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IFB100 Emergency


Brian Whitlock
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PLEASE can it be standard to have reverse polarity and  over-voltage protection  in location audio gear. Stuff happens. This is needlessly stressful for an end-user, for want of a 79 cent diode.

 

(end rant)

 

Peter

Try $0.03 for a 3 Amp silicon diode, $0.20 for a Schottky with less than a quarter of a Volt drop. Polyfuses are about $0.50. Over voltage is a bit more expensive.

Best,

Larry F

Lectro 

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I would be interested to hear from Larry and Glenn and other knowledgeable pros about the effectiveness of reverse polarity protection for solid state equipment in use today.

 

Some years ago I had a client who plugged an Eclair ACL into a reverse polarity source. The motor was fried instantly and had to be replaced at a cost of several thousand dollars. It was a lot of money at the time.

 

When I asked the Eclair distributor about a fuse or protective circuit, I was told that the damage occurred so quickly that it wasn't possible or, at least, wasn't practical to incorporate effective protection. Anything that would react quickly enough would be so hair trigger that it would be constantly tripping at the wrong time.

 

Of course, that was some years ago and circumstances may have changed considerably in the interim.

 

Is it now possible to reliably protect against damage from reverse polarity? Are there conditions where it isn't possible or practical? (A camera motor, after all, requires considerably more power than most audio circuits.)

 

David

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[snip]

Some years ago I had a client who plugged an Eclair ACL into a reverse polarity source. The motor was fried instantly and had to be replaced at a cost of several thousand dollars. It was a lot of money at the time.

 

When I asked the Eclair distributor about a fuse or protective circuit, I was told that the damage occurred so quickly that it wasn't possible or, at least, wasn't practical to incorporate effective protection. Anything that would react quickly enough would be so hair trigger that it would be constantly tripping at the wrong time.

[snip]

 

David

A manufacturer should assume that their customer is not always perfect or that other circumstances may cause reverse power. Or as our cynical production manager puts it, "Design something to be absolutely idiot proof and Mother Nature will design better idiots."

Reverse protection today can be done easily with a low cost silicon diode, literally for pennies. On battery operated gear, the 0.7 Volt drop will lower efficiency. For instance, with a 9 Volt battery, the 0.7 Volt drop will reduce battery life about 8% since the internal switching power supply (pro gear) will draw 8% more current to compensate. However, more modern Schottky diodes have a 0.3 Volt drop, reducing battery life by 3%. If you are running off a bag battery with other gear, that loss will be even less in the aggregate. It is also possible to use a shunt diode across the power input with a autoreset polyfuse (or just a common fuse) in series before the shunt and not have any power loss at all. The shunt diode shorts out a reverse power connection and the polyfuse opens before the diode can overheat and let out the magic smoke. Narrowing the copper traces on the circuit board for a small length leading to the shunt diode can also provide protection against huge fault currents in the years before polyfuses became readily available. We've even used, prior to polyfuses, small wire jumpers that melt at high current levels. This is still cheaper to replace than an entire board (motor).

For higher current devices like the camera you mention, in the days of yore before cheap diodes were available, a polarized relay that would only close with the application of the correct polarity would work. This would handle even high currents with no voltage drop. A standard relay could have even been polarized with a small cheap diode in the relay coil circuit. All this to say, there is no valid reason IMHO that modern gear shouldn't be protected against reversed batteries or DC power. That salesman was basically saying that his company didn't want to spend the money to protect the camera and depended on the customer being perfect.

Best Regards,

Larry Fisher

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PLEASE can it be standard to have reverse polarity and  over-voltage protection  in location audio gear. Stuff happens. This is needlessly stressful for an end-user, for want of a 79 cent diode.

 

(end rant)

 

Peter

 

yes!

I zotzed a 7" LCD on my cart this way. Came home from a long day on set to my new monitor and tried to solder up a power feed to an XLR4... and got it backwards. Fortunately the people I bought it from repaired it for me, but it also meant another week of shooting without my new monitor. All in all, I still felt lucky. 

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If you have 2 Volts or more than the device needs from your DC power source, a silicon diode is your simplest and best bet. A 1N4003 is the size of a fat 1/4 Watt resistor, cost pennies (more at Radio Shack) and will handle up to 1 Amp continuous. In general, use a diode rated at two times or more than your continuous load. The diodes will handle a single surge at 10 times their rated continuous current so start up surges are not a problem.

Best,

Larry F

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PLEASE can it be standard to have reverse polarity and  over-voltage protection  in location audio gear. Stuff happens. This is needlessly stressful for an end-user, for want of a 79 cent diode.

 

(end rant)

 

Peter

 

 

+1 Peter, thanks!!!!!

 

While your at it Glen, how about an on/off switch on this piece of gear as well, I can't think of any other gear that I have used in the past 20 years that did not have one.   It would stop a lot of us from having to pull power out of the side of it just to shut it down.  

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