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2465B DMM cal
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So I finally found a transformer that will give me the 500 Volts I need to calibrate the DMM on the 2465BDM scope. The part that I'm having problems with is the fact that it appears to change the impedance of the test leads when it goes in to actual testing. For example I may set to 190.00 volts AC and then when it goes in to cal "busy" mode it jumps to 220+ volts. I can only assuming it's doing something to change the impedance of the leads. It passes on to the next test however the calibration is way off when I finally get to use it. Has anyone had any experience with this type of calibration. I have the scopes DMM in parallel with a reasonably calibrated DMM to act as a transfer standard. I'm making the assumption that the scopes DMM will not change it's input impedance..clearly falsely.
Thanks, Jeff |
¿ªÔÆÌåÓýYou need to put your 190 volts inside a regulated loop to get the effective
source impedance way down.
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Or, you could set it to some lower voltage like 150.xx so that when the
calibration takes place, the correct input is seen.
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Tom
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OK, pretty much the conclusion I was coming to as well. I just didn't expect it to be changing the impedance when the test commences. I really didn't expect it to increase, decrease I could see if it was switching in the 10 Meg impedance of the meter...but decrease..I'm a bit confused by that. I guess I can measure the impedance and see what's it's going to and from. If I know that I can predict the values. I'll post what I find to perhaps help someone out along the way.
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Thanks, Jeff On 12/29/2012 10:31 PM, Tom Miller wrote:
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Jeff,
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Is this a standard HV power transformer? I have never calibrated a 2465B DMM but the things I have calibrated use frequencies around 1 KHz or higher. Two thoughts. Since it is a transformer and a 'current' device, I would think that you would need an appropriate 'load' (noticeably lower than the input impedance of the DMM's) to keep the output voltage 'stable' and, assuming this is a resistor, with a power rating of V^2/R. If there are two DMM's in parallel, you should be OK working as a 'transfer standard' unless the meters make some different internal connections (zero sampling, etc.) as part of the measurement process whereby the DMM automatically disconnects and reconnects itself as part of the measurement process. The 3457A does this. You might want to consider a high power HiFi amplifier and an audio signal generator to generate some AC voltages. For me, the AC standards were the hardest. DC, Current and Resistance were much easier. Good luck. Joe -----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Jeff Machesky Sent: Sunday, December 30, 2012 1:33 AM To: TekScopes@... Subject: Re: [TekScopes] 2465B DMM cal OK, pretty much the conclusion I was coming to as well. I just didn't expect it to be changing the impedance when the test commences. I really didn't expect it to increase, decrease I could see if it was switching in the 10 Meg impedance of the meter...but decrease..I'm a bit confused by that. I guess I can measure the impedance and see what's it's going to and from. If I know that I can predict the values. I'll post what I find to perhaps help someone out along the way. Thanks, Jeff On 12/29/2012 10:31 PM, Tom Miller wrote:
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That would be great Jeff. Good luck, hope it works.
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Tom ----- Original Message -----
From: "Jeff Machesky" <jeff@...> To: <TekScopes@...> Sent: Sunday, December 30, 2012 2:32 AM Subject: Re: [TekScopes] 2465B DMM cal OK, pretty much the conclusion I was coming to as well. I just didn't expect it to be changing the impedance when the test commences. I really didn't expect it to increase, decrease I could see if it was switching in the 10 Meg impedance of the meter...but decrease..I'm a bit confused by that. I guess I can measure the impedance and see what's it's going to and from. If I know that I can predict the values. I'll post what I find to perhaps help someone out along the way. |
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On Sat, Dec 29, 2012 at 7:38 PM, Jeff Machesky <jeff@...> wrote:
So I finally found a transformer that will give me the 500 Volts I needHow about a little more information about what you are doing, something like: I have a ??? connected to a ??? transformer driving the scope The ??? produces ??? volts at ??? Hz |
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David, I was just about to do just that.
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A brief history to start, a while back the NVRAM in my 2465BDM had been lost. I have since replaced the Dallas chip with a Maxxim chip and successfully and fairly accurately re-calibrated the scope portion of my 2465BDM. The frequency counter I calibrated using 2.5, 5 and 10 Mhz WWV signals off shortwave with the beat freq method. The DMM portion I was able to calibrate all the DC portions with useable (while not up to spec) accuracy. I had used a switch mode circuit I build up to produce the 1000 volts DC needed. The DC appears fairly accurate. The AC calibration had eluded me because I didn't have anything floating around to produce 500 volts AC. Yesterday I had found a transformer that was from an AC unit, it has taps for 200, 230 and 460 volts in and 24 v AC out. Knowing I could feed 30 volts or so in the secondary and get my 500 V AC out the primary I did so. The problem was I needed to trim that voltage to precise values. The 200 V tap was good for the 190V cal, the 460V tap was good for the 500V cal. From that I made a resistor divider using the two meters as part of the divider. This kept the current down and allowed me to use a 10 turn trim pot to obtain exact voltages. The waveforms were staying in pretty good shape. Well this seamed like a good idea...however every time the Tek's DMM goes in to it's sample mode to actually do the calibration the voltages I was seeing on my reference meter were shooting up. Based on the circuit layout this was indicating to me an increase in impedance on the meter side. So I think at this point I may have to use a resistor divider network that has more current capability or control the voltage feeding the transformer with either a triac, rheostat or a variac. I just now found out that the input impedance of the Tek's DMM is going from Gigaohm range to 1.000 Meg while it's taking it's sample. I was obviously mistaken about the dividers behavior. Another observation, if the calibration fails with an out of limits message it appears the impedance stays at the 1 Meg, this could potentially allow me to re-adjust my input and try the step again. So in a sense I'm only seeing the impedance of the transfer meter until the Tek decides to sample and then I'm seeing around 909k. Ohh and I'll be the owner of an HP 3457a on Thursday, not that it'll help me much with the higher voltages. Thanks, Jeff On 12/30/2012 10:12 AM, David DiGiacomo wrote:
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I've not used the DMM on my 2465 all that much, however I found out something pretty scary. When the scope turns on the DMM does self checks and one of those checks puts the DMM in current mode. So if you happen to have any circuit connected and power cycle the scope you'll end up with a short on the DMM. Has anyone else noticed this behavior ?
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Thanks, Jeff On 12/30/2012 8:56 AM, Tom Miller wrote:
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So I thought I would share with the group the techniques I ended up using to calibrate at least the AC voltage side of the 2465BDM's DMM option.
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For the 190v and 500v 60 Hz I used a 460v to 24v transformer feed in reverse at around 30v or so to obtain just over 500 volts. From that I feed in to a resistor divider with 1 Meg on one side and a series of 100k trimmer pots on the other. This combination allowed me to adjust enough to get exactly 190.00v or 500.00v. I had both my transfer standard meter and the scopes DMM connected at the same time. The voltage would increase during the test, but when it was all said and done everything appeared to be in spec. For the 50 Khz test signals I had two options. Using my sig gen I first tried the step up transformer I had been using and much to my surprise it peaked out in coupling around 20 Khz @ 235 volts RMS with only 10 volts input. Increasing to 50 Khz it fell down to only around 19 volts. So while technically this would work it wasn't an ideal solution. I like to have a little margin. I looked around the mess I call my shop and noticed I had a collection of PC related power supply transformers and figured..hey that's an idea. So using the same technique with the PC power supply transformers I easily maxed out 50 volts / div at several Mhz. I would need 100x probes to see how far it would go. I was able to ascertain at least 400+ volts RMS from the bench meter @ 50 Khz. The other one that appears to work really well at high frequencies is the back lighting transformers found in laptop and LCD monitors. They can produce some crazy high voltages at several hundred Khz. The PC power supply transformers should also work for current calibrations by using them as they were designed. You'll end up with low voltage but higher currents. The signal generator output can be used to adjust that current. They won't work for the 60 Hz portion however. For that regular transformers would need to be used, or in some cases just a signal generator. Hope this helps anyone down the road. Thanks, Jeff On 12/30/2012 10:12 AM, David DiGiacomo wrote:
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