Measuring DC current: why no current transformer, and what instead
Anyone searching for “DC current transformer” or “DC CT 4–20 mA” runs into a physical problem: a current transformer is a transformer. It only transfers AC quantities, because only a changing primary current induces a voltage in the core. With DC the core saturates and nothing flows on the secondary side. CELSA therefore manufactures current transformers exclusively for AC – and for DC a measuring chain of shunt and display.
The shunt resistor is in series with the DC circuit and delivers a fixed voltage drop at rated current of 60 mV, 80 mV, 100 mV, 150 mV or 300 mV, in class 0.5 for 1 A to 4000 A. These millivolts are measured either by an analogue moving-coil ammeter PQ or PAQ or by a digital panel meter with shunt input. The digital M series instruments display the current, monitor limits and, as an option, pass it on via an analogue output – e.g. 4–20 mA – to PLC or control system.
For DC voltage up to 600 V DC there are digital displays with direct input, such as the M3.96-145 DC4 with optionally two analogue outputs, RS232/RS485 and up to eight setpoints. And if you already have a standard signal, the DTVU20 isolating amplifier isolates and converts it (input 0–10 V, 0–20 mA, 4–20 mA) – a 60 mV shunt signal, however, is not accepted directly by the DTVU20.
Step 1: Choose the shunt by current and meter
Choose the shunt rating just above the highest operating current, the rated voltage to match the meter input – 60 mV is the standard of the analogue ammeters and the digital SR displays, which also take 150 mV. Form A covers 1 A to 250 A, form B 250 A to 1000 A, form C 4000 A.
Step 2: Choose the display: analogue or digital
Analogue: moving-coil ammeter PQ (90° scale) or PAQ (240° scale), class 1.5, scale in amperes to the shunt, no auxiliary supply. Digital: M1.48-104, M1.72-144 and M1.96-144 SR2 as 4-digit displays for 60 mV and 150 mV with limit monitoring; M2.96-145, M3.48-105 and M3.96.2-145 SR4 5-digit with options such as analogue output, digital input and setpoints.
Step 3: 4–20 mA for PLC or control system
If the DC current is to continue as a standard signal, the analogue output of the digital meter takes the role of the “DC transducer”: shunt → display with analogue output → 4–20 mA to the controller. If a standard signal already exists, the DTVU20 isolating amplifier isolates it galvanically and converts it to 0–10 V, 2–10 V, 0–20 mA or 4–20 mA.
Step 4: Measuring DC voltage
DC voltage needs no shunt: the displays of the DC voltage/DC current group measure directly, the M3.96-145 DC4 up to 600 V DC. Analogue moving-coil voltmeters PQ and PAQ show DC voltage.
Calculators and related pages
Typical Applications
- Battery and UPS monitoring
- Rectifiers, electrolysis, electroplating
- Test benches, DC drives and charging equipment
Recommended Products
FAQ
Are there current transformers for DC?
No, not as a transformer: a current transformer only transfers AC, with DC the core saturates. CELSA manufactures current transformers for AC and measures DC with shunt and display.
What replaces the DC current transducer with 4–20 mA?
A digital panel meter with shunt input and analogue output: the shunt delivers 60 mV, the display shows the current and passes it on as 4–20 mA. An existing standard signal is converted by the DTVU20 isolating amplifier.
Which shunt do I need for 100 A DC?
A 100 A/60 mV shunt of form A if the meter has a 60 mV input – which the analogue ammeters PQ and PAQ and the digital SR displays have. The scale or display is then referred to 100 A.
Can I measure DC with a Rogowski coil?
No. Like a current transformer, a Rogowski coil only measures the change of current, i.e. AC. For DC it remains the shunt.
How accurate is measurement with a shunt?
CELSA shunts are class 0.5, the moving-coil displays PQ and PAQ class 1.5. The accuracy of the chain is determined by shunt, display and, with analogue instruments, the resistance of the measuring lead.