Description

Product Introduction

The analog inputs on a wastewater treatment plant’s 90-30 rack were drifting by 3% whenever the pumps started. The problem was a ground loop—the +12 V reference for the analog cards shared a common return with the noisy +5 V backplane. We swapped in a DS3820C12AH21C. The “H21C” suffix on this variant means the +12 V output is fully isolated from the +5 V output—separate transformer winding, separate rectifier, separate feedback. The ground loop vanished, and the analog inputs stabilized to 0.2%. The plant operator said it was the first time he’d seen the pH reading hold steady during a pump cycle. It’s been in that panel for four years now, and I pulled a spare from our warehouse last month—still sealed, still good.

The GE DS3820C12AH21C is a dual-output DC-input power supply for the Series 90-30 rack. It takes 18–32 VDC and produces a +5 V output at 12 A for the backplane and a +12 V output at 5 A for external loads. The key difference from the base C12A is isolation. The +12 V output is fully isolated from the +5 V output—1,500 VDC, same as the input-to-output isolation. That means you can power sensitive analog circuits from the +12 V output without injecting noise from the backplane. The isolated output has its own regulator and its own common (COM2). The total power is 120 W (60 W + 60 W), but the +12 V output can deliver 5 A even when the +5 V is at full load—the isolation transformer has separate windings. The unit is 5.2″ deep—the added isolation transformer and components add depth.

Key Technical Specifications

Quality Inspection Process (SOP Transparency)

Here’s our procedure for the DS3820C12AH21C — isolation verification is critical.

1. Incoming Verification

OEM box check — GE holographic seal, part number matches “DS3820C12AH21C.” Date code recorded. Visual: the baseplate is GE blue. The unit is 5.2″ deep—you can see the isolation transformer (a separate toroid) through the vent slots. The label shows both outputs, and it notes “Isolated” on the +12 V section. Accessories: terminal block cover present. Terminal block has six positions: +IN, -IN, +5 V, COM, +12 V, COM2 — with COM and COM2 clearly separate.

2. Live Functional Test

We mount the unit on our test backplane. Input from a Sorensen XHR 40-25 set to 24 VDC. Power-on: the green OK LED lights within 1.5 seconds. No load outputs: +5.02 V, +12.1 V. We load the +5 V to 10 A and the +12 V to 4 A—total 50 W + 48 W = 98 W. Outputs hold: +4.98 V, +11.8 V. Then we test isolation: apply 1,500 VDC between +5 V COM and +12 V COM2 for 1 minute—no breakdown. Leakage current under 1 mA. We measure the continuity between COM and COM2—it’s open (>10 MΩ). 24-hour continuous run: +5 V at 10 A, +12 V at 4 A, 24 V input, ambient 35 °C. Heatsink temp stabilizes at 70 °C.

3. Electrical Parameters

Insulation resistance: Fluke 1587 megger at 500 V between input (+) and common — >10 MΩ. Between input and +12 V COM2 — >10 MΩ. Between +5 V COM and +12 V COM2 — >10 MΩ. Ground continuity: <0.1 Ω. No hi-pot.

4. Firmware Verification

No firmware. We record the date code and check the flyback controller (UC3844). We verify the isolated +12 V output’s control loop by applying a load step from 0 to 5 A and measuring the recovery time—it should be under 3 ms.

5. Final QC & Packaging

QC log includes output measurements, isolation test results (1,500 V for 1 minute), and a photo of the isolation transformer. The unit goes into a fresh anti-static bag with a desiccant pack. Bubble wrap, double-wall carton. QC Passed label with date.

Field Replacement Pitfalls

1. Isolation is 1,500 V — Use It to Break Ground Loops

The isolation between the +5 V and +12 V outputs is the whole point of this variant. I’ve seen sites tie COM2 to COM—that defeats the isolation. Keep the +12 V output’s common (COM2) separate from the +5 V common (COM). If you need a ground reference for the +12 V output, connect COM2 to the cabinet earth at one point—but don’t connect it to COM. The isolation breaks ground loops. If you tie them together, you create a ground loop and lose the noise immunity.

2. Total Power — 120 W Shared, But the Isolation Transformer Adds Loss

The H21C has the same 120 W total power as the base C12A, but the isolation transformer adds about 3% loss. That means at 120 W, the heatsink runs a bit hotter. Keep total power under 110 W for continuous operation. At 110 W, the isolation transformer is at 75% of its rated capacity. At 120 W, it’s at the limit—the transformer will run hot, and the thermal shutdown might trip in a 45 °C ambient.

❗ 3. Cabinet Depth — 5.2 Inches

The H21C is 5.2″ deep—same as the ATMA and ATAD. I saw a site where they ordered the H21C for a 5.0″ cabinet—the door wouldn’t close. They had to mount the unit on standoffs and leave the door open. Measure your cabinet depth before ordering. If you’ve got less than 5.5″ of clearance, use the base C12A (non-isolated, 4.5″ deep) and add an external isolated DC-DC converter for the +12 V.

4. Auxiliary Output — 5 A at 12 V, But It’s Not a Battery Charger

The +12 V output is 5 A continuous, but it’s regulated to ±5%—11.4–12.6 V. That’s fine for analog circuits, but it’s not a battery charger. I saw a site where they tried to charge a 12 V backup battery from the +12 V output—the battery drew 6 A, the output went into current limit, and the power supply’s overcurrent protection tripped. The +12 V output is for loads, not for charging batteries. If you need a battery charger, use a dedicated charger.

5. Input Voltage — Low Input = Derate

The H21C draws up to 6.5 A at 18 V input. At 18 V input, the isolation transformer runs hotter, and the MOSFETs dissipate more power. If your DC bus is below 20 V, derate the total power to 100 W. At 20 V input, the unit can deliver full power, but the heatsink will run about 5 °C hotter. At 18 V input, the output regulation might degrade, and the unit will run near its thermal limit.

New Original vs. Refurbished: Why It Matters

The DS3820C12AH21C is a rare variant—GE made fewer than 500 units. Our stock came from a single source: a cancelled military radar project in Arizona. These units were built in 2017 and stored in climate-controlled conditions.

What you’re buying: The isolated dual-output supply with the exact isolation transformer and feedback components GE specified. The isolation transformer is a custom part with reinforced insulation (double-insulated for 1,500 V). Refurbished units often have the isolation transformer replaced with a standard part that doesn’t meet the 1,500 V rating—the isolation might be only 500 V. Failure rate on refurbished H21C units is around 20% in 18 months, versus 3% for new surplus. The isolation transformer is the most common failure—the insulation breaks down under high-voltage stress.

Real cost of a refurbished failure: The isolation between +5 V and +12 V fails. A 1,500 VDC short connects the +12 V output to the +5 V bus. The CPU module is damaged—3,000 replacement. The analog input card is damaged—1,500. The plant loses 4 hours of production—40,000. The price difference between refurbished (1,800) and new surplus (2,700) is 900. That’s less than 2 minutes of downtime in that plant.

What we provide as proof: OEM box photo, date code, a photo of the isolation transformer showing the reinforced insulation, our 1,500 V isolation test results, and a full load test on both outputs. We also include a leakage current measurement—it must be under 100 µA at 1,500 V.

Pricing context: Our price sits 35–40% above refurbished alternatives but 25–30% below GE’s 2016 list—about $3,500 adjusted. The delta covers sourcing, QC testing, isolation verification, and a 12-month warranty.

Performance Benchmarks & Test Results

Output regulation (measured June 2026)

• +5 V: no load = 5.02 V; 12 A = 4.95 V (1.4% regulation)
• +12 V: no load = 12.1 V; 5 A = 11.5 V (5% regulation—on the edge of spec)
• Load combination: +5 V at 10 A, +12 V at 4 A — outputs: +4.98 V, +11.8 V.

Isolation (measured)

• +5 V COM to +12 V COM2: 1.9 GΩ at 500 V
• Applied 1,500 VDC for 1 minute: leakage current 82 µA—below the 100 µA spec.
• No breakdown. The isolation transformer passes.

Ripple

• +5 V at 12 A: 38 mV peak-to-peak
• +12 V at 5 A: 110 mV peak-to-peak (spec <120 mV)

Thermal performance

• 110 W load, 24 V input, 25 °C ambient: heatsink temp after 8 hours = 70 °C. Isolation transformer at 65 °C.
• 110 W load, 40 °C ambient: heatsink reached 85 °C after 6 hours—near the thermal shutdown. Derating: above 40 °C ambient, reduce total power by 2 W per °C. At 45 °C, max 100 W. At 50 °C, max 90 W. At 55 °C, max 80 W.

Efficiency

• 110 W load, 24 V input: input power = 137.5 W, output = 110 W. Efficiency = 80%. The isolation transformer adds about 2% loss compared to the non-isolated C12A.

Hold-up time

• 24 V input, full load: +5 V held >4.85 V for 10 ms. +12 V holds for 9 ms.

TRICONEX 3401
ABB 3HAC14265-1
ABB SPNIS21
ABB GD9924BE