Rate my continuity tester input protection - Page 1
Forum section: electronics / projects

author: andybrandi
author_rank: Contributor
timestamp: October 06, 2021, 07:36:19 pm
content: I'm currently trying to build a battery-powered continuity tester. One of my requirements is to not have it blow up when I accidentally probe voltages of up to 450V. I came up with a cute input protection circuit built around an nFET and an optocoupler. So far it simulates fine but I'm unsure whether I missed any pitfalls or if there is a simpler design I didn't think of, so I'd be glad to get some input (pun intended) from you.EDIT: no idea why I can't post inline images; it's down in the attachmentsThis is the relevant part of the input stage. Vtest is regulated (via Reg+, Reg-) such that there is a 0.3V drop across the probes with no load, but I left the regulator part out for now since I don't think it is relevant to judge the input protection.R1, R2 and R3 form a high-impedance divider to regulate Vtest accordingly. R2 has to be small enough to keep the voltage drop at a safe level even for very high input voltages.D3 blocks current flow for positive input voltages.D1 goes reverse-breakdown if Vin < -15V. The resulting current flow will activate the Q1-Q2 Darlington pair via U1 and shunt Cgate to ground, thus turning off M1.R5 pulls Cgate high during normal operation and also keeps M1 off for a certain amount of time after a fault. (I added Cgate to give the nFET a larger (and well-defined) gate capacitance to arrive at a suitable RC time constant.)According to LTSpice M1 turns off in a couple of microseconds, although so far I only used generic or placeholder components and did not account for possible inductances.Eventually I'll add an actual fuse as fallback (and possibly a 450V MOV across the inputs?).Any suggestions are welcome. Thanks!

author: Gyro
author_rank: Super Contributor
timestamp: October 06, 2021, 08:10:52 pm
content: Sorry, I'm not sure I get it.If it's battery powered, surely the whole thing will be in a suitably insulated plastic case with only the two probe sockets and a well insulated switch passing through it (with suitable care about not changing the battery without disconnecting the probes).I'm not sure what the opto buys you. Maybe in the context of the rest of the circuit it would make more sense. PTCs (backed up by fusible resistors) and clamps would seem to make more sense.You haven't mentioned whatsortof 450V you're thinking about, ie. CAT ratings. Accidentally putting a [Edit: dedicated] continuity tester across a 450V supply sounds a little too careless for your continued survival.

author: Manul
author_rank: Super Contributor
timestamp: October 06, 2021, 08:30:08 pm
content: Frankly, I do not like this. I would choose passive protection. Active protection, like swtching a mosfet is not adequate as a primary overload protection (too slow, path through mosfet capacitance, only blocking one way because of body diode, etc). It just feels like "ehh".I would strongly suggest to rethink it into passive overload protection solution.

author: 2N3055
author_rank: Super Contributor
timestamp: October 06, 2021, 08:45:51 pm
content: https://www.welectron.com/Brymen-BT-75EU-Voltage-TesterThat's the thing to have continuity tester that you can stick to 450 V and not hurt it or yourself..I understand learning but this sounds a bit dangerous at this level of knowledge.I would suggest to start with continuity tester that survives 24V and then go from there.And seriously, purchasing one of those testers and opening them and reverse engineering the input stage would be a safe and not very expensive way to learn..And safe to use too..

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2021, 12:10:15 am
content: For a protected continuity tester, I would use a high frequency AC current for excitation so the output can be capacitively coupled and low frequency AC would be rejected by the AC coupling capacitor. This considerably eases the requirements on the protection circuits.Of course this would fail in the sense that capacitors would show as shorts. If that is not acceptable, then I would use high voltage depletion mode power MOSFETs to protect the DC current output.

author: ledtester
author_rank: Super Contributor
timestamp: October 07, 2021, 02:37:32 am
content: This design uses a Bourns TBU-CA050 for circuit protection:- The Ultimate Continuity Tester - Leo's Bag of Tricks(link jumps directly to the schematic)https://youtu.be/N2M-p-OGvPg?t=9m5s

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: October 07, 2021, 03:18:24 am
content: As you come from Germany, you should have heard of the famous Unitest V1x200 (the continuity tester which makes those meowing noises when you take the test leads into your fingers and play with them a bit.)It is protected against 600V and doesn't waste much components to protection only. Even the glow lamp across the input doubles as indicator. No ICs, no Mosfet, just 2 or 3 standard BJTs. Maybe it uses one fuse resistor and it is coupled to the test circuit with at least one high-value resistor. They even sell it in kit form, so there should be a circuit diagram somewhere. If not, redocument it - it is really simple!

author: bdunham7
author_rank: Super Contributor
timestamp: October 07, 2021, 03:44:47 am
content: Download the manual for a Fluke 8840A or 8842A and look at the ohms current source protection behind the diode (not the MOVs and stuff). I think their idea is similar to yours. Like yours it simply uses a 1N4007 diode to completely block positive overvoltages, but then it just uses three HV BJTs in series to limit the current for negative voltages. The 350V 2N6520 transistors they used are still available, although they need to be somewhat beta-matched in their circuit. That gives you a 1000V+ withstand capability with just a few part, no clamping and very little dissipation.

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2021, 04:24:19 am
content: Quote from: bdunham7 on October 07, 2021, 03:44:47 amThe little known diode bridge circuit for protecting low impedance inputs and outputs from high voltage works like that but for both positive and negative voltages. I have always wondered why ohms converter circuits do not do it that way with at least one high voltage MOSFET. They used to make high voltage JFETs which could do it but they were never commonly available.I have been thinking about using that method for protecting the outputs of an LCR meter or network analyzer.