09-06-2024, 03:02 PM
We need to sense the load current at the supply side because we might have a device that takes two HT voltages and only has one common return connection.
Looking at all the cheap combo voltage & current meters on Amazon etc it seems the 10A shunt is obvious, so I will cut it and power module from 5V or 12V (maybe 5V to reduce waste). Then I will measure what current gives 10.0 on the scale. I will call that 10mA.
So I will have a switchable 100 Ohms or 10 Ohms in series with HT to load. That will be 1V at 10 mA or 100mA (which the supply won't do, but it might manage more than 10mA). A 1/4 of an LM3900 will measure the voltage and with a PNP transistor on the output it will be a current source such that with shunt on meter cut, 10mA through the 100 Ohm resistor gives 10.0 on the display.
The voltage part is apparently 100V max. However I will scale the HT, by measuring volt meter input resistance, by a series resistor so that 300V reads 30.0 V. Maybe I can blank the dot, or just write x10 beside it.
Loads of people sell these and the sellers may be optimistic about the voltage the circuit can use (I'll assume 5V to 12V rather than claimed 4V to 30V. The voltage measurement, module supply and common 0V are on a 3 pin connector with a supplied plug having black, yellow and red wires. I'll use 5V to verify black + yellow is power. The current uses a fatter red and black wire and the thick 10A shunt is obvious beside the connector.
The only kind of affordable DC current at a floating HT is a battery powered DMM. So the series sense resistor and amp to current convert is the simplest. A voltmeter would only save a PNP transistor and two resistors.
Looking at all the cheap combo voltage & current meters on Amazon etc it seems the 10A shunt is obvious, so I will cut it and power module from 5V or 12V (maybe 5V to reduce waste). Then I will measure what current gives 10.0 on the scale. I will call that 10mA.
So I will have a switchable 100 Ohms or 10 Ohms in series with HT to load. That will be 1V at 10 mA or 100mA (which the supply won't do, but it might manage more than 10mA). A 1/4 of an LM3900 will measure the voltage and with a PNP transistor on the output it will be a current source such that with shunt on meter cut, 10mA through the 100 Ohm resistor gives 10.0 on the display.
The voltage part is apparently 100V max. However I will scale the HT, by measuring volt meter input resistance, by a series resistor so that 300V reads 30.0 V. Maybe I can blank the dot, or just write x10 beside it.
Loads of people sell these and the sellers may be optimistic about the voltage the circuit can use (I'll assume 5V to 12V rather than claimed 4V to 30V. The voltage measurement, module supply and common 0V are on a 3 pin connector with a supplied plug having black, yellow and red wires. I'll use 5V to verify black + yellow is power. The current uses a fatter red and black wire and the thick 10A shunt is obvious beside the connector.
The only kind of affordable DC current at a floating HT is a battery powered DMM. So the series sense resistor and amp to current convert is the simplest. A voltmeter would only save a PNP transistor and two resistors.
