Mr Carlson's Lab patreon

Sharing more of my Circuit Designs with you! Here are some Test Equipment Protection Devices.

🕑 Added 2018-01-17 02:41:12 +0000 UTC

Comments

Mitch Blades

Richard, do you happen to have the schematic and or board diagram(s) for your version?

Tom Pratt

Would the DC blocking version be a good choice for a spectrum analyzer?

Gary Nations

I really like the DC block, thanks.

Gary Nations

Why not add a 28 or 48 volt gas discharge tube that would shunt to ground? On the first one.

LoSaYa

Nice information

Timothy M Tapio

Thanks!

Paul Coaker

Hammond on Digikey

Ryan Dunbar W1RMD

Not a bad idea. I'll try that too!

Timothy M Tapio

Where do you find those little boxes to build your circuits in? I need some of those in various sizes as well as small metal ones to provide shielding for wideband LNAs, filters, etc.

John Scoles

bigger is better

Brad Shugart

Anyone know if the wattage rating for the Zeners is important? Two choices are 1/2 watt or 1.3 watts.

Brad Shugart

I think the design intent is to make much easier replace the components when they sacrifice their lives. It's much more work to open up the scope and work on the board. In my case, I refurbished an RCA WO-91A and the only place to mount the board was underneath. The access to work on the board is more difficult than where Paul placed it in the RCA Wo-31A.

Richard Kelly

Returning to an old thread, but a good one. I have just built the curve tracer, albeit a SMD version with an audio op-amp as a split rail producer from a single rail supply. I see the benefit of the protection and was about to order some 15V zeners when I had a thought. My question is: if we have rails at say 15V, then shouldn't we be using a pair of back-to-back zeners whose combined fwd and reverse voltages fall under the rail voltage? I seem to recall we should protect op-amps from an input which exceeds the supply rail, or at least no more than 0.3V wide of it?

MarkM

I already built an Octopus curve tracer a couple of years ago. I will add this protection circuit to the input stage of mine. That back-to-back Zener trick blew me away! I even have 15V @1W Zeners! I've had them for decades and thought I'll never be able to use them in any projects. Glad to see I was wrong. Love your channel. Very glad I joined.

Björn Ahlström

Bruce Dow

Brilliant! :-)

Edward Bones

Still, I would like to know why original equipment builders do not add the dc block inside the cabinet of their equipment. I realize it adds cost, but on an expensive signal generator or spectrum analyzer it seems like a no brainer. Is it because ultimately it affects input impedance, causes attenuation, etc. and would be hard to give output spec’s at all conditions? Is it purely cost savings? Or as you say,”what were the engineers thinking?”

Kevin Carson

I know this is an old thread, I am unsure if anyone will see this. But, Can you not just add the 250ma fuse into the D/C block circuit between the 56k resistor and the probe side and have both d/c block and OV protection in one box? I am going to build it that way unless It would interfere with the curve tracer. Thanks

Tom Hamernik

I'm curious, why isn't this protection built directly into the CT PCB?

Adriaan Diedericks

Hi Paul. Could i use one of these circuits to protect my bench amplifier when doing AF signal tracing? I currently have a 0,1uf HV capacitor in series with a 10k ohm feeding two anti- parallel diodes. The signal to the amp is then taken across the two silicon diodes. I was thinking of using your circuit and changing the zeners to to si diodes. I like the smaller cap in yours and the 10M bleeder resistors. Like to hear your thoughts.

Mr Carlson's Lab

Hi Jason. Taking into consideration the speed of a Varistor, these parameters would have to be tested before I would say "OK."

Jasin Colegrove

Hi Mr. Carlson, I was wondering if you could replace the two zener diodes with one varistor with a breakdown voltage just above the 10 volts? wouldn't this serve the exact same purpose as the two zener protection diodes just in a single package? I was just curious, given the characteristic curve of a varistor it seems very plausible, but i'm no expert. Kind regards

Mr Carlson's Lab

Thats a perfectly valid question Stephen. It all depends on the project. In this case, the 10 Meg resistors are 1/2 Watt. Normally, the Wattage is chosen as to the amount of current the resistor has to deal with. The Wattage of the 10 Meg resistors in this case is just chosen due to the amount of them that I have on hand :^)

Stephen Martin

Hoping this isn't a dumb question... Paul, when you use components ie. resistors/capacitors/etc,, what wattage rating are they? e.g. You mention using a 10k resistor, but is it a 1/8watt, 1/4watt, 1/2watt, ??? rated component? Thx.

Mr Carlson's Lab

Ok. The circuit you have made is essentially a DC block then. The diodes will clip the signal at the forward ratings of the diodes. If that signal has any "push" behind it, the diodes may get hot if they are actively clipping the signal. the resistor you have added will take some of the load though.

Joe Eide

When I say that I can watch the signal, I mean that I can watch the S-meter along with a spectrum display. The spectrum display on the receiver not only shows the wanted signal, but also unwanted near by. As to the RF signal present, the old VFO's for example can generate several volts. Hope that helps. Joe

Joe Eide

I will use it to sniff circuits in the 1.8 to 30 mhz area primarily. However, if I accidently make contact with B+, I don't want to damage the input of my receiver. I do not plan to transmit through this! I work on a lot of old boatanchors (vfo's, receivers & cw transmitters, etc). The tranceiver makes a great signal tracer because I can listen and watch the signals and even "auto tune" the receiver to the signal under test. It works great for checking squirrelly vfo's etc. It is used as a relative signal strength monitor, so some loss and tilt through the spectrum under test is not critical. Thanks again.

Mr Carlson's Lab

That looks good Joe. What is this device going to isolate you from? (this is important, so give me a good description) How much RF is present? Is it just a signal generator? Is this going to be installed in an RF line where 100 Watts is present? Do you intend this to act like a switch? These are important things for me to know.

Joe Eide

Okay Paul, here is a link. <a href="https://www.dropbox.com/s/62eord9xi1xuhih/KB9R%20-%20Rcvr%20Protect.JPG?dl=0" rel="nofollow noopener" target="_blank">https://www.dropbox.com/s/62eord9xi1xuhih/KB9R%20-%20Rcvr%20Protect.JPG?dl=0</a> Thanks again. Joe

Mr Carlson's Lab

I would need more information to make any suggestions. Does this box actively switch RF? Is it in line when RF transmit power is present?

Mr Carlson's Lab

Hi Joe. If you have a link to some pictures, I will take a look.

Joe Eide

Paul, I don't know if I can post a picture, but I built my circuit in a Pomona 2391 metal enclosure. The antenna (probe in my case) connector goes through a 22 ohm resistor to 4ea. - 1N4148 diodes (2 in series forward and 2 in series reversed) to ground. From the junction of the diodes and the resistor, a .01ufd at 1kv is connected to the receiver connector. I don't remember why I chose the 22 ohm value. So, I'm sure that you could suggest an improved circuit to protect my test receiver. Thanks for considering my query. Joe - KB9R

Joe Eide

Hi Paul. I have a modern ham radio on the bench that has multiple antenna inputs (BNC's). I use one connected through a box containing a capacitor to block DC on the center conductor. The box is mounted under a shelf so it makes a convenient place to connect a probe for sniffing circuits. Can you suggest revisions to your protection boxes to give me further protection for the RF input of my tranceiver for test purposes? Thank you so much. Joe - KB9R

Mr Carlson's Lab

You're Welcome!

David Wiszczor

I am going to build this, ordered parts today! Thank=you!

Mr Carlson's Lab

Hi Randy, here is a list: <a href="https://rebelba42.lima-city.de/img/mcl/" rel="nofollow noopener" target="_blank">https://rebelba42.lima-city.de/img/mcl/</a>

Randy Wilson

Hi Mr Carlson's just signed up for class where do I start?

KellAudio

Hey the life you save may be your own. Or your scope's.

KellAudio

No Isolation, no current limiting... that rare floating open mains neutral does strange.

KellAudio

Seems to be a damn Good theme, there's no time like Now to think protection. I was excited with a new score on the bench, a superamp, Heathkit AA-1800 to finally get my hungry JBL's feed. Suspicious, that nothing seemed wrong with with it; staring at my pride Tek scope something was wrong. Never ignore those .... Wait, what is lacking?

Mr Carlson's Lab

Hi Bob, if you short the input jacks, you will see a result. You are correct about the DC Block, we don't care so much whats on the screen, as we are using the device as a signal source only.

Bob Snyder

Why doesn't the curve tracer show the capacitance and resistance in the DC block circuit? I guess we really don't care in this use case since we're just using it as a signal source. But I expected to see that show up on the screen.

Mr Carlson's Lab

You're welcome Jeff! Thanks for your kind words!

Jeff Becker

I just signed up. Besides the fact that you are such a wonderful teacher, Paul, the chief reason I did so was to commit myself to getting my workspace, parts, and gear organized and actually doing the projects I have been watching on the vids. Kind of like telling everyone you know about a diet/exercise New Year's resolution. Well, perhaps that is a bad example. Anyhoo, thanks for lighting a fire under my backside.

Mr Carlson's Lab

You're no burden in any way Edward. Thanks for taking the time to write, and for your suggestions as well!

Edward LeDoux

All your videos are very educational. Better than any I've seen. Someday, if you are so inclined and have the time, would be nice to know the basics of testing vacuum tubes, DIY Tube Tester perhaps. If not, I can weasel it out for myself using mfg specs. Your Knight VTVM video showed how to approach VTs in circuit. Am retired and VT stuff's my hobby. Have 35mhz DT scope, S260 VOM, DMM, 2Mhz Function Gen, toroidal iso xfmr (2a), Heathkit im-13. and Type 10c variable xfmr. Also built a Greinacher cascade (225v) for my nixie tubes. My Hakko SS is not as good as yours. No reply needed. Ignore me if I am a burden in any way.

Mr Carlson's Lab

You're welcome Colin!

col

thanks paul good stuff as aways

Mr Carlson's Lab

I look forward to seeing your results!

George Myers

Looks like 3-4 people saw this video and jumped on ebay. This protection device will be the first thing I build. Going by my past experience in hooking up stereo gear I know I will need it.

Michael Lynch

I saw that one as well. Pretty rich price for an old machine like that. And one that will very likely need an overhaul. I like Mr. Carlson's $5 pricetag better.

Reb Elba

US $368.99 plus Shipping: $85.75, oh boy thats expensive! The Only disadvantage of Pauls course is, every time I see older test gear were powered on due taking a picture for an auction, my face is looking like a soldiers face, while he recognizes that the thing he's throwing away was not the grenade :D

Bruce Davis

Hoky Smokes, Bullwinkle - your $5 oscilloscope is now going for at least $300. <a href="https://www.ebay.com/itm/112758977999?" rel="nofollow noopener" target="_blank">https://www.ebay.com/itm/112758977999?</a>

Mr Carlson's Lab

Glad you enjoyed Fred!

Fred Smead

Thank you for sharing your protection circuits with us. Sometime ago, I lost the output chip in my audio generator. Using your circuit, with a change of diode values, I could have prevented this from occurring. Fred

Mr Carlson's Lab

What kind of oscilloscope do you have David?

Mr Carlson's Lab

Glad you enjoyed Ralf!

Michael Lynch

I am building the circuit tracer as a stand alone, since I do not have a $5 oscilloscope to dedicate to the task. So I am putting this on a board and adding this to the stand alone curve tracer box.

Reb Elba

That's the beauty of your circuit design: It's, simple, only a few components needed and the result is very useful in many applications. Your explanation was absolut clear and easy to follow and I really like starting with a clean paper sheet. This gives me the chance to make my own thoughts during your design process. Thanks for sharing this Paul, this was the best thing on my day!

Guido Zijlstra

Really like the 'theory to practical' videos like this. Keep up the good work.

DAVID MCKEE

Wish I had this a few months ago. Blew out CH 1 on my oscilloscope when a lead brushed across a power supply. I'll be building these. Maybe you could do a video on o-scope front end repair:)

Mr Carlson's Lab

Looks Good Yannick! Just add the "K" after the 56 for the resistor. The only issue with having this in one box with a switch... If you forget to flip that switch..... bad things happen.

Mr Carlson's Lab

Hi Bob, glad your here! I will save the Bio for a video one day. Feel free to ask questions, that's why I'm here :^)

Bob Vines

Paul, Yet another great teaching video - I'll definitely be building these protection circuits, even though I haven't found a WO-33A 'scope (or two) to modify into a curve tracer. I really want to add a curve tracer to my small (so far) collection of test gear.What type of electronics training did you have? I've been piddling around with electronics for ~50 years, most of the time having no clue what I was really doing and without training. In college around 1980 or so, I did take the "mandatory" circuits analysis class & an electronics lab class (that were required for an ME student), but never really "got into" electronics until the mid-'90s when I adopted a ~'76-vintage minicomputer that "needed help." But I had been out of college for ~10-years by then and had no easy way to "officially" take electronics training. One reason that I'm one of your Patreon supporters is just to get some of that practical electronics training. I just wish that I could afford a higher level of support for you.

Yannick Turcotte

Realy Nice Paul thank you. Is it a good idea to put this inside like that ? Just did this schematic: <a href="https://1drv.ms/u/s!AnKLPDy3pII_gkWc1bxvx6rodk2g" rel="nofollow noopener" target="_blank">https://1drv.ms/u/s!AnKLPDy3pII_gkWc1bxvx6rodk2g</a>

Kenneth Graham

Great Paul , This will be a much needed addition . Thanks , Ken

John Harbor

really enjoying this stuff.....

Anders "SM7HCE"

Nice little gadgets Paul, as you said it´s also excellent for other test gear to add a DC-block, just make sure that you dont poke around in power transformers with AC guys :-):-)

Allan Mitchell

Nice one mr Carlson big thumbs up mate

Nick Vacalo

Thanks Mr. Carlson this is awesome

Michael Barg

Hi Paul: Yes, please share as many of your designs with us as you wish! They are highly instructive and useful along with your clear and insightful discussions.

Mr Carlson's Lab

LOL, That one isn't mine.... But it sure looks impressive!

Bill Basch

hey Paul - could we get a full review of this amp? - <a href="https://i.imgur.com/gB7NX52.jpg" rel="nofollow noopener" target="_blank">https://i.imgur.com/gB7NX52.jpg</a>