Current limit existing power supply

[Karl Bongers]

Hi Everyone,

I'm looking for a way to add current limit to an existing power supply
so I can use it on protos or breadboarding without worrying too
much about blowing things up. Mostly I'm interested in low
current stuff, up to 100ma for example.

At work we have some fancy power supplys that you can set the current
limit on, very nice. I've often wanted to build my own, but never
find the time. One thing I use all the time is just a DC power pack
with a 7805 soldered on and some wire out to my breadboard.

I'm thinking a pchannel mosfet in series with a power supply could
somehow work? Measure the drop across it somehow? Trip the mosfet
to off? Is that called a crowbar circuit?

I needed 12v/5v the other day, extended my PC power supply out
and put a couple small 4 ohm resistors in series. Figured it
would be better to burn these up on rather than
arc weld with my PC. Kindof a cheap fuse, but I hate fuses,
you never have the right size no matter how many you have.

Any idea?

 

[Robert C Monsen]

Hi Everyone,

I'm looking for a way to add current limit to an existing power supply
so I can use it on protos or breadboarding without worrying too
much about blowing things up. Mostly I'm interested in low
current stuff, up to 100ma for example.

At work we have some fancy power supplys that you can set the current
limit on, very nice. I've often wanted to build my own, but never
find the time. One thing I use all the time is just a DC power pack
with a 7805 soldered on and some wire out to my breadboard.

I'm thinking a pchannel mosfet in series with a power supply could
somehow work? Measure the drop across it somehow? Trip the mosfet
to off? Is that called a crowbar circuit?

I needed 12v/5v the other day, extended my PC power supply out
and put a couple small 4 ohm resistors in series. Figured it
would be better to burn these up on rather than
arc weld with my PC. Kindof a cheap fuse, but I hate fuses,
you never have the right size no matter how many you have.

Any idea?

An easy 'floating' current limit would be to use an LM317 as a current
limit. The LM317 tries to keep the voltage of its adjust pin at 1.25V below
its output pin. Thus, if you put a resistor after it and connect the adj pin
after the resistor, it'll limit the current to 1.25/R. Unfortunately, this
will also mess with the actual voltage output, dropping it from 0 to 1.25V,
depending on the current.

There are nice plans you can use to build current limiting power supplies,
but doing it, buying meters and parts, doing the enclosure, its hard to beat
a cheapo PS built in China.

You can buy a really nice cheapo adjustable power supply, with voltage and
current limits, for around $60, from these guys:

http://store.yahoo.com/webtronics/index.html

I just bought one, and its quite nice not having to diddle pots to adjust
voltages. The one I got has LCD readouts for voltage and current. Max is 18V
at 2A. They have some kind of special on, where they'll send you a crappy
DVM if you order. The DVM is really crappy, but for a free spare, I'll take
it.

This place also has them:

http://www.mpja.com/listitems.asp?dept=2&main=1

Regards,
Bob Monsen

 

[Karl Bongers]

Hi Robert,

An easy 'floating' current limit would be to use an LM317 as a current
limit. The LM317 tries to keep the voltage of its adjust pin at 1.25V below
its output pin. Thus, if you put a resistor after it and connect the adj pin
after the resistor, it'll limit the current to 1.25/R. Unfortunately, this
will also mess with the actual voltage output, dropping it from 0 to 1.25V,
depending on the current.

Yes, That's what I always had in mind to build, but I'm always too lazy
to find a box, transformer, etc. And it seems I'm always surrounded by
existing power supplys(PC, old junk parts).

I'm looking at resettable fuses in the Digikey catalog, Raychem
RXE series. On the low amp end they have:
..05A hold/.10A trip, 7.3 to 11 ohm,
..10A hold/.20A trip, 2.5 to 4.5 ohm, ...

I'm thinking these might be a decent solution. Not perfect, the
voltage would droop some, but it would be a step up from my 4ohm
resistors that will literally burn up. They don't cost much, $0.40.

There are nice plans you can use to build current limiting power supplies,
but doing it, buying meters and parts, doing the enclosure, its hard to beat
a cheapo PS built in China.

You can buy a really nice cheapo adjustable power supply, with voltage and
current limits, for around $60, from these guys:

http://store.yahoo.com/webtronics/index.html

Thanks for the links, yes these look nice, I like the idea of being able
to see current draw on a LCD meter. Sure would beat the "hold your
finger on the 7805 and see how hot it gets method"(Ouch!) Or the hunt
for a small resistor then measure voltage drop and calculate method.

I'm thinking of places that would be nice to use these resettable fuses.
Like in my DVM, seems I always burn out the fuse in these, then never
have replacement fuses. Hmmm, a cheap DVM could be used to display your
amp use, considering how inexpensive some of these are now,
it could be an afordable method.

It would be neat to dream up a simple add on circuit to an existing
power supply that would give you a variable current limit.
I'm trying to think up some simple way to detect the small drop
on the high side(pchannel mosfet) and get that to latch the mosfet
off after some trip point.

Karl.

 

[Walter Harley]

Hi Everyone,

I'm looking for a way to add current limit to an existing power supply
so I can use it on protos or breadboarding without worrying too
much about blowing things up. Mostly I'm interested in low
current stuff, up to 100ma for example. [...]

Think in terms of current limit, not fuse. I've built and used power
supplies both ways. The fuse approach is a pain in the proverbial, because
if you have an electrolytic bypass cap across your power line in your
circuit, the startup current surge will blow the "fuse". What you want is
something that reduces the voltage proportionately if the current draw is
higher than expected, not a crowbar.

 

[Tony]

An easy 'floating' current limit would be to use an LM317 as a current
limit. The LM317 tries to keep the voltage of its adjust pin at 1.25V below
its output pin. Thus, if you put a resistor after it and connect the adj pin
after the resistor, it'll limit the current to 1.25/R. Unfortunately, this
will also mess with the actual voltage output, dropping it from 0 to 1.25V,
depending on the current.

Use the 317 as Bob stated but install it in the existing supply before the
supplies regulator to eleiminate the voltage drop problem. Install a switch to
bypass it when not needed. May want to use the 317L for the low currents you
want.

Tony

 

[[email protected]]

Karl,

"Tony" took the approach that I have used - place the 317 circuit before
the 7805-type regulator. Works slick.

By-the-way: "Hi" from a former JGED co-worker!

-Rob

 

[Walter Harley]

"Tony" took the approach that I have used - place the 317 circuit before
the 7805-type regulator. Works slick.

Does it? Have you looked at the behaviour right around limiting? I think
some regulator chips aren't very well-behaved with input voltages that are
just below their dropout voltage. I wonder if you don't get some nasty
oscillations.

 

[roma]

Hi Everyone,

I'm looking for a way to add current limit to an existing power supply
so I can use it on protos or breadboarding without worrying too
much about blowing things up. Mostly I'm interested in low
current stuff, up to 100ma for example.

At work we have some fancy power supplys that you can set the current
limit on, very nice. I've often wanted to build my own, but never
find the time. One thing I use all the time is just a DC power pack
with a 7805 soldered on and some wire out to my breadboard.

I'm thinking a pchannel mosfet in series with a power supply could
somehow work? Measure the drop across it somehow? Trip the mosfet
to off? Is that called a crowbar circuit?

I needed 12v/5v the other day, extended my PC power supply out
and put a couple small 4 ohm resistors in series. Figured it
would be better to burn these up on rather than
arc weld with my PC. Kindof a cheap fuse, but I hate fuses,
you never have the right size no matter how many you have.

Any idea?

http://members.shaw.ca/roma/supply.html

 

[Tony]

Does it? Have you looked at the behaviour right around limiting? I think
some regulator chips aren't very well-behaved with input voltages that are
just below their dropout voltage. I wonder if you don't get some nasty
oscillations.<<

The effect seems to be that the final output does become somewhat unstable
around the limiting point -- it drops just like its supposed to. Oscillation or
not the average current is limited. If he wanted a precision high performance
supply then other means would be better and more costly/complicated. He just
wants to prevent accidents when tinkering.

Tony

 

[Fred Bloggs]

I'm looking for a way to add current limit to an existing power supply
so I can use it on protos or breadboarding ...[ snip blah blah blah...]

A little "lab" box w/clip leads- other variations left as exercise for
the student:

Please view in a fixed-width font such as Courier.


V(+) >------------------------------------> (+)


9V BATT
| 100K L
+--|>|---| |--/\/\----+
| 1N4148 | | O
| |
+---------+ +-----+ A
| | | |
| e c | D
| \ / G
| NPN ----- ----- N-CH
| | | ^ |
V(-) >--+----/\/\----+------+ +--------> (-)
Rc S D
0.6
<---- Ilim=---
Rc

 

[Karl Bongers]

Hello Fred,

blah, blah-blah, blah-blah

Nice circuit, works great.
Can I get rid of the diode? Does it serve a purpose?

And a tougher question, how do I get rid of the battery, make it
self powered?

I'm looking for a way to add current limit to an existing power supply
so I can use it on protos or breadboarding ...[ snip blah blah blah...]

A little "lab" box w/clip leads- other variations left as exercise for
the student:

Please view in a fixed-width font such as Courier.


V(+) >------------------------------------> (+)


9V BATT
| 100K L
+--|>|---| |--/\/\----+
| 1N4148 | | O
| |
+---------+ +-----+ A
| | | |
| e c | D
| \ / G
| NPN ----- ----- N-CH
| | | ^ |
V(-) >--+----/\/\----+------+ +--------> (-)
Rc S D
0.6
<---- Ilim=---
Rc

 

[Karl Bongers]

Think I figured it out, no battery, seems to work.
Thanks for the help!


Rc

V(+) >-+-/\/\----+-----------+ +-----------> (+)

| |b s| |d p-ch

Vin | --- pnp ------- mosfet Vout limited

| c / \ e |g

+------+ +----------+--/\/\---+ Ilim= 0.6/Rc

100k |

|

GND--------------------------------------+---> GND

I'm looking for a way to add current limit to an existing power supply
so I can use it on protos or breadboarding ...[ snip blah blah blah...]

A little "lab" box w/clip leads- other variations left as exercise for
the student:

Please view in a fixed-width font such as Courier.


V(+) >------------------------------------> (+)


9V BATT
| 100K L
+--|>|---| |--/\/\----+
| 1N4148 | | O
| |
+---------+ +-----+ A
| | | |
| e c | D
| \ / G
| NPN ----- ----- N-CH
| | | ^ |
V(-) >--+----/\/\----+------+ +--------> (-)
Rc S D
0.6
<---- Ilim=---
Rc

 

[Karl Bongers]

By-the-way: "Hi" from a former JGED co-worker!

Hi Rob, Good to hear from you, send me your email address.

 

[Fred Bloggs]

Think I figured it out, no battery, seems to work.
Thanks for the help!


Rc
V(+) >-+-/\/\----+-----------+ +-----------> (+)
| |b s| |d p-ch
Vin | --- pnp ------- mosfet Vout limited
| c / \ e |g
+------+ +----------+--/\/\---+ Ilim= 0.6/Rc
100k |
|
GND--------------------------------------+---> GND

I'm looking for a way to add current limit to an existing power supply
so I can use it on protos or breadboarding ...[ snip blah blah blah...]

A little "lab" box w/clip leads- other variations left as exercise for
the student:

Please view in a fixed-width font such as Courier.


V(+) >------------------------------------> (+)


9V BATT
| 100K L
+--|>|---| |--/\/\----+
| 1N4148 | | O
| |
+---------+ +-----+ A
| | | |
| e c | D
| \ / G
| NPN ----- ----- N-CH
| | | ^ |
V(-) >--+----/\/\----+------+ +--------> (-)
Rc S D
0.6
<---- Ilim=---
Rc

Congrats- that is the complementary version. The battery powered version
will not depend on your supply voltage so that you could use it in an
adjustable power supply situation where you work at low or high voltage.
You can put the N-CH version in series with the high side too- it
installs like an ammeter-observe the +/- polarities. Since the battery
supplies current only when there is a fault, it will last forever. A
version with a fault indicator LED, and some other protection features,
would be like so:
Please view in a fixed-width font such as Courier.



+----------------------------------------------+
| |
| 9V BATT |
| | 100K |
| +----|>|--| |---+------/\/\--+ |
| | 1N4148 | | 1K | |
| | +-|>|--/\/\--+ |
| | LED | |
| | | |
| | | |
| +----------------+ +-----+ |
| | | | | |
| | e c | IRFIB7N50A |
| | 2N3904 \ / G |
| | ----- ----- N-CH |
| | 5.6 | | ^ | |
(-)---------+--------/\/\-------+------+ +----+--------(+)
| | | S D | |
| | + | | |
| +-----------||------+ | |
| | 1000U 10WVDC | |
| | | |
| +------------------|>|--------------+ |
| 1N4007 |
| |
| |
+-------------CURRENT LIMIT 100MA--------------+

 

[Fred Bloggs]

Ooops- interchange c-e connections on your PNP....