Power Supply for Active Mini Whip Antenna?

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ultravista

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My mini whip antenna requires a 12-15V 100-150ma power supply.

Around the house, I have many power supplies that are labeled as 12-15V and 100-150ma but when measured, are nowhere near the printed output. All are over voltage and current. Not one was true to the label. Not one was under voltage or current.

For example, one that is labeled 9V 500ma - it measures 13v 2.5a, it is 2000ma higher than the label ...

An Archer Universal Adapter that goes from 3-12V in 6 steps is actually 3 (5.2V), 4.5 (6.9V), 6 (8.8V), 7.5 (10.6V), 9 (12.6V) and 12 (16V).

With that said, where can I find a true 12V 100mA power supply
 

slicerwizard

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My mini whip antenna requires a 12-15V 100-150ma power supply.

Around the house, I have many power supplies that are labeled as 12-15V and 100-150ma but when measured, are nowhere near the printed output. All are over voltage and current. Not one was true to the label. Not one was under voltage or current.
Yes, welcome to the world of unregulated power supplies. Most equipment is designed to tolerate them.


For example, one that is labeled 9V 500ma - it measures 13v 2.5a, it is 2000ma higher than the label ...
You did not pull 32 watts out of that adapter. Measuring adapters with no load or into a dead short is meaningless. And a good way to destroy them.


An Archer Universal Adapter that goes from 3-12V in 6 steps is actually 3 (5.2V), 4.5 (6.9V), 6 (8.8V), 7.5 (10.6V), 9 (12.6V) and 12 (16V).
So connect it to your antenna and use the setting that provides 12-15 volts under load. The 12V setting sounds like it'll be fine, but work your way up from the low end.


With that said, where can I find a true 12V 100mA power supply
I doubt you need one (your antenna DC power specs even say so), but if you're really set on having one, look for a regulated 12V supply. Lots of them out there.
 

ultravista

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slicerwizard, what about the current?

The power feed unit for this active antenna requires 12-15V and 100maH. When I connected one of the power supplies rated a 9V 500maH, I smelled smoke. Smoke poured out of the power feed unit when I opened the case. The inductor and diode between the + of the power and + of the antenna lead were damaged.



These are a 680 uH inductor and 1N4148 diode. I am not an electronics person, please bear with me.

The power supply caused the damage, but I do not know why. Can you explain? I assume either over voltage or current ... what can the 680 uH inductor and 1N4148 diode tolerate?

One last thing please.

I found an RCA labeled power supply, 9V 100maH. When tested with the multi meter it was > 9V and somewhere near 500maH. How does one measure the output to not overload and damage the device?
 

DJ11DLN

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Current output is dependent on device demand. A 500 mA power supply will not force-feed a device that entire half-amp if the device only draws 100, or 50, or 10 mA. If it worked that way, every time you turned on your car stereo, which is powered by a battery able to supply 700+ amps, supplemented by an alternator able to output 100+ additional amps, it would go up in smoke (or the fuse would blow, one would hope). It doesn't, because the radio only draws what it needs to operate.

I can't help with your blown diode and capacitor; many things could have caused that. There could have been a fault in the device. Or reversed polarity could have done it. Or as you conjecture, over-voltage. Not over-amperage, as I tried (successfully, I hope) to explain above, it does not work that way. But any good regulated power supply that has a continuous rating that meets or exceeds the device requirements and displays the proper voltage under load will work just fine.
 

ultravista

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This is the power supply that caused the damage.


Here are three other power supplies that might be acceptable.




Please excuse my ignorance but what which is most important, the voltage or current? Meaning, will the device under perform if <100mA using these adapters?
 

M105

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Make sure the voltage is in range. The current doesn't matter as long as it is greater than or equal to 100ma. A 12V linear regulated supply would be my preferred power source but the 9V unregulated supply you tried should work ok.
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I am curious as to how you measured the current on these supplies. Do you own a variable load resistor?
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I don't know what caused the smoke but if your schematic is correct my guess would be a short in the antenna unit or cable going to it. The drawing shows a fuse not a diode. If you had a 0.2 A fuse there it would have just popped it and not smoked the RF choke.
 
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ultravista

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Make sure the voltage is in range. The current doesn't matter as long as it is greater than or equal to 100ma. A 12V regulated supply would be my preferred power source.
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I am curious as to how you measured the current on these supplies. Do you own a variable load resistor?
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I don't know what caused the smoke but if your schematic is correct my guess would be a short in the antenna unit or cable going to it. The drawing shows a fuse not a diode...
I measured with a multi meter attached to the power supply plug - nothing else.

The maker of the power feed unit says these two components are a 680 uH inductor and 1N4148 diode.

Here's a photo of the two. There appears to be burning on the bottom of the inductor.


The 9V 500ma power supply was attached to the power feed unit without the antenna. I attached the multi meter to the antenna side of the power feed unit. Immediately after applying the meter to the power feed unit, I began smelling smoke.

Would this mean that resting, or no meter, the power feed unit when energized was OK but putting the meter to the output caused it to blow?

What role does the current play here?
 

jackj

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You measure voltage by connecting the meter across the power supply. You measure current by connecting the meter in series with the power supply and load so that the current the load draws from the power supply passes through the meter. If you put the current meter across the power supply then you have placed a short across the power supply. That is what over heated the choke and took out the diode.
 

ultravista

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jackj - thank you. The choke, is that the inductor?

Can you explain where current comes into play here? Meaning, what is the 100mA for?

Also, in the above three power supplies, are the 'regulated' - in other words, how do you tell regulated vs. otherwise?
 
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M105

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I measured with a multi meter attached to the power supply plug - nothing else.

The maker of the power feed unit says these two components are a 680 uH inductor and 1N4148 diode.

Here's a photo of the two. There appears to be burning on the bottom of the inductor.


The 9V 500ma power supply was attached to the power feed unit without the antenna. I attached the multi meter to the antenna side of the power feed unit. Immediately after applying the meter to the power feed unit, I began smelling smoke.

Would this mean that resting, or no meter, the power feed unit when energized was OK but putting the meter to the output caused it to blow?

What role does the current play here?
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Current is measured by placing the meter (set to current scale) in series with a load. It is not possible to measure it without a proper load. Placing a meter in current mode across the power supply as you did represents a near dead short to the power supply and a risk of popping the fuse inside your meter. Current or ma in this case is the quantity of electrons, voltage is the pressure of electrons. As long as you have enough quantity available at the rated pressure you can power the circuit. In this case a 1000 amp supply at 12-15 volts works just as well as a 100ma supply at 12-15V. The circuit draws no more quantity than it needs so as long as 100ma is available you are good.
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In your photo the 680 MH choke appears to have burned. That probably shorted the windings which would render it useless. You need to replace it and the diode. It is what isolates your antenna signal from the power line and prevents RF noise from entering the circuit.
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Check that antenna connector the diode connects to for a short. Also check the capacitor that connects to it and goes to the transformer for a short. Both should be open (near infinite resistance). Please note! If you connected a meter in current mode across that antenna connector while the power supply was connected then YOU created the short. NEVER use your meter in current mode unless you understand what you are doing. That is always done in series with the power supply and load...NEVER across the line!
 
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ultravista

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M105 - thank you for the detailed information.

If I understand, regardless if the power brick is 100 or 1000ma, the circuit will only draw what it needs? How does it do that?

In this situation, the importance is the voltage, not the current?

What is the best way to in-series test the output of the power supply? Can you provide a simple way?
 

prcguy

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I agree that using the meter in current mode across the antenna inputs probably roached the components. I also see a problem that the diode should not be on the antenna jack side of the circuit. The diode is there to prevent an accidental reverse connection of power to the antenna but a diode in this configuration can also rectify large signals and cause lots of intermodulation problems. The diode should be on the power jack side feeding through the inductor which would connect right to the antenna jack.
prcguy
 

M105

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M105 - thank you for the detailed information.

If I understand, regardless if the power brick is 100 or 1000ma, the circuit will only draw what it needs? How does it do that?

In this situation, the importance is the voltage, not the current?

What is the best way to in-series test the output of the power supply? Can you provide a simple way?
Think of it as a water hose. If you have a sprinkler that needs 1 gallon per minute at 60psi that is is your load. If you feed it with a small hose capable of delivering 1 gal/minute at 60 psi the sprinkler works fine. If you feed it with a large hose capable of delivering 100 gal/minute but also pressurized to 60 psi the sprinkler also works fine. It only uses 1 gal/min at that pressure even though the hose can supply much more. The key here is the pressure forcing the water through the sprinkler not the amount of water the hose is capable of supplying. With electricity voltage is equivalent to pressure and amperage is equivalent to quantity. As long as a power supply can supply the minimum amount or greater current required by the load at the required voltage you are good.
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To measure current:
You simply place the meter in series with the supply and load. Positive lead of the power supply to red lead on meter. Black lead of meter to positive lead of load. Negative lead of load to negative lead of power supply. The current then flows through the meter to the load which allows the meter to measure the quantity of electrons (amps) flowing through it. The meter itself in current mode has a very low resistance of around one ohm or less and actually measures the voltage drop across that shunt resistance. It then uses ohm's law to calculate the current. So when you place a meter in current mode directly across a power supply you are essentially shorting the power supply out. When measuring current use a meter setting that is higher than the current you expect to see. The meter has an internal fuse that will blow if you pass more current through it than it is rated for.
 

jackj

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Current vs Voltage

jackj - thank you. The choke, is that the inductor?

Can you explain where current comes into play here? Meaning, what is the 100mA for?

Also, in the above three power supplies, are the 'regulated' - in other words, how do you tell regulated vs. otherwise?
As others have said, voltage is the equivalent of water pressure while current is the equivalent of water flow. Current is what does the work, voltage is how much force is behind the current. A given load will draw the amount of current dictated by the load's resistance. Resistance, in the case of DC circuits, is measured in Ohms and will be constant. The higher the voltage applied to the load, the more current it will draw.

There are two basic types of power supplies, switching and analog. A switching power supply will be smaller and lighter than the same capacity analog supply. Switching power supplies are always regulated while analog supplies may or may not be. Unless the label of an analog supply says it is regulated, it will be hard to tell by looking. The rating on the label of an unregulated supply will give the voltage under it's specified load but that voltage will vary under various loads. It will be about 1.4 times it's rated voltage under no load so if you plug in the power supply and measure it's voltage without a load, you can tell if it is regulated. A regulated 12 volt power supply will measure 12 volts while an unregulated one will measure close to 17 volts.
 

ultravista

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A regulated 12 volt power supply will measure 12 volts while an unregulated one will measure close to 17 volts.
Is it reasonable to then check the voltage output of a power supply to validate whether or not it is over/under the labeled rating?

If the max voltage is 15V, and it receives 17V from a power supply labeled as 12V, then it may potentially fail due to over voltage?
 

KC4RAF

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No to your last question. The reading you are getting from the power supply at 17 volts is not under any type of load. Once you place the supply to the device, it should show the proper voltage.
All wall warts will show higher voltage than what is on the label when the supply is NOT plugged into the device.
I use a wart for my HTs; the label says 10 volts @ .5a, but checking the voltage when not connected to the HTs, the voltage shows about 14v. BUT when plugged into the HTs, the meter shows ~10v.
Once I connected the wart through my meter in series with the HT and read about .33 amps ( 33 miliamps).
As others have posted, follow what they are saying.
 
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KC4RAF

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To your first question; I have well over 50 wall warts from other devices that finally gave up their life or usefulness. I will use my volt meter to see if they are giving a voltage, but I don't check for accuracy of the voltage, ( like it says 12 volts @ 50 milliamps ).
If power supply shows no voltage, then it has an open somewhere in it.
As long as it shows any where between 4 and 7 volts over, then I know its good. ( That is just a rough statement about 4 and 7. It could be a little lower or a little higher. )
Hope I've not confused you any more. Stuff like this can be very confusing to a person that's new to this.
 

prcguy

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In addition to what others have mentioned, you do not want to use a switching supply on an active preamp antenna. Switching supplies produce a lot of RFI and when used to power an active antenna will usually put some of that noise right into the active antenna circuits. Find an appropriate old fashioned analog wall wart to power your antenna.
prcguy
 

M105

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In addition to what others have mentioned, you do not want to use a switching supply on an active preamp antenna. Switching supplies produce a lot of RFI and when used to power an active antenna will usually put some of that noise right into the active antenna circuits. Find an appropriate old fashioned analog wall wart to power your antenna.
prcguy
I'll second that suggestion. Avoid switching supplies around RF applications. There are some good switching supplies but most are cheap Chinese versions with very little filtering or shielding. They actually radiate RF noise in the local area. I recently scoured my junk box and came up with a huge conventional 5V regulated wall-wart to power the USB hub my SDR dongles are plugged into. The little switching supply that came with the hub was an RF noise machine!
 

jackj

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What prcguy says about switching power supply noise if correct....but

Some well designed switching supplies are as quiet as a church mouse. It takes a lot of filtering and isolation to produce a quiet DC voltage out of a switching supply. That makes the good ones expensive, a lot more than the couple of bucks the eBay bargains cost.
 
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