Power Supply Question... ?

[Mikejo]

Power supply question:
I have a small Samlex SEC-1223 13.8v Power supply running my Yaesu FT 857D.

I decided to measure the output amperage using a multi meter in series with the positive lead from the power supply unit going out to the radio to see what my output amperage would be when the radio was transmitting on FM at the full 100 W it’s capable of.

What I noticed was, the output from the power supply was measured at approximately 4. 5 amps.
When I keyed down, I watched the cross needles on the SWR/power meter on my MFJ-941E and noticed I was getting the full 100 W output, with an SWR lower than 2.0.

Why was I only reading approximately 4.5 A, should it not have been closer to the 22 A the radio requires for output?

 

[WA0CBW]

What amp meter were you using?

 

[Mikejo]

Just using your standard multi meter with the negative probe plugged into the “com” Jack and the positive plugged into the “10 A” Jack set to measure amperage.

 

[WA0CBW]

Were you measuring with the meter "in series" in the positive lead to the radio?

 

[prcguy]

Its impossible to produce 100w of RF power with only 4.5A of current at 13.8V, your meter or measurement has some problems. I've tested several FT-857s in the past and at 100W in FM mode they should be around 18A. Most HF rigs will be under 20A even though their instructions say 22A, which is a safe number for worst case planning.

 

[wscranston]

Not a ham but...since the jack in the meter is marked "10A" I would assume that means 10 amps is the max it can handle? IOW maybe you're overloading the meter?

 

[prcguy]

And another thing, when measuring current with a hand held multimeter, do not use its supplied leads for measuring anything over a few hundred milliamps of current. The supplied leads have very small wire that can add up to a lot of resistance and that will throw off your current readings due to some power going up in heat inside the leads. For making current measurements up to the 20A level (if the meter is rated for that) use a short run of at least 10ga wire fitted with banana plugs rated for the current. Or use a dedicated current meter with low loss test cables.

 

[Mikejo]

If the leads were a problem with measuring something rated for “10 A“, why did I even have the jack on the multimeter in the first place if the wires on the multimeter can’t handle it?

 

[Mikejo]

To answer another question above, read my post it tells you how I connected the multimeter between the radio and the power supply unit.

 

[Mikejo]

And yes PRC guy, I figured I should be seeing something between 18 V and 22 V when transmitting through the microphone, without saying anything or whistling, and that’s why I brought this issue to the forum. I did try to use a different multi meter, and got roughly the same results that’s why am so puzzled?

 

[Mikejo]

Correction: “18 A” to “22 A”

(don’t beat me up, it was just a typo!)

 

[Mikejo]

Also, I’ll take your advice and make up a couple of 10 gauge wires with the banana plug on one end and alligator clips on the other end and give that a try! (If I below the cheap multi meter, no problem there)!

 

[prcguy]

The meter was designed and tested to measure whatever its rated for as a stand alone unit. The test leads are a 5 cent accessory and not necessarily intended to be used for all types of measurements like extremely low resistance measurements or high current measurements. That requires leads or cables rated for those measurements. Then you have to make measurements within the design range of the meter and trying to measure 18 or 20 amps on a meter designed and calibrated to 10 amps will produce errors or may damage the meter.

If the leads were a problem with measuring something rated for “10 A“, why did I even have the jack on the multimeter in the first place if the wires on the multimeter can’t handle it?

 

[mmckenna]

Just using your standard multi meter with the negative probe plugged into the “com” Jack and the positive plugged into the “10 A” Jack set to measure amperage.

Multimeters with amperage reading capability will have fuses on those ports. If it shows "10A" on it and you hook it up to something you expect to be pulling in the 20 amp range, you're going to eventually pop the fuse.
On the Fluke meters rated for 10 amps, it has an 11 amp fuse. Those fuses are a couple of bucks each.

An 11amp fuse isn't going to open the second you hit it with 11.01 amps, but hit it with 20 amps and it's going to go in a few seconds. Plus, your meter isn't going to read correctly if you exceed it's rated capacity.

 

[Mikejo]

Well, I can always set the radio to 40 W or something like that and expect to get something 10 A or under With bigger cables to carry the current. If I’m still getting the same readings, who cares I’m putting out 100 W! That’s good enough for me.

i’ll consider this thread closed.

 

[majoco]

The proper way to do it is with a current shunt - basically a low value resistor. At it's rated current value it will produce the stated voltage. This one appears to give 100millivolts for 20Amps.

 

[prcguy]

Yes! Everyone who intends on doing accurate low voltage medium to high current measurements should have a current shunt in their box of goodies. Shunts can be had new and cheap on that auction site.

The proper way to do it is with a current shunt - basically a low value resistor. At it's rated current value it will produce the stated voltage. This one appears to give 100millivolts for 20Amps.

View attachment 96509

 

[vagrant]

A DC clamp meter is a fun tool to have!