From: QS@nospam.quantsuff.com
"IB" wrote in message news:wo%Vk.8212$hv.5308@newsfe13.ams2...
>
> "geronimo" wrote in message
> news:6l6bi4lurf1lvj9m54i0udmh0hf3062uro@4ax.com...
>> 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
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
