#2 . . . "Protection from Improper Power Supply Connections"

by Joe Everhart, N2CX

Here's another circuit "quickie" from my grab bag. It's for homebrewers and QRP'ers who suffer from temporary lapses of sense when they hurriedly hook up a battery or power supply without double checking polarity. The laws of probability say that you have a 50/50 chance of getting it right, but our friend Murphy tips the scales so that 90% of the time the polarity is wrong. The result seems to be usually that you end up frying something. And if the rig has unusually expensive or hard-to-get parts, they are just what goes up in smoke!

There is a simple fix for this malady -- the dummy diode. That's not a new type of semiconductor device form Motorola, but an ordinary diode to protect dummies from themselves. (One ham friend of mine has a much more anatomically descriptive term for it.)

Figure 1 shows this device in use. It's merely a garden variety rectifier diode hooked up with reverse polarity across the DC power input to your rig. If the DC power is hooked up as it should be, the diode is reversed biased and will draw only microamps of leakage current. But if you accidentally reverse the positive and negative power leads, the diode conducts and acts as a short circuit to ground. I highly recommend adding the in-line fuse shown in the figure. If the fuse is in place, the short circuit will blow this fuse, protecting the rig and telling you that you goofed. Without the fuse, several outcomes are possible:

1) If the DC source is a current limited power supply, the supply will just reduce its output voltage low enough to supply the rated short circuit current. The rig will see a reverse polarity voltage of only about 0.7 to 1 volt (one diode drop). It will probably survive.

2) If the DC source is a non-current-limited supply to a high current battery, a large short circuit will be drawn. Again, there are several possible sub-outcomes:

a) If the current is much above the diode's forward current rating, it will eventually overheat and fail. When it fails it will probably become a open circuit. When this happens (within a fraction of a second) there is no longer any reverse polarity protection and your rig will fry. Therefore the diode did you no good in the first place.

b) If the current is less than the diode's rating, the power wiring and/or source will go up in smoke. Again, the diode did you no good.

c) If you are extremely lucky, the diode, the power source and wiring will not instantaneously self-destruct and you will notice that something is over-heating. You may have enough time to remove power and correct the reversed polarity, but don't count on it!

The bottom line is, use a series fuse. It should be large enough to pass the highest current you would normally draw, but small enough to blow if there is a short circuit or reverse polarity condition.

What kind of diode should you use? well, that depends. For low power transmitters and such, I usually use a 1N4000 series diode. The 1N4001 will withstand a back voltage of 50 volts so the usual 12 or 24 volt DC supply will be unaffected. If your rig normally draws less than an amp, use a 1 amp series fuse for protection. For higher power rigs, both diode and fuse should be upgraded accordingly.

For even better protection, you can use a transient suppressor diode. It hooks up the same way as a regular diode, but acts like a high power zener. Figure 2 shows its use. The diode shown is specially designed to to have low leakage for a DC reverse voltage of up to 15 volts or so, as you would see in 12 volt applications even including auto battery voltage. Above 18 volts, it conducts heavily to protect electronic equipment from overvoltage surges. And of course it acts like a regular diode if the power is backwards, to provide reverse polarity protection. The series fuse is still needed to provide a "safety valve" if overvoltage or reversed power make the protection diode conduct.

I highly recommend the transient suppressor diode if you can get one. The part number I have used successfully is a General Semiconductor industries 1.5KE18A. Motorola also manufactures the same device under the part number 1N6277. I can't guarantee a source for the diode, but I know that some branches of Active Electronics used to carry the GSI part. Their head office is at 133 Flanders Road, Westborough, MA 01581. They have a toll-free telephone number, 1 (800) 677-8899.

Whichever scheme you use, I think you should consider adding a protective diode and fuse to your DC-powered QRP gear. This little "quickie" can save you big trouble and a lot of bucks if you should slip up and swap positive and negative power leads. Just think of it as a prophylactic measure to help you practice safe QRPing!

PS: Why don't I just use a series diode? Well, I could but don't for two reasons. First, I don't like to lose the 0.7 volt diode drop. When I'm using batteries, I want every last bit of efficiency I can. And secondly, using a series diode adds a nonlinear impedance to the power supply. That can make either a receiver or transmitter oscillate unpredictably. Only a dummy would use a series diode!

You can contact Joe at n2cx@voicenet.com or send mail to: Joe Everhart, 214 New Jersey Road, Brooklawn, NJ 08030

Last Modified: Nov 30, 1997