometimes multi-tap to support different output voltages). A 120V in -> 24V out would use a 5:1 step-down ratio, and the output voltage would vary in direct proportion to the output: 117V in would yield 23.4V out; 123V in would give 24.6V out... all in all, not much variation.

 

Keep in mind that AC voltage is usually rated and expressed in RMS (root-mean-square) voltage, which is 0.707 of peak-to-peak voltage... if your meter reads P-P, it's going to read 1.414 times higher than the "actual" RMS voltage: 24Vrms = 33.Vp-p. Basic multimeters usually give sort of an average between the two, so unless the meter states that it reads "true RMS", it's not unusual for it to read high, without the voltage actually being too high.

 

All that aside, most 24VAC and dual-voltage cameras are rectifying and regulating the voltage internally, probably down to 12VDC or less (if memory serves, the dual-voltage CNBs state that they'll work with anything from 10-30V AC or DC), so even with your "over" reading, it SHOULDN'T be the power supply that's killing the cameras.

[eztech 0]

Then I am at a loss. We install these CNB cams ( CNB-BBM-24F and CNB-V2815NVR) all the time. We use these power supplies all the time. Anyone have a clue what might be happening here? This always happens surrounding inclement weather, yet none of the equipment is getting wet, no lens fogging, nothing.

 

If anyone here has any solutions or avenues to search please let me know at your discretion, Thanks

 

Here is an image of the issues I'm having

 

Here is the Layout of the Installation

 

www.indecomaz.com/Files/CCTVPlan2.pdf

Edited December 14, 2011 by Guest

[Soundy 1]

Are these cameras only dying temporarily, or permanently? Have any of them gone back to CNB to see if they can determine what is actually failing in them? Is it possible power is getting fed into the video output? How are they wired - are you using coax, or UTP+baluns? If the latter, are you terminating in keystones? I've seen a particular keystone design that allowed wire ends to touch and fried a couple cameras when they got full PoE power fed into the NIC.

[eztech 0]

They were cabled with coaxial originally, we took a shot in the dark and re cabled to cat5e with baluns about 2 months ago. the Baluns use one pair connected to terminals on the baluns. The system generally has no video distortion and no ground loops. It varies, the 2 with no video in the pic are probably permanently damaged. Some will go on and off for a day and then be fine. Sometimes a month goes by without any issues.

[eztech 0]

ok, so I disassembled the box cam that recently failed and was surprised to find this.

 

The yellow wire was pinched between the power board and the housing

 

Notice the melted bulge in the power supply

 

Here is a pic of the housing where the conductor eventually meted through its sheathing

 

Here is another pic of the housing to see the manufacturer name

l

[sjfshaun 0]

Keep in mind that AC voltage is usually rated and expressed in RMS (root-mean-square) voltage, which is 0.707 of peak-to-peak voltage... if your meter reads P-P, it's going to read 1.414 times higher than the "actual" RMS voltage:

 

Matt, clearly you are knowledgeable in your field. However, I must inform you that AC RMS voltage is not 0.707 of peak to peak voltage. It is 0.707 of peak to ground voltage.

 

Therefore:

AC RMS = 120V

AC peak to ground = ~170V

AC peak to peak = ~340V

 

Or, as in your formula:

 

AC RMS = 24V

AC peak to ground = ~34V

AC peak to peak = ~68V

 

http://en.wikipedia.org/wiki/Root_mean_square

 

EZtech, I can tell by your caring for you customer's needs and attention to detail that you are a woman of great skill and knowlege, and have provided your customer a fine installation.

[Soundy 1]

Ah yes, my bad... although I'd term it simply "peak voltage" (or maybe "zero to peak") rather than "peak to ground", since there is no ground reference on the secondary side of a transformer.

 

In any case, the idea is the same: your average multimeter doesn't read "true RMS" and thus will often read higher than the rated RMS voltage.

[Soundy 1]

ok, so I disassembled the box cam that recently failed and was surprised to find this.

 

The yellow wire was pinched between the power board and the housing

 

Wow.

 

I think I'd be sending that back to CNB and *insisting* on a replacement.

[NotAfraid 0]

Eztech, I stand in admiration of your courage. Being female the world must be a scary place for you. I can only imagine the sense insecurity you must be feeling by the loss of one of your cameras. You must have a million of those cameras around your house. Perhaps next time you put a camera in your shower you might want to make sure it is properly suited for wet environments. The detergents in bubble bath have been notorious for consistent power board failure. Just some things to keep in mind. If I were going to place a camera in my shower I would make it IP66 rated to possibly avoid some of these issues in the future. Its not easy being a woman in a mans world but I guess you're just gonna have to keep on truckin.

[eztech 0]

Ok Adam

[Ken Layton 0]

If you have one of those 24 volt AC power boxes with a circuit board and terminal strip, be sure to pull that circuit board/terminal strip assembly out and eyeball it good for cold solder joints.

 

Also, if you crimped on fork terminals onto the power wires, are the crimps nice and tight?