Unison 901 & 801 Solid State Relays: Choosing the Right 45 mm SSR
The workhorse relays behind heater banks, furnaces and sealing jaws across Indian industry.
Unison Controls builds its solid state relays in Ahmedabad, and the 45 mm 901 and 801 series are the ones most panel builders reach for. Both take a 4–32 VDC signal from a PLC or temperature controller and switch an AC load silently at the zero crossing of the mains wave. This guide explains how they differ, how to read the part number and how much current they really carry.
901 vs 801 at a Glance
| Feature | Unison 901 | Unison 801 |
|---|---|---|
| Footprint | 45 mm industry-standard, screw mount on heat sink | 45 mm, screw mount or DIN adapter |
| Control input | 4–32 VDC, 4–16 mA, green LED | 4–32 VDC, 4–16 mA, green LED |
| Output voltage | 24–480 VAC (48 code); 24–600 VAC on 60-code 50/90 A models | 24–480 VAC, 800 V peak inverse |
| Ratings made | 25, 40, 50 and 90 A | 16, 25 and 40 A |
| Switching | Zero-cross (ZDA) or random (RDA) versions | Zero-cross |
| Protection | Snubber and TVS; 6–8 kV input–output isolation | Built-in snubber and TVS; 6 kV input–output isolation |
| Approvals | EN 60947-5-1; UL-marked versions in the range | EN 60947-5-1 |
| Stock at Industrypapa | 25 A, 40 A, 50 A | 16 A, 25 A |
From Unison 801 and 901 datasheets. Always confirm the full code printed on the relay label.
Decoding a Unison SSR Code
901 / 801
The series. It sets the housing size, mounting and the family of ratings.
Z · D · A
Z = zero-cross (R = random), D = DC control input, A = AC output. A DC-output relay ends in DD instead.
48 · 25 · 01
48 = up to 480 VAC output (58 = 580 V, 60 = 600 V), 25 = nameplate amps, then the variant code. AL5 is a suffix on current 901 production — quote it in full when reordering.
Example: 901 ZDA 48 40 01 AL5 is a 40 A zero-cross relay, DC control, 24–480 VAC output.
Nameplate Amps vs Real Amps
| 901 model | No heat sink | With G-68 heat sink | Larger heat sink |
|---|---|---|---|
| 25 A | about 10 A | about 16 A | — |
| 40 A | about 12 A | about 18 A | — |
| 50 A | about 12 A | — | up to about 32 A (B-48) |
| 90 A | — | — | about 65 A (B-48 / A-100) |
Figures from the Unison 901 datasheet. An SSR turns roughly 1–1.5 W per amp into heat; the heat sink, not the label, sets the continuous current.
Where 45 mm Unison SSRs Work Best
Plastic extrusion and moulding
Barrel and die heaters switched every few seconds by a PID controller — no contacts to wear out.
Packaging sealers
Sealing jaws on FFS and pouch machines, where silent, fast cycling keeps seal temperature steady.
Furnaces and ovens
Resistive heater banks in curing ovens, dryers and textile stenters, often three relays per zone.
Unison 45 mm SSRs in Stock
901 series
Unison 901 ZDA 48 25 01 AL5
25 A, zero-cross, 24–480 VAC
Rs. 350
Unison 901 ZDA 48 40 01 AL5
40 A, zero-cross, 24–480 VAC
Rs. 450
Unison 901 ZDA 60 50 01 AL5
50 A, zero-cross, up to 600 VAC
Rs. 650
801 series
Wiring a 901 or 801 Correctly
- Control side. Terminal 3 (+) and 4 (−) take the 4–32 VDC signal from the PLC or the SSR output of a temperature controller. The green LED confirms the input.
- Load side. Terminals 1 and 2 go in series with the heater on one phase. For a 3-phase heater, use one SSR per phase or a 3-phase relay.
- Heat sink first. Apply a thin layer of thermal compound and tighten both mounting screws evenly before powering the load.
- Fuse the output. Use a fast semiconductor fuse sized to the load; an ordinary MCB is too slow to save a shorted thyristor.
- Leave air space. Mount heat sinks vertically with fins upright and keep 20–25 mm between them so air can rise freely.
Unison 901 & 801 FAQs
What is the difference between Unison 901 and 801?
Both are 45 mm zero-cross DC-to-AC relays with the same control input. The 901 range goes higher — up to 90 A and 600 VAC — and has random and UL-marked versions; the 801 is a compact, economical range from 16 to 40 A.
What does ZDA mean in 901 ZDA 48 25 01?
Z is zero-cross switching, D is a DC control input and A is an AC output. 48 means up to 480 VAC and 25 is the nameplate current.
Can I run a 25 A SSR at 25 A?
Only with a heat sink large enough. Without a heat sink a 25 A 901 carries about 10 A; on a G-68 heat sink about 16 A.
Why is my SSR hot even at low current?
A solid state relay always drops about 1–1.5 V, so it heats up in proportion to current. That is normal; use the correct heat sink and ventilation.
Can a Unison SSR switch a motor?
Use a random-type or motor-rated relay and size it well above the motor’s starting current. For heaters and resistive loads, zero-cross is the right choice.
Are Unison SSRs available with GST invoice?
Yes. In-stock Unison relays dispatch the same day from Noida with a GST invoice.
Choose the Right Unison SSR
Tell us the load type, voltage, current and how often it switches. We will recommend the SSR and heat sink and quote from stock.
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