Looking for a 10mA constant current source.

[Dave]

Hi,
I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to
drop 20V or so across a resistor and constant current source. The input
voltage will be pretty stable (28 +/- 0.2V), so it does not need to work
over a wide range.

I was thinking about an LM334 with the temperature compensation diode
added. The only problem is 10mA is on its limit, and might cause
significant internal heating, so it will be difficult to keep the
temperature compensation diode at the same temperature as the regulator
IC. Why the hell NI did not build the diode into the package I do not know.

The 28V input is actually switched, the frequency of which I do not
know. But I don't think it will be too high, and so the rise/fall time
of the constant current source should not be an issue.

Any better suggestions?


HP / Agilent's 346B noise source

http://www.home.agilent.com/USeng/nav/-536888365.536880068/pd.html

is what I am trying to copy, but a look at the manual (page 36 of 81)

http://www.home.agilent.com/cgi-bin/pub/agilent/reuse/cp_ObservationLogRedirector.jsp?NAV_ID=-536888365.536880068.03&LANGUAGE_CODE=eng&CONTENT_KEY=1000002290-1%3aepsg%3aman&COUNTRY_CODE=US&LINKTYPE=LitStation&CONTENT_TYPE=AGILENT_EDITORIAL

shows a degree of complexity I can't really understand. They use the 28V
to make an oscillator, regulate that and use that to drive the constant
current source. I can't understand the logic for just not using the 28V.

 

[Jamie]

"Dave" <nospam@nowhere.com> wrote in message news:42dfdb96@212.67.96.135...

Hi,
I'm looking for a constant current source that must

a) Be floating.

Most are

b) Provide 10mA - exact value is not too critical.

tol of 10%?

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

15 to 30 is a very small range, is this part of 10% or in addition too?

Add a balanced bridge or switch to a constant voltage source and use
bipolars to add drive current.

Any parts count limitations?

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to
drop 20V or so across a resistor and constant current source. The input
voltage will be pretty stable (28 +/- 0.2V), so it does not need to work
over a wide range.

I was thinking about an LM334 with the temperature compensation diode
added. The only problem is 10mA is on its limit, and might cause
significant internal heating, so it will be difficult to keep the
temperature compensation diode at the same temperature as the regulator
IC. Why the hell NI did not build the diode into the package I do not
know.

The 28V input is actually switched, the frequency of which I do not
know. But I don't think it will be too high, and so the rise/fall time
of the constant current source should not be an issue.

Any better suggestions?


HP / Agilent's 346B noise source

http://www.home.agilent.com/USeng/nav/-536888365.536880068/pd.html

is what I am trying to copy, but a look at the manual (page 36 of 81)


http://www.home.agilent.com/cgi-bin/pub/agilent/reuse/cp_ObservationLogRedirector.jsp?NAV_ID=-536888365.536880068.03&LANGUAGE_CODE=eng&CONTENT_KEY=1000002290-1%3aepsg%3aman&COUNTRY_CODE=US&LINKTYPE=LitStation&CONTENT_TYPE=AGILENT_EDITORIAL

shows a degree of complexity I can't really understand. They use the 28V
to make an oscillator, regulate that and use that to drive the constant
current source. I can't understand the logic for just not using the 28V.

 

[Dave]

Jamie wrote:

"Dave" <nospam@nowhere.com> wrote in message news:42dfdb96@212.67.96.135...

Hi,
I'm looking for a constant current source that must

a) Be floating.


Most are

Thinking about it, there is no need for it to be floating.

b) Provide 10mA - exact value is not too critical.


tol of 10%?

Yes, as long as it stays stable. Absolute number is not critcal, but
stability is.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.


15 to 30 is a very small range, is this part of 10% or in addition too?

The exact current is not critical. It will affect the operation of the
diode and change the noise output, but the noise output will be calibrated.

What I don't want is the current changing once the unit has been
calibrated, so the current must remain fixed with temperature.

Add a balanced bridge or switch to a constant voltage source and use
bipolars to add drive current.

I'm not sure what you mean here. I don't have a constant voltage source
- only +28V pulses, but I can use a regulator internally.

This will be connected to an HP 8970A noise figure meter, which puts
out 28 +/- 0.1 V according to the spec. Knowing HP, it will be a lot
better than +/- 0.1 V, but I assumed +/- 0.15V as there will be drops
across the cable/connectors. Not much I agree at 10mA over a couple of
metres.



I guess I should find out how much current I can take from that 28V
supply. I need to about 10mA for the diode, but whether the noise meter
can source 20 mA, 50 mA or 100mA I do not know.

Any parts count limitations?

I was hoping for somthing I can stick on a bit of stripboard no bigger
than about 50 x 70 mm. Would rather avoiding having to make a PCB, but
it would not be the end of the world. Hence componenets only avialable
in SM would not be an ideal choice.

 

[Jon]

Dave,

Many semiconductor houses, for example Microsemi, have a line of
constant current diodes. They are essentially depletion mode FETs that
have the Drain and Gate connected together. You can construct one
yourself. Pick a FET that has an Idss close to the value of current
that you need. You will need to do some selection.
Regards,
Kral

 

[Bob Monsen]

Dave wrote:

Jamie wrote:

"Dave" <nospam@nowhere.com> wrote in message
news:42dfdb96@212.67.96.135...

Hi,
I'm looking for a constant current source that must

a) Be floating.

If you don't need a floating source, you could perhaps use a TL431. Use
a 1% 249 ohm resistor from ref to anode, ground the anode, and connect
your diode between ref and cathode. Then, use a resistor from Vcc to the
cathode to get the current into the ballpark + 10%. You can read the
voltage across the diode at the cathode.

The TL431 is temperature compensated.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877

 

[John Larkin]

On Thu, 21 Jul 2005 18:29:58 +0100, Dave <nospam@nowhere.com> wrote:

Hi,
I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to
drop 20V or so across a resistor and constant current source. The input
voltage will be pretty stable (28 +/- 0.2V), so it does not need to work
over a wide range.

I was thinking about an LM334 with the temperature compensation diode
added. The only problem is 10mA is on its limit, and might cause
significant internal heating, so it will be difficult to keep the
temperature compensation diode at the same temperature as the regulator
IC. Why the hell NI did not build the diode into the package I do not know.

The 28V input is actually switched, the frequency of which I do not
know. But I don't think it will be too high, and so the rise/fall time
of the constant current source should not be an issue.

Any better suggestions?


HP / Agilent's 346B noise source

http://www.home.agilent.com/USeng/nav/-536888365.536880068/pd.html

is what I am trying to copy, but a look at the manual (page 36 of 81)

http://www.home.agilent.com/cgi-bin/pub/agilent/reuse/cp_ObservationLogRedirector.jsp?NAV_ID=-536888365.536880068.03&LANGUAGE_CODE=eng&CONTENT_KEY=1000002290-1%3aepsg%3aman&COUNTRY_CODE=US&LINKTYPE=LitStation&CONTENT_TYPE=AGILENT_EDITORIAL

shows a degree of complexity I can't really understand. They use the 28V
to make an oscillator, regulate that and use that to drive the constant
current source. I can't understand the logic for just not using the 28V.

Sounds like all you need is a single resistor. Add an SSR in series if
you want to chop it not too fast.

John

 

[Dave]

John Larkin wrote:

On Thu, 21 Jul 2005 18:29:58 +0100, Dave <nospam@nowhere.com> wrote:


Hi,
I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to

Sounds like all you need is a single resistor. Add an SSR in series if
you want to chop it not too fast.

John

A resistor is certianly not a bad idea, but I don't think it is accurate
enough. HP certinaly do not use a simple resistor in their noise sources.

There are reasons for this.

1) Diode noise output changes with current - both in terms of amount and
the frequency spectrum.

2) Diode voltage changes with temperature.

3) The output voltage of the noise meter is speced as 28 +/- 0.1 V.

4) A 10% change in current will typically cause a 0.5dB change in Excess
noise ratio (ENR). But the ENR will need to be measured to much greater
accuracy than that.

 

[Dave]

Jon wrote:

Dave,

Many semiconductor houses, for example Microsemi, have a line of
constant current diodes. They are essentially depletion mode FETs that
have the Drain and Gate connected together. You can construct one
yourself. Pick a FET that has an Idss close to the value of current
that you need. You will need to do some selection.
Regards,
Kral

I just looked at Microsemi and they have one at 10mA. But there are no

specs for tempearature stability.

 

[Dave]

Bob Monsen wrote:

Dave wrote:

Jamie wrote:

"Dave" <nospam@nowhere.com> wrote in message
news:42dfdb96@212.67.96.135...

Hi,
I'm looking for a constant current source that must

a) Be floating.




If you don't need a floating source, you could perhaps use a TL431. Use
a 1% 249 ohm resistor from ref to anode, ground the anode, and connect
your diode between ref and cathode. Then, use a resistor from Vcc to the
cathode to get the current into the ballpark + 10%. You can read the
voltage across the diode at the cathode.

The TL431 is temperature compensated.

I'm not sure if you would say I need it floating or not now. First I
thought yes, then no. But I think I was right the first time.

But I don't think the TL431 can be used.

I need to ground (0V) the anode of my noise diode. Anything else can be
grounded too if need be. The cathode must be connected to the 28V supply
via the current control element.

Would you call that floating or not? I think it does need to be floating.

The data sheet on the TL431 does show a precision current sink, but that
would require that I lift the anode lead of my noise diode off of the 0V
line. That is not possible

I'm now thinking about perhaps using the TL431 or similar to regulate
the 28V +/- 0.1 V to say 24.000 V, then use a resistor in parallel with
a thermistor.

Pehaps this is why HP use an oscillator and rectify that. At first I
thought it was a bit odd, but I must admit I can't think of anthiing
that will be accurate.

I did wonder about measuring the temperature coefficient of the diode,
which should be linear over the small range of interest (only around
room temperature). Then using a series resitors, with a thermistor
fitted somewhere such that the current is kept constant at the two
temperature extreams (15 and 30 deg C). Hopefully it would not very too
much in between, although I would have to model it to be sure. A bit
messy though.

 

[Jim Thompson]

On Thu, 21 Jul 2005 23:19:50 +0100, Dave <nospam@nowhere.com> wrote:

Bob Monsen wrote:
Dave wrote:

Jamie wrote:

"Dave" <nospam@nowhere.com> wrote in message
news:42dfdb96@212.67.96.135...

Hi,
I'm looking for a constant current source that must

a) Be floating.




If you don't need a floating source, you could perhaps use a TL431. Use
a 1% 249 ohm resistor from ref to anode, ground the anode, and connect
your diode between ref and cathode. Then, use a resistor from Vcc to the
cathode to get the current into the ballpark + 10%. You can read the
voltage across the diode at the cathode.

The TL431 is temperature compensated.


I'm not sure if you would say I need it floating or not now. First I
thought yes, then no. But I think I was right the first time.

But I don't think the TL431 can be used.

I need to ground (0V) the anode of my noise diode. Anything else can be
grounded too if need be. The cathode must be connected to the 28V supply
via the current control element.

Would you call that floating or not? I think it does need to be floating.

[snip]

Is current FORWARD or REVERSE thru the noise diode? Is 28V supply
+28V or -28V?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Fred Bloggs]

I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to
drop 20V or so across a resistor and constant current source. The input
voltage will be pretty stable (28 +/- 0.2V), so it does not need to work
over a wide range.

I was thinking about an LM334 with the temperature compensation diode
added. The only problem is 10mA is on its limit, and might cause
significant internal heating, so it will be difficult to keep the
temperature compensation diode at the same temperature as the regulator
IC. Why the hell NI did not build the diode into the package I do not know.

The 28V input is actually switched, the frequency of which I do not
know. But I don't think it will be too high, and so the rise/fall time
of the constant current source should not be an issue.

Any better suggestions?

You can make a good current reference out of the LM185. The typical
performance on temperature stability is 30ppm calculated over the entire
operating range, but these parts are optimized for nearly zero tempco
in the vicinity of 25oC, and should be very much less over 15o-30o like
3ppm average or less.
View in a fixed-width font such as Courier.

28V>--|>|-------+--------------+
| |
LM185 | [120]
_|_/ |
// \ ------------+
--- |
| e
| |/
| --|
| | |\
| e c
| |/ |
+-------| |
| |\ |
| c |
[270K] | |
| ----+
| |
| [R] R~120 ohm
| |
| |
| |
| -
| ^ ND
| |
+------+-------+
|
gnd

 

[John Larkin]

On Thu, 21 Jul 2005 22:07:46 +0100, Dave <nospam@nowhere.com> wrote:

John Larkin wrote:
On Thu, 21 Jul 2005 18:29:58 +0100, Dave <nospam@nowhere.com> wrote:


Hi,
I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to

Sounds like all you need is a single resistor. Add an SSR in series if
you want to chop it not too fast.

John


A resistor is certianly not a bad idea, but I don't think it is accurate
enough. HP certinaly do not use a simple resistor in their noise sources.

There are reasons for this.

1) Diode noise output changes with current - both in terms of amount and
the frequency spectrum.

2) Diode voltage changes with temperature.

3) The output voltage of the noise meter is speced as 28 +/- 0.1 V.

4) A 10% change in current will typically cause a 0.5dB change in Excess
noise ratio (ENR). But the ENR will need to be measured to much greater
accuracy than that.

If the 28 volts is stable to 0.1 volt, and the zener changes by less
than 0.1 volt over temp (probably less in real life) that's +-0.2
volts out of 28, or +-0.7 per cent max.

You've got to calibrate the zener's enr anyhow.

John

 

[RST Engineering (jw)]

A resistor is certianly not a bad idea, but I don't think it is accurate
enough.

Why would you think this? Remember the cardinal engineering law - KISS.


HP certinaly do not use a simple resistor in their noise sources.

And your point is?

There are reasons for this.

1) Diode noise output changes with current - both in terms of amount and
the frequency spectrum.

Yes.

2) Diode voltage changes with temperature.

Over your given temperature range it won't change squat.

3) The output voltage of the noise meter is speced as 28 +/- 0.1 V.

And this means???

4) A 10% change in current will typically cause a 0.5dB change in Excess
noise ratio (ENR). But the ENR will need to be measured to much greater
accuracy than that.

How you would get 1% error is beyond me, but if you want to futz around with
exotic solutions, have at it.

Jim
>

 

[Tony Williams]

In article <42E02BCC.10000@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:

28V>--|>|-------+--------------+
| |
LM185 | [120]
_|_/ |
// \ ------------+
--- |
| e
| |/
| --|
| | |\
| e c
| |/ |
+-------| |
| |\ |
| c |
[270K] | |
| ----+
| |
| [R] R~120 ohm
| |
| |
| |
| -
| ^ ND
| |
+------+-------+
|
gnd

Hello Fred. What does the 'R~120 ohm' do?

Thanks.

--
Tony Williams.

 

[Fred Bloggs]

Tony Williams wrote:

In article <42E02BCC.10000@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:


28V>--|>|-------+--------------+
| |
LM185 | [120]
_|_/ |
// \ ------------+
--- |
| e
| |/
| --|
| | |\
| e c
| |/ |
+-------| |
| |\ |
| c |
[270K] | |
| ----+
| |
| [R] R~120 ohm
| |
| |
| |
| -
| ^ ND
| |
+------+-------+
|
gnd


Hello Fred. What does the 'R~120 ohm' do?

Thanks.

It's a feel good:
1) off-loads some power dissipation from the xstrs;
2) additional limit against turn-on transients (28V is switched);
3) HF isolation of regulator reactance from ND RF ckt.

 

Dave wrote:

Fred Bloggs wrote:

You can make a good current reference out of the LM185. The typical
performance on temperature stability is 30ppm calculated over the entire
operating range, but these parts are optimized for nearly zero tempco
in the vicinity of 25oC, and should be very much less over 15o-30o like
3ppm average or less.
View in a fixed-width font such as Courier.

28V>--|>|-------+--------------+
| |
LM185 | [120]
_|_/ |
// \ ------------+
--- |
| e
| |/
| --|
| | |\
| e c
| |/ |
+-------| |
| |\ |
| c |
[270K] | |
| ----+
| |
| [R] R~120 ohm
| |
| |
| |
| -
| ^ ND
| |
+------+-------+
|
gnd


I think you have the device wrong there - the LM185 is a
2 terminal device and you seem to be showing 3. I assume
the transistors are both PNP.

The LM185, unlike the LM185-xx parts, is a 3-terminal,
adjustable voltage device. The LM185-xx parts are fixed-voltage,
2-terminal versions of the same part. For your temperature range,
the LM385 commercial version suffices.

One might consider increasing Fred's feel-good resistor to take
more of the 28V supply dissipation--120R only drops 1.2V.

Lastly, have you measured your noise diode's output power
vs. drive current? You might only need the resistor, and
that would be simpler now, wouldn't it?

Cheers,
James

 

[Tony Williams]

In article <42E0DA88.2000903@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:

Hello Fred. What does the 'R~120 ohm' do?

It's a feel good:
1) off-loads some power dissipation from the xstrs;
2) additional limit against turn-on transients (28V is switched);
3) HF isolation of regulator reactance from ND RF ckt.

Cheers.

--
Tony Williams.

 

[Fred Bloggs]

dagmargoodboat@yahoo.com wrote:

For your temperature range, the LM385 commercial version suffices.

Right-

One might consider increasing Fred's feel-good resistor to take more
of the 28V supply dissipation--120R only drops 1.2V.

Good- make that 1.2k

Lastly, have you measured your noise diode's output power vs. drive
current? You might only need the resistor, and that would be simpler
now, wouldn't it?

HP has a detector in there somewhere- the 5.6KHz is used to
chop/amplify a noise sample and apply feedback to the DC supply.

 

[budgie]

On Thu, 21 Jul 2005 18:29:58 +0100, Dave <nospam@nowhere.com> wrote:

Hi,
I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to
drop 20V or so across a resistor and constant current source. The input
voltage will be pretty stable (28 +/- 0.2V), so it does not need to work
over a wide range.

I was thinking about an LM334 with the temperature compensation diode
added. The only problem is 10mA is on its limit, and might cause
significant internal heating, so it will be difficult to keep the
temperature compensation diode at the same temperature as the regulator
IC. Why the hell NI did not build the diode into the package I do not know.

The 28V input is actually switched, the frequency of which I do not
know. But I don't think it will be too high, and so the rise/fall time
of the constant current source should not be an issue.

Any better suggestions?

I'd be inclined to investigate the LM317 family. Your temp range isn't going to
challenge a modest tempco. 10mA is a doddle for even the 317LZ (TO-92) although
to limit self-heating I'd contemplate the TO-220 pack.

 

[ehsjr]

Dave wrote:

Hi,
I'm looking for a constant current source that must

a) Be floating.

b) Provide 10mA - exact value is not too critical.

c) Have as low a temperature coefficient as possible. Operating range
15-30 deg C.

It is to be used to provide current to an avalanche noise diode. The
input voltage will be 28V, the diode will drop about 8 V, so I need to
drop 20V or so across a resistor and constant current source. The input
voltage will be pretty stable (28 +/- 0.2V), so it does not need to work
over a wide range.

I was thinking about an LM334 with the temperature compensation diode
added. The only problem is 10mA is on its limit, and might cause
significant internal heating, so it will be difficult to keep the
temperature compensation diode at the same temperature as the regulator
IC.

So put two LM334/resistor/diode 5 mA circuits in parallel.

Ed



Why the hell NI did not build the diode into the package I do not know.

The 28V input is actually switched, the frequency of which I do not
know. But I don't think it will be too high, and so the rise/fall time
of the constant current source should not be an issue.

Any better suggestions?


HP / Agilent's 346B noise source

http://www.home.agilent.com/USeng/nav/-536888365.536880068/pd.html

is what I am trying to copy, but a look at the manual (page 36 of 81)

http://www.home.agilent.com/cgi-bin/pub/agilent/reuse/cp_ObservationLogRedirector.jsp?NAV_ID=-536888365.536880068.03&LANGUAGE_CODE=eng&CONTENT_KEY=1000002290-1%3aepsg%3aman&COUNTRY_CODE=US&LINKTYPE=LitStation&CONTENT_TYPE=AGILENT_EDITORIAL


shows a degree of complexity I can't really understand. They use the 28V
to make an oscillator, regulate that and use that to drive the constant
current source. I can't understand the logic for just not using the 28V.