Overvoltage protection design ideas

[Dannyx]

Good day folks. Some of the wonderful people here may know that a while back I mucked about with one of those Ebay PSU modules and I'm just about ready to wrap the build up now, having completed a nice case for it, until I thought of something: so I'm powering it with a UPS transformer which puts out 36v, which is rectified then fed into the input pins of the module. The module says it can handle up to 50v, which I can only hold as correct, since the input does get fairly close to that (around 48-49v) and I haven't had any funny issues during testing. However, I definitely don't want to push it and I imagine if the mains input to the transformer goes up slightly (say 240v instead of 230v), the output would also increase since there's no regulation on the output of that rectifier.

So I had a look around to see how I could avoid such issue and found this....Seems simple enough and I actually thought of something like that even before running actual searches: get a zener close to the intended maximum voltage and a FET and build up that little circuit. If the output gets over the zener threshold, the zener conducts, the FET closes and blows the fuse...would this work ? Trouble is I'm not sure how stable my mains voltage really is, which means that if I set the "trip" voltage (Vz) to be too "strict", this setup would blow fuses all the time whenever the unregulated output goes a hair over Vz and the FET becomes conductive which would be a pain....imagine that happening when you're powering something on the bench and it just cuts out on you like *poof*... If Vz is too high, the output is already too high for the module also and the circuit becomes useless since it trips too LATE....

Approach no. 2 is this: I also thought of modifying this circuit a bit and build a regulator to ensure the supply NEVER goes above 50v like this: feed the unregulated 50v into a transistor, then put the zener on the base, so now, the transistor is conducting all the time under the value dictated by the zener - something like 48v (if such a value exists). I know that there must be a minimum current flow through the zener in order for it to regulate, so if my input is 48v, the zener value will have to be lower than that, but I just chose a round value for argument's sake. Problem no.2 is that the transistor would have to be HUGE. Correct me if I'm wrong, but at 15A which the maximum possible current draw through this PSU module (let's just leave it at that, since it may not be achievable in reality) the series transistor would dissipate over 700W (48v*15a = 720w !), so it's not practical in the slightest. It's unlikely I'll ever need that kind of power, plus I doubt the module could deliver it reliably to begin with, but like I said: worst case scenario for example purposes.

Which of these two would be most useful/reliable ? Probably neither, so suggestions are welcome as always Cheers and thanks.

[stj]

Re: Overvoltage protection design ideas

that's a modern version of a classic crowbar circuit using a thyrister and zener.

[Dannyx]

Re: Overvoltage protection design ideas

Even with the first example, the transistor would still need to be a fairly beefy one, since it has to pass the full current when it becomes conductive, even if very briefly....it may not survive it. A 15a fuse would mean 750 watts of power at 50v.

[Dannyx]

Re: Overvoltage protection design ideas

Had another brainfart for a more sophisticated and precise approach: how about an op-amp ? (gotta love that schematic I drew in like 1 minute and took another 5 to edit in MS paint because I forgot the dropping resistor for the zener as always )

Create a low and stable reference voltage on the inverting input using a zener ZD, then fit a resistor divider (R3 and R4) on the output I want to "monitor" and hook that to the non-inverting input. That way, when the output exceeds the threshold set by the reference, the non-inverting input becomes higher than the inverting input, the op-amp turns the transistor on and it shorts the rail, killing the fuse. How's that ?

[budm]

Re: Overvoltage protection design ideas

Instead of using crowbar protection, why not use latch disconnect instead to turn off the relay until the fault is clear?

"Correct me if I'm wrong, but at 15A which the maximum possible current draw through this PSU module (let's just leave it at that, since it may not be achievable in reality) the series transistor would dissipate over 700W (48v*15a = 720w !)," the power dissipation of the pass Transitor = Vdrops (between E and C of the Transistor) x current flow.

[Dannyx]

Re: Overvoltage protection design ideas

Instead of using crowbar protection, why not use latch disconnect instead to turn off the relay until the fault is clear?

"Correct me if I'm wrong, but at 15A which the maximum possible current draw through this PSU module (let's just leave it at that, since it may not be achievable in reality) the series transistor would dissipate over 700W (48v*15a = 720w !)," the power dissipation of the pass Transitor = Vdrops (between E and C of the Transistor) x current flow.

You mean "A relay", since there currently isn't one Good idea, though it clutters things up a bit. So I could still use the op-amp idea then, just with a relay on its output instead of the transistor ? Clever, I must say and makes a whole lot of sense How would I achieve latching though ? By looping the output back to the NI input or by adding a cap to the output ? Not sure the op would be able to deliver the current required to charge the cap AND flip the relay tho.....

Now about transistors used in linear regulators: yes, you're right, I see it now. I ALWAYS screw this up: it's the dissipated voltage times the current, so If I have 50v and regulate it to 47v, it's 3v times 15a = 45W....hm...seems doable then

[budm]

Re: Overvoltage protection design ideas

Use SCR (or SCR equivalent circuit using Transistors) as the latching device in DC circuit.
Example of how it is used:
http://www.electronics-tutorials.ws/...r-circuit.html
https://dmohankumar.wordpress.com/20...esign-note-21/

[Dannyx]

Re: Overvoltage protection design ideas

Use SCR (or SCR equivalent circuit using Transistors) as the latching device in DC circuit.
Example of how it is used:
http://www.electronics-tutorials.ws/...r-circuit.html
https://dmohankumar.wordpress.com/20...esign-note-21/

I see: a combination of relay and SCR. I assume in the schematic in the second example link, my "fault" signal would go into that free "input voltage" pin. I just ran into another issue of obtaining a small sample from a 48v rail, since a resistor divider would have to be "oversized" to drop it down to 2-3v. I may just go with the zener again.

[budm]

Re: Overvoltage protection design ideas

What do you mean by over size resistors? You can have resistor and pot to adjust the trip point, we are talking about very low current to trip the gate.

[Dannyx]

Re: Overvoltage protection design ideas

What do you mean by over size resistors? You can have resistor and pot to adjust the trip point, we are talking about very low current to trip the gate.

I was thinking of the op-amp idea: since the output voltage is almost 50v and I'd set a low reference voltage on the - input of the op-amp like I discussed, it would take a resistor divider with very high values to step it down from 50v to like 2-3v close to the reference, but I actually calculated the values shortly after and they could be like 100k and 5k...

I'm not sure whether to go with the linear regulator idea or with the " full stop" idea.

[Dannyx]

Re: Overvoltage protection design ideas

How about using a resettable fuse for this purpose ? Trouble is, I already drilled holes for the traditional cartridge-style fuse holders, so what would happen if I had a PPTC AND a glass fuse in series ? Which would blow first ? I assume the glass one would still go, thereby making the resettable one useless....

[sam_sam_sam]

Re: Overvoltage protection design ideas

It would depend on what the rating on the resettable fuse there are two rating one working amperage and max rating if you hover around the working amperage might not trip for a long time where as regular fuse will blow and no power will be flowing where as a resettable fuse will still have current flowing through it when it trips

So I would use both but what you have to figure out is what the regulator fuse amperage needs to be to protect the power supply or the equipment in case the resettable fuse is to slow to trip

I have used resettable fuses before in power supply but I select one where the trip point is and use them in switching power supply where when it over current shuts down the power supply ( or the device that it is powering) the only problem with this is if the switching power reset itself and put power on it again)

I have a USB power supply that has 5 amp power supply where the USB cord shorted out at tip
that did not have a resettable fuse but after that happened I put in a 2 amp trip current which is about a little more than one amp working amp which lowers the current a little bit because the resistance goes up when being over loaded and heat up a lot just before it trips

I cooked the USB connector that was being over current and next USB connector was damaged if I had a resettable fuse it would have damage the resettable fuse instead

We have some crane battery operated magnet for pick up metal plate the 3.15 amp glass fuse would some time blow for unknown reason this has a transformer in it

Well I finally figured why this was happening it was that the extension cord that they were plugged into the connectors contact were worn out

I ended up hard wiring outlet near where they are sitting to be charged

I was going to put resettable fuse in line with the glass in these magnets if had not figured out what was happening

One note they do make 250 volt ac resettable fuses the maximum rating that I have seen is 3 amps and lower amperage one as well

One note it is some what hard to 15 amp resettable fuses that rated over 16 volts

Here is a data sheet for one these resettable fuses

https://cdn.badcaps-static.com/pdfs/...c3444c1a36.pdf


I hope this helps

[Dannyx]

Re: Overvoltage protection design ideas

I just measured my rectifier's output again to establish once and for all what I'm working with exactly and it's around 48v, since I wasn't sure exactly what it was. It's lower than the maximum input of that Ebay module (50v), but still a bit close for comfort....ok, realistically it can probably take more than 50v for brief moments, but I'm not quite willing to test it The PSU runs fine, I tested it, it's just that I'm thinking of spikes in the AC line causing spikes on the secondary too.

The original idea was to simply cut the voltage entirely when the threshold is reached, but then I started thinking more and more about some sort of active regulator that "clips" the excess voltage if it goes above 50v. Trouble is I couldn't find a viable solution that can operate at such high current and voltage (50v at 15a would be very demanding and space is also scarce so there's little room for ventilation and heatsinking). Assuming there WAS room for all that, I thought about using a zener to drive a MOSFET, but a gate voltage of 47v (the closest zener value) would exceed the maximum VGS of most FETs, making the task impossible.

There's another idea I had which still uses a zener, but couldn't find such design anywhere, so either I'm a genius and a pioneer or, OBVIOUSLY, the idea wouldn't function and is bad to begin with, hence why it's not presented anywhere: instead of an "N" series pass element, how about a "P" type and a zener AFTER it. When the voltage exceeds 50v, the excess Vin-Vz closes the base/gate of the pass element, shedding away that excess voltage....not sure if this would work in the slightest - there are probably many many things wrong here which would result in immediate destruction or no response at all.

I also thought about Op-amps for a while, but I ran into the same issue: driving the series pass element at considerable voltage, plus no op amp I researched can go any higher than 35v...

Switching regs are the norm nowadays, obviously...

[stj]

Re: Overvoltage protection design ideas

you can get circuit-breakers that fit into the same hole as a fuseholder.
or you could use the hole for a trip indictator lamp.

[Curious.George]

Re: Overvoltage protection design ideas

Good day folks. Some of the wonderful people here may know that a while back I mucked about with one of those Ebay PSU modules and I'm just about ready to wrap the build up now, having completed a nice case for it, until I thought of something: so I'm powering it with a UPS transformer which puts out 36v, which is rectified then fed into the input pins of the module. The module says it can handle up to 50v, which I can only hold as correct, since the input does get fairly close to that (around 48-49v) and I haven't had any funny issues during testing. However, I definitely don't want to push it and I imagine if the mains input to the transformer goes up slightly (say 240v instead of 230v), the output would also increase since there's no regulation on the output of that rectifier.

What is the nature of your load as it will also impact the instantaneous voltages that you see on the power supply rail?

So I had a look around to see how I could avoid such issue and found this....Seems simple enough and I actually thought of something like that even before running actual searches: get a zener close to the intended maximum voltage and a FET and build up that little circuit. If the output gets over the zener threshold, the zener conducts, the FET closes and blows the fuse...would this work ? Trouble is I'm not sure how stable my mains voltage really is, which means that if I set the "trip" voltage (Vz) to be too "strict", this setup would blow fuses all the time whenever the unregulated output goes a hair over Vz and the FET becomes conductive which would be a pain....imagine that happening when you're powering something on the bench and it just cuts out on you like *poof*... If Vz is too high, the output is already too high for the module also and the circuit becomes useless since it trips too LATE....

See the above question.

Approach no. 2 is this: I also thought of modifying this circuit a bit and build a regulator to ensure the supply NEVER goes above 50v like this: feed the unregulated 50v into a transistor, then put the zener on the base, so now, the transistor is conducting all the time under the value dictated by the zener - something like 48v (if such a value exists).

You may not be able to do this without carefully selecting a transistor for a high enough beta as the zener will limit the amount of base current you can provide to the pass transistor. Remember, the zener now has ~48V across it so any current through it (multiplied by 48) will determine the amount of power that IT dissipates! This puts a limit on the high end for the zener current.

I know that there must be a minimum current flow through the zener in order for it to regulate, so if my input is 48v, the zener value will have to be lower than that, but I just chose a round value for argument's sake. Problem no.2 is that the transistor would have to be HUGE. Correct me if I'm wrong, but at 15A which the maximum possible current draw through this PSU module (let's just leave it at that, since it may not be achievable in reality) the series transistor would dissipate over 700W (48v*15a = 720w !), so it's not practical in the slightest. It's unlikely I'll ever need that kind of power, plus I doubt the module could deliver it reliably to begin with, but like I said: worst case scenario for example purposes.

The transistor dissipates whatever the drop across it is times the current through it. So, if you're only dropping a few volts across it, then the power dissipated is "a few volts * 15A". The only time it would dissipate ~700W would be if the drop across it was ~48V (i.e., if the output was shorted!)[/QUOTE]

You appear willing to tolerate the power source being disconnected in the event of an OV condition (instead of trying to clamp it to a safe voltage).

Why not put a "switch" in series with the supply and open it as soon as the voltage climbs out of spec? Of course, a relay would be slow but you can use a pass transistor or a FET for the job and get near instantaneous disconnect.

Note that you have to also consider that the voltage on the output isn't always the same everywhere along the wire. There are effectively propagation delays et al. caused by wire inductance, wire resistance, etc. that cause the voltage seen at point A to differ from that at point B -- potentially significantly! So, the point at which you are monitoring may not reflect the voltage seen by some critical bit of circuitry that you are presumably trying to protect. And, the voltage you see now may not reflect the voltage that is seen after the switch is opened (esp if you are "dumping" -- the sudden disconnection of -- an inductive load).

[Dannyx]

Re: Overvoltage protection design ideas

When talking about said load, are we referring to what's attached to the transformer or to what's attached to the PSU module that's ALREADY attached to the transformer ? If we're talking about the transformer, then the load is the module itself and the bulk caps, so I believe it's a capacitive load IMO.
If we refer to the load of the module, then it's intended to power various devices, ranging from inductive ones like coils and motors to resistive ones. Come to think of it, I really should check if the module has a diode on its output to protect it from coils and stuff trying to discharge back into the supply....

Either way, I can imagine that if the power module is powering something and it suddenly gets disconnected, it can cause a spike to show up at the input (at the bridge rectifier's output that is - the very first stage of the DC section). Is this why you were asking ?

[Curious.George]

Re: Overvoltage protection design ideas

When talking about said load, are we referring to what's attached to the transformer or to what's attached to the PSU module that's ALREADY attached to the transformer ?

For the purposes of the protection circuit discussion, the load is whatever is "downstream" from your switch. When you abruptly open the switch, you are "dumping" the load. This can have repercussions for the "supply" (whatever is upstream from the switch) and the "load".

If the supply is a "commercial power supply/module", it may not be designed to tolerate such a dramatic change in the load (this is a separate design criteria besides volts, amps, weight, operating temperature range, etc.) E.g., some supplies are designed with a minimum load in mind and if that load is not present, the supply misbehaves.

The load can also see unexpected conditions on the power rails if the supply suddenly "disappears".

Folks thinking about designing automotive products often think the electrical "noise" (spark plugs, etc.) is the challenging aspect of that environment (ignoring the temperature range). But, in fact, the "load dump" that occurs every time you start the car (and the starter motor STOPS spinning) is the real hazzard. Instead of designing for ~16V (a 14.4V charged battery), you have to deal with this huge inductive voltage spike appearing on the "12V" power rail. If the battery is brand-spanking new (low output impedance), it can help tamp this down (assuming the wiring takes this into consideration). But, as the battery ages, its output impedance increases to the point where it has less effect on damping that spike.

[The battery typically won't care about the spike but the electronics in your infotainment system probably will!]

Either way, I can imagine that if the power module is powering something and it suddenly gets disconnected, it can cause a spike to show up at the input (at the bridge rectifier's output that is - the very first stage of the DC section). Is this why you were asking ?

I would imagine disconnecting the input to the power module would be have little consequences. But, you'll have to explore that possibility by looking at the characteristics of the "load" that the module presents to whatever supplies it with power. Likewise, you have to consider whether you are using high-side switching or low-side and any sneak paths that can remain when the switch(es) are open.

[Note I'm speaking in generalities and not addressing the specifics of your design]

[Dannyx]

Re: Overvoltage protection design ideas

[Note I'm speaking in generalities and not addressing the specifics of your design]

Correct, I know that's your style

The automotive power system discussion you initiated, despite digressing slightly from the original topic, is VERY interesting, as me and my colleague often tackled it and had slightly different views. For instance, I must admit I'd have lost the argument, because I used to believe that when the car starts, the huge load caused by the starter makes the overall voltage drop (which it does, no doubt) but never considered that it actually "kicks back" and causes a "+" surge. We'd always discuss the reason of fried poorly designed car accessories, especially cigarette lighter chargers, and I'd always claim that there's no way they got killed by OVERvoltage, since it doesn't (shouldn't) occur, but apparently there is some truth to this so I should reconsider....makes a whole lot of sense now. I believe it widely depends on the car, and we assume modern ones have more sophisticated power systems in them, though even older ones SHOULD have.....something to at least reduce this effect - think of a diode across the starter...stupid and utterly rubbish, of course, but just for the sake of argument something along those lines

[petehall347]

Re: Overvoltage protection design ideas

auto-transformer . check min against max of supply and working voltages of unit .

[Curious.George]

Re: Overvoltage protection design ideas

[QUOTE=Dannyx;794687]The automotive power system discussion you initiated, despite digressing slightly from the original topic, is VERY interesting, as me and my colleague often tackled it and had slightly different views. For instance, I must admit I'd have lost the argument, because I used to believe that when the car starts, the huge load caused by the starter makes the overall voltage drop (which it does, no doubt){/QUOTE]

Considerably. And, the drop can vary with the state of the battery, ambient temperature (cold weather halves the battery's capacity and doubles the apparent size of the engine, behind the starter), integrity of connections (think: corrosion), etc.

but never considered that it actually "kicks back" and causes a "+" surge. We'd always discuss the reason of fried poorly designed car accessories, especially cigarette lighter chargers, and I'd always claim that there's no way they got killed by OVERvoltage, since it doesn't (shouldn't) occur, but apparently there is some truth to this so I should reconsider....makes a whole lot of sense now.

This is also true when driving relays/solenoids, etc. (think: hood/trunk releases, door locks, etc.)

I believe it widely depends on the car, and we assume modern ones have more sophisticated power systems in them, though even older ones SHOULD have.....something to at least reduce this effect - think of a diode across the starter...stupid and utterly rubbish, of course, but just for the sake of argument something along those lines

A one-size-fits-all solution isn't practical. Cars vary. Cars are "maintained" by folks with a variety of different skillsets, etc. And, owner/operators aren't particularly saavy, either (ever see someone "jump" a car by putting the cables on BACKWARDS?? If the stuck vehicle's battery is truly shot, it won't put up much of a fight against the fully charged battery that's trying to reverse it!)

And, failure of one "massive snubber" would be a disaster for all the other bits of kit that could be compromised by a voltage surge (newer vehicles have electronics IN each headlight, ECU, ABS, window controller, etc.)

So, when designing for that environment, you condition power coming into your "module" to protect against the crap that could exist outside. Then, protect yourself from any bad things you might do within your module (e.g., RCD snubbers across all of the door lock solenoids that you are driving)

Then, all you have to worry about is temperature, corrosive fluids, someone accidentally wacking your module with a hammer, ... fun stuff!