Topic: ZTEX USB-FPGA Modules 1.15x and 1.15y: 215 and 860 MH/s FPGA Boards - page 46. (Read 182450 times)

I just did a quick survey of all the "wall wart" power supplies I could find laying around my house. I looked at 6 different ones all from very different backgrounds. It appears that 5 out of the 6 were regulated, and output a voltage right near the rated voltage with no load. On a scope, the ripple is basically 0.

The odd one was rated for 9V and 600 mA, but the open circuit voltage measured was 12.85 V, so I assume it is unregulated. On the scope, the ripple was on the order of a half volt, and the peak voltage was at most about 13.1 V. I didn't measure it under load.

Without being able to measure gusti's power supply, we can't really conclude anything from this, but it was an interesting experiment!


To me, the dangerous thing is that charged up output capacitor. If that is sitting at a relatively high voltage and has a high capacitance, I could see that potentially causing some damage.

On the other hand, if it has an output capacitor, then the maximum voltage that the capacitor could be charged up to when gusti plugged in the board is not much higher than his measured 13.5 V. This doesn't seem like a dangerous situation.

If it doesn't have an output capacitor, then the higher voltage (definitely not higher than 21 V) will be applied for an incredibly short time. This doesn't seem dangerous to me, either.

I'm no expert on these power supplies, though. Is there someone reading that might have some experience with these and can correct me if I'm wrong?

I meant 5) said what you said: If there is an output capacitor the mean voltage will be close to the peak voltage.

First of all, whether or not it has a large capacitor on the output, it is not an ideal full wave rectifier.

Second, it either has an output capacitor and the ratio of the peak to average voltage is much smaller than pi/2, or it doesn't have an output capacitor and 5 is irrelevant. It can't be both. Or am I missing something?

Like I said before, I'm just trying to understand the cause of the problem and whether or not I need to be worried about something like this happening to our boards. Thanks for your help with this.

see 5)

That is why I'm assumung Gusties power supply has no filter capacitors, i.e. an unregulated 12 V power supply with filter cpapcitors would have an much higher idle voltage (and a regulated power supply would deliver about 12V).

This calculation only applies to an ideal full-wave rectifier. With filtering it is very different situation. If you have an output capacitor, then the average will be much closer to the peak.

According to Gusti, this thing: http://articulo.mercadolibre.com.ar/MLA-129889415-transformador-3-com-para-hub-swicht-officeconnect-3c16744a-_JM

It seems to be an unregulated supply with an idle mean voltage of 13.5V. The peak voltage should be about 21V, see https://bitcointalksearch.org/topic/m.637394 (2112, thanks for your calculations)

What power supply was used, anyway?  Brand/model #?

If you would have accepted the possibility that you choose an improper power supply you would have one running board and a 30% discount for a replacement order.

Sure, all is perfect designed and tested on your side, and the customer (me) is the stupid who cannot choose a proper power supply.
Customers, beware.

In the name of science I'm answering:

1) cheap multi-meters measure the average voltage
2) int sin(t),t=0..pi = 2
3) thus peak to average ratio for a full-wave rectifier is pi/2 ~= 1.570
4) 12V * pi/2 ~= 18.9V, 13.5V * pi/2 ~= 21.21V
5) unregulated but filtered power supply will charge its filtering capacitors to the peak voltage in (4)
6) a silicon device designed for 16V maximum input may release its magic smoke if supplied with the unfiltered rectified waveform with peaks as high as (4)
7) if the power supply was first plugged in to the mains side and then to the load side then the filtering capacitors will get partially discharged from the state (5) to their designed average
8) the safer way of powering up is first to plug in the load side of the supply to the device then plug in the supply to the mains outlet.

The above is what science says.

But the whole thread is an example why selling electronics to consumers demands sandbagging: both financial markups to cover losses caused by pissed of consumers and to safeguard devices from the accidental abuse by the uneducated consumers.

Short circuits or other random errors like ESD or defect parts (with an unacceptable life time -- all boards are functionally tested) can be excluded since both board are damaged in the same way: both voltage regulators on both boards are destroyed. The damage on the 3.3V regulators is even visible.

  If nothing else, atleast some good came out of this experience so far.  Aye, people may not read it but it grants you the ability to point it out when they blow their junk up. ;p

O.k. I will add power supplies for EU to the shop at begin of next year. There exist small variants which can shipped per airmail and which I can offer for about 10 EUR.

As I wrote, switching supplies are save because they are always regulated. I.e. in order to find one you just need to enter "switching power supply 12v" on ebay:
http://www.ebay.com/sch/i.html?_nkw=switching+power+supply+12v

They cost about $10. If I would offer them in my shop they would cost about $30 to $40 including additional shipping costs. (Because they either need to be imported (high shipping costs) or they I have to purchase expensive international variants)-

I will add the hint to purchase switching supplies from Ebay within the next days to http://www.ztex.de/usb-fpga-1/usb-fpga-1.15x.e.html#con5. The problem: almost no one reads it.

If an over voltage spike occurs, the switching regulators breaks through, i.e. you have the input voltage on the 1.2V and 3.3V rails. This immediately destroys most chips and capacitors.


A bulk capacitor is on the FPGA board. The job of input capacitors it to eliminate spikes (ceramic types) and to reduce slower peaks. Without that capacitors the board would be even more sensitive.

According to Gusti the damage occurred in idle mode.


This is usually stated on the label. If not, and it is a transformer based thing, you have to be careful. Switching regulators are always regulated.

The weird thing is two board and destroyed the same way.

Maybe the fryer could post a picture of power supply label.  Reverse polarity was the first thing I thought of but the fryer claims it was proper polarity.

We aren't living in the past already :)
Most modern switching power supplies are designed for 100-240 range or even better (sometimes marked as "autovoltage"), this covers the entire world, only some cheap physical plug adapters may be needed.

Well, there is was a regulator as the part of the device.

I can't imagine how this damage could have happened if he used correct polarity and the board was good at arrival. I doubt that the PS was really that bad.
I can think of shorting something at the back of the board by stray screw laying on the table, but ztex says that this regulator is overcurrent-protected and that would be strange to fry both boards same way...

There really isn't a reliable way to know outside of testing with a multimeter.   A well regulated 12V supply shouldn't spike above 13-14V unloaded, and most will read slightly below the rated voltage, somewhere around 10-11.5V.

Generally cheap wall-worts that come with other gadgets (switches, routers, etc etc) are unregulated, the regulation circuit is part of the device they're intended to power.  A bad unregulated 12V supply can often spike to 20V+ unloaded, and the same supply can dip well below 12V under moderate load.  This is often the bane of those beginning in microelectronics (amtel/arduinos/etc).

The only way to know for sure that the PSU the buyer is using is going to work and not fry the board is to supply it yourself.

Yeah I agree selling kits would be worthless.  If power supplies are offered they should be separate and optional.

Another option would be to find a large retailer (amazon, digikey) that has a compatible PS. 
Ztek could test it an then provide a link to it from the product page.

Like I said in the other thread (now locked), that's the voltage when there is no load. I don't understand how voltage without current can do damage to the board. As soon as there is some current draw, it should drop to acceptable levels. If it had a huge output capacitor, then I could see it potentially doing some damage. But ztex, you said it probably didn't have one?

I'm only asking in the name of science here, not to slander ztex or his design or to take the side of the customer. I also want to make sure this type of thing doesn't happen to a customer using one of our boards. I've definitely lost some sleep worrying about something like this happening, and now that it's happened to someone else, I'm even more worried. If it turns out that using an unregulated "12 V" supply can really do this much damage, I don't want to ship a single board to a customer until I feel that our customers are sufficiently well informed not to do this.

Now, can someone tell me how one can tell that a supply is unregulated or not? Other than measuring it with a multimeter, of course. As far as I know, there is nothing on the label to indicate this.

Those who are unsure about what PS to use, can't test voltages, etc etc, may still prefer the piece of mind buying an overpriced PS from you.

Don't bother selling 'kits', sell verified power supplies separately and allow the buyer to decide which type they need.  Even if you only stock power supplies for 60hz 120V US and 50hz 220V EURO you'll cover most of your bases.  Absolutely charge extra shipping, and absolutely mark up the price to cover your extra costs.

Out of 100 single-board orders you sell, maybe 10% of those buyers will know the difference between regulated and unregulated power supplies.  Cover your ass on the other 90% who will just try plugging in whatever fits in the socket.

[edit] While, yes, there are a large number of possible combinations if you wish to cover all countries, offering A, C, and G transformers will cover almost all of the major ones.

http://en.wikipedia.org/wiki/Mains_electricity_by_country