Power Supply Series and Parallel Operation Limits
Do not approve power supplies for series or parallel operation from their nominal output ratings alone. For an OEM purchase, the deciding evidence is the exact model’s permitted connection method and operating limits, followed by verification of the complete circuit. More terminals, identical voltage labels or spare physical space on a rail do not establish that two outputs may be joined.
The WEHO NDR-75-24 provides a concrete boundary: the retained original specimen is marked 24 V, 3.2 A, with two −V and two +V output terminals. The supplied material does not establish approved series operation or a current-sharing function for this exact revision. Treat those arrangements as unqualified until the manufacturer provides matching instructions.
State which problem the extra supply is meant to solve
Buying a second supply may be intended to increase voltage, increase current, isolate a noisy branch or maintain operation after a failure. These are different architectures, with different acceptance tests. Write the intended outcome in the request for quotation before proposing a connection. A supplier should be able to explain how the offered configuration meets that outcome and what happens when one unit is off or faulty.
| Buyer objective | Evidence required before approval |
|---|---|
| Higher output voltage | Explicit series permission, allowable output-to-earth voltage, startup/shutdown and reverse-voltage behavior |
| More current at the same voltage | Explicit parallel permission, load-sharing method, combined rating and protection coordination |
| Continuity after one source fails | Defined redundancy architecture, fault isolation, remaining capacity and failure monitoring |
| Independent machine branches | Separate load allocations and a documented return, signal and grounding arrangement |
The numbers 24 + 24 = 48 and 3.2 + 3.2 = 6.4 are arithmetic, not manufacturer ratings for a combined NDR-75-24 system. Do not put those calculated values on a machine specification until the proposed arrangement and all operating conditions have been qualified.
Parallel permission is not the same as useful current sharing
Small differences between regulated output voltages can make one source carry more of the load. The referenced explanation of paralleling and redundancy distinguishes the ability to connect outputs from controlled load sharing and from fault-tolerant redundancy. That is a general engineering distinction, not permission to connect this WEHO model.
Ask the supplier for the allowed number of units, required settings, wiring arrangement and load-sharing tolerance across the intended load and temperature range. Confirm how overload protection behaves when one source reaches its limit first. Do not infer a sharing input from a status indicator, duplicate output terminal or unfamiliar terminal name on another series.
Series approval must cover more than the total voltage
For a proposed series arrangement, request the permitted output-to-earth potential and applicable isolation limits for the exact supply revision. The equipment designer must also assess receiver insulation, external connections, accessible voltage and the effect of grounding one point of the combined circuit. A general statement that an AC-to-DC supply is isolated is not a complete series-operation instruction.
Define the sequence when one supply starts first, shuts down first or enters protection. Ask how reverse voltage and externally stored energy are handled and whether additional protection is required. These conditions belong in a supplier-approved circuit and test plan; they should not be improvised by connecting two outputs on a workbench.
Keep the NDR-75-24’s actual interface in the comparison
The original unit has a metal case, a narrow blue front, four upper output clamps and three lower input clamps marked protective earth, N and L. Its label states 100–240 VAC, 1.6 A, 50/60 Hz and 24 V, 3.2 A. The complete drawing gives a 32 × 125.2 × 102 mm envelope and a rear rail-mounting assembly. These facts identify the component, not an approved multi-unit topology.
Do not substitute a generic family description for this photographed revision. For example, extra output connections on the front are not evidence of independently isolated channels. Obtain the offered unit’s terminal assignment, mechanical instructions and matching technical sheet, and resolve inconsistencies before the sample build is frozen.
Compare alternatives before adding integration work
A single supply with the required output rating can avoid some coordination questions. Separating independent loads across two supplies may solve a capacity problem without joining outputs, provided shared signals and returns are designed appropriately. Where continuity is essential, ask for a documented redundant supply and decoupling arrangement rather than treating ordinary parallel wiring as redundancy.
Compare total installed cost: extra rail space, distribution, protection, fault monitoring, wiring, validation and spare-part control. A lower unit price is not a saving if the combined circuit lacks an agreed acceptance basis or requires repeated commissioning changes.
Put the accepted configuration in the purchasing record
Provide the supplier with the load profile, input conditions, enclosure temperature and required failure response. Request a written configuration drawing and limits for the exact model and revision. Agree tests for normal load, load steps, one-source loss, recovery and relevant fault conditions with the equipment designer; these checks must be performed through a safe, approved test procedure.
Record supply revisions, settings, external devices and wiring with the accepted sample. For repeat orders, require change notification for anything affecting output control, protection or permitted interconnection. An apparently equivalent replacement should not inherit the original configuration’s approval without review.
Buyer questions
Do two +V terminals mean the supply supports parallel operation?
No. Duplicate terminals alone do not establish a current-sharing controller or an approved interconnection with another supply. Use the exact terminal documentation and manufacturer permission.
Can two identical units be assumed safe in series?
No. Matching model labels do not establish the allowed output-to-earth voltage or startup, shutdown and reverse-voltage behavior. Ask for explicit series-operation instructions or select a different qualified architecture.
Send WEHO the required voltage, current and failure-response objective and request a supported single-source or multi-source proposal. Include sample quantities, expected production demand and documentation requirements; no price, availability or delivery commitment is implied by the component example.
