Electronics > Beginners

Help with overload protection

(1/3) > >>

Jwillis:
I think this is right but still not sure. Overall the circuit works well without protection but I was wondering if the load protection transistor needs to be very powerful. I am considering a 2N5551https://www.onsemi.com/pub/Collateral/2N5550-D.PDF

fourfathom:
Some comments:
* The current-limit transistor needn't be a large one, as it only has to shunt the base current of the Darlington pass transistor stage.
* That 1K resistor feeding the Darlington base should be moved to the output of the opamp.  As it is, your over-current and over-voltage clamp transistors are directly shunting the opamp output.  Moving the resistor makes this shunting more effective.  These two transistors should still connect to the Darlington base.
* You'd better study that bias network at the over-voltage transistor base.  It is nominally sitting at around -9V, which will probably break down the transistor base-emitter junction, and in any case will not provide useful OV protection.  And why are you sensing the +44V input, and then shutting down the regulated output?  This makes no sense to me.  Think hard about this OV-protection circuit.
* With that 0.1 Ohm transistor, your current-limit threshold will be about 6A.
* As the OV and CL thresholds are dependent on the base-emitter junction thresholds, the settings will be relatively unprecise, and will vary significantly over temperature.  This may not be a problem for you.

I have not reviewed your components for voltage limits or power dissipation.  You should do this carefully and design in plenty of margin.  Your pass transistor will dissipate up to 44W when the output is shorted (assuming a 1A current-limit, which at the moment you do not have).

Ian.M:
@O.P: You have found a crappy regulator design there.  The nominal 0-24V output range is laughable.  For 44V in I would expect a max output of 40V, and it certainly can't get down to 0V, as that topology cant deliver less than the reference voltage except if the protection is activated.  Replacing the 5K resistor with a 3K3 one would increase its max output voltage to around 40V.

IMHO running a simple low voltage Zener reference from a raw rectified unregulated supply is asking for trouble.   Low voltage Zeners have notably soft knee so the variation in current through it from input voltage ripple will result in significant reference voltage ripple, which is then amplified by the factor the output voltage is greater than the reference by.  One quick and easy improvement would be to use a TL431 for the reference voltage and feed it from a regulated supply - nothing fancy, just a 12V Zener and a couple of resistors.

FourFathom has already commented on the protection circuit  deficiencies.   I don't much like the protection circuit either.  Although the OPAMP in question is rated for indefinite short circuit, as FourFathom has already mentioned, it would make more sense to move the 1K resistor to between the OPAMP output and the junction of the pass transistor Darlington pair base with the two protection transistor collectors.  FourFathom has mentioned the excessive current limit - a 0.47R current sense resistor would make more sense for a 1A supply.  Also, there should be a resistor in series with the over-current protection transistor base to limit the current through its B-E junction in the event the output is suddenly shorted.  1K should be satisfactory.  With these mods, any small signal  NPN, hFE>100 should be satisfactory, as worst case it only has to handle under 50mA Ic and about 3V Vce.

Jwillis:
OH OK  Super. I'll most definitely make the appropriate  changes.
 As for the out put voltage the numbers I gave are arbitrary. I really only need a constant 12 or 24 volts to power a few cooling fans.I haven't decided on that so I left it variable for now. All the parts are what I have on hand.
Your right about the pass transistor. One my E load at 1 amp it gets to 130 Degrees C. But should only need 500 to 600 mA.

Thanks guys this is great .

Ian.M:
If I were you, I wouldn't start from here!
You have a classic X-Y problem going on.

The objective is to get rid of excess heat.
The intention is to use 12V or 24V cooling fans - so far so good.
However your reasoning then all goes pear shaped!

I assume the input voltage is >24V, probably >35V or you'd be using an off-the-shelf 12V regulator.   If the input voltage is 45V, then to run a 12V fan you need to drop 33V.  If the fans draw 600mA, that's approx. an extra 20W of heat to get rid of!  Not Clever.  |O

The way out of the hole you have dug is to use a switching regulator.  Tell us the abs. max. input voltage and whether you are able to do a PCB layout for a high frequency part or need an 'old skool' low frequency part that can be prototyped successfully on perfboard and we can come up with some suggestions.

Also, what's the input supply from? - your current circuit shows a -15V supply for the OPAMP, which if its beefy enough would be fairly close to what you need to run 12V fans.  Add a string of 3x 1N4001 diodes to drop 2V . . .

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod