EEVblog #376 - Multimeter Fuse Diode Followup

[EEVblog]

Dave.

[Wytnucls]

Thanks Dave, very informative video on an obscure but vital subject.
I see you're extrapolating below the graph at times. Are you sure that is a valid procedure in this case?
Temperature inertia would possibly modify the curve below its published performance.

[EEVblog]

I see you're extrapolating below the graph at times. Are you sure that is a valid procedure in this case?
Temperature inertia would possibly modify the curve below its published performance.

Sure. But in this case it's not going to suddenly shoot off horizontal the moment it gets below the graph. So it's safe to extrapolate and have bit of a guestimate if needed.
You don't get an accurate number, but you can get a ballpark.

Dave.

[JackOfVA]

One small point is that 0.6 or 0.7V DC drop across a typical general purpose silicon diode such as a 1N400x series is a valid approximation only for current in the 100mA or so range. At 10A forward current, a typical 1N400x diode has around 1.2V drop.

With 6 diodes in series at 10A, the net voltage drop will be about 7.2V, so the 5 ohm resistor shunted by the diodes will dissipate a bit over 10 watts.

Since the fuse will blow in a few milliseconds, the resistor will safely handle this brief overload. 

[LaurenceW]

Hmm, I guess practical experience bears this out (You're not a MAN until you've dropped your nice new Fluke across the 240V AC mains on the 400mA current range, AND survived), but such behaviour can cause maybe several HUNDRED amps to flow.

OK, the fuse will blow in fractions of a mS, but the poor diodes still have to cope for this time. Are we all fairly comfortable to extrapolate the graphs in this way? Is the basic failure mode of the diode still and only due to thermal effects (in which case we might get away with it), or are other electromagnetic or other breakdown effects at work here, in a few microseconds of gross overload?

Nice cheap PUCKER fuses available from CPC in the UK, by the way. http://cpc.farnell.com/1/1/13932-fuse-test-equipment-440ma-dmm-b-44100-bussmann.html

[ttp]

Hmm, I guess practical experience bears this out (You're not a MAN until you've dropped your nice new Fluke across the 240V AC mains on the 400mA current range, AND survived), but such behaviour can cause maybe several HUNDRED amps to flow.

Back in the 80's I had Fluke 77 that ended up across 240V AC with switch set to VAC but the probe still plugged into 10A terminal. Resulting arc melted about 3mm of one probe, as far as I remember the fuse inside just exploded into pieces. The meter was dead, dead beyond economical repair. Good input protection is good, prevention is even better. It's almost 30 years since then and I'm still very careful to unplug the probe from current terminal as soon as I'm finished with current testing . I can say I've learned something from that experience.

[MartinX]

I think I remember older Fluke77 that said UNFUSED next to the 10A jack, I definitely know some old Beckman multimeters were unfused on the 10A range because I have one myself.

[Poe]

The Littelfuse specs don't look so good.  Although admittedly an unlikely scenario, am I correct in assuming that double the diode's max continuous current could be applied for 16 minutes?

The Bussman fuse appears to be fairly well matched to the diode.  It might blow slowly (hundreds of seconds), but only at slightly greater than the max continuous current of the diode (~120%).

[Dread]

One very common thing is using the dmm in checking switch mode power supplies and those graphs did not look very good at higher frequencies. Any thoughts on this?

[nitro2k01]

The Littelfuse specs don't look so good.  Although admittedly an unlikely scenario, am I correct in assuming that double the diode's max continuous current could be applied for 16 minutes?

Yes, it's a specialty fuse for multimeters. It\s meant to protect against gross overload conditions. The current rating is not the smallest current that the fuse is guaranteed to blow at, but the largest current that the fuse is guaranteed not to blow at.

The Bussman fuse appears to be fairly well matched to the diode.  It might blow slowly (hundreds of seconds), but only at slightly greater than the max continuous current of the diode (~120%).

You're forgetting half of the circuit, namely the shunt resistor. If you go by the voltage drop and current of the diode, and the resistance of the shunt, you get that you need 3.6 V / 5 ohm = 720 mA before the diodes (nominally) turn on. Of course, there's a transition range of current between maybe 500 mA and 1.5 A where the current is shared between the shunt and the diodes. Lower than that and the diodes won't come on so all current goes through the shunt. Higher than that and the diodes will conduct all of the current, and clamp the voltage to protect the shunt.

Consider the following possibility: You have a load which continuously draws say 1.25 A. Pretend half of that goes through the shunt, which produces a bit under the nominal 4W of power. The diode is also below its rated continuous current. Not a problem anywhere, and this condition can exist basically indefinitely without damage. The fuse will still blow in ~5 minutes, but it doesn't really have to at that point. At the 1.5 A point, the fuse goes down in the 1 s and lower range. The shunt and diodes should still be able to withstand that basically indefinitely, but that's when the fuse starts to blow quickly. I think you'll find there's no range of currents  that won't either be safe for the components or blow the fuse quickly.

[nitro2k01]

One very common thing is using the dmm in checking switch mode power supplies and those graphs did not look very good at higher frequencies. Any thoughts on this?

The diodes will turn on quickly but recover slowly. And if you have enormous and fast current spikes, (on the milliamps range) those will be caught by the capacitor (C51, 10 nF), and still heat up the fuse. The meter will be safe, though in the absolute worst case scenario the fuse might blow unnecessarily.

If you're probing high frequency currents like that, your bigger problem is probably getting accurate readings, even on the true RMS version of the meter. Then you need a proper power analyzer (or alternatively better input and output filtering on the power supply, if you're just probing the inputs and outputs.)

[Dread]

One very common thing is using the dmm in checking switch mode power supplies and those graphs did not look very good at higher frequencies. Any thoughts on this?

The diodes will turn on quickly but recover slowly. And if you have enormous and fast current spikes, (on the milliamps range) those will be caught by the capacitor (C51, 10 nF), and still heat up the fuse. The meter will be safe, though in the absolute worst case scenario the fuse might blow unnecessarily.

If you're probing high frequency currents like that, your bigger problem is probably getting accurate readings, even on the true RMS version of the meter. Then you need a proper power analyzer (or alternatively better input and output filtering on the power supply, if you're just probing the inputs and outputs.)

Good point, I did not think about the caps.
My orginal thought was having the Dmm set to read Ma by mistake and then touching the positive probe on the HV side while the other is grounded. 

[ttp]

I think I remember older Fluke77 that said UNFUSED next to the 10A jack, I definitely know some old Beckman multimeters were unfused on the 10A range because I have one myself.

I've seen some images of Fluke 77 with UNFUSED written on the case, I've also found images with FUSED on the case. Mine must have been the later case as I rememeber hoping a new fuse and inside cleanup will restore my Fluke to life again... Service manual from late 80's has 15A fuse on 10A input.

[ModemHead]

I think I remember older Fluke77 that said UNFUSED next to the 10A jack, I definitely know some old Beckman multimeters were unfused on the 10A range because I have one myself.

You remember correctly.

According to the service manual, the first version only had a 630mA ceramic fuse on the 300mA input jack, while the 10A jack is wired straight to its shunt.  Pictured here is the second version, which added a 3A HRC fuse in series with the small fuse for the 300mA input jack, and the 10A jack is still unfused.

[JackOfVA]

I ran a test to determine the voltage drop across six series 1N4007 diodes with currents between 250 mA and 11A. Test setup block diagram, photo of test board, oscilloscope grab and plot of V versus I below.

Data pretty well track the data sheet, 10A corresponds to about 1.25V drop per diode, or 7.5V for 6 in series.

Jack
 

[robrenz]

Very nicely done!

[EEVblog]

I ran a test to determine the voltage drop across six series 1N4007 diodes with currents between 250 mA and 11A. Test setup block diagram, photo of test board, oscilloscope grab and plot of V versus I below.

Data pretty well track the data sheet, 10A corresponds to about 1.25V drop per diode, or 7.5V for 6 in series.

Nice test! Thanks for sharing.

Dave.

[robrenz]

Wow that Kepco BOP is quite a bit of kit!  I didn't know such a thing existed.

[JackOfVA]

I have three of them, +/- 20V at 10A,  +/-100V at 2A and +/- 1KV at (if I recall correctly) 100 mA.

You can use them as a conventional power supply with an adjustment knob on the front, or drive them from a function generator, in which case they work as a voltage (or current) amplifier.

Frequency response isn't fantastic, depends on the model, but they are in the 10-15 KHz range.

Jack

[robrenz]

Are they fairly repairable, if I see a "not functioning" cheap on ebay is there hope or are they full of unobtanium?

[JackOfVA]

The main problem is a lack of service info - I've been unable to locate a schematic source other than from Kepco, and I recall they asked about $100 for a service manual.

One item of note is that these power supplies have a card edge connector extending from the rear panel. Normally there's a connector that plugs into the connector with certain pins jumpered. If that connector is missing, the power supply will not function. Hence, there's a reasonable chance that one identified as "not working" may just be missing the connector with jumpers. You can download the normal operations manual from Kepco and it has details on the pin jumpers and block diagrams. But no schematic. If you don't care about using the card edge connector for remote analog programming, you should be able to just solder the appropriate pins. (I also recall there may be a 10K resistor used between a couple of pins, but it's been too long since I built a connector up that I could be mistaken.)

Out of curiosity, I asked Kepco for a repair quote - this was a couple years ago - and their answer was a flat rate equal to 50% of the price of a new supply.

Looking inside the two low voltage units I have, the parts look more or less standard. It's all analog and seems to have common garden variety op amps and the like, all through hole types. They should be repairable. Have not had the HV version open, but I suspect it's similar. (The HV model I have uses vacuum tube regulators, and but a later PCB revision has transistor series regulators .)

Jack

[robrenz]

Thanks for the info JackOfVA. I was thinking of this for a amplifier for my function gen but the frequency is kind low and may end up more expensive than a amp specificaly designed for a FG.

[JackOfVA]

Thanks for the info JackOfVA. I was thinking of this for a amplifier for my function gen but the frequency is kind low and may end up more expensive than a amp specificaly designed for a FG.

That depends on your needs, of course. I use the lower voltage, high current supplies to drive inductors through their B-H curves and the higher voltage supplies to drive capacitors through their C-V curves. 1 KHz is usually adequate.

Rigol makes a 10 watt amp for its function generators, with a response up to a MHz or perhaps more. Probably will add one of those to my collection as well.

[robrenz]

@ JackOfVA,

Thanks to you I scored a brand new Electronic Measurements Inc. BOSS20-10-1-D-PM Bipolar Operational Source/Sink Power Supply ±0 - 20V, 0 - 10A  on Ebay.  It looks to have very similar specs to the Kepco BOP. They became part of Lambda and the BOS/S are not in production any more. Now my FG will finaly have some power.

EDIT:
Talked to TDK Lambda service and they are sending me the schematics (free) that the techs used for these units with all their markups. including the digital control interface unit. (which my unit does not have).

[JackOfVA]

Looks to be a nice unit and you'll find use for it, I'm sure.

Jack