+/- 1200 VDC Resonant Converter and a few questions. - Page 1
Forum section: electronics / projects

author: baltersice
author_rank: Regular Contributor
timestamp: February 22, 2017, 10:59:39 pm
content: I am trying to turn my Keithley 236 into a 237High VoltageSource Measure Unit. As a first step I build the low noise, low EMI/RFI dc-to-dc converter module according to their schematic.Their circuit descriptionQuoteOf course it doesn't work right away. The node consisting of the Q7 transistor collector and U5A Pin 2 is at .5V permanently and I think that is why the 74HC123 monostable multivibrators aren't emitting any pulses. I am a bit puzzled by that transistor in general, does anybody see its purpose?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2017, 12:08:47 am
content: Seems to be a cascode, so the sum currents from R12 (a minimum 'idle' current?) and R11 (whenever U4's output is above about 6.2V) are put into C13.There's no other load on C13, so it will charge at a proportional rate, until U5B + CR6 triggers (I guess) and pulls it down.U5A I don't think is a Schmitt trigger input, and if it were, it wouldn't be a well defined threshold either (the usual CMOS 30-70% range). But with everything in a loop, as long as it has enough range, that shouldn't be terrible. And as long as the capacitor charges at a not-terrifically-slow rate, the lack of Schmitt trigger shouldn't be a show stopper.Tim

author: baltersice
author_rank: Regular Contributor
timestamp: February 23, 2017, 08:09:19 am
content: Quote from: T3sl4co1l on February 23, 2017, 12:08:47 amThat's it, thank you so much! Now it makes senseI solved the problem by replacing both 74LS123 and am now getting +/- 140 V on the output with a random transformer from the graveyard as T2.The transistors are getting very warm tho ...The correct transformer is only given as an internal part number, so I can't get it, but I have a couple of E E "wind it yourself" transformers and would like to try that.In fact I have already measured the pule frequency: 20 kHzAnd the resonance cap is 220 nF.Solving f = 1 / 2*pi * sqrt(L*C) for L = 300 µH for resonance10 µH are already there, so I'd aim for 290 µH primary and then trial and error with the secondary until I have + / - 1200 V ...?Would that work?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2017, 09:05:25 am
content: The inductance of the transformer is very much damped be the load on the output side. So there is no such simple calculation for T2. The main parameters should be the turns ratio and winding numbers to avoid saturation. In first approximation the turns ration sets the output voltage - shifting the frequency is more like fine tuning to that. The adjustment range is usually rather limited.My best guess would be to run a simulation of the circuit (at least the transformer part) and from there on get an idea for the right parameters for T2. It would also give you an idea how to adjust when the transformer is not right.

author: Zero999
author_rank: Super Contributor
timestamp: February 23, 2017, 09:20:04 am
content: Quote from: baltersice on February 23, 2017, 08:09:19 amNote that the schematic specifies the 74HC123, which has totally different electrical characteristics to the LS version. The HC variant is CMOS, rather than TTL and has a much higher input impedance and different logic thresholds.

author: baltersice
author_rank: Regular Contributor
timestamp: February 23, 2017, 09:25:23 am
content: Quote from: Kleinstein on February 23, 2017, 09:05:25 amIn LTSpice? That would be the only simulator I have ever used ... a while agoI'd probably replace everything to the left of the LC circuit with a PULSE voltage source and play around with the values looking for ... what? A sinusoidal waveform? Correct output voltage?Quote from: Hero999 on February 23, 2017, 09:20:04 amWhoops, typo! Got the right ones, but was looking at the LS datasheet. Good catch tho

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2017, 09:43:20 am
content: LTSpice was my idea for a simulation too. I would think about starting at T1 with the simulation. The point would be to get the right wave forms and idea for the correct turns ratio. For comparison (to see how the signal should look like) one might have to to look up a few texts about such resonant converters too.

author: TiN
author_rank: Super Contributor
timestamp: February 23, 2017, 11:04:19 am
content: Subscribed. At least somebody building things here, not just talking like me.I'd like to learn a bit how to simulate transformers for same purpose as well, as I'm interested in such design too, as a HV power supply for DAC output stage..

author: baltersice
author_rank: Regular Contributor
timestamp: February 23, 2017, 02:40:21 pm
content: VictoryWell almost ... Voltage is displayed with a 10:1 divider, so +/- 1200 are there. Gotta do a "load test" to see if it can deliver the 10 mA, that the Keithley 237 needs.Then I'd also like to some fine tuning, to get the waveform (see scope) more sinusoidal. The first sentence in the circuit description calls this alow EMI/RFIdc-to-dc converter after all.I didn't solve this with a simulation, I had just made another mindless mistake earlier.This article was very helpful tho:http://www.eetimes.com/document.asp?doc_id=1273871

author: kridri
author_rank: Contributor
timestamp: February 26, 2017, 05:22:31 pm
content: Hi SirDo you have the entire service manual with schematics? Because I can't find it online.Kris

author: PointyOintment
author_rank: Frequent Contributor
timestamp: February 28, 2017, 09:04:01 pm
content: Very interesting! One thing I'm not sure about is that clear box in the middle of your photo. What's it for? (Also, what's that cool-looking instrument with the colorful banana jacks at the top?)

author: TiN
author_rank: Super Contributor
timestamp: March 01, 2017, 04:50:23 am
content: Clear box is his voltage divider. You can see blue MOX high-value resistors on banana jacks. Box is to reduce charge pickup on divider and protect from contamination (It's easy to scare all those picoamps).

author: baltersice
author_rank: Regular Contributor
timestamp: March 02, 2017, 08:11:53 pm
content: Quote from: PointyOintment on February 28, 2017, 09:04:01 pmThats an Oltronix Power Supply B703, made a video about it a while ago...

author: Hydron
author_rank: Super Contributor
timestamp: February 22, 2022, 10:55:33 pm
content: Inspired by baltersice, I've had a go at this too. In my case I've ordered boards rather than home etched - with the photos online (special thanks to monolith here:https://www.eevblog.com/forum/testgear/playing-around-with-an-old-keithley-236-source-measure-unit/) I've managed to nearly exactly clone it:I've mostly gone with the original parts Keithley used - the main exceptions being the mystery transformer (I used baltersice's choice of core and winding ratio/number), the trimpot (have added an alternative footprint as the original is a less common one), a 1uF 100V cap (added an alternative much more common lead spacing), and I've skipped Kiethley's crazy use of a unavailable pre-aged zener diode as a voltage reference (though the spaces for the zener and resistor are still there if needed).Keithley's parts choices in general are a bit nuts - lots of unnecessary E96 1% resistors, 1kV caps where 10V would do etc. I considered doing a bunch of changes to make it cheaper but I would have been lucky to save $10-20 so I just didn't bother in the end, and went for an ~exact replica.The zener reference however was particularly silly given the supply is only loosely regulated (feedback isn't from the final output but instead from a separate multiplier branch) and there is a 5V reg which is totally adequate for the task right there asking to be used. I changed R14 to 44.2k (one of the silly values they've used elsewhere) and put a link to 5V in instead of the zener, so that I get a similar adjustment range (biased to slightly lower output voltages than the original but with enough range to hit +/-1200V still - 42k would be a better value).This has been tested (unloaded only, need to build a dummy load now), and worked first try which was a pleasant surprise - the project of upgrading my 236 to a 237 has been enough of a chore already!Will report back once I've done a proper test and built an enclosure etc (gonna use it as a learning exercise in sheet metal bending) - assuming it works fine under load I will likely have a few spare boards to give away if anyone else is crazy enough to do this (and post the design files too, though they are pretty rough!)I've also got the conversion of the rest of the SMU to complete and report back on - right now the hold-up is some missing precision resistors (3x 1M 0.1% 5ppm/C PTF651M0000BZBF) and relay K7 (COTO HV relay #1240-0229), which are not fitted on most 236 analogue boards (baltersice was obviously lucky with his - later units like mine are missing these). The relay is of course unobtanium; there is possibly a work-around involving swapping it for one in a less critical one (K5) and replacing that one with something else, but thankfully I've managed to secure another kind forum member's spare, so I don't need to risk this bodge.Edit: no issues under load, other than VR3 gets pretty toasty - it's a 1W 20V zener used to drop the 50V supply down to ~30V to feed a LM317, and has to support all the current for the control circuitry. I've replaced it with a 5W zener and it still gets hot (like ~100C with full load on the converter)!Ideally the layout should have a decent sized copper fill on the board connected to each lead of the zener to heatsink the leads, but as keithley didn't do it I didn't consider it either when laying out the board. Pics of keithley original boards show discolouration under this part, but I missed that unfortunately. For lack of anything better I've used a much chunkier zener and raised it above the board a bit to increase dissipation. I've also raised up off the board the 2M2 resistors on the output (hiding behind the orange caps in the pic) for similar reasons - they run with 1200V across them (= 650mW), yet keithley only specifies 1/2W resistors?!? (thankfully I used 1W resistors)

author: MaxFrister
author_rank: Regular Contributor
timestamp: November 14, 2023, 03:27:55 am
content: I'm also considering modifying my 236 but do not have the high voltage relay installed. The 3 installed relays are marked Coto 1240-0229.I've not been able to find any specs or datasheet for these (a modern one with the same number is available). I've located a number of "1240" relays but with a variety of suffixes. Does anyone know how to decode the suffixes? Any suggestions for a similar modern and/or available relay. I'm not too worried about the footprint as I could make an adapter if necessary.

author: Hydron
author_rank: Super Contributor
timestamp: November 15, 2023, 12:54:58 pm
content: Istolerepurposed the relay from the (LOW) remote sense circuit to use in the HV position as a temporary measure (that's been temporary for 18 months now...). It's the same part, and the LOW remote sense relay honestly seems over-specced for where it's located (LOW Sense appears to be clamped to within 15V of LOW Force, so contact-contact rating doesn't need to be high unlike in the HV position), so that relay can probably be replaced with something else that meets the coil-contacts isolation requirement, e.g. 1500V (even guarding/shielding is probably unnecessary given it's on the low rather than high side).At the very least I'll do a document dump shortly - the PSU board design files at a minimum will be a massive time-save for someone trying to repeat this, though it's still a big project and maybe trawling ebay for a 237 to repair is a better use of time. Also check before starting whether the 500V P-channel FET is available - there is precisely ONE option in production. All that said, I have more bare PSU boards and a spare 237 output board I picked up off ebay (with one dead P-fet), so I'm still tempted to try and find another 236 to convert as it'd be a lot easier with what I have now.Edit: to clarify I checked the schematics and it looks like I moved relay K5 into the K6 position and jumpered the now-missing K5 (losing remote sense in the process, but it could be replaced with a different relay to regain it).

author: ZGoode
author_rank: Frequent Contributor
timestamp: May 22, 2025, 05:47:10 pm
content: I saw you posted the gerber files in my FS 236 post a while back. Do you happen to have the BOM anywhere so myself (and anyone else who tries to replicate your work) ensures we get the right parts/footprints for it?

author: Hydron
author_rank: Super Contributor
timestamp: May 26, 2025, 11:32:59 pm
content: I'm not sure how coherant a BoM I have sorry - I bought bits from a bunch of places (and my pre-existing parts stash) and tracked it on paper for the most part. I probably have some of it in a spreadsheet (or digikey/mouser order) though - will post what I have if I find it.