ESR Project Build - Part 6 of 6

This build is not done until the unit is assembled in its cabinet. I thought this last step would be a slam dunk. But I was terribly mistaken!

The W2AEW design called for a 5 volt regulator. Since the parts used didn't require 5V specifically, I tested it using a 6.5V switching supply that I had handy. After all the wiring was done, I merrily built this 7805 circuit on a tiny proto PCB assuming all would be well. Of course, once it was all nicely soldered into place the unit didn't function!  I could not get the meter to deflect with the leads shorted.  Wah, wah, wha... epic fail!

I soon discovered that if I raised the voltage, things returned to normal. So I tested it on a variable supply and found that it returned to normal over 6V. Being too lazy to rework the PCB much, I simply put a pair of diodes in the ground circuit to raise the output to 6.2V (Figure 1). If I was less lazy, I would have run a filter cap over the diodes also, but found that I could get away without.

Figure 1. 7805 regulator circuit used (for 6.2V)

My BNC test connector cannot be grounded, so I found a piece of saved orange crate plywood, painted it black and drilled a hole for it. I eventually got the unit pretty much done without the external case but ran into another problem (more about this later).

Figure 2 shows the internals of the unit -- large enough for an electrical outlet for the wall wart which delivers about 10 VDC without a load. 10V incidentally, is too much voltage for low ESR readings. So a regulator was needed.

Figure 2. Internals of the ESR unit. The PCB hanging down has the 7805 regulator.

The problem was the Fine/Coarse switch that I added to the front panel. When that switch was on Fine, a 47 ohm resistor was placed in parallel with the 10 ohm, to reduce the resistance ever so slightly from 10 ohms to 10||47=8.25 ohms. This provides wider spaced low resistance readings on the meter.

The problem turned out to be that the old switch I had used had an on resistance of several ohms! This drove me batty until I finally took the meter and read the switch resistance. So even with the 47 ohm resistor switched in, it ended up having no effect. After rummaging through the junk box again and measuring the on resistance this time, the issue was resolved with a better switch.

Figure 3 shows the unit re-assembled in the Sencore transistor tester case.

Figure 3. Assembled ESR meter. Zero pot at left, BNC for test leads in middle, power switch at upper right and Coarse/Fine switch below the power switch. BNCs at bottom are NC.

I suppose the lessons of the day includes:

• Don't assume the last part of the build will be trivial
• Don't assume switches will all measure 0 ohms, especially old ones!
• Test with the intended supply voltage up front.

Thanks for reading.