---
seo_title: "Control an Electronic Load with Python - instro"
myst:
  html_meta:
    description: "Control programmable electronic loads from Python with instro: set CC, CV, CR, or CP mode and level, enable the input, and read voltage and current."
---

# Electronic Load (ELoad)

{.lead}
Using InstroELoad for SCPI-based programmable electronic loads

`InstroELoad` provides a unified interface for programmable electronic loads. This class is initialized with a vendor-specific driver (`BK85XXB`, …), and provides the vendor-agnostic API (`set_mode`, `set_level`, `get_voltage`, …).

## Creating an InstroELoad

```python
from instro.eload.drivers.bk_85xxb import BK85XXB
from instro.eload import InstroELoad

eload = InstroELoad(
    name="myELoad",
    driver=BK85XXB(visa_resource="USB0::0x0614::0x0960::8514B12345::INSTR"),
)
```

## Supported Vendors

:::{driver-cards} eload
:::

- **Virtual**: no hardware or server; the load and its source simulated in-process (`VirtualELoad`, see [Virtual instruments](/library/simulated-instruments.md#virtual-instruments))

If your vendor or model is not listed, see [Custom Driver Development](/library/custom-instruments.md#electronic-load-eload), or open a [Driver Request](https://github.com/nominal-io/instro/issues) issue on GitHub.

## Example

More examples found in [Examples](/examples/eload/index.md)

## Details

The following presents details about the `InstroELoad`. Specific driver details can be found on [their pages](#supported-vendors).

### Driver Composition

An `InstroELoad` is built from a concrete driver:

```
InstroELoad("name", driver=BK85XXB(visa_resource="USB0::..."))
```

- **`BK85XXB`** owns the connection setup and vendor-specific command mapping.
- **`InstroELoad`** owns the category-level workflow: measurements, commands, publishers, the background daemon.

### Lifecycle

The typical InstroELoad workflow:

1. **Construct**: instantiate the vendor driver and pass it to `InstroELoad`.
2. **`open()`**: establishes the VISA connection.
3. **Configure and measure**: set operating mode, level, range, and read measurements.
4. **`start()`**: begins a periodic background daemon. (Optional)
5. **`stop()`**: ends the background daemon (if started).
6. **`close()`**: disconnects from hardware.

### Operating Modes

Electronic loads can operate in four modes:

- **CC (Constant Current)**: the load draws a constant current regardless of voltage
- **CV (Constant Voltage)**: the load maintains a constant voltage across its terminals
- **CR (Constant Resistance)**: the load simulates a constant resistance
- **CP (Constant Power)**: the load draws constant power

:::{note}
**Mode Configuration Required**

The operating mode must be set using `set_mode()` before you can configure the level or range. Not all electronic loads support all four modes. Consult your instrument's manual.

`set_range()` support depends on the driver. `BK85XXB` supports CC and CV, and auto-ranges CP and CR from the level value. `RigolDL30xx` supports CC, CV, and CR. In other modes, `set_range()` raises `NotImplementedError`.
:::

### Init from a Config File

`InstroELoad` can also be constructed directly from a JSON config file, which removes the need to write any Python setup code:

```python
from instro.eload import InstroELoad

eload = InstroELoad(config="bench_eload.json")
```

Where `bench_eload.json` contains:

```json
{
  "version": 1,
  "instrument": "InstroELoad",
  "device": {
    "name": "bench_eload",
    "description": "Bench electronic load",
    "manufacturer": "B&K Precision",
    "model": "8514B"
  },
  "driver": {
    "connection_type": "visa",
    "name": "BK85XXB",
    "visa": {
      "visa_resource": "ASRL3::INSTR",
      "serial_config": {
        "baud_rate": 9600,
        "data_bits": 8,
        "parity": "N",
        "stop_bits": 1
      }
    }
  },
  "load": {
    "mode": "CC",
    "level": 1.5,
    "range": 5.0,
    "slew_rate": {
      "direction": "BOTH",
      "rate": 0.5
    }
  },
  "timing": {
    "poll_interval": 1.0
  },
  "publishers": [
    {
      "type": "NominalCorePublisher",
      "dataset_rid": "<dataset_rid>"
    }
  ]
}
```

To try a config with no hardware, use the virtual driver: `"driver": {"connection_type": "virtual", "name": "VirtualELoad"}`. Optional `source_voltage` (V, default 12) and `source_resistance` (ohms, default 0.5) set the simulated source it draws from.

See [Config Files](/library/config-files.md) for the full field reference.

## Custom Driver Development

For more information on writing a custom driver, see [Custom Driver Development](/library/custom-instruments.md#electronic-load-eload).