7805 regulators are regulating at 7 volts?

[geronimo]

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".

 

[PeterD]

On Thu, 20 Nov 2008 11:27:27 -0600, geronimo <Jamesw@grandecom.net>
wrote:

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".

You do realize your DPM requires an isolated power source, right?

Show us a schematic of what you are doing, or have done.

 

[IB]

"geronimo" <Jamesw@grandecom.net> 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.

 

[Quantsuff]

"IB" <ib@ib.com> wrote in message news:wo%Vk.8212$hv.5308@newsfe13.ams2...

"geronimo" <Jamesw@grandecom.net> 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

 

[Brian Gregory [UK]]

"PeterD" <peter2@hipson.net> wrote in message
news:6cfbi4tnrtm5bc5aa0fpprvr5d0l9pjhvo@4ax.com...

You do realize your DPM requires an isolated power source, right?

Show us a schematic of what you are doing, or have done.

If you are trying to use the 7805 to power the meter at the same time as
using the meter to check the output voltage of the 7805 you're likely to
have problems, and maybe even to cause damage to the meter.

Is that what you mean?

--

Brian Gregory. (In the UK)
ng@bgdsv.co.uk
To email me remove the letter vee.

 

[geronimo]

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"
<QS@nospam.quantsuff.com> wrote:

"IB" <ib@ib.com> wrote in message news:wo%Vk.8212$hv.5308@newsfe13.ams2...

"geronimo" <Jamesw@grandecom.net> 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

 

[John Fields]

On Thu, 27 Nov 2008 06:30:45 -0600, geronimo <Jamesw@grandecom.net>
wrote:

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.

1. Please don't top post.

2. If you want to post binaries to USENET, use an alt.binaries.*
newsgroup, like alt.binaries.schematics.electronic. If you don't,
chances are your post won't make it.

Alternatively, do schematic capture with LTspice:

http://www.linear.com/designtools/software/ltspice.jsp

and post the .asc file.

JF

 

[geoff]

geronimo wrote:

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.

Possibly a 'fake' device.

geoff

 

[Brian Gregory [UK]]

"geoff" <geoff@nospam-paf.co.nz> wrote in message
news:BuCdnb3MquEiya7UnZ2dnUVZ_tTinZ2d@giganews.com...

geronimo wrote:
Hello all, it was just a bad batch of 7805s!
Possibly a 'fake' device.

I've a vague idea that I remember a 7905 accidentally fitted instead of a
7805 did something very similar in something I was working on.

--

Brian Gregory. (In the UK)
ng@bgdsv.co.uk
To email me remove the letter vee.

 

[Jasen Betts]

On 2008-12-02, Brian Gregory [UK] <ng@bgdsv.co.uk> wrote:

"geoff" <geoff@nospam-paf.co.nz> wrote in message
news:BuCdnb3MquEiya7UnZ2dnUVZ_tTinZ2d@giganews.com...
geronimo wrote:
Hello all, it was just a bad batch of 7805s!
Possibly a 'fake' device.

I've a vague idea that I remember a 7905 accidentally fitted instead of a
7805 did something very similar in something I was working on.

hmm, yeah, given a 12V supply and no load you'd get that.

 

[Wolfgang Rompel]

"Jasen Betts" <jasen@xnet.co.nz> schrieb im Newsbeitrag
news:gh5j7d$8e3$2@reversiblemaps.ath.cx...

<ng@bgdsv.co.uk> wrote:
"geoff" <geoff@nospam-paf.co.nz> wrote in message
news:BuCdnb3MquEiya7UnZ2dnUVZ_tTinZ2d@giganews.com...
geronimo wrote:
Hello all, it was just a bad batch of 7805s!
Possibly a 'fake' device.

I've a vague idea that I remember a 7905 accidentally fitted instead of a
7805 did something very similar in something I was working on.

hmm, yeah, given a 12V supply and no load you'd get that.

Looks like operating the 7805 as pulse with modulator / switching regulator.
Some digital voltmeters show unreal values, when the DC inputs are driven by
pulses. Oscillation usually happens, when the input and output capacitors
are missed.
Place at least 1ľF/16Volts electrolytic cap's from input terminal to GND
Pin and also from output terminal to GND Pin. Place the capacitors as close
as possible to regulators case. Please report the voltage reading.
w-ro