Peak Reading Watt Meter

 

My homebrew 3-1000Z amp has a couple of loose ends to tie up before it will be ready to go on the air.  One issue is the "output" meter circuit.  Tom, W0IVJ, suggested I look at the circuit he used with an LT1014 quad op-amp chip that provides linear peak readings to a 1ma meter movement.  Ted, K9IMM, noted I might use an MCP6004 in the circuit which is a bit better on leakage current and runs at 5vdc.  After "dead bug" construction and testing it on an old Hunter Model 26 meter, I built another prototype on perf board and used a 1ma movement meter from the junk box.  The results were most gratifying.

By now it was obvious that this circuit could be used in my application within an amplifier, retrofitting older SWR meters and as a stand alone unit with an appropriate sensor device (and maybe audio applications?).  I made some additions to the original circuit and had PC boards produced.  Here's a picture of the completed board with the zener, 50 ohm input, 1N23E diode and DC power options.

Schematic Drawing

 

 

 

This is the Board layout.

Construction Notes

If you are mounting the board on a meter, first measure the distance between the meter terminals and divide the distance by two. Call this distance D.  Next, lay a straight edge between W6 and W7, and measure the distance D to each side of the board from the the trace that is half way between W6 and W7.  Start by drilling small holes and then use progressively larger bits to achieve the correct size for the meter terminals. 

Decide on what power source you will use.  If you use 6 to 14vac, wire the AC source to W1 and W3.  Install a 1 or 1.5a full wave bridge at D4 noting the polarity..  Make C3 10uf or larger.  If you use 9 to 18vdc source , wire the plus  to W1 and the common to W2.  Place a jumper between the indicated square solder hole and the one below it in D4. For reference, the square hole of D3 is connected to the plus side of C3, and the hole below the square hole is connected to W1.   In this case C3 can be small or even eliminated.

Decide what diode you will use for D2.  If you use a 1N34A, or equivalent, install a jumper between the wire points below D2 and D3.  If you use a physically larger diode such as a 1N23E, install it between those wire points and place a jumper where D2 is shown.

D3 is an optional 5.1v zener if you feel input protection is necessary.

R4 is an optional 50 ohm resistor if you desire to keep the RF feed at 50 ohms.

RF (input) is wired with the shield to W5 and center conductor to W4 using RG174 coax.

If the unit is mounted directly to the meter, diode meter protection and RF suppression on the meter terminals is probably not needed.  One can solder those components directly to the meter pads if needed.

It may be a good idea to spray the board with a protective lacquer after completion.

The last step is to adjust R3 for an appropriate value on your meter.  I will set mine so 1500 watts PEP is about 75% of the meter scale.  Other know values of power can be used to complete the meter scale.  The most obvious way to accomplish this is to use a reliable watt meter in series with this unit and a dummy load.  Transmit CW RF and note the value on the watt meter and the reading you get on this meter.  Noting the readings for 100, 500, 1000 and 1500 watts should establish sufficient points on this meter to be adequate.  After all, the readings are pretty much relative when we are tuning up with the exception of 1500 watts.  We want to know that value precisely. 

You should also be aware that like any other meter, the values are only accurate when measuring a matched 50 ohm circuit.

 

 

EST 3/26/08