Voltage regulator/ reference

[George Herold]

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.

 

[George Herold]

On Friday, May 10, 2019 at 2:19:04 PM UTC-4, piglet wrote:

On 10/05/2019 18:56, George Herold wrote:
OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.



TL431?

Dirt cheap, available thru-hole, relatively robust, available worldwide.
Either supply via a resistor or 21mA current source. If the load current
is likely to fluctuate a lot during the experiment then I guess
self-heating induced TC drift could be a problem. A PNP shunt booster
might solve that.

piglet

Hi piglet. OK that was the other reference I found on Digikey.
I've hardly ever used a shunt reference (LM4040 a few times)
(oh and the LM399.. but that's a little over the top.)
It's certainly cheaper than the LT1460! The load should not be
changing by much. I'll add some to DK order,
Thanks

George H.

 

[John Larkin]

On Fri, 10 May 2019 10:56:00 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.

Most opamps are current and thermal limited.

The classic fix for c-load stability adds a resistor in series with
the opamp output, and a cap to ground, both of which further protect
the opamp.

They could always blow it up by connecting it to some weird voltage.
That would need more serious protections.

Data sheets often specify DO NOT EXPOSE TO STUDENTS.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Piglet]

On 10/05/2019 18:56, George Herold wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.

TL431?

Dirt cheap, available thru-hole, relatively robust, available worldwide.
Either supply via a resistor or 21mA current source. If the load current
is likely to fluctuate a lot during the experiment then I guess
self-heating induced TC drift could be a problem. A PNP shunt booster
might solve that.

piglet

 

[George Herold]

On Friday, May 10, 2019 at 2:32:45 PM UTC-4, John Larkin wrote:

On Fri, 10 May 2019 10:56:00 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.


Most opamps are current and thermal limited.

The classic fix for c-load stability adds a resistor in series with
the opamp output, and a cap to ground, both of which further protect
the opamp.

They could always blow it up by connecting it to some weird voltage.
That would need more serious protections.

Right. The c-loaded opamp worked fine. But there are all sorts of
potential (NPI) voltages available... what happens if the output of
an opamp (+/-15V rails say) gets connected to +30 or -30 V?

I was playing around with depletion fets and the DN2540 with ~100 ohm
of Gate- Source resistance makes an OK ~20 mA current limit*.
I could put that on the output of the opamp and feedback around it.
(But I'm not sure how well it would work.. and lotsa testing takes
time.)
I thought about protection diodes on the output to the power rails...
but then maybe the students blow up the power supply.. which would be
worse that frying an opamp.

Data sheets often specify DO NOT EXPOSE TO STUDENTS.
Grin, Well some of the professors are worse than the students.

George H.
*50 ohms-> Vgs= 1.5V, 33mA
100 ohms-> Vgs=1.7V, 17mA

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

On Friday, 10 May 2019 19:51:17 UTC+1, George Herold wrote:

On Friday, May 10, 2019 at 2:32:45 PM UTC-4, John Larkin wrote:
On Fri, 10 May 2019 10:56:00 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.


Most opamps are current and thermal limited.

The classic fix for c-load stability adds a resistor in series with
the opamp output, and a cap to ground, both of which further protect
the opamp.

They could always blow it up by connecting it to some weird voltage.
That would need more serious protections.
Right. The c-loaded opamp worked fine. But there are all sorts of
potential (NPI) voltages available... what happens if the output of
an opamp (+/-15V rails say) gets connected to +30 or -30 V?

I was playing around with depletion fets and the DN2540 with ~100 ohm
of Gate- Source resistance makes an OK ~20 mA current limit*.
I could put that on the output of the opamp and feedback around it.
(But I'm not sure how well it would work.. and lotsa testing takes
time.)
I thought about protection diodes on the output to the power rails...
but then maybe the students blow up the power supply.. which would be
worse that frying an opamp.

Data sheets often specify DO NOT EXPOSE TO STUDENTS.
Grin, Well some of the professors are worse than the students.


George H.
*50 ohms-> Vgs= 1.5V, 33mA
100 ohms-> Vgs=1.7V, 17mA

diodes, resistors. Zener on psu too.


NT

 

[John Larkin]

On Fri, 10 May 2019 11:51:12 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Friday, May 10, 2019 at 2:32:45 PM UTC-4, John Larkin wrote:
On Fri, 10 May 2019 10:56:00 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.


Most opamps are current and thermal limited.

The classic fix for c-load stability adds a resistor in series with
the opamp output, and a cap to ground, both of which further protect
the opamp.

They could always blow it up by connecting it to some weird voltage.
That would need more serious protections.
Right. The c-loaded opamp worked fine. But there are all sorts of
potential (NPI) voltages available... what happens if the output of
an opamp (+/-15V rails say) gets connected to +30 or -30 V?

I was playing around with depletion fets and the DN2540 with ~100 ohm
of Gate- Source resistance makes an OK ~20 mA current limit*.
I could put that on the output of the opamp and feedback around it.
(But I'm not sure how well it would work.. and lotsa testing takes
time.)

That depletion fet thing should work fine. I'd trust a Spice sim and
just go for it.

Do clamp the opamp output to the rails (or ground) so that 20 mA has
somewhere to go, especially with power off.

We use DN2530s, SOT-89s, good for 3 watts on a copper pour.

There are also some self-protecting bidirectional solid-state relays
that can be fun as current limiters, but at higher currents.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Friday, May 10, 2019 at 4:03:57 PM UTC-4, John Larkin wrote:

On Fri, 10 May 2019 11:51:12 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Friday, May 10, 2019 at 2:32:45 PM UTC-4, John Larkin wrote:
On Fri, 10 May 2019 10:56:00 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.


Most opamps are current and thermal limited.

The classic fix for c-load stability adds a resistor in series with
the opamp output, and a cap to ground, both of which further protect
the opamp.

They could always blow it up by connecting it to some weird voltage.
That would need more serious protections.
Right. The c-loaded opamp worked fine. But there are all sorts of
potential (NPI) voltages available... what happens if the output of
an opamp (+/-15V rails say) gets connected to +30 or -30 V?

I was playing around with depletion fets and the DN2540 with ~100 ohm
of Gate- Source resistance makes an OK ~20 mA current limit*.
I could put that on the output of the opamp and feedback around it.
(But I'm not sure how well it would work.. and lotsa testing takes
time.)

That depletion fet thing should work fine. I'd trust a Spice sim and
just go for it.

Do clamp the opamp output to the rails (or ground) so that 20 mA has
somewhere to go, especially with power off.

Oh, put the diode clamps between opamp output and current limit...
Then the most the power supply has to suck up is ~20 mA...
That's smart. I could have wimpier diodes then.

The DN2540 comes in TO-220 packs! I could probably handle a
couple of hundred volts, Oh boy, now I want to try bullet proofing
the opamp. I'll think about it.

The weekend is going to be filled with trying to fix the brakes on my
old tractor. It's my son's idea. We've got a month till
"Drive your tractor to school day" and I told him he could drive it if
we can get the brakes working... big disk brakes, I've never had them
apart.

How we have fun in the country. :^)
https://www.facebook.com/PioneerCentralHighSchool/videos/drive-your-tractor-to-school-day-2018/10155877084347400/

George h.

We use DN2530s, SOT-89s, good for 3 watts on a copper pour.

There are also some self-protecting bidirectional solid-state relays
that can be fun as current limiters, but at higher currents.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Friday, May 10, 2019 at 4:51:37 PM UTC-4, George Herold wrote:

On Friday, May 10, 2019 at 4:03:57 PM UTC-4, John Larkin wrote:
On Fri, 10 May 2019 11:51:12 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Friday, May 10, 2019 at 2:32:45 PM UTC-4, John Larkin wrote:
On Fri, 10 May 2019 10:56:00 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.


Most opamps are current and thermal limited.

The classic fix for c-load stability adds a resistor in series with
the opamp output, and a cap to ground, both of which further protect
the opamp.

They could always blow it up by connecting it to some weird voltage.
That would need more serious protections.
Right. The c-loaded opamp worked fine. But there are all sorts of
potential (NPI) voltages available... what happens if the output of
an opamp (+/-15V rails say) gets connected to +30 or -30 V?

I was playing around with depletion fets and the DN2540 with ~100 ohm
of Gate- Source resistance makes an OK ~20 mA current limit*.
I could put that on the output of the opamp and feedback around it.
(But I'm not sure how well it would work.. and lotsa testing takes
time.)

That depletion fet thing should work fine. I'd trust a Spice sim and
just go for it.

Do clamp the opamp output to the rails (or ground) so that 20 mA has
somewhere to go, especially with power off.

Re power off state: I've tested that some.. and near as I could figure
over-voltage (at the input) started to power on the circuit, but with a
current limit it's not a problem.

What's the Vgs voltage spread on the DN2530 like?
I've got ten DN2540's, I've tested 4 and about the same..
~+/- 100mV.. but factor of 2 higher than the spec sheet.
Are parts in batches/ on tape fairly similar? It would
be fine if I could measure some, and pick a resistor for the
reel.

George H.

Oh, put the diode clamps between opamp output and current limit...
Then the most the power supply has to suck up is ~20 mA...
That's smart. I could have wimpier diodes then.

The DN2540 comes in TO-220 packs! I could probably handle a
couple of hundred volts, Oh boy, now I want to try bullet proofing
the opamp. I'll think about it.

The weekend is going to be filled with trying to fix the brakes on my
old tractor. It's my son's idea. We've got a month till
"Drive your tractor to school day" and I told him he could drive it if
we can get the brakes working... big disk brakes, I've never had them
apart.

How we have fun in the country. :^)
https://www.facebook.com/PioneerCentralHighSchool/videos/drive-your-tractor-to-school-day-2018/10155877084347400/

George h.


We use DN2530s, SOT-89s, good for 3 watts on a copper pour.

There are also some self-protecting bidirectional solid-state relays
that can be fun as current limiters, but at higher currents.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Jon Elson]

George Herold wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements
could run over a day or two and the voltage needs to be stable over that
time.

If you really need a stable 5 V, use an LM4140-2.5 and a 2:1 amplifying
circuit with an AD706 op-amp. For 20 mA load, you may need a current
boosting transistor after the op-amp. For best results, the LM4140 should
be run off about 4 V, dropping the input with a resistor is fine. It also
needs a cap at input and output.

This is probably way overkill, this is what I did when we needed microvolt
stability, with absolute minimum 1/f noise.

Jon

 

[Phil Hobbs]

On 5/10/19 1:56 PM, George Herold wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.

The old standby for a 'reference-class regulator' is the LP2951.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Friday, May 10, 2019 at 10:29:54 PM UTC-4, Jon Elson wrote:

George Herold wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements
could run over a day or two and the voltage needs to be stable over that
time.
If you really need a stable 5 V, use an LM4140-2.5 and a 2:1 amplifying
circuit with an AD706 op-amp. For 20 mA load, you may need a current
boosting transistor after the op-amp. For best results, the LM4140 should
be run off about 4 V, dropping the input with a resistor is fine. It also
needs a cap at input and output.

This is probably way overkill, this is what I did when we needed microvolt
stability, with absolute minimum 1/f noise.

Jon

Thanks Jon, I'll try and remember that if I need it.

George h.

 

[George Herold]

On Friday, May 10, 2019 at 4:51:37 PM UTC-4, George Herold wrote:

On Friday, May 10, 2019 at 4:03:57 PM UTC-4, John Larkin wrote:
On Fri, 10 May 2019 11:51:12 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Friday, May 10, 2019 at 2:32:45 PM UTC-4, John Larkin wrote:
On Fri, 10 May 2019 10:56:00 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.


Most opamps are current and thermal limited.

The classic fix for c-load stability adds a resistor in series with
the opamp output, and a cap to ground, both of which further protect
the opamp.

They could always blow it up by connecting it to some weird voltage.
That would need more serious protections.
Right. The c-loaded opamp worked fine. But there are all sorts of
potential (NPI) voltages available... what happens if the output of
an opamp (+/-15V rails say) gets connected to +30 or -30 V?

I was playing around with depletion fets and the DN2540 with ~100 ohm
of Gate- Source resistance makes an OK ~20 mA current limit*.
I could put that on the output of the opamp and feedback around it.
(But I'm not sure how well it would work.. and lotsa testing takes
time.)

That depletion fet thing should work fine. I'd trust a Spice sim and
just go for it.

Do clamp the opamp output to the rails (or ground) so that 20 mA has
somewhere to go, especially with power off.
Oh, put the diode clamps between opamp output and current limit...
Then the most the power supply has to suck up is ~20 mA...
That's smart. I could have wimpier diodes then.

The DN2540 comes in TO-220 packs! I could probably handle a
couple of hundred volts, Oh boy, now I want to try bullet proofing
the opamp. I'll think about it.

Beer check!
So here's my 'bullet proof opamp.
https://www.dropbox.com/s/n5nxwxbez81ruat/Bullet-proof%20opamp.JPG?dl=0

The current limit drops a little less than 2 Volts... Hence the ~8V
zener.

Do you see any blunders? I tested this at ground and +/-40 V volts on the
output. Everyone seemed happy.

George H.

snip tractor video
George h.


We use DN2530s, SOT-89s, good for 3 watts on a copper pour.

There are also some self-protecting bidirectional solid-state relays
that can be fun as current limiters, but at higher currents.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Friday, May 10, 2019 at 10:57:19 PM UTC-4, Phil Hobbs wrote:

On 5/10/19 1:56 PM, George Herold wrote:
OK this is an easy problem, and I'm mostly just trolling
for maybe another idea/ IC.
So I need a 5 V bias supply. Current ~20 mA (A little less depending
on how I load it.) The voltage doesn't need to be particularly accurate,
but it does need to be stable. Some of the experiments/ measurements could
run over a day or two and the voltage needs to be stable over that time.
How stable you ask? I'm not sure, I could do a few measurements.. but I first
built the supply from a three terminal regulator (lm317l) And there was some
drift that might have been the regulator.
So my second idea was to use a voltage reference (already used in the circuit
elsewhere) and buffer that with an opamp. (See my C-loaded opamp thread
of a week or two ago.) This seems fine, but a secondary concern with the
circuit is that it's going to be hooked up by students and I worry about some
one hooking it up wrong and cooking the opamp. So I've been noodling about
different protection circuits.. but I don't know if they'll work.. they would
need testing.

The 'problem' with the opamp is that it's a surface mount and if it needs
to be replaced that is harder for some tech to do locally.

So I was searching digikey today for a 5 V reference in through hole
with I supply >20 mA. And found the LT1460
https://www.analog.com/media/en/technical-documentation/data-sheets/1460fc.pdf

It comes in a TO-92 that can be easily replaced.

Thoughts?
Thanks for reading
George H.



The old standby for a 'reference-class regulator' is the LP2951.

Got it, Thanks Phil.
GH

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

On Monday, 13 May 2019 20:36:39 UTC+1, George Herold wrote:

So here's my 'bullet proof opamp.
https://www.dropbox.com/s/n5nxwxbez81ruat/Bullet-proof%20opamp.JPG?dl=0

The current limit drops a little less than 2 Volts... Hence the ~8V
zener.

Do you see any blunders? I tested this at ground and +/-40 V volts on the
output. Everyone seemed happy.

George H.

no diodes for inductorproofing. Little in the way of input protection. Any power supply protection? i_limit is an unknown. It might do what you need but it's not bulletproof.


NT

 

[George Herold]

On Monday, May 13, 2019 at 4:08:52 PM UTC-4, tabb...@gmail.com wrote:

On Monday, 13 May 2019 20:36:39 UTC+1, George Herold wrote:

So here's my 'bullet proof opamp.
https://www.dropbox.com/s/n5nxwxbez81ruat/Bullet-proof%20opamp.JPG?dl=0

The current limit drops a little less than 2 Volts... Hence the ~8V
zener.

Do you see any blunders? I tested this at ground and +/-40 V volts on the
output. Everyone seemed happy.

George H.

no diodes for inductorproofing. Little in the way of input protection. Any power supply protection? i_limit is an unknown. It might do what you need but it's not bulletproof.


NT

Hmm inductor proofing... I hadn't thought about that. (does that mean
I owe you a beer?) At 5 V and ~20 mA max out I think it will be OK.

Yeah the input is set by me.. so needs no protection.
There is the issue of picking G-S resistors for the dn2540's.
The ten I have here all have about the same curve... but?

George H.

 

On Monday, 13 May 2019 21:17:12 UTC+1, George Herold wrote:

On Monday, May 13, 2019 at 4:08:52 PM UTC-4, tabby wrote:
On Monday, 13 May 2019 20:36:39 UTC+1, George Herold wrote:

So here's my 'bullet proof opamp.
https://www.dropbox.com/s/n5nxwxbez81ruat/Bullet-proof%20opamp.JPG?dl=0

The current limit drops a little less than 2 Volts... Hence the ~8V
zener.

Do you see any blunders? I tested this at ground and +/-40 V volts on the
output. Everyone seemed happy.

George H.

no diodes for inductorproofing. Little in the way of input protection. Any power supply protection? i_limit is an unknown. It might do what you need but it's not bulletproof.


NT

Hmm inductor proofing... I hadn't thought about that. (does that mean
I owe you a beer?) At 5 V and ~20 mA max out I think it will be OK.

if kids are handling the output they'll do their best to destroy it. They'll connect everything including as many batteries as they can find. 2 diodes is all it costs for inductorproofing, plus series R & Z for batteryproofing.

Yeah the input is set by me.. so needs no protection.
There is the issue of picking G-S resistors for the dn2540's.
The ten I have here all have about the same curve... but?

George H.

depends how many you're making, and thus how you're assembling them. Cuttable wire links are popular in consumer equipment. If that's too imprecise or large, parallel pads could enable slicing the original R's track and adding a new one after construction.

Enjoy the beer.


NT

 

[Phil Hobbs]

On 5/13/19 4:17 PM, George Herold wrote:

On Monday, May 13, 2019 at 4:08:52 PM UTC-4, tabb...@gmail.com wrote:
On Monday, 13 May 2019 20:36:39 UTC+1, George Herold wrote:

So here's my 'bullet proof opamp.
https://www.dropbox.com/s/n5nxwxbez81ruat/Bullet-proof%20opamp.JPG?dl=0

The current limit drops a little less than 2 Volts... Hence the ~8V
zener.

Do you see any blunders? I tested this at ground and +/-40 V volts on the
output. Everyone seemed happy.

George H.

no diodes for inductorproofing. Little in the way of input protection. Any power supply protection? i_limit is an unknown. It might do what you need but it's not bulletproof.


NT

Hmm inductor proofing... I hadn't thought about that. (does that mean
I owe you a beer?) At 5 V and ~20 mA max out I think it will be OK.

Given that creative fellow who blew out your 78xx current source by
connecting a Helmholtz coil to it, I suspect you need better notes.

Cheers

Phil Hobbs
(Who has similar war stories to tell--we should meet at the Dayton Hamfest)

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Monday, May 13, 2019 at 6:32:30 PM UTC-4, Phil Hobbs wrote:

On 5/13/19 4:17 PM, George Herold wrote:
On Monday, May 13, 2019 at 4:08:52 PM UTC-4, tabb...@gmail.com wrote:
On Monday, 13 May 2019 20:36:39 UTC+1, George Herold wrote:

So here's my 'bullet proof opamp.
https://www.dropbox.com/s/n5nxwxbez81ruat/Bullet-proof%20opamp.JPG?dl=0

The current limit drops a little less than 2 Volts... Hence the ~8V
zener.

Do you see any blunders? I tested this at ground and +/-40 V volts on the
output. Everyone seemed happy.

George H.

no diodes for inductorproofing. Little in the way of input protection. Any power supply protection? i_limit is an unknown. It might do what you need but it's not bulletproof.


NT

Hmm inductor proofing... I hadn't thought about that. (does that mean
I owe you a beer?) At 5 V and ~20 mA max out I think it will be OK.

Given that creative fellow who blew out your 78xx current source by
connecting a Helmholtz coil to it, I suspect you need better notes.

Better notes...? OK my notes stink mostly. I keep notes, but they are
useless to anyone but me.
Right. inductors, I'm a slow learner and sometimes it takes me
more than once to learn something. :^)

This supply will have something like a (RC) 100 ohm x 0.1uF tc.
Inductor-wise I think the 100 ohm in the current limit will help
save me.
(Kids/students blew up this 'bullet proof' transistor
which JT (RIP) said was the same silicon as the LM317, but different
metalization. I can't recall the part number... We still use it
a darlington with current and thermal overload protection.
With a 1N400x across the output, that's a bullet-proof output
too. (so far :^)

Cheers

Phil Hobbs
(Who has similar war stories to tell--we should meet at the Dayton Hamfest)

Dayton Hamfest? Ohio? Soon?
Maybe Sphero, or others could have a meet-up?

George H.

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Klaus Kragelund]

Put a diode at the input of the supply, so it’s inpossible to hook it up wrong

Active diode if you cannot live with a diode drop

Cheers

Klaus