Author Topic: Instek GPD-3303S Power Supply Overshoot - Opinion wanted  (Read 14699 times)

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Offline Gary350z

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Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« on: January 21, 2017, 01:28:25 pm »
I recently purchased an Instek GPD-3303S Power Supply.
It works good with the following exception.

The channel 3 output, which is switchable between 5, 3.3 and 2.5 volts, has a very large overshoot when the output is switched on as shown in the attached image.

The peak value of the overshoot is about 9.2V with the output loaded or unloaded. The overshoot is the same regardless if the output is set to 5, 3.3 or 2.5 volts.

The pulse is narrow, about 40us, so it probably has a low energy content.
A 470uf cap across the output will eliminate the overshoot.

My question: Is this overshoot acceptable or bad? If bad, how bad is it? Will it blow out circuits attached to it?

Thanks for any advice.
 
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Offline henken

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #1 on: January 21, 2017, 03:16:46 pm »
I'll just add some data from my unit, for comparison.

My unit's channel 3 overshoots to approximately 7 V, regardless of which output voltage is selected. For me, the overshoot is ~30µs long (time with Vout > 5) with the channel set to 5 V.
 

Offline Fgrir

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #2 on: January 21, 2017, 04:57:45 pm »
I've confirmed the same effect on my GPD-4303S which is the 4-channel version of that supply.  On this model channels 3 and 4 are fully adjustable, but I get almost the same transient at start-up on channel 3 at any voltage setting.  Interestingly my channel 4 shows no overshoot at any setting.
As far as damaging circuits, I think it will depend on the circuit.  I know I'll keep this in mind before I connect anything sensitive to channel 3 from now on - maybe I need to print up a warning sticker.
 

Offline Kleinstein

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #3 on: January 21, 2017, 05:05:50 pm »
This is a significant overshoot. It really should not happen. If a 470 µF or similar capacitor can fix it, it would be a good idea to add one (maybe a little smaller so the overshoot is limited to 100-400 mV).
 

Offline bitseeker

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #4 on: January 21, 2017, 10:34:01 pm »
That startup waveform looks really bad. Not only does it severely overshoot, but it doesn't even rise smoothly afterward. Interestingly, Fgrir's captures of channels 1, 2 and 4 show proper and smooth startups. Something's definitely wrong with channel 3 on these supplies.

Hopefully, a not-too-big capacitor will smooth things out. Don't make it too big, though, because it does store energy, energy which will be suddenly released (i.e., with no current limiting) if your circuit ever has a short in it during operation. So, the capacitor should be just big enough, but not larger.

Of course, the ideal solution is to fix the regulation problem.
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Offline djnz

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #5 on: January 22, 2017, 05:50:17 am »
Apparently Siglent SPD3303() power supplies also have / used to have this problem, and some fixes are discussed here:

https://www.eevblog.com/forum/testgear/siglent-spd3303d-review/25/

Depending on how close the designs of these supplies are, it could be useful to read that thread.
 
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Offline bitseeker

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #6 on: January 22, 2017, 07:45:20 am »
Thanks for digging that up, djnz. And here's the post with the details of Siglent's fix:

https://www.eevblog.com/forum/testgear/siglent-spd3303d-review/msg299246/#msg299246
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Offline Fgrir

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #7 on: January 22, 2017, 08:31:17 pm »
OK, warranties are for losers so I cracked my brand-new GPD4303S open to see why this spike is happening.

First theory was that the service manual schematic has R727 going to +5VCC instead of +5VC as it should.  This would have eliminated the intended 7mV offset in the voltage feedback signal and would prevent them from setting the voltage setpoint down to zero, or even better below zero to ensure that the integrator stage would saturate low instead of high when Q706 is engaged to disable the output.  No joy here though, probing the board it appears that R727 is actually connected to +5VC as it should be and this is either a typo or an old schematic that has been fixed already.  :(

So now I am pretty sure it is just a bad design of their disable circuit.  With the unit powered on but output disabled, on my channel 3 I am seeing both V and I integrator outputs (U705) saturated high.  This means that when the Q706 clamp is released at turn-on the main output FET will be turned hard on until either of the integrators can slew down into regulation.  Looking at the inputs to the voltage integrator I see the feedback signal at ~5mV and the control DAC output at ~6mV.

On channel 4 the added offset to the feedback voltage is larger (9mV) due to the higher feedback gain.  Looking at my channel 4 while still disabled the DAC output for the voltage control actually sits at about 2.5mV which is below the feedback signal of 9mV so it is able to keep the voltage feedback integrator saturated low when the output is disabled.  I think this is why my channel 4 doesn't glitch at startup - the voltage control loop is starting from a disabled state rather than the enabled state that channel 3 was starting from.

The datasheet for the DAC (AD5643R) shows a 2mV typical and 10mV max for the zero-scale output, so it could be a part-to-part variation between my channels, but I think it is more likely a firmware thing that they don't drive channel 3 to a hard zero when off but they do for channel 4.  It makes me wonder if a firmware update could fix the problem on the 4303S.  If not that then I think a slight increase in the voltage feedback offset voltage might cure the glitch at the expense of a small setpoint error.  Changing R727 from 1M to something about 700K would add 3mV to the feedback offset which should be enough to keep the integrator low when disabled.  I think I might be willing to trade 3mV offset error for getting rid of this glitch.

However, for the 3303S there is no DAC for channel 3, just a set of resistor dividers to choose the setpoint and a saturated 2N3904 to clamp the setpoint while the output is disabled.  I don't expect that transistor will be able to hold the voltage anywhere near the 7mV offset on the feedback signal, so for this model I expect that the integrator will always be saturated high when coming out of the disabled state.  I can't really see a simple fix for this model.

Now to decide if I want to solder on my brand-new power supply or just put the case back on and live with the glitch...  :-//
 

Offline bitseeker

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #8 on: January 23, 2017, 01:40:47 am »
Interesting investigation, Fgrir. Thanks for the sharing. As a first step, you could temporarily put a resistor in parallel with R727 to confirm the fix. Then, decide if you want to make it permanent. :-/O
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Offline Kleinstein

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #9 on: January 23, 2017, 08:26:48 pm »
There is 2nd thread on the GPD3303 supply  rather similar. https://www.eevblog.com/forum/testgear/thoughts-on-gw-instek-3303s-supply/msg1118794/#msg1118794

I there suggested a possible "fix", by making the control slightly faster. This way the existing 200 µF cap at the output would be big enough to absorb the than 5-10 times smaller peak. It is not a true fix for the root problem but could have other advantages too.

I very much support the analysis of Fgrir: the clamping transistors can not bring the voltage down to below about 50 mV (CE saturation) and thus the integrator goes to the positive limit. One possible way around would be to replace Q707 and Q706 with N MOSFETs, that would allow a lower clamping and thus likely negative saturation (if the OPs offset is not too large). Does not work with channel 4 though, as there is no second transistor.

Another option might be a (PNP) transistor and maybe a diode on the collector side in parallel to c717 with base control from behind the diodes D704/D705. This would limit overshoot to about 1.4 V. It is not perfect anti windup, but would also work with CC to CV transitions.
 
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Offline Fgrir

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #10 on: January 23, 2017, 09:37:47 pm »
Yes, I hate that this got split into two threads but here seemed more specific to the overshoot problem.

If I was going to make my mod I was leaning towards adding a resistor from the cathode of ZD703 to pin 3 of U704.  This would have a similar effect to changing the value of R727 while in the disabled state, but with a much smaller error when enabled.  However I am a bit less enthusiastic about my idea for ensuring the integrator stays low while disabled since I realized that in this state the op-amp output is basically short-circuited.  On my channel 4 (which as-built enters this state) I am measuring -1.6V at U805A's output.  The TL072 can theoretically handle this, but with a 15V rail and the typical short-circuit current of 40mA it is dissipating over 500mW which would raise its temperature by 67C over ambient using the 125C/W published in it's datasheet.  For the worst-case 60mA short-circuit current we are looking at a 100C rise above ambient.  I don't have any thermometers handy, but touching U805 confirms it is working very hard in the disabled state.

I think all of Kleinstein's suggestions make sense as well, but for me these solutions are becoming more complicated than I want to try to hack in to a $500 power supply.  I am leaning towards putting the case back on my unmodified unit and waiting to see if channel 4 dies an early death before I consider any changes to fix the glitch on channel 3.  A warning sticker on channel 3 should be enough to keep me from connecting it to anything that can't handle an 8V spike.

Mostly I am going to add this to my pile of reasons^H^H^H^H^H^H^Hexcuses to pay more for top-name gear.  If I had been doing paying work instead of troubleshooting this thing I could have easily spent twice as much  ::)
 

Offline bitseeker

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #11 on: January 23, 2017, 10:08:14 pm »
Mostly I am going to add this to my pile of reasons^H^H^H^H^H^H^Hexcuses to pay more for top-name gear.

Nah, not an excuse -- just prudent shopping. Of course, not everyone lives in an area with good access to top-name gear at good prices, so that's a factor too. In my case, all my power supplies are HPAK or Power Designs, except one -- a Mastech rebrand that behaves badly on shutdown. But that's a topic for another thread.
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Offline Kleinstein

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #12 on: January 24, 2017, 01:13:53 pm »
Having the OP to go into negative saturation could in deed be a problem. I agree that adding true anti windup (or changing to MOSFETs would be a bigger modification.
My first suggested "fix" by making the control faster should is relatively straight forward and simple.

As in the plan the control of channels 3 and 4 is rather slow and is missing the usual phase boost one needs in a LDO type regulator.

According to my simulation, the unmodified circuit should show quite some ringing when there is a significant low ESR capacitor (e.g. 470 µF) at the output and a step to low load current. It depends on the ESR of the existing 200 µF capacitor at the output how bad things get. So just adding a capacitor to the output is not such a good idea.

Adding an RC combination parallel to R712 (for channel 3) gives phase boost and is the usual way to dampen that kind of ringing. Channels 1 and 2 also have it, though in a slightly different configuration. As a side effect the speed of regulation gets higher and by this way the overshoot is reduced. In the simulation something like 1,5 K and 50 nF worked well, but the values chosen are not that critical. Already 3.3 K and 10 n gives a significant reduction in the peak and dampen ringing to an acceptable level. So one can chose on how much faster it should get. As this is only AC, it should not have an influence on the set voltage and voltage reading.
 
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Offline Fgrir

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #13 on: January 24, 2017, 06:26:16 pm »
Even with no external capacitor the transient control on channel 3 is not very good.  Channel 3 recovery from overcurrent is a disaster, and while it is better it's not even very good for channel one.  I am pretty much set at this point give up on improvements and to consider my GPD-4303S as a low-quality dual output power supply and only use channels 3 and 4 in cases of dire need.  It is nice having the 1mA set and readback, but as a power supply this thing leaves a bit to be desired.

A few overcurrent recovery test results:
Load is Aim-TTI LD400P set for transient between 0A and 1A, f=0.5Hz, Duty = 99% at 0A, Slew = 4A/ms
In all cases the power supply is set to 5V 0.9A output.
For comparison (and my own comfort) I also tested my trusty old Topward 6303A which has been powering my prototypes for the last 15 years.  IMHO this is what an overcurrent recovery should look like, and this wasn't a particularly expensive supply. :-+
« Last Edit: January 24, 2017, 06:28:41 pm by Fgrir »
 
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Offline FastEddieFelson

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #14 on: January 24, 2017, 06:53:01 pm »
Even with no external capacitor the transient control on channel 3 is not very good.  Channel 3 recovery from overcurrent is a disaster, and while it is better it's not even very good for channel one.  I am pretty much set at this point give up on improvements and to consider my GPD-4303S as a low-quality dual output power supply and only use channels 3 and 4 in cases of dire need.

Thanks for the additional info Fgrir. Really appreciate it. I had been looking at getting one of the GPD-3303S units myself as a first benchtop PSU since the ITT liquidation has made them cheap and widely available. Just pulled the trigger yesterday and bought one NIB for $200. Now I'm not sure I made the right call. Can you help a newbie out and put your findings into a beginner's context for me?

Is this still acceptable for a first benchtop PSU? I had been looking at the cheaper Chinese made units and figured an Instek would be better quality for roughly the same $$. Should your findings scare someone off like me who's just starting out and ready to get their first dedicated PSU or are these findings more for the hardcore "pixel peepers" or whatever the EE equivalent is? If I leave Channel 3 set to 5V and never move it while I've got a circuit hooked up I should be OK with overvoltage issues right? What about overcurrent recovery? Is there a similar technique I can use to mitigate the effect you've found or is the overcurrent a dealbreaker for a newbie? Thank you again.
 

Offline bitseeker

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #15 on: January 24, 2017, 07:13:02 pm »
Can you help a newbie out and put your findings into a beginner's context for me?

I would summarize it as follows. Avoid using channel 3 on anything that can't withstand 8-9 volt spikes. If unsure, don't connect it to channel 3. When using channel 3, it's safer to enable the output first, then attach the device or circuit. Although this doesn't mitigate every issue, it should cover most bases. When in doubt, leave it out (of channel 3).

Quote
Is this still acceptable for a first benchtop PSU?

It'll work, but it's unfortunate that channel 3 has many caveats. It's extra cognitive load for a beginner. Perhaps stick a warning note on channel 3 as Fgrir mentioned in one of his posts.

Quote
I had been looking at the cheaper Chinese made units and figured an Instek would be better quality for roughly the same $$.

That tends to be the case, but as shown here regardless of the name on the badge, there's no substitute for testing/verifying the actual operation and quality of a device.
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Offline Fgrir

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #16 on: January 24, 2017, 07:57:21 pm »
I'll second most of what bitseeker advised - I think it can be a decent supply if you understand it's limitations and for $200 new I doubt you'll find anything much better.  I think channels 1 and 2 are not too bad and I wouldn't be too worried about using them in most situations.  Channel 3 is pretty nasty and I think I would avoid using it except in cases where the load can tolerate the expected overvoltage, for example a board that has it's own voltage regulator or a load that is tolerant of short spikes.  I wouldn't use channel three to directly power a 3.3V microcontroller or logic circuit since I think that might not survive many turn-on cycles.
 

Offline Kleinstein

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #17 on: January 24, 2017, 09:19:13 pm »
Adding the RC combination in parallel to R712 should also improve the CC-CV mode transitions: In the simulation it does it, if the voltage goes significantly down for some time (e.g. ms range) before coming back from the CC mode. There is a little overshoot when the voltage only drops slightly, but usually not that much, as the CC mode control is also rather slow.

This same limitation (poor reaction after a short phase of CC mode with the voltage not going down significant), may apply to other supplies too. Often it is a kind for RC combination in the feedback path (like the supposed addition) that makes a relatively slow recovery from a lower voltage and still a fast recovery from normal load changes.

To get a significant better CC to CV mode transition, a kind of true anti windup might be needed. Many commercial supplies get away without this and thus might have similar limitations for a short time current limit.

I have an idea how to add true anti windup (described in an earlier post), but this only works really well if the OPs output is always higher than the input (=ADC output here). It depends on the on the power MOSFET gate threshold whether this condition is met. So this type of anti-windup would get difficult or not very accurate (need the extra series diode).

At least in the simulation, an extra RC parallel to R712 to a large part fixes the problems (turn on, CC-CV transition and ringing with capacitive load) and I don't see to much possible downsides.

The simulation showed a possible problem of the circuit not directly related:
If the OPs go significantly negative there can be a significant current flow through the small PNPs (e.g. Q701) base that drive the MOSFETs gate. This might even violate there specs (base current) and thus could be a potential reliability issue. It could also explain why the OP might get quite warm to touch - though under normal conditions this should only happen for a short time during transients. It could be longer time if an external voltage source is applied though. So it might be a good idea to add an extra series resistor to Q701,Q801,Q102,Q402.
 

Offline henken

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #18 on: January 24, 2017, 10:26:31 pm »
I made a quick attempt at adding the RC combination in parallel to R712 as proposed by Kleinstein.

However, this turned out to be a bit too much for my soldering skills.

R712 turns out to have a 0603-sized package, and the only 1.5k resistors I have are 0805. I also have no 50nF on hand, so I tried to combine the 0805 resistor with two 100nF 0603 capacitors, in a kind of lopsided Pi-shape on top of R712. This was a bit too difficult for me to pull off at the moment. Maybe there's an easier way to do this.  :-DD
 

Offline Kleinstein

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #19 on: January 24, 2017, 11:11:36 pm »
A resistor of only 0603 size is making soldering a little difficult in deed. My idea was still old style through hole. :palm: Now I understand Fgrir hesitating.

However I doubt there will be an easier solution than adding just 2 small parts with 2 connections to the board.

The free standing version with 3 parts directly on top of a 0603 could be tricky. Even just 2 parts is not easy to solder.

It does not have to be directly at the resistor: one side would also be at the negative output and other bigger parts (e.g. D703). So it could be a fine wire to the right point.

The values are not that critical - 100 nF or 2.2 K should be OK too.
 

Offline bitseeker

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #20 on: January 24, 2017, 11:38:25 pm »
You could probably use a small piece of protoboard for the RC filter and then wire it to R712. If it works well, mount the filter board with Kapton tape, hot glue, etc.
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Offline henken

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #21 on: January 25, 2017, 02:35:10 am »
I gave it another shot and managed to get the tiny components stacked over R712.  100nF, 100nF and 1.5k?. I measured the connections after soldering to verify.

The spike is reduced but still present (expected), but the reduction appears to be less than in your simulation. The spike now reaches only 3.24 volts on this unit.

I don't know why I don't get a reduction as significant as in the simulation, but for me this might be good enough (I mainly need 3.3V and 5V).

 

Offline bitseeker

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #22 on: January 25, 2017, 03:35:37 am »
That doesn't sound too bad, henken. Congrats on getting the spike under control. Post a pic of your handiwork?
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Offline Kleinstein

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #23 on: January 25, 2017, 05:22:10 pm »
There are a few uncertainties and simplifications in the simulation. For the peak on enable, I did not include the current limiting and I don't think it will have an influence. Also the ESR of the output capacitor can have an important influence: to little ESR might cause instability (also for the unmodified circuit) and higher ESR allows a higher peak. Similar the resistance of the shunt and the connecting wire and it's inductance has an influence. The extra resistance from the wire can help a little. So simulations one power supply circuit have it's limitations.

In my first simulations I had used a slightly small FET and this way got a smaller peak.

p.s.:
To make sure there is no problem with stability one should do a few tests with the modified supply: Something like load transients, maybe just with a switch and resistor.
« Last Edit: January 25, 2017, 05:25:35 pm by Kleinstein »
 

Offline Fgrir

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Re: Instek GPD-3303S Power Supply Overshoot - Opinion wanted
« Reply #24 on: January 25, 2017, 06:52:11 pm »
A resistor of only 0603 size is making soldering a little difficult in deed. My idea was still old style through hole. :palm: Now I understand Fgrir hesitating.
I probably should have mentioned that - if it was thru-hole I'd have been modding the crap out of this thing  :-/O

I might still have a go at it in the future, but not until I finish the job this was bought for.  That 1mA/1mV set/readback seems pretty unique at this price point and will make some testing I'm going to do much easier.  I'm glad that henken was braver than I so we could see that your suggestion offers a significant improvement to the size of the spike.  A 3.24V spike is still not ideal, but I can't imagine very many cases where that would cause damage.

Of course this whole exercise already has me eyeing more expensive supplies.  I may need to check into that Test Equipment Anonymous thread...
 


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