Help with overload protection - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: September 27, 2019, 05:25:33 am
content: 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

author: fourfathom
author_rank: Super Contributor
timestamp: September 27, 2019, 06:07:12 am
content: 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).

author: Ian.M
author_rank: Super Contributor
timestamp: September 27, 2019, 06:25:48 am
content: @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.

author: Jwillis
author_rank: Super Contributor
timestamp: September 27, 2019, 06:45:39 am
content: 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 .

author: Ian.M
author_rank: Super Contributor
timestamp: September 27, 2019, 08:01:27 am
content: If I were you, I wouldn't start from here!You have a classicX-Y problemgoing 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.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 . . .

author: Jwillis
author_rank: Super Contributor
timestamp: September 27, 2019, 09:49:43 am
content: Quote from: Ian.M on September 27, 2019, 08:01:27 amAgreed .I understand that I could solve the problem I have much simpler. I could simply buy a few AC fans. Or use a separate 12V source . But I have what I have and didn't want to purchase any more if I can solve issues with what I have on hand . I realize that some approaches are unorthodox but with limited resources I do what I can. Heck I don't even have any tl431's even in the scrap pile.My schematics aren't perfect and neither are my calculations because I'm trying to learn.This circuit is simply to cool the heat sink of a high voltage high amperage power linear supply and I'm tapping the positive and negative source from that.And it's -15V because I'm running a 7915 regulator off of one half of the rectifier.This runs the other opamps for the power supply circuit.I could go with a larger heat sink but again I have what I have.I try my hardest to wrap my head around the math and I preform experiments to verify.I, not getting frustrated with something I enjoy doing. I enjoy it because it re leaves my chronic anxiety and depression. I realize that this whole project has become a monster and I expect it to to get even more complex when I start adding digital control.The supply I'm building is based on the infamous 0-30v TL 081.In the diagram many of the values are incorrect to give the expected output but I did that intentionally until I can reconfigure the bread board to handle higher amperage. The tracks can only handle a maximum of maybe 10 amps before things start to melt.At 12 amps things start going south really badly . So I have to fashion some rails for the high current . Also the electronic load was only capable of 150W But I have 800W at least with 40 volts and 20 amps .So I modified the Electronic load to handle more.Anyway at at 10 amps the heat sink for the power transistors is hitting around 70 degrees C .Anyway with the filter caps I have the voltage will probable drop off at full load. 40000uF is probably not near enough to what is needed.As you see I'm trying to solve problems as they arise with knowledge that I'm really pushing the envelope with limited resources.After some thought I was wondering if tapping off half the rectifier with a 7812 regulator would work to power a few fans.hmm

author: Ian.M
author_rank: Super Contributor
timestamp: September 27, 2019, 11:18:00 am
content: Tapping off for the fans similar to the -12V rail isn't a good idea. Single ended charge pumps like that are practical for a few tens of mA, maybe 100mA on the high end. The RMS ripple current in the capacitor is approx three times the DC output current, and large caps that can handle high ripple currents don't come cheap.I assume there aren't any other suitable secondaries or you'd be using them for the -15V rail. Does the 30VAC secondary have a center tap? If so, then you can get half the DC bus voltage for the fans, which would reduce the dissipation in a linear regulator to about 5W for 600mA @12V out. Still not great, but well within what a well heatsinked 7812 can handle.Otherwise its bite the bullet and either add a switching regulator that can handle the input voltage, add a separate supply for the fans or buy those expensive AC fans.

author: AVGresponding
author_rank: Super Contributor
timestamp: September 27, 2019, 12:48:18 pm
content: The OP mentions using "a few cooling fans".If you wire them in series, you can power them off the input without any regulation, ie 3 x 12v fans in series will cope with about 48v or so before they start to freak out overmuch.They can also be under-run for a quieter life, ie 4 x 12v fans in series will run slightly slow on 44v, but will still be effective.

author: Jwillis
author_rank: Super Contributor
timestamp: September 27, 2019, 05:24:01 pm
content: Quote from: Ian.M on September 27, 2019, 11:18:00 amBummer about the rectifier tap. I check on the scope for the amount of ripple pulling 12volts and 500mA through a 7812 and the ripple is insane.Rats.I could rewind a small core I have kicking around for the 12 volts . Or add a secondary to the existing core .Looks like there may be room for that .Probably the best route to take so I'm not compounding problems. I can also use it to run the digital readouts. And probably less risky if things go wrong.The transformer I'm using can be wired to have a centre tap but that would sacrifice the current output.What if I wound another 15 or 20 volt secondary in series with the 30V to get a negative rail with a double bridge rectifier .Then I could run the op amps and fans off the lower voltage negative rail.Something like the drawing below.

author: Ian.M
author_rank: Super Contributor
timestamp: September 27, 2019, 09:32:14 pm
content: If you need asymmetric positive and negative rails, you cant put the secondaries in series before the bridge rectifier. Symmetrical rails from a bridge rectifier + center tapped winding is a special case. Therefore you need to use a second bridge rectifier and put the bridge rectifier outputs in series.Edit: just seen your diagram of exactly that.It would be helpful if you can tolerate a lower voltage negative regulated supply for the OPAMPs (in the -8V to -10V range) so you can use the unregulated auxiliary supply for the 12V fans without over-voltaging them too much or needing to waste power in diode droppers or a regulator for the fan supply. If you *NEED* a full 12V, then you'll have to use a LDO regulator for it and only have just enough headroom to cope with a +/-10% line voltage variation, to minimise the power wasted dropping the voltage for the fans.

author: Jwillis
author_rank: Super Contributor
timestamp: September 27, 2019, 11:42:16 pm
content: Sorry that this thread changed from the original subject .Quote from: Ian.M on September 27, 2019, 09:32:14 pmI plan to scrap the original negative rail configuration for the newer approach. I haven't wound a new secondary yet so I'll have to work out an appropriate voltage to work with .This will simplify the negative rail and be a little more stable.As you suggested I do have LDOs that just may work.Thanks for the input and advice .It's really appreciated.