The Hoffman Controls 905-120/277 SCR Heater Power Switch is designed for resistive heating applications. These SCR (triac) controllers fire at zero voltage crossing and are used for connecting resistive loads to line voltage. Firing at zero voltage reduces the radio frequency interference (RFI) from these units.
A master unit (905-M) can directly accept proportional analog signals. It includes a 901-E Multiple Input Logic card mounted on the master for driving the power switch. Additional circuits that are controlled by the same input can be connected to a slave unit (905-S) that accepts the pulse width modulated signal provided by the master.
These units can be used in any connection arrangement or combination of phases without regard to phase order. They can be used in three phase applications, in-line, Delta, or Wye. The 905 series comes in a single phase (one power switch) and three phase (two power switch) version. Various models are rated at currents up to 48 amps and voltages up to 480 volts.
(SCR Replacement for 900-120/277)
The design of the heat sink is critical to the reliability of the product. The most efficient shape for dissipating heat is an “H type” profile. This profile is incorporated in this Series. This design allows equal flow of the dissipation of heat from the SCR component. This occurs because the component heat generating surfaces are located symmetrically around the center axis of the “H” profile. The ability of the heat to be uniformly rejected around 360 degrees, provides equal distribution of heat at the lowest allowable temperature. In addition to the heat sink’s superior ability to reject heat, another unique design feature is inherent in the “H” design profile. By providing a louvered opening at the top and bottom of the heat sink side exposed to the electrical control panel, ambient air is allowed to draft (vent) over the SCR due to thermal convection that naturally occurs. This design of venting of ambient air through this section of the “H” heat sink, further reduces the level of the junction temperature of the SCR; making the controller more reliable.
Hoffman Controls 905-120/277 Features:
- Each power switch incorporates its own zero-firing logic, providing any combination or connection arrangement for single or three phase applications.
- Up to eight power switches may be controlled from one 901-E Series Logic.
- The 901-E included with the master unit accepts voltage or current input.
- No phasing required
Hoffman Controls 905-120/277 Specifications:
- Master (accepts 0-10V DC, 4-20mA or 135 ohms) – 905-M
- Slave (accepts pulse width) – 905-S
- Multi Voltage, Range – 208 to 480V AC
- Range – 208 to 600V AC
- Current, @ rated ambients – 25A/48A/74A
- Ambient, Components, Min/Max – 0°F/32°F Min. 75°C/167°F Max.
- Heat Sink, Max. – 50°C/122°F
- Humidity – Non-condensing
- Heat Sink, configuration – “H”
- Feature – Ventilated
Dimensions (L x W x H):
- Size 25A – 5” x 5.85” x 3”
- 48A – 10” x 5.85” x 3”
- 74A – 10” x 5.85” x 3”
Dimensions (L x W):
- Cutout 25A – 4.85” x 3.25”
- 48A – 8.75” x 3.25”
- 74A – 8.75” x 3.25”
- Input, Master – 0-10V DC, 4-20mA, 135 ohm
- Slave – Pulse Width Modulated
- Indicator, 905-M, 905-S, 901 Series – LED
- De-ratings – See I&O Instructions
- SCR Connection Requirements, line voltage
- Maximum Wire,direct connected to SCR – 8 AWG
- Add box lug under terminals; Maximum wire size. – 4 AWG