Help Driving a HP-87106A - Coax switch from a MCU  - Page 1
Forum section: electronics / beginners

author: bingo600
author_rank: Super Contributor
timestamp: January 22, 2023, 02:38:52 pm
content: I got a HP-87106A - Coax switch locally for $20.And i would like to control it from a MCU Ie. Arduino Nano (5V) or STM32 (3.3V) ... (I might have to do some level xlation for the MCU to ULN drive , expects 5v)The switch requires a 20..32V PSU, and the "latching" relays are driven (200mA) by connecting the respective pin to ground (min 15ms).I'm a bit of an analog electronics noob.So i thought i'd ask how to drive it, and read the positions here.I have The "No" options (No TTL Drive), and the 16-pin IDC interface.And i'll only drive/close one relay at a time.My plan was to maybe to use a "spare laptop PSU" , i have 19v & 20v versions (i expect it'll work even if the ULN is darlington).Then connect Coax Relay IDC pin :Code:1 : To the 24v (20v) VCC15 : To GNDAndThe Coax Relay drive pins to the output pins of the ULN2803A3,5,7,9,11,13 - To ULN OutputsULN GND to GND , and ULN COM(mon) to VCC (protection diodes ??).I'll prob. avoid to use Coax Relay Pin16 (Release all relays) , as it could draw 200mA per relay to release , and i'm not sure if the ULN can handle that.The "Connect pin to ground" confuses me a bit ??Would the ULN or a NPN/FET with the Relay pin connected to the collector/source switch to ground ?I think so , as "open" it would be "VCC" or ??Q1:Can i use an ULN2803A or ULN200xA to drive the relays ??https://www.sparkfun.com/datasheets/IC/uln2803a.pdfQ2:The Coax Relay has position indicator pins with a Common (pin 2)Code:Pin 2 - Position CommonPin 4.6.8.10.12.16 - Position indicatorsCould i connect the indicator common to the MCU VCC ie 5v ?And then the position indicators would be 5v ?Or should i expect troubles here ?Can anyone deduct that from the attached pdf , or should i just try , and measure w. a DMMPs: I have ULN's in the drawer, so using those is preferred.TIA/Bingo

author: berke
author_rank: Frequent Contributor
timestamp: January 22, 2023, 04:35:36 pm
content: Yes that ULN2803A seems like a good interface. Connect your microcontroller outputs (5V or 3.3V should both work) to the ULN inputs (e.g. pin 1B). Connect the ULN open collector outputs (e.g. pin 1C) to the drive pins. Connect the ULN "COM" to the 24V VCC, this is for flyback protection.When you set the µC pin high, it will put the transistors into saturation. The output transistor will pull current from its output pin and drive the voltage to ground (almost...). When you release it, the diodes connected to COM will serve as an alternate path for the flyback current and prevent the voltage from rising. The ULN should sink less than 700 µA per pin from your microcontroller ((3.3-1.4)/2700). In principle you don't need a base resistor as it is included, nor a pull-down.Looks like 20V should work.For the position indicator, my guess is that the relays are DPST so you can connect things like you say, but you should verify first, buzz it.It's preferable to include series resistors to protect the microcontroller I/O pins. It depends on your GPIO specs and the voltages on your board. I would try 1k on the outputs and 10k on the inputs.If you turn on all channels simultaneously the ULN might get hot, keep an eye on that. At 1.2 A it's within spec if it stays cool.

author: bingo600
author_rank: Super Contributor
timestamp: January 22, 2023, 06:14:04 pm
content: @berkeThank you so much.Quote from: berke on January 22, 2023, 04:35:36 pmI'll do thatQuote from: berke on January 22, 2023, 04:35:36 pmAccording to the DS , the relays are latching , and will "drop loading" as soon as it has reached the "position".Does that mean it'll send a "Coil kick" through the ULN flyback diode on every "Activation" ?I suppose it's kind of a "Flip-Flop" open/close that have made, how else to switch state ...QuoteHmm ... According to this, the previous "path" are automatically opened, when closing a new one .... Fits me fine.Quote from: berke on January 22, 2023, 04:35:36 pmI might end up using a STM "Black pill", since i'll "wire this up" ... Then i have USB-->Serial, for sending commands + enough ports to avoid using shift registers.I'll prob. install a push buttons, for manual port selection & for "activating" the selection (long press).Quote from: berke on January 22, 2023, 04:35:36 pmHP writes it's OPTO SSR's on the position indicators , so prob. a string of optocouplers with collector connected to the common pin 2Quote from: berke on January 22, 2023, 04:35:36 pmI'll try with something like thatQuote from: berke on January 22, 2023, 04:35:36 pmSince i'll use it for switching DUT's to a measuring device (Ie. GPSDO's to a Counter), i'll never intentionally connect more than 1 port to the "comon"./Bingo