Re: L/C Meter Project


Roy <roy.dyball@...>
 

Hi Jim

Thank you for the prod. You were correct the first time.

When I checked the series current at the battery it was 1.6A. No wonder it was hot. I cut the trace from the regulator output on the board and measured the current with the load removed, with just the regulator in circuit. The regulator was only pulling 2MA and happily sitting on 4.98V. The L/C meter had to be pulling 1.599A. It hit me immediately what was going on and it was my fault.

When I built the kit there was 3 X 100k resistors missing. Also a mystery 1.5K resistor was left over. I assumed it was to go in the position for R1 because there was no resistor supplied for this, R1 showed on the circuit as 15K. This resistor goes to pin 15 of the LCD and controls contrast. I had compared other circuits on the net and saw that a pot had been used on pin 15 to control the contrast. It seemed plausible because a non standard value of resistance was specified that someone had just measured the pot and substituted a resistor to keep the cost down. So I just used the mystery 1.5K in its place and when the project worked so well and the contrast looked good I did not give it much of a second thought.

The first thing I did was to unplug the LCD display module and measure the current drawn by only the processor and oscillator, it was under 10MA.  The next thing was to disconnect the mystery resistor and reconnect the display. The current drawn from the battery now read 100MA and of course the display was blank. This wrong choice of resistor was responsible for chewing up 1.59A. I cobbled together a 6.8 and 1.5k in serries to make a 7.3k resistor as I did not have two 15k to put in parallel. The current still read 100MA and the display looked terrific and had good contrast.

I still was not happy because the regulator was slightly warm and the specs said it should only draw 10MA and my kit was still using ten times the current. The suppliers had noted that they had supplied a bonus back lit LCD. I found the series resistor that feeds the LED (R5) a 10 ohm resistor and immediately thought that the value was too low for a serries LED resistor at 5V. I disconnected it and the total current drawn by the fully working L/C meter was 10MA. I like the back light but I think I will switch it and experiment with a different value for R5 to reduce the current. The regulator is now running as cool as a cucumber and I am happy with the 160 times power reduction.

Roy.

 

 

 


--- In ultralightdx@..., "jim_kr1s" wrote:
>
>
> --- In ultralightdx@..., "Roy" roy.dyball@ wrote:
> >
>
> >
> >Maybe its oscillating, they are prone to this.
>
> Yep! I use either a 0.33-uF 35-V Tantalum or a plain 1-uF electrolytic
> mounted right at regulator inputs, and a good 0.1-uF right on the
> outputs. A 9-V battery can look like a high impedance on the input. But
> the circuit board should have all the necessary bypassing already.
>
>
> > 5V computer gear battery operated is not a good match. But you are
> > correct I should measure the current before and after the reg.
>
> Worth a look. I have a 78L08, low-power (100 mA) 8-V regulator sourcing
> about 60 mA in my regen and it's cool as can be. Same I/O differential
> as yours. I once made a regulated 6.2-Vdc filament supply for a BC-221
> freq meter, using the old TO-3 LM309K with a couple of diodes in the
> common lead. The regulator was on a fairly large heat sink but it still
> got hot.
>
> Regulators want to have a load connected when powered up, but if you put
> a 6-V bulb or something on the output you can try it by itself and see
> what's happening. Could be a floor-sweeping IC, too.
>
> Speaking of rafts of equipment, I was looking at my notebook last night.
> My signal generator and the HP selective voltmeter together cost me less
> than the PL-380 + shipping. KR1S shop:
> http://qrp.kearman.com/images/kr1slab0608.jpg
> Except for the DDS on
> top of the scope, a little SWR meter, and maybe the counter, nothing on
> the back row of the bench is less than 30 years old, but it does the
> job.
>
> 73,
>
> Jim, KR1S
> http://qrp.kearman.com/
>

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