EEVblog #376 - Multimeter Fuse Diode Followup - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: October 26, 2012, 12:15:22 am
content: Dave.

author: Wytnucls
author_rank: Super Contributor
timestamp: October 26, 2012, 09:00:43 am
content: 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.

author: EEVblog
author_rank: Unknown
timestamp: October 26, 2012, 09:30:44 am
content: Quote from: Wytnucls on October 26, 2012, 09:00:43 amSure. 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.

author: JackOfVA
author_rank: Unknown
timestamp: October 26, 2012, 02:54:29 pm
content: 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.

author: LaurenceW
author_rank: Frequent Contributor
timestamp: October 26, 2012, 09:37:25 pm
content: 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 ofgrossoverload?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

author: ttp
author_rank: Regular Contributor
timestamp: October 27, 2012, 10:39:26 am
content: Quote from: LaurenceW on October 26, 2012, 09:37:25 pmBack 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.

author: MartinX
author_rank: Regular Contributor
timestamp: October 27, 2012, 03:37:04 pm
content: 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.

author: Poe
author_rank: Frequent Contributor
timestamp: October 27, 2012, 10:20:11 pm
content: The Littelfuse specs don't look so good. Although admittedly an unlikely scenario, am I correct in assuming thatdoublethe 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%).

author: Dread
author_rank: Regular Contributor
timestamp: October 28, 2012, 03:08:21 am
content: 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?

author: nitro2k01
author_rank: Frequent Contributor
timestamp: October 28, 2012, 05:15:08 am
content: Quote from: Poe on October 27, 2012, 10:20:11 pmYes, 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 guaranteednotto blow at.Quote from: Poe on October 27, 2012, 10:20:11 pmYou'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.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: October 28, 2012, 05:30:23 am
content: Quote from: Dread on October 28, 2012, 03:08:21 amThe 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.)

author: Dread
author_rank: Regular Contributor
timestamp: October 28, 2012, 05:50:16 am
content: Quote from: nitro2k01 on October 28, 2012, 05:30:23 amGood 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.

author: ttp
author_rank: Regular Contributor
timestamp: October 29, 2012, 02:43:36 am
content: Quote from: MartinX on October 27, 2012, 03:37:04 pmI'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.

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 29, 2012, 04:18:11 am
content: Quote from: MartinX on October 27, 2012, 03:37:04 pmYou 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.

author: JackOfVA
author_rank: Unknown
timestamp: October 30, 2012, 11:35:30 pm
content: 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

author: robrenz
author_rank: Super Contributor
timestamp: October 31, 2012, 01:21:34 am
content: Very nicely done!

author: EEVblog
author_rank: Unknown
timestamp: October 31, 2012, 01:44:14 am
content: Quote from: JackOfVA on October 30, 2012, 11:35:30 pmNice test! Thanks for sharing.Dave.

author: robrenz
author_rank: Super Contributor
timestamp: October 31, 2012, 02:46:30 am
content: Wow that Kepco BOP is quite a bit of kit! I didn't know such a thing existed.

author: JackOfVA
author_rank: Unknown
timestamp: October 31, 2012, 09:49:00 pm
content: 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

author: robrenz
author_rank: Super Contributor
timestamp: November 01, 2012, 12:17:34 am
content: Are they fairly repairable, if I see a "not functioning" cheap on ebay is there hope or are they full of unobtanium?

author: JackOfVA
author_rank: Unknown
timestamp: November 01, 2012, 01:06:57 pm
content: 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

author: robrenz
author_rank: Super Contributor
timestamp: November 01, 2012, 01:28:07 pm
content: 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.

author: JackOfVA
author_rank: Unknown
timestamp: November 01, 2012, 03:20:17 pm
content: Quote from: robrenz on November 01, 2012, 01:28:07 pmThat 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.

author: robrenz
author_rank: Super Contributor
timestamp: November 05, 2012, 03:37:46 am
content: @ 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).

author: JackOfVA
author_rank: Unknown
timestamp: November 06, 2012, 03:07:09 pm
content: Looks to be a nice unit and you'll find use for it, I'm sure.Jack