HV Configuration

The HV Config page (/hv) appears on builds with an HV generator and voltage feedback: ESPGeiger-HW and the esp8266_hv, esp8266oled_hv and esp32_hv builds. Custom builds with an HV generator but no feedback show frequency and duty on the Config page instead.

The HV stage is a Pulse-Width Modulated boost converter circuit. By controlling the PWM duty cycle we produce a stable HV DC driving voltage for the Geiger-Muller tube - typically ~400 V depending on tube spec.

ESPGeigerHW HV

HV Settings

The /hv page edits the espghw prefs:

Setting Key Type Default Builds Description
PWM Frequency espghw.freq Int 100-9000/40000/80000 6000 all PWM frequency in Hz. ESPGeiger-HW max 9000 Hz; esp8266_hv max 40000 Hz; esp32_hv max 80000 Hz.
PWM Duty espghw.duty Int 1-600/1023 280 all PWM duty (1-1023, 10-bit). Each step is ~0.1 % of period.
ADC VD Ratio espghw.ratio Int 0-50000 1035 HV ADC builds Voltage divider scale factor - converts ADC counts to displayed volts. Calibrate with an external HV meter. 0 disables the ADC readout.
ADC VD Offset espghw.offset Int -100 to 100 0 HV ADC builds Fixed offset added to the displayed voltage after scaling.
HV target espghw.target Int 0-500/700 0 HV ADC builds Auto-trim target voltage (0 = open loop, disabled). When non-zero, firmware trims duty by a few steps to hold target. See Closed-Loop Trim.

When changing settings:

  • Always disconnect the Geiger tube before adjusting frequency or duty
  • After submitting changes, the firmware briefly drops PWM low to avoid voltage spikes during the transition
  • The closed-loop trim is suppressed for 30 seconds after changes and for 5 minutes after boot while HV settles, so it doesn’t chase the transient

Protection

  • ESPGeiger-HW caps duty at 600, well above its usual ~320, so a typo can’t saturate the coil.
  • ESPGeiger-HW drops duty to minimum if HV reads over 530 V for 3 seconds. It stays there until settings are saved or the device restarts and /hv shows the cut-off.
  • On ESP8266 the HV pin is held low if the PWM generator stops.

Calibration with a Multimeter

⚠️ HV is dangerous. 400 V at the tube is well above the let-go threshold and the boost-cap can hold charge after PWM stops. Wear gloves where practical, keep one hand in your pocket and discharge the output cap with a 1 MΩ resistor across it before touching the rail.

A standard digital multimeter has roughly 10 MΩ input impedance. Probing the HV rail directly will load it down hard - you’ll read maybe 200 V on a supply that’s actually delivering 400 V into the tube - and the meter sees its full input rating rather than a scaled-down reading. The fix is a high-value series resistor in front of the meter that turns the meter input into the low side of a voltage divider.

Recommended probe: a single 1 GΩ resistor in series with the meter’s positive lead. With a typical 10 MΩ DMM the divider works out to:

V_HV / V_meter = (1 GΩ + 10 MΩ) / 10 MΩ ≈ 101

So HV ≈ DMM reading × 100 (close enough - the 1% offset is well within tube tolerance). For a 10× reduction (e.g. for a high-impedance bench meter) use a 100 MΩ series resistor instead. If your DMM input impedance is something other than 10 MΩ, plug it into the formula above.

Build the probe on an offcut of perfboard or inside a marker-pen case - keep the resistor body well clear of any conductive surface, since 400 V across a 1 GΩ part will arc to anything close enough. Use HV-rated wire (silicone test-lead wire is fine) and finish with proper meter probes at the user end.

Calibration procedure:

  1. Disconnect the Geiger tube. The tube is a current sink at threshold and will load the rail asymmetrically; calibrate against the bare boost output, not against the tube
  2. Connect your DMM (with 1 GΩ series resistor) between the HV rail and ground, set to DC volts, 20 V or 200 V range
  3. Open /hv. Set Ratio = 1 and Target = 0 (open-loop, no trim chasing)
  4. Adjust Duty until the DMM reads close to the voltage you want (HV/100 - so 4.00 V for 400 V)
  5. Note the raw ADC counts shown on /hv and your measured HV (DMM × 100)
  6. Set Ratio = measured_HV × 1024 / raw_ADC. The displayed reading on /hv should now match your DMM × 100
  7. Once /hv agrees with the DMM at one operating point, sweep duty over a couple of points to confirm the displayed voltage tracks the meter - if it does, the divider is calibrated
  8. Reconnect the tube. Read the now-calibrated voltage on /hv directly - no probe needed for routine checks
  9. (Optional) Set Target to your desired voltage to enable the closed-loop trim

If the displayed voltage drifts noticeably when you reconnect the tube, that’s normal - tubes draw a few µA at threshold and most home-built boost stages have non-zero output impedance. Set the open-loop Duty to give your target voltage with the tube connected, then enable Target to track it through temperature and age.

Generic HV Builds (esp8266_hv / esp32_hv)

  1. Flash esp8266_hv or esp32_hv and wire your boost circuit (transistor, inductor, diode, cap, feedback divider). See Build Targets.
  2. HV starts inactive. On the Config page, System tab, HV Hardware, set HV PWM Pin to the GPIO driving your transistor, save and reboot.
  3. Open /hv to set frequency, duty and, after calibration, the target.

The feedback input is A0 on ESP8266 and GPIO 36 on ESP32.

Closed-Loop Trim

With Target above 0, the firmware moves duty by 1 step every 10 seconds when the average of the last 8 readings is outside a band around the target, up to ±5 steps from your saved duty. The band is 2 ADC counts either side, about ±2 V on ESPGeiger-HW. Set duty so HV is already right with the tube connected and Target = 0; the trim only tracks drift.

If the duty field shows (trim +5) or (trim -5), the trim has hit its limit: move duty by hand in that direction until the trim sits near 0. The trim resets on reboot and on any duty or frequency change.

This site uses Just the Docs, a documentation theme for Jekyll.