HS100 30R F Precharge Resistor
Terms and Conditions
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An Ohmite HS100-30R-F precharge resistor: 30 ohms, 100 watts, wirewound in an aluminium housing for heatsink mounting. The resistor that stands between your pack and a welded main contactor.
This part exists because of one specific failure. An inverter's DC link is a large capacitor, and at rest it is empty. Close a main contactor straight onto it and the pack sees a near short circuit for a few milliseconds. The inrush is enormous, it arcs across the contactor tips as they touch, and it welds them. A welded main contactor means a pack that can no longer isolate itself, which is the worst failure a high voltage system can have.
Precharge fills the capacitor through this resistor instead. The 30 ohm value limits the current to something the resistor can absorb, the DC link comes up to pack voltage over a second or so, and only then does the main contactor close, onto a link that is already at the same potential as the pack. There is nothing left to arc.
Mount it on a heatsink with thermal compound. The aluminium housing is designed to conduct its heat away into metalwork, and the 100 W rating assumes you have done that. Left in free air it will dissipate a fraction of that figure. Precharge is a short duty, but a controller that retries repeatedly can put real energy into it.
Key specifications
| Manufacturer part | Ohmite HS100-30R-F |
|---|---|
| Resistance | 30 ohms |
| Power rating | 100 W, heatsink mounted |
| Construction | Wirewound, aluminium housed |
| Mounting | Heatsink, with thermal compound |
| Standards | MIL 18546 and IEC 115 |
| Weight | 0.1 kg |
Features
- Wirewound, which handles the high energy pulse of a precharge event without degrading
- Aluminium housed for heatsink mounting, so the 100 W rating is achievable
- Linear power dissipation, stable at elevated ambient temperature
- Built to MIL 18546 and IEC 115
- Also suits inrush limiting and braking duty
Compatibility
For precharge circuits in EV traction packs, and for inrush current limiting generally.
Check 30 ohms suits your pack and your DC link. The resistance sets the precharge time, and the precharge time has to be long enough to fill your capacitance but short enough that the controller does not close the main contactor early. If you are using a controller with a fixed precharge time, the resistor and the capacitance together have to fit inside it.
It is not optional and it is not a fuse. A precharge resistor carries current only during precharge; if yours is getting hot in normal running, the precharge contactor is not dropping out and something is wrong.
In the box
- 1 x HS100-30R-F precharge resistor
Downloads
A 3D CAD model in STEP format is in the Documents tab, for checking the mounting footprint against your heatsink.
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