inrush surge

It's unlikely on a ReVox deck, but some more advanced equipment I've worked on over the years has had multiple relays or SCRs in a timed power up sequ ence. One RPTV I worked on years ago would occasionally blow fuses on star t (but never when running), and it turned out to be a stuck relay (prevente d one SMPS from being power up until the first one did).

But although I c> Back around 1980 I had to repair a ReVox deck. It kept blowing its fuse -

-

t the

few

the

."

Add an anti-inrush NTC thermistor made for the purpose, assuming no actual fault situation .

That, of course, was the issue. There wasn't anything obviously wrong with the unit.

Burning the house down is a bit extreme, but such burnt offerings have been known to appease the Gods of Electronics.

When all else fails, a measurement or three might be a good idea. Put a small value resistor (0.1 ohm) in series with the AC line, and measure the inrush current and duration. Then compare with the fuse ratings and see if the selected fuse type and value are sane.

I've run into cute tricks like a big Lambda 12V power supply, that had a 10 ohm resistor in series with the AC power lead. When the power supply output finally stabilized, a relay would short out the 10 ohm resistor so that full power could be delivered. That works great until the relay contacts arced closed, causing the PS to suck full current on startup and blowing the fuse. It took a while to find that one.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Skype: JeffLiebermann     AE6KS    831-336-2558

It's a Lambda LFS-48-24. Schematic is on Pg 6.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

"William Sommerwerck"

** That is just what a FAST fuse does when subjected to current surges. ** Fuses in the AC supply always need to be "slo blo" or "T" types.

There are darn few exceptions to this rule.

BTW

Fuses with a flat metal strip inside are inherently slow and made of tin not copper too.

.... Phil

The version of this I've seen is on the DC input (274 V nominal, 100 A) to a big variable-frequency drive. There are two relays; the "little" SPST one that operates first has about a 10 ohm ceramic resistor in series with the contacts. The "big" one that operates second has no resistor, and is DPST, for both sides of the DC bus. You can hear the "tick, tick" of both relays operating in a second or less when you power on the machine, and the "tick" of the big one releasing when you power it off.

There is also a 125 A fuse on the DC input. It is rated at 100 sec max at 250 A (2x rated), 15 sec max at 375 A (3x), and 1 sec max at

625 A (5x).

Matt Roberds

Or, the resistor is on the DC side of the bridge, and they have an SCR triggered by the startup of the switching supply to short the resistor. If the SCR fails to turn on, the resistor goes up in flames.

Jon

Could it be the fuse was rated for 240 volt operation (at 1/2 the value) and being used on 120 volts?

I often see this - 240 volt operation ha a fuse 1/2 the value of that for

120 volt operation.

Mark Z.

"Mark Zacharias"

** I see the reverse sometimes, mainly with 1960s Fender guitar amps.

The AC tranny has been replaced long ago to suit use in Australia - but the fuse holder labelling is still for the USA.

... Phil

That's the same idea as the one I described. It's useful for power supplies that have relay driven power on/off circuits, where one doesn't want the full AC power drain going through the on-off switch as in TTL/CMOS on-off. Relays or an SCR are about the only way to simulate the function of inrush current limiter. Were a big power supply to use an NTC thermistor for the function, it would probably explode. As mentioned, the problem is when the contacts short, the full inrush current appears on turn-on, blowing the fuse.

The PS I was playing with only had one click. I think it was about

1-2 seconds after power was applied.

Yikes, that's big. I don't think a fuse that big actually "blows". More like "melts".

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

The more I think about it (and look at the wiring diagram), the more I think the "big" relay is really two SPST relays operated at the same time.

In this case, the operator position is not convenient to where the relays need to live, so the relays get switched by 12 VDC control signals. There are some other safety functions between the operator and the relays as well; the computer can decide that bad things are happening and drop out the control signals to the relays.

I think this system is set up to fail safe if one set of contacts gets welded. The normal startup sequence is something like

1. Close main relay in - line. VFD should not yet have DC.
2. Close pre-charge relay in + line. VFD has current-limited DC. Input caps charge.
3. Close main relay in + line. VFD has current-unlimited DC.
4. Open pre-charge relay in + line.

and the normal shutdown sequence is something like

1. Open main relay in - line. The input caps should start discharging, and the voltage at the VFD should start dropping.
2. Open main relay in + line.

If the pre-charge relay contacts or main relay + line contacts are welded, then the VFD will see DC as soon as it does startup step 1. It can then open the main relay - line and be safe.

If the main relay - line contacts are welded, it probably can't detect it at startup, but it can detect it at shutdown when the voltage fails to drop off; it can open the main relay + line and be safe.

Roughly 34.5 kW at nominal voltage. The motor that this thing drives is rated at 33 kW, or just a touch under 25 hp.

Physically, the fuse is only about 1.5" (38 mm) long and maybe 0.75" (19 mm) diameter, with big lugs sticking out of the ends to bolt it down. In use, it lives behind a polypropylene cover that you have to remove an interlock and a screw to get at.

Well, technically, all fuses just "melt". :)

Matt Roberds

In the Lambda LFS-48-24 power supply I mentioned, only one set of contacts are required. The sequences is roughly:

1. Apply AC power (there's no on-off switch in the PS) with a 10 ohm resistor in series with the AC line. BFC's (big fat caps) charge slowly, limiting inrush current.
2. When the voltage across the BFC's is at full charge and stable, short the 10 ohm resistor with an SPST (actually DPST with both contacts in parallel) to provide full power.

Shutdown is something like:

1. Remove AC power. Relay remains closed until BFC's are nearly discharged. This is to prevent momentary power glitches from producing much longer duration power drops at the output.
2. Relay opens when BFC's are discharged.

Ummm... my guess is that it should be:

1. Open main relay in + line only when caps are fully discharged. In this case, the idea is to NOT open the contacts under load to prevent arcing.

If there was something to detect input current, it could detect a much faster than normal rise in input current, and use that to shut down the PS. The problem is that by that time, it would be too late, and with welded contacts, there would be no way to shut it down. Argh.

Ummm... that's a bit larger than the typical commonly available power supply. Well, it's larger than anything I've ever seen. My closest experience is with solar powered battery controllers and inverters.

Is the fuse cartridge filled with sand?

I think of melting as a slow motion explosion. I was playing with a

5V 300A computer power supply. It had a 250A fuse in the output. So, I decided to see what would happen if I shorted the output with a screwdriver. (Do not try this at home). There was very little spark as the screwdriver welded itself to the copper bus bar contacts. I watched the fuse slowly melt into a puddle of tin(?). It took about 5 seconds. The screwdriver was quite hot and had to be removed from the contacts with a cold chisel and hammer. Apparently, the screwdriver was too high a resistance to "blow" the fuse and NOT trigger the over current detection circuit. I later tried to reproduce this with a similar power supply, but couldn't find the correct resistance screwdriver.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Part of the reason the system I am describing has more than one set of contacts is that switching the "line" power is also its job; it's not like your power supply where the user is expected to provide the line power switching.

The VFD, here, has the ability (and authority) to command the motor to zero speed, so it can arrange for there to be no load before it starts trying to open the relays. Both + and - lines float with respect to "ground", so it doesn't really matter which one it opens.

Good point. There is a current transformer on the "line" side of the DC input, before any relay contacts. I don't know if it uses that in its startup/shutdown strategy, but there isn't any reason why it couldn't.

I can guarantee that you've seen one of these, just not at this level of detail. They sold a lot of them in sunny California.

Dunno. It's got a label or wrapper around the whole thing, and I've never peeled it back. I've never blown one, and I've never unbolted one from its holder to shake it. It has a 2,000 A "breaking capacity", if that implies anything about its construction.

Charles Darwin never met *you*, apparently. :D

Somehow, that's not usually specified for screwdrivers, unless they are deliberately sold with a very high resistance.

Matt Roberds

all the lambda stuff I've come across is bizarre in design and construction.

I don't recall the model, but one was just a plain switching supply of a few hundred watts with a gigantic wirewound power resistor in it. Nobody else does that, nor do most places change their name every 5 weeks either- lambda, nemic, veeco, tdk, wtf?