On a sunny day (Fri, 7 Jan 2022 21:05:50 -0000 (UTC)) it happened "Arnie Dwyer
(ex Jan Frank)" <spamme@not.com> wrote in
<XnsAE18A2F78FF27idtokenpost@144.76.35.252>:
>Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>[...]
>
>> One common approach is to do resistive or capacitive current limiting
>> inside a feedback loop. The two fight each other, and the point where
>> the feedback loop loses is the current limit.
>
>Thanks, but sounds complicated. All I need is protection against accidental
>transient short circuits. The old ua723 Vbe limiter seems to be the optimum
>solution.
>
>I don't know of any other current limiter that can handle 30mA at 330V, at a
>price I can afford:)
?
No idea about this project but I used BU??? HV 400V power MOSFETs in the eighties
to do just that, on a big heatsink.
Reply by Arnie Dwyer (ex Jan Frank)●January 7, 20222022-01-07
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
[...]
> One common approach is to do resistive or capacitive current limiting
> inside a feedback loop. The two fight each other, and the point where
> the feedback loop loses is the current limit.
Thanks, but sounds complicated. All I need is protection against accidental
transient short circuits. The old ua723 Vbe limiter seems to be the optimum
solution.
I don't know of any other current limiter that can handle 30mA at 330V, at a
price I can afford:)
> Cheers
>
> Phil Hobbs
>
Reply by Arnie Dwyer (ex Jan Frank)●January 7, 20222022-01-07
John Larkin <jlarkin@highland_atwork_technology.com> wrote:
> On Fri, 7 Jan 2022 12:40:07 -0000 (UTC), "Arnie Dwyer (ex Jan Frank)"
> <spamme@not.com> wrote:
[...]
>>This causes a serious droop in the +HV supply. As the constant current
>>demand increases, the +HV drops significantly. This makes it impossible
>>to reach the desired 20 mA maximum with a large load impedance.
>
> Sure. It depends on what the current limit is for. If it's to prevent
> explosions if the hv supply gets grounded, there's probably a value
> that works.
It would still increase the impedance of the +HV supply and degrade the
voltage regulation under load. I need the current source to supply as high
a voltage as is reasonable in order to drive current into a high
resistance load at max current. As it is, the voltage drops from 330V to
286V at 20mA, which is acceptable.
This limits the maximum load resistance to 286/20e-3=14,300 ohms at max
current. This is acceptable, since the area of the electrodes will be
fairly large in order to achieve a low current density. The cell will be
the largest I have ever built, so there is some uncertainty in the
results.
During the electrolysis process, the conductance of the cell will increase
by a factor of 10 or 20 as silver ions enter the solution. The current is
limited to restrict the combination of silver and hydroxide ions in the
Nernst Diffusion layer to produce silver hydroxide, AgOh. This is
undesirable, since it limits the concentration of silver ions to low
values, such as 4-5 ppm. It takes care to reach 18 ppm, which is about the
best you can do at home.
The current supply will be connected to an external electrolysis cell, so
the leads will be exposed. The current limit is to protect the supply in
the event the positive lead accidentally strikes ground during the
connection.
Normally the supply will be turned off and the capacitors discharged while
it is being connected, but there could be some other accident that shorts
the supply. For example, a DVM could be connected to monitor the voltage
across the load to check for saturation. The leads could be connected
together for an instant and short out the supply.
Also, the 1N4007 rectifier diodes have a peak surge current of 30A, but
I'd rather not deal with that kind of current even for a half-cycle. The
power supply switch is a miniature DPDT and I'm certain the contacts would
not handle it. So the old uA723 style Vbe current limiter seems the best
choice. It is only there for transients, so I don't need to go into
voltage foldback and all its complications.
Thanks for your help.
Reply by John Larkin●January 7, 20222022-01-07
On Fri, 7 Jan 2022 12:40:07 -0000 (UTC), "Arnie Dwyer (ex Jan Frank)"
<spamme@not.com> wrote:
>"Arnie Dwyer (ex Jan Frank)" <spamme@not.com> wrote:
>
>> jlarkin@highlandsniptechnology.com wrote:
>>
>> [...]
>>
>>>>> If you size C4 right, it can be the current limiter. Caps don't get
>>>>> hot!
>>>>
>>>>You need the current to be adjustable. The 8N60 mosfets are insulated
>>>>and mounted on a heatsink. For any load within the operating range, the
>>>>current is adjusted for maximum voltage across the load. This minimizes
>>>>the heat generated in the mosfets and minimizes the cooling needed.
>>>>
>>>>The upper mosfet is only for brief shorts that may occur.
>>>
>>> That's the one I meant. C4 could be picked to limit the current
>>> available from the voltage doubler.
>>
>> Brilliant! Thanks
>
>Well, it kind of works. Dropping C4 from 22uF to 1uF did indeed drop the
>short-circuit current to approximately 30 mA, the same as before. However,
>it greatly increases the supply impedance.
>
>This causes a serious droop in the +HV supply. As the constant current
>demand increases, the +HV drops significantly. This makes it impossible to
>reach the desired 20 mA maximum with a large load impedance.
Sure. It depends on what the current limit is for. If it's to prevent
explosions if the hv supply gets grounded, there's probably a value
that works.
--
If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon
Reply by Phil Hobbs●January 7, 20222022-01-07
Arnie Dwyer (ex Jan Frank) wrote:
> "Arnie Dwyer (ex Jan Frank)" <spamme@not.com> wrote:
>
>> jlarkin@highlandsniptechnology.com wrote:
>>
>> [...]
>>
>>>>> If you size C4 right, it can be the current limiter. Caps don't get
>>>>> hot!
>>>>
>>>> You need the current to be adjustable. The 8N60 mosfets are insulated
>>>> and mounted on a heatsink. For any load within the operating range, the
>>>> current is adjusted for maximum voltage across the load. This minimizes
>>>> the heat generated in the mosfets and minimizes the cooling needed.
>>>>
>>>> The upper mosfet is only for brief shorts that may occur.
>>>
>>> That's the one I meant. C4 could be picked to limit the current
>>> available from the voltage doubler.
>>
>> Brilliant! Thanks
>
> Well, it kind of works. Dropping C4 from 22uF to 1uF did indeed drop the
> short-circuit current to approximately 30 mA, the same as before. However,
> it greatly increases the supply impedance.
>
> This causes a serious droop in the +HV supply. As the constant current
> demand increases, the +HV drops significantly. This makes it impossible to
> reach the desired 20 mA maximum with a large load impedance.
>
> The other problem is the ripple on the +HV supply increases dramatically.
> The TL431 would probably clean it up, but it is a bit disappointing.
>
> The original circuit using the Vbe of a 2N3904 is quite a bit more accurate
> and holds the +HV constant until the current limit is reached. Back to the
> original version.
>
> Thanks for the help! A great idea and probably useful in some applications.
>
>
>
One common approach is to do resistive or capacitive current limiting
inside a feedback loop. The two fight each other, and the point where
the feedback loop loses is the current limit.
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.nethttp://hobbs-eo.com
Reply by Arnie Dwyer (ex Jan Frank)●January 7, 20222022-01-07
"Arnie Dwyer (ex Jan Frank)" <spamme@not.com> wrote:
[...]
> The other problem is the ripple on the +HV supply increases
> dramatically. The TL431 would probably clean it up, but it is a bit
> disappointing.
Oops. There used to be a TL431 in the current source but it was replaced with
a LM358. I will have to correct the title.
Designing electronics is HARD:)
Reply by Arnie Dwyer (ex Jan Frank)●January 7, 20222022-01-07
"Arnie Dwyer (ex Jan Frank)" <spamme@not.com> wrote:
> jlarkin@highlandsniptechnology.com wrote:
>
> [...]
>
>>>> If you size C4 right, it can be the current limiter. Caps don't get
>>>> hot!
>>>
>>>You need the current to be adjustable. The 8N60 mosfets are insulated
>>>and mounted on a heatsink. For any load within the operating range, the
>>>current is adjusted for maximum voltage across the load. This minimizes
>>>the heat generated in the mosfets and minimizes the cooling needed.
>>>
>>>The upper mosfet is only for brief shorts that may occur.
>>
>> That's the one I meant. C4 could be picked to limit the current
>> available from the voltage doubler.
>
> Brilliant! Thanks
Well, it kind of works. Dropping C4 from 22uF to 1uF did indeed drop the
short-circuit current to approximately 30 mA, the same as before. However,
it greatly increases the supply impedance.
This causes a serious droop in the +HV supply. As the constant current
demand increases, the +HV drops significantly. This makes it impossible to
reach the desired 20 mA maximum with a large load impedance.
The other problem is the ripple on the +HV supply increases dramatically.
The TL431 would probably clean it up, but it is a bit disappointing.
The original circuit using the Vbe of a 2N3904 is quite a bit more accurate
and holds the +HV constant until the current limit is reached. Back to the
original version.
Thanks for the help! A great idea and probably useful in some applications.
Reply by Arnie Dwyer (ex Jan Frank)●January 7, 20222022-01-07
jlarkin@highlandsniptechnology.com wrote:
[...]
>>> If you size C4 right, it can be the current limiter. Caps don't get
>>> hot!
>>
>>You need the current to be adjustable. The 8N60 mosfets are insulated
>>and mounted on a heatsink. For any load within the operating range, the
>>current is adjusted for maximum voltage across the load. This minimizes
>>the heat generated in the mosfets and minimizes the cooling needed.
>>
>>The upper mosfet is only for brief shorts that may occur.
>
> That's the one I meant. C4 could be picked to limit the current
> available from the voltage doubler.
Brilliant! Thanks
Reply by ●January 7, 20222022-01-07
On Fri, 7 Jan 2022 04:06:15 -0000 (UTC), "Arnie Dwyer (ex Jan Frank)"
<spamme@not.com> wrote:
>jlarkin@highlandsniptechnology.com wrote:
>
>> On Fri, 7 Jan 2022 00:33:11 -0000 (UTC), "Arnie Dwyer (ex Jan Frank)"
>> <spamme@not.com> wrote:
>>
>>>Here are the documents for a short-circuit protected constant current
>>>supply for making silver ions and other uses. The current can be
>>>adjusted from 100uA to 20mA. The compliance range is 320V. The polarity
>>>swap is on a separate pcb not shown here.
>>>
>>>The zip contains the LTspice ASC and PLT files, and the Diptrace
>>>schematic and pcb files.
>>>
>>>The pcb is hand wired for the moment, so the nets are shown in different
>>>colors according to their function.
>>>
>>>https://drive.google.com/file/d/1KzJKoNSXNhtRJ7mhsGU4WRb2KdW76GVz/view?
>>>usp=sharing
>>>
>>>https://tinyurl.com/yskm9hzp
>>
>> If you size C4 right, it can be the current limiter. Caps don't get
>> hot!
>
>You need the current to be adjustable. The 8N60 mosfets are insulated and
>mounted on a heatsink. For any load within the operating range, the current
>is adjusted for maximum voltage across the load. This minimizes the heat
>generated in the mosfets and minimizes the cooling needed.
>
>The upper mosfet is only for brief shorts that may occur.
That's the one I meant. C4 could be picked to limit the current
available from the voltage doubler.
--
I yam what I yam - Popeye
Reply by Arnie Dwyer (ex Jan Frank)●January 7, 20222022-01-07
jlarkin@highlandsniptechnology.com wrote:
> On Fri, 7 Jan 2022 00:33:11 -0000 (UTC), "Arnie Dwyer (ex Jan Frank)"
> <spamme@not.com> wrote:
>
>>Here are the documents for a short-circuit protected constant current
>>supply for making silver ions and other uses. The current can be
>>adjusted from 100uA to 20mA. The compliance range is 320V. The polarity
>>swap is on a separate pcb not shown here.
>>
>>The zip contains the LTspice ASC and PLT files, and the Diptrace
>>schematic and pcb files.
>>
>>The pcb is hand wired for the moment, so the nets are shown in different
>>colors according to their function.
>>
>>https://drive.google.com/file/d/1KzJKoNSXNhtRJ7mhsGU4WRb2KdW76GVz/view?
>>usp=sharing
>>
>>https://tinyurl.com/yskm9hzp
>
> If you size C4 right, it can be the current limiter. Caps don't get
> hot!
You need the current to be adjustable. The 8N60 mosfets are insulated and
mounted on a heatsink. For any load within the operating range, the current
is adjusted for maximum voltage across the load. This minimizes the heat
generated in the mosfets and minimizes the cooling needed.
The upper mosfet is only for brief shorts that may occur. It is not
intended for long-term misconnections. Without it, of course, a short would
destroy the voltge doubler diodes and source resistor.
I had to adjust Vto in the 8N60 model to 4V to reflect the datasheet value.
Thanks for taking a look at the circuit.