over current protection

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seyyah

Joined: 07 Oct 2001
Posts: 518
Helped: 1

12 Feb 2003 22:28 over current protection

I need a simple and reliable over current protect circuit. Circuit must cut the line current when it exceeds 2A @28V.

Encrypted

Joined: 28 May 2001
Posts: 146
Helped: 6

12 Feb 2003 22:36

A fuse?

flatulent

Joined: 19 Jul 2002
Posts: 4849
Helped: 289
Location: Middle Earth

12 Feb 2003 22:47 supply

Can you do any changes to the soiurce of the 28 V? Many power supply ICs have over current built in. Look at the data sheet for the IC in your supply and it will tell you what resistors to put on which pins.

Here is a link to a current limiting circuit http://www.maxim-ic.com/appnotes.cfm/appnote_number/173

froggy

Joined: 06 Jul 2001
Posts: 92
Helped: 2

12 Feb 2003 23:01

Why not use a PolySwitch device? They are used in almost every PC nowdays to protect the ps from shorts on the keyboard/mouse connector, scsi, usb and so on. The main advantage over a fuse is that they reset by themself after they have tripped (when voltage is removed for a few seconds).

See www.polyswitch.com for examples.

/Froggy

jzo777n

Joined: 10 Sep 2002
Posts: 113
Helped: 2

13 Feb 2003 9:19

You have similar device from Bourns, which they call Multifuse.
http://www.bourns.com/2/mf.html

Some ting’s to remember for Polyswitch/Multifuse
You have a small hold current that’s flows trough the device.
There is a maximum interrupt current/voltage.
It’s has a temperature dependent trip current

jzo

jzaghal

Joined: 31 Jul 2001
Posts: 213

13 Feb 2003 11:13

Hi,

What sort of PSU do you use ??
If you want to build it from scratch, I would use the linear regulator
type 723. This chip is very reliable and very cheap !!

Bye.

flatulent

Joined: 19 Jul 2002
Posts: 4849
Helped: 289
Location: Middle Earth

15 Feb 2003 19:04 summary

As you can see there are a variety of methods. Each has cost and performance combination different from any other.

The selection depends upon how fast must the load be disconnected and how critical is the current threshold.

seyyah

Joined: 07 Oct 2001
Posts: 518
Helped: 1

15 Feb 2003 23:02 Re: summary

Actually i won't cut off the main current. I only control one of the terminals. At first i thoguht a transistor based solution. Then i decided to use opamp. But i couldn't be sure and asked you. This polyswitch or similars looks like to be suitable if they are fast and cheap enough.

seyyah

Joined: 07 Oct 2001
Posts: 518
Helped: 1

16 Feb 2003 0:30 Re: summary

I think polyswitches or similars aren't fast enough. They need seconds to cut off the current. (am i wrong?) Are there any other suggestions or circuits?

flatulent

Joined: 19 Jul 2002
Posts: 4849
Helped: 289
Location: Middle Earth

16 Feb 2003 0:37 right

You are right. Things that require heat take time to work. You might want to try an ordinary fuse. They can blow in milliseconds if they are fast acting. Watch out for the load initial current if you have to charge capacitors in it. In that case a slow blow fuse will work.

Tell us more about your load and source. Will the load only draw large currents when some bad thing happens? Can your source put out more current for a short time?

seyyah

Joined: 07 Oct 2001
Posts: 518
Helped: 1

16 Feb 2003 16:27 Re: right

Source can supply more than 4A steadily and put up with larger currents for a short time. And it's protected by two different types of fuses which activate at 4A . Load draws small currents in normal operation. If something goes wrong it can draw 2,3... times more than normal current or there can be short circuit. What i want to do is, allowing faults to a certain limit. When limit is reached, circuit must cut off that channel's current without affecting other channels. I want this protecting circuit to be resettable so i can't use a fuse.

flatulent

Joined: 19 Jul 2002
Posts: 4849
Helped: 289
Location: Middle Earth

16 Feb 2003 18:12 stuck with electronic

In this case you are stuck with the electronic method. Can the reset be a bag of fuses near by? You might look into circuit breakers. I have only seen these on the power mains and do not know if they are available for lower voltages and currents.

jzo777n

Joined: 10 Sep 2002
Posts: 113
Helped: 2

17 Feb 2003 10:00

You can look at Hot Swap IC from manufactures like Maxim, Linear, and TI.

There is a simple current limiter with two transistors.
The first transistor you connect the base and emitter? over a current shunt resistor.
This transistor shunts the second transistor which is your switch.

Or a simple crowbar with a tyristor and a polyswitch.

Sebi

Joined: 02 Jan 2002
Posts: 63
Location: Hungary

19 Feb 2003 13:06

And when You put a small thyristor between pass transistor base and GND, triggered by sense transistor collector, this circuit remain off state after overcurrent.
Possible reset push button parallel the thyristor anode-cathode.

Tornado

Joined: 26 Apr 2002
Posts: 340
Helped: 1

19 Feb 2003 18:40 Overcurrent protection

Hi Seyyah, can you please post some schematic in PDF or GIF or
BMP format and I will help you.

Regards
Tornado

Sebi

Joined: 02 Jan 2002
Posts: 63
Location: Hungary

19 Feb 2003 19:28

Here is my circuit...

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MCMC

Joined: 02 Feb 2002
Posts: 239

20 Feb 2003 6:03

Hi flatulent Very Happy

Ok I upload again!!

I work in the tutorial in how to calibrate it, and some modification for more current !!!! 5 to 6 amp

in this new schematic i make a correction, in the power transformer.

this power supply is for shortcircuit overcurrent protection ideal for breadboard use, I use it and work fine.
mcmc

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Last edited by MCMC on 21 Feb 2003 6:03; edited 1 time in total

MCMC

Joined: 02 Feb 2002
Posts: 239

20 Feb 2003 16:46

Hi all

This is the instructions and MOD for the Power Supply

Regards

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seyyah

Joined: 07 Oct 2001
Posts: 518
Helped: 1

20 Feb 2003 22:07 Re: Overcurrent protection

This is the general schematic for Tornado

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seyyah

Joined: 07 Oct 2001
Posts: 518
Helped: 1

21 Feb 2003 17:07 Re: Overcurrent protection

how is this? I think it can work and it is simple. What do you think? Does this have any weakness according to you or does it need a revision?

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Sebi

Joined: 02 Jan 2002
Posts: 63
Location: Hungary

21 Feb 2003 17:26

Current limit2.gif produced about 5V on 3ohm resistor.

seyyah

Joined: 07 Oct 2001
Posts: 518
Helped: 1

22 Feb 2003 9:55

I think it needs some change on value of components but the importnat question is: Am i on the right way?

Sebi

Joined: 02 Jan 2002
Posts: 63
Location: Hungary

22 Feb 2003 10:11

Without some bistabil element(e.g.thyristor or flip-flop) You can build only current limiter not a fuse. Have You seen my small circuit (fuse.pdf)? It conduct up to 2A, when overloaded fired the thyristor and remain in this state. After resetting conduct again. Try out and apply serial with each outputs.

seyyah

Joined: 07 Oct 2001
Posts: 518
Helped: 1

22 Feb 2003 17:20

Yes i've seen it and you are right. First i thought it is not important to cut off the current if we can prevent it to increase over 2A. But when i saw transistor is burning with good smoke and smell, i decided that cutting off is a must.

Smanetto

Joined: 04 Apr 2002
Posts: 17
Location: Italy

15 Mar 2003 3:30

Sebi, are you sure your circuit works?
The current-sensor transistor should be a PNP, with the emitter on the left and the base on the right.
In this way when the base goes 0.7V under the emitter, the collector source corrent to the resistors. With a NPN, the current pass from the base to the collector through the PN junction, and this is not correct.

Also you are using the other transistors as a darlington emitter follower, in other words a configuration that put the last emitter at 1.4V under the first base. It is ok if you want a power supply, but in this case the transistors have to be used as a switch and for this application it is used a PNP transistor (or a P channel Mosfet).

Polyswitch and similar are fine, but they are slow and you loss a few volts (they have to heat for breaking the circuit).

Seyyah, think about a circuit that switch off the power and try to restart every few seconds.

Pay attention the limiter is able to charge the capacitors of the circuit you are powering, otherwise it will not start.

Sorry for my English

sick_man
Guest

15 Mar 2003 3:52

for over current i use an scr and a zener too set a threshold
using a .22R resistor in series with v+ and the scr gets clamped too gnd the zener sets and threshold on a pot this gives a good protection

and blows a fuse if the scr fires

punny attampts at current limit are not worth the equipment

the best is a scr crowbar

the fuse can be replace by a semiconductor driven resetable fuse
or use an r2r ladder and comparitor too get steped current
using then 1ma and 1000 ma scaled pots also gives a voltage offset this can be used to give a current display 4 digit

a good psu is a 5 amp 0 - 30v 0 - 5000ma digital psu from rs
it allows for many adjustments
and has also some nice features

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