Hello all, it was just a bad batch of 7805s! Now I have the DPM
powered with good 5V DC, but the DPM still doesn't work right. See new
thread on this I started below. Thanks for the advice.


On Sat, 22 Nov 2008 18:47:10 -0500, "Quantsuff"
wrote:


"IB" wrote in message ...

"geronimo" wrote in message
...
I ordered eight TO-220 7805ABC regulators from Mouser
electronics. The 0-20 V digital LED panel meter I got didn't have the
needed regulator power supply, so I had to build it. The DPM is for
monitoring a 12 volt battery, and so the input sensing wire to the DPM
also just connects to the same 12v battery voltage that the 5 volt
supply circuit does. Well, I built up a 5V supply on a tiny perf
board, and when I powered up the DPM it would flash random normal
voltages, from around 2 to 12 volts for a brief second and then show
'the overrange indication. So I checked the output of the 7805
regulator, and it's exactly 7 volts, not five. No wonder the DPM isn't
working right. So I tried the other 7805 ICs and get the same exact 7
volts output. How can this possibly be? Its a high end fluke DMM, I am
confident the voltage is not off! I don't think there is even a 7807
regulator, is there? They are indeed marked "7805ABC".


The most likely explanations (in order of probability):

1. You are not pulling the minimum current - try loading it with a 500R.

2. Your layout is causing the 7805 to run instable. Have you looked at the
output with a scope? Do you have decoupling caps close to it? (.1uF)

3. Your DVM is incorrect. Have you given it a sanity check (perhaps by
measuring a new 1.5V battery)?

4. You have a bad batch of 7805's. Try some from a different source.



Also, some DPM's need an isolated supply: try the 7805 with a separate
supply. Or test that the DPM read zero when you leave the input lines
shorted and disconnected from the 12v.

qs