Hi Guys,
I have a mesa 7i77 card i'm using with linuxcnc to run a plasma cutter table. I burnt two cards so far. card is like 300$ a piece.
I am now trying to isolate it from all the other electrical stuff around. I know I have to go over all the wiring and all that but I thought why not add additional security and isolate the analog outputs and inputs of the card. I asked the major AI guys and they suggested I make my own board. I have never designed a board but I am good with a soldering iron. I did emphasize that I want a through hole diy board and even asked AI to put socket for components to make replacing them easier. I also looked on the net for solution but they are almost as expensive as the mesa board itself. I did find an aliexpress din rail isolator but the response time is 10ms. I bought one to test though. The modded plasma table I retrofitted to use linuxcnc dates back to 1995. not sure the drives will tolerate a 10ms lag.
back to my project. here is what the AI guys advised:

2 channels isolatation board:

Category,Qty,Reference,Value / Part Number,Package,Description,Unit Cost Est.,Line Cost Est.
Power,1,U3,TPS7A3901DGNR,VSSOP-24,Dual ±15V Voltage Regulator,4.20,4.20
Power,2,L1, L2,10µH,7x7mm,Power Inductor,0.50,1.00
Power,4,C_IN,22µF 25V X7R,1210,Regulator Input Cap,0.15,0.60
Power,4,C_OUT,10µF 25V X7R,1210,Regulator Output Cap,0.12,0.48
Isolation ICs,2,U1, U2,HCPL-7840-000E,SOIC-8,Isolation Amplifier,3.50,7.00
Gain Setting,8,R_GAIN,10.0kΩ 1%,0805,Gain Setting Resistor,0.02,0.16
Filtering,4,C_FILT,100pF NPO,0805,Input Filter Capacitor,0.03,0.12
Decoupling,14,C_DEC,100nF X7R,0603,Power Decoupling Cap,0.01,0.14
Bulk Decoupling,2,C_BULK,1µF X7R,0603,Bulk Decoupling Cap,0.02,0.04
Protection,2,TVS_IN,SMBJ12CA,SMB,Bidirectional TVS (±12V),0.25,0.50
Protection,1,F1,1812L150/60DR,1812,Resettable PTC Fuse (1.5A),0.50,0.50
Protection,1,TVS_PWR,SMBJ28CA,SMB,Unidirectional TVS (28V),0.25,0.25
Indicator,1,LED1,Green LED,0805,Power Indicator LED,0.10,0.10
Indicator,1,R_LED,2.2kΩ,0805,LED Current Limit Resistor,0.02,0.02
Connectors,3,CONN,3-pin Screw Terminal,5.08mm,Signal/Power Connector,1.10,3.30
PCB,1,-,PCB 2-Layer,80x100mm,Custom Circuit Board,5.00,5.00

And the board design instructions:

=========================================== LINUXCNC PLASMA CUTTER - 2-CHANNEL ANALOG ISOLATOR =========================================== BOARD CONNECTORS: ----------------- PWR_IN (2-pin): PIN1 = +24V_IN, PIN2 = PGND CH1_IN (3-pin): PIN1 = CH1_IN+, PIN2 = CH1_IN-, PIN3 = GND_CTRL CH2_IN (3-pin): PIN1 = CH2_IN+, PIN2 = CH2_IN-, PIN3 = GND_CTRL CH1_OUT (3-pin): PIN1 = CH1_OUT+, PIN2 = CH1_OUT-, PIN3 = GND_ISO CH2_OUT (3-pin): PIN1 = CH2_OUT+, PIN2 = CH2_OUT-, PIN3 = GND_ISO PWR_OUT (2-pin): PIN1 = +15V_ISO, PIN2 = -15V_ISO POWER SUPPLY WIRING: -------------------- PWR_IN: PIN1 --> F1(FUSE) --> +24V_F +24V_F --> TVS_PWR:ANODE TVS_PWR:CATHODE --> PGND +24V_F --> L1 --> U5:TPS7A3901(PIN1,VIN) PGND --> U5(PIN3,GND) U5(PIN8,VOUT1) --> +15V U5(PIN18,VOUT2) --> -15V +15V --> U6:LM7805(IN) U6(OUT) --> +5V U6(GND) --> GND_CTRL +15V_ISO --> U7:LM7805(IN) U7(OUT) --> +5V_ISO U7(GND) --> GND_ISO CHANNEL 1 (U1 - HCPL-7840) WIRING: ----------------------------------- [CONTROL SIDE] CH1_IN: PIN1 --> R1(10k) --> U1(PIN3,IN+) CH1_IN: PIN2 --> R2(10k) --> U1(PIN2,IN-) U1(PIN2,IN-) --> R3(10k) --> GND_CTRL U1(PIN3,IN+) --> C1(100pF) --> GND_CTRL CH1_IN: PIN1 --> D1:TVS(ANODE) CH1_IN: PIN2 --> D1:TVS(CATHODE) +15V --> U1(PIN8,VCC1) +5V --> U1(PIN1,VDD1) U1(PIN4,GND1) --> GND_CTRL +15V --> C5(100nF) --> GND_CTRL [at U1 Pin8] +5V --> C6(100nF) --> GND_CTRL [at U1 Pin1] [MACHINE SIDE] U1(PIN6,OUT+) --> R4(10k) --> CH1_OUT: PIN1 U1(PIN5,OUT-) --> R5(10k) --> CH1_OUT: PIN2 U1(PIN5,OUT-) --> R6(10k) --> GND_ISO +15V_ISO --> U1(PIN7,VCC2) +5V_ISO --> U1(PIN5,VDD2) U1(PIN5,GND2) --> GND_ISO +15V_ISO --> C7(100nF) --> GND_ISO [at U1 Pin7] +5V_ISO --> C8(100nF) --> GND_ISO [at U1 Pin5] +15V_ISO --> C19(1uF) --> GND_ISO CHANNEL 2 (U2 - HCPL-7840) WIRING: ----------------------------------- [Follow CHANNEL 1 pattern with these substitutions] U1 -> U2 CH1_IN -> CH2_IN CH1_OUT -> CH2_OUT R1-R6 -> R7-R12 C1 -> C2 C5-C8 -> C9-C10 C19 -> C20 D1 -> D2 ADDITIONAL DECOUPLING CAPACITORS: --------------------------------- +15V --> C11(100nF) --> GND_CTRL -15V --> C12(100nF) --> GND_CTRL +15V_ISO --> C13(100nF) --> GND_ISO -15V_ISO --> C14(100nF) --> GND_ISO POWER INDICATOR: ---------------- +15V --> R9(2.2k) --> LED1:ANODE LED1:CATHODE --> PGND CRITICAL LAYOUT RULES: ---------------------- 1. GND_CTRL and GND_ISO must NEVER connect electrically 2. Maintain ≥8mm gap between GND_CTRL and GND_ISO copper 3. Place 100nF capacitors within 3mm of IC power pins 4. Route IN+/IN- and OUT+/OUT- as parallel trace pairs 5. Keep input and output sections physically separated VERIFICATION CHECKLIST: ----------------------- [ ] All HCPL-7840 have correct power: VCC=±15V, VDD=+5V [ ] Input bias resistors (R3,R6,R9,R12) connect to correct ground [ ] TVS diodes oriented correctly (cathode band to negative) [ ] No shorts between GND_CTRL and GND_ISO [ ] Isolation barrier clearly defined in PCB layout [ ] All connectors labeled correctly NOTES: ------ - GND_CTRL connects to Mesa card ground - GND_ISO connects to servo drive grounds - Board provides two fully isolated ±10V analog channels - Use shielded cable for analog signal connections

So what am I looking for? I'd like to have your opinion whether it's a good idea to go this route, learn some stuff and build this board and be able to repair it when needed, If I order it online I will have a minimum of 5 boards. This gives me 4 spare boards in case of future failures. Or forget about it and go the expensive option? I am not in a hurry and I'm not losing any money going the DIY option by the way. The setup is already working as is but I want to add protection.
I am going to play with a Claude Kicad MCP to see what I can get out of this.

 

±10V analog isolators are not common:
Here (Figure 12) is a reference on how to build one using two op amps and two linear opto isolators.
Of course isolation requires no common ground connection, and two separate, ±12-15V isolated power supplies for the input and the output. Can you do that in your setup?

Does that sound like something you would want to do?
If so, we can help with the design.

 

Thanks for the quick reply.
Yes. What I want to achieve is isolate the analog output and input pins from any external electrical component. I will look into the link you posted and get back; Thanks again.

 

what you should consider is ?? Is there extra lattency (delay) introduced by the added isolation unit ?? does it matter

if your control chain is not like
send relative/absoluye move/shift coordinates state of cutter to equipment ? wait complete/ready for next command to be sent
e.g.
if your "control board" is somehow dynamically a part in the cutter position adjustment loop - several problems may arise

the probability of the latter option is suggested by "ANALOG isolator" !!!???

 

signal isolators and conditioners are very common. any decent electrical supplier will have some, and if you are in the urban area, there will be 20+ of suppliers within couple of miles from you. also online electronics retailes have dedicated section for this, here is DigiKey.


and here are several that will do +/-10V to +/-1-V and the prices and in Canadian $. some of products are universal and allow you to choose independently input and output type, range and scaling.

 

if considering DIY, i would rather make something that has several channels and use ADC/DACs that are isolated or, use common ones (without isolation) then add your own isolation on a digital bus.

for example:
MCU (your choice), say RP2040 or whatever.
use bus of your choice (such as I2C).
optionally add multiplexer so that same bus addresses are not an issue (TCA9548 or whatever)
on "side A" side add ADC or DAC or both using chosen kind of bus, for example ADS1115 for analog inputs, MCP4728 for analog outputs
on "side B" side add ADC or DAC or both using chosen kind of bus, for example ADS1115 for analog inputs, MCP4728 for analog outputs.
add I2C isolator like ISO1540DR or whatever between MCU, side A and side B.
use ready made DCDC isolated converters that give you +/-12V or +/-15V. you need one for each side that has DACs.
use OpAmps like OPA2186 to scale DAC signals to +/-10V.
add protection and also anything else you think could come in handy.

 

For grins, I generated a modified Figure 17 circuit from my posted reference which uses just one opto.
Below is the LTspice sim:
The bottom op amps provide a small, stable current to bias the isolator at the center of its range for a plus and minus input and output.
It has a -3dB rolloff of about 15kHz.
It's not necessarily optimized for your requirements.

 

what you should consider is ?? Is there extra lattency (delay) introduced by the added isolation unit ?? does it matter

if your control chain is not like
send relative/absoluye move/shift coordinates state of cutter to equipment ? wait complete/ready for next command to be sent
e.g.
if your "control board" is somehow dynamically a part in the cutter position adjustment loop - several problems may arise

the probability of the latter option is suggested by "ANALOG isolator" !!!???

Here is the analysis of my setup requirements:


Here is a summary of what your system needs versus what the isolator options provide:

Parameter	Your System's Requirement	HCPL-7840 (Simple)	IL300 (Complex)	Verdict
Required Bandwidth	> 500 Hz (5-10x servo rate)	100 kHz	200 kHz	Both exceed by 200-400x
Max Allowable Latency	< 100 µs (10% of servo period)	~10 µs	< 10 µs (circuit dependent)	Both meet easily
Required Slew Rate	~0.5 V/µs (for 10V step in <20µs)	~8 V/µs	Circuit Dependent	HCPL-7840 meets
Critical Need	Stability, noise immunity, simplicity	Good CMRR, integrated	Excellent linearity, discrete	Simplicity wins

 

signal isolators and conditioners are very common. any decent electrical supplier will have some, and if you are in the urban area, there will be 20+ of suppliers within couple of miles from you. also online electronics retailes have dedicated section for this, here is DigiKey.
View attachment 360757

and here are several that will do +/-10V to +/-1-V and the prices and in Canadian $. some of products are universal and allow you to choose independently input and output type, range and scaling.

View attachment 360755

These devices are more expensive than the board I'm trying to protect
The board is a mesa 7i77 by the way.

 

For grins, I generated a modified Figure 17 circuit from my posted reference which uses just one opto.
Below is the LTspice sim:
The bottom op amps provide a small, stable current to bias the isolator at the center of its range for a plus and minus input and output.
It has a -3dB rolloff of about 15kHz.
It's not necessarily optimized for your requirements.

View attachment 360761

looks nice even if I don't really read schematics
Can this be made in a finished project to be ordered online?
Does it need any tuning before wiring it (once it is manufactured)?

 

The HCPL-7840 does require an output differential to single-ended configured op amp or instrumentation amp to get the ±10V output you want, but that should still make a reasonably simple and low-cost circuit.

Likely better than using the IL300.

 

Burr-Brown (now a part of Texas Instruments) and Analog Devices both are big in the signal isolation field, with analog and digital signal isolators, opamps, ADCs, etc.

ak

 

Can this be made in a finished project to be ordered online?

Not without paying an arm and a leg.

But the HCPL-7840 can't either.

If you want cheap, you likely have to build it yourself.

 

I have a leg and an arm to spare Let me call UPS

 

There are many different types of analog isolators like this ACPL-C87xx.
I have many of these parts on the shelf. And a number of competitors parts from a project two years ago.
I found them to be better than the TL300. I know I should have a tube of parts from TI also.
I think some have an input voltage of 0.2V or 2V or? I can't find it now, but I think there is one for +/- 2V.

I made a scope probe out of an isolator. 1000V isolation, 100khz bandwidth. I had a variable gain amp on the front end. I needed to measure signals with only some pF back to the scope.

 

Ron; I am curious.
What did you use to power the isolated VDD1?
Batteries? or you used one of those isolated board-mounted DC/DC converters?

 

What did you use to power the isolated VDD1?

There is an entire class of small DC to DC with isolation power supplies. This one is 5Vin 5Vout 1W. I needed 1000s of volts of isolation so I used larger ones where there is more separation between input and output. Look at some data sheets. Most have a part number like NME for which model, 1 for watts, 05 for input and 05 for output.
Outputs are often 3.3, 5, 6, 12, 24 and inputs are often 5, 12, 24. So I was using 12405 or 22405.

Look for RFM-0505s
NMV1s0505
RO-0515S

 

there is literally tons of them, most are using standard pinout which makes them interchangeable.

couple notes:
you will want one with dual output either +/-12V or +/-15V since signal output is supposed to be +/-10V. also you may want to dig a bit more and pick variant that has higher switching frequency. i usually aim for 500kHz or higher. this is easier to filter. also pay attention to maximum capacitive load. you do not want your filter to make the regulator fail to run. since your analog is voltage type, current draw is tiny (easier to filter). or just use +/-15V so your supply filters can have higher voltage drop.

about cost of industrial signal conditioners - i hear you, but it was just an example that those things are not unicorns. plus, i have developed surprisingly high tolerance for what other people pay...

but i did offer another DIY path using digital conversion and multiple channels. that brings per channel cost down and ... it is digital, 3-way isolated etc. with PCBA from JLC or example it would cost some $100... for 5 boards and extra channels. just saying.

 

Thanks Ron; I have used those too in several projects.
In one of them though, at low loads the switching went into a skip-cycle mode and the ripple voltage increased significantly.
A simple led indicator load was enough to fix the issue.

 

So how do I go about my DIY route which will be an interesting learning route? Something I can either solder myself or order online. I'd start with a two channel board to order online so that I can get 5 boards for a total of 10 channels. Something that does not require tuning once created.