Rotek Model 350A high current adaptor for 3900 series calibrator - Page 1
Forum section: electronics / general

author: trobbins
author_rank: Frequent Contributor
timestamp: August 21, 2025, 02:41:58 am
content: I've just started to check out a Rotek Model 350A High Current Adaptor that is referenced in the 3900 series Precision Calibrator manual (on-line at xdevs). The 3900 manual makes reference to a 350A manual, which is what I am hoping someone may have available to scan, as I can't locate anything on-line.The adaptor can output up to a stable 10Adc or ac, or up to 50Aac through separate front panel terminals. For 10A range it uses (I think) 3x// MJ2501 and 3x// MJ3001 darlingtons to swing/set at a current sensed by a four terminal Dale resistor, from a hefty raw dc bipolar supply using 76mF 15V e-caps. For the 50A range it uses the toroidal transformer (presumably as a 5:1 turns ratio) by driving the primary. The control pcb includes 2x AD741, and 2x AD301, and appears to take just a current control level signal from the 3900 calibrator, and a high current 50A range signal.I'm hoping that the adapter can be configured to use a separate stable control level signal to achieve a similar calibrated output to what it was set up for originally. The original AC spec relates to frequencies from 40 to 1kHz at 10A, and to 400Hz at 50A, and it would be interesting to see if those frequency limits can be extended.Ciao, Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 21, 2025, 12:27:24 pm
content: Tim and the group,I have a very similar transconductance amplifier in my collection, the Rotek 650A. Only last week I was thinking the same thing, build an interface so that I can use it.The 650A is compatible with 600 and 3900 calibrators. I used to have a 600, 20 years ago.I found this in the 3900 manual:Here are pictures of the 650A:The board is similar to that in the 350A, hopefully the interface is similar or the same.The power section and chassis layout are the same as the 350A.I think my first step is to make a breakout board for the connector.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 21, 2025, 02:55:30 pm
content: Tim and the group,I have drawn up a breakout board. I will make this over the next few days.Schematic:PCB:Regards,Jay_Diddy_B

author: trobbins
author_rank: Frequent Contributor
timestamp: August 22, 2025, 12:07:12 am
content: Jay_Diddy_B, nice to have a 'partner in crime' for this topic.The pcb in my 350A has no connection to HZ_X_1000 (J11/4). J11/6 does go to a bjt so does appear to be a controlling state input for 50A function. The only other control input is J11/14, along with guarding pins.I had initially seen 'factory's post from Nov 10 2022 (https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg4515002/#msg4515002, but missed his later post with photos (https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg4527716/#msg4527716) of an earlier 350 model. There certainly has been some development tweaking of the 350, 350A, and 650A going on over time, but it does mainly look like equipment and pcb layout changes rather than major redesign.In lieu of a manual showing up, I'll do a schematic sketch of the 'I_in' circuit path on my pcb to better appreciate its function. My 350A, and factory's 350, has a front panel label 'current setting by 1000', and factory has confirmed that the 'I_in' control line does function as a 0-10mA FS current loop for the standard 10A FS range output, but he hasn't tested the 'Hi_I' output range yet.Like you, I'd be hoping that a retrofit local reference, with say a front panel control setting dial, and perhaps a set of terminals for /1000 current loop metering (using say a 6-digit meter 10mA range) or a reference voltage output, could make this a stand-alone high current source.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 22, 2025, 01:28:43 am
content: Hi Tim,What you are saying makes a lot of sense. I have traced this so far:The HI_I is a 5V logic signal. I am not sure of the function or the polarity.Correct me if I am wrong.1) The unit needs a current input2) 0-10mA becomes 0-10A, a gain of 10003) The input is applied between I_IN and I_GUARD?I know that the 50A ac output is enabled by shorting the two red terminals on the front panel. There is no logic control for that.I will get to this on the weekend.I am not optimistic that the performance will be great. Mine contains only LM741 op-amps. It could be a useful source of current.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 22, 2025, 01:44:19 am
content: Hi,From the post by Factory:This makes sense.Jay_Diddy_B

author: trobbins
author_rank: Frequent Contributor
timestamp: August 22, 2025, 06:18:01 am
content: My pcb has 'I-in' (P4/14) returning loop current to 'PG' (P4/18), with I_in generating a 4V FS signal across a 400R 0.05% with // 50k trim, that appears to be unity gain buffered by 0.05% resistors using the first 741 (no null cctry). That buffered output appears to then be compared to the output shunt voltage (1V FS for 10A 0.1R) with 0.05% resistor scaling using the 2nd 741, whose open loop gain output then drives both output stage darlington bases. I_in is shorted with two series contacts in the relay, or connected through, with relay coil control via LG and PG.So far I can't identify the current control signal sent from the 3900 calibrator in the 3900 schematics - I thought it would be related to either output sense schematic (pdf page 108) or current sense schematic (pdf page 120), but I can't see anything relevant.

author: factory
author_rank: Super Contributor
timestamp: August 22, 2025, 01:27:43 pm
content: Here are some pictures I didn't upload previously, of the board in the 350 I have, seems to have the same part number as the 650A one but looks quite a bit different in layout. There are a few modifications to this board from a previous owner? and some of the wires are missing from the ribbon cable, I can't get a better picture of the rear of the BNC, due to the inner box chassis.For reference, with nothing connected it powered up with both LED indicators lit.It tested fine with both AC & DC input, I didn't test the 50A AC only output, as I didn't have the Supercon plugs at the time, I now have a set of these, but haven't yet found a roundtuit.The one at work was a variant of the 650 (with 100A AC option) as I now remember the text about "use with 600 or 3900" above the power switch, they had many 600 series in the past and one 700 series which was the main calibrator until 1997, when they bought a secondhand 5500 from brand F. Sadly the last Rotek was skipped last year, along with everything else in the cal lab :'(, nothing I could have done, as they didn't want to sell anything and I was off work recovering from a major health malfunction. This still annoys me I haven't got a stable enough AC source.The 100A option that work had used the same 50A Supercon plugs, I checked this using the blunt end of some drill-bits to gauge the size. And the guy that used to do the electrical calibration (when it was in-house) complained a lot about the unreliability of the Rotek calibrators.P.S. The 710 manual is available from the web archive here;https://archive.org/details/manualsplus_00425David

author: trobbins
author_rank: Frequent Contributor
timestamp: August 22, 2025, 01:53:12 pm
content: Biff Wheeler has kindly sent me the manual for the 650 high current amp - link below. I'll check it tomorrow for similarity to 350A. There does seem to be info on the 650B version, which adds functionality to act as a standalone current generator with front panel demand voltage input.https://dalmura.com.au/static/Rotek%20650%20amp.pdf

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 22, 2025, 02:23:43 pm
content: Tim,That document is fantastic for my unit!! The control board that I have matches the PCB in that manual. The one I have is partially stuffed. It does not include the components for stand alone operation. I can add the required components.My chassis matches the one in the manual too. There are extra holes in the metal for the front panel 0-10V input.I will modify mine.Thanks again!!Jay_Diddy_B

author: trobbins
author_rank: Frequent Contributor
timestamp: August 23, 2025, 04:26:15 am
content: Biff has done it again - link to Rotek series 600 calibrator manual - thanks Biff !https://dalmura.com.au/static/Rotek%20600%20calibrator%20manual.pdf

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 24, 2025, 02:53:05 pm
content: Hi,I can report some progress converting my 650A into a 650B. It was made a lot easier with the documents that were found by Tim.I have done most of the modifications and performed a quick test. I am waiting on some semi-precision resistors and the push buttons from Digikey. They should arrive tomorrow.I did find one error on the PCB that I have. U2-pin2 is connected to ground. It should be connected to U2-pin1.Modified Control PCBOutput Shorting MOSFETsFront panel input binding postsRegards,Jay_Diddy_B

author: trobbins
author_rank: Frequent Contributor
timestamp: August 25, 2025, 02:02:19 am
content: Good stuff Jay_Diddy_P - that was quick!I can't say much for their pcb version markings - they are the same on your pcb!Your 650 pcb has the ADG201 cmos switch to isolate circuitry sections around AR1 (741) and it seems to disconnect the 4V FS current input signal developed across the 400R R18, and replace it with the external control 4V FS from the retrofitted J11-J10 front panel terminals. Is 40k R21 disconnected by the cmos switch to just leave the //10pF around AR1 (Edit I see that it is always on!) ? My pcb has no traces going to J3/A,B for that external control 4V, but at least it has pads.The 100k R46 back to J3/K has been snipped off (?) on my pcb, so there is no feedback to the front panel J7 terminal. Is that feedback R fitted on your pcb?The output and drive circuitry is the same on my pcb (although C5 seems to be 10nF not 100nF). Wrt your comment on U2, the 350A pcb doesn't include the FET clamp or related logic circuitry.I note my 'Error Warning' indicator is 'Over compliance' on the 650 ! And it looks like the larger 0.05% 500k R20 up near the P4 connector on your pcb is a more benign 511K 1/4W RN55 on my pcb.There is a note on the 650 schematic about setting the output stage bias to a range from 10-25mV across the emitter resistors - I guess that accounts for the likely variation between the 6 bjt's.Ciao, Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 25, 2025, 03:21:49 am
content: Quote from: trobbins on August 25, 2025, 02:02:19 amThis is the schematic:IC1 switches between the signal from the calibrator and the front panel input. The ADG201 is a normally closed SPST switch. The switch connected to R21 has its input grounded. This means that it is always ON. This is done to provide compensation for the switch ON resistance. The input resistance is 40K + Rswitch. The feedback is also 40K + Rswitch. Rswitch is around 90 Ohms.Jay_Diddy_B

author: trobbins
author_rank: Frequent Contributor
timestamp: August 25, 2025, 03:22:51 am
content: Ta, just edited my comment while you were posting!!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 25, 2025, 03:34:35 am
content: Quote from: trobbins on August 25, 2025, 02:02:19 amThere is no track on my PCB from pin K on the edge connector, I can't find R46 on the board.Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 25, 2025, 03:39:20 am
content: Quote from: trobbins on August 25, 2025, 02:02:19 amThe large 500K 0.05% resistor is R20. It is part of the gain trim from the calibrator input:Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 25, 2025, 03:45:13 am
content: Quote from: trobbins on August 25, 2025, 02:02:19 amThe amplifier is biased in class AB mode. The instructions are to set the bias to between 100mA and 250mA. The emitter resistor are 0.1 Ohms. This very typical in an audio power amplifier.Jay_Diddy_B

author: trobbins
author_rank: Frequent Contributor
timestamp: August 25, 2025, 03:55:58 am
content: Yeh the choice of R20 (large 0.05% or 1/4W circa 1%, and 500k or 511k) is a bit strange. My pcb has no room for anything larger than 1/4W, and even the next door 0.05% resistors are a bit sandwiched in as the 10M-10pF network has been factory retrofitted as a mod. Maybe they only had 500k in that size, as it almost looks too large for your pcb.I now have enough information to retrofit a front panel control input for AC or DC external input, and I'll just use a 2-pin header to allow external or internal reference selection. I was also going to retrofit a simple stable, low tempco 4Vdc FS reference using the pcb dc supply and a front panel 10T pot to allow it to set the output dc current for standalone operation.So far I've confirmed the large e-caps, and the power transformer and mains are ok, so checking the standby bias is next and whether it needs any added temp comp.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 25, 2025, 04:18:41 am
content: Quote from: trobbins on August 25, 2025, 03:55:58 amThe key specification on the 500k resistor is the temperature coefficient. The manual specifies wire wound 5 ppm / C.I don't think the 5 ppm specification is needed.I am using modern metal film resistors on the external input. These have a tempco of 15 ppm /C.I am doing this as a project. I also have a Fluke 5220A.Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 26, 2025, 01:55:54 am
content: Hi, I have finished the conversion of my Rotek 650A to the B version.PCBFront PanelI used a 3D printer to make the two buttons.I have given it a quick adjustment. I will do a more careful adjustment when I have more time.Regards,Jay_Diddy_B- Amp-nuts go with the flow

author: trobbins
author_rank: Frequent Contributor
timestamp: September 03, 2025, 10:17:16 am
content: Nice conversion indeedI went with a simple local 5V voltage reference, connected through a setting resistor to the 400R R18 that provides a 4V FS signal input to the control pcb. The setting resistor is either 100R (for nominal 10A demand setting), or an additional two 10T Helipots of 1k and 10k in series to provide a trimmable demand setting down to 0.44A. TheMAX5225MAX6250 is simply powered from the pcb +15V rail. I can add an inverting buffer for -5V, and have some front panel switches for setting convenience.Initial testing at 1A demand showed one of the MJ2501 is thermally walking away. That darlington returned to 'norminal' with its base resistor increased from 47R to 360R. All the base 47R 3W resistors had drifted high by 20-25% - so now replaced. I've relocated one of the string of 1N914 bias setting diodes to the chassis/heatsink to see if that can provide some thermal compensation, but so far testing for 2 hours has shown there is still latent thermal walkaway with the identified poor MJ2501 (perhaps it had a hard life). I'll continue along the thermal compensation control path to hopefully reach a bias equilibrium, then put the top cover on and turn the fan on and monitor the poor darlington emitter current when testing at up to 10A rating.So far the constant current is somewhat stable, with short duration fluctuations of about 10ppm, but some longer duration shifts of circa 50ppm, which don't appear to be tempco related. Still need to confirm more using available 6.5d DMMs and shunt.A demanded dc output current also comes with an AC component (0.17Aac for 1Adc) which I'll look at later, but should be rectified ripple. Perhaps that ripple can be neutralised - one for the 'to do' list.

author: trobbins
author_rank: Frequent Contributor
timestamp: September 06, 2025, 01:43:37 pm
content: I was able to confirm ok constant current operation up to rated 10Adc from this adapter. The 'poor' MJ2501 showed stable thermal behavior with 2x 1N914 relocated to the heatsink, even at 3A with no fan.As far as logging of a HP 34401A, and a Picotest M3510A, the adapter current was stable to within metering jitter and external influences.I had noticed an AC current level in the DC output when using my Keithley 197, but I've confirmed that is a quirk of my 197 when selecting AC current.And now I've just managed to blow the 15A fuse on my Picotest M3510A - the foibles of a continuous 10A test where I was trying to clarify settling times and drift. The Pico is not as robust as one could hope - with the fuse holder melting and making a difficult fuse extraction - so now I'll have to dream up a 10A fused work-around, which may not be pretty.