Homemade high-voltage probe not dividing the voltage by proper amount

Question

I have tried to build a homemade high-voltage probe, as commercial ones are too expensive.

Firstly, I connected a 1 MΩ resistor to a ~1.4 V battery which gave an output voltage of ~0.2 V. By using this formula (where V_R = output series voltage, R = resistance of resistor, and V_B = voltage of battery):

$$R_M=\frac{V_R\cdot R}{V_B-V_R}$$

I managed to find the internal resistance (~1.4 MΩ) of the multimeter. I then multiplied it by 9 to get the resistance needed to divide the incoming voltage by 10.

Next, I connected the completed resistors in series with my multimeter and a 12 V power supply.

This was the result:

Instead of reading an expected voltage of 1.2 V, it read close to ~3 V (decreasing only by a factor of 4).

My first thought was that there was a bad connection when soldering the resistors, but it was indeed 1.4 MΩ when measured with a multimeter.

I am unsure as to how to move forward, and I am open to any suggestions.

Answer 1

This meter claims to be \$ 20k\Omega\ per\ volt\$ when measuring DC voltage.
For AC voltages, ohms-per-volt is less (can't make out the value 3k/V? 5k/V?)

However, on its most-sensitive voltage range, it would seem that 0.25V corresponds to 100uA...so on that particular scale the meter would be \$ 2.5k\Omega\ per\ volt\$ instead - an anomaly.
I would expect higher DC voltage ranges would more-closely correspond to the 20k/V spec.

A 20k/V DC voltmeter would have an internal resistance that changes for each switch position:

• 0.5V scale: 10k
• 2.5V scale: 50k
• 10V scale : 200k
• 50V scale: 1MEG
• 250V scale: 5MEG
• 1000V: 20MEG (probe?)

You should really read the manual - it is unclear what "probe" means, written above the 1000V scale.
If you really wanted to extend this meter to 10kV for example, you might choose the 250V scale, and add a high-voltage-compatible series resistor of 195 MEG to the 5 MEG resistor already inside the meter. Full-scale would correspond to 10kV DC, so you'd actually read the scale marked "0 - 10" and interpret that as "thousands-of-volts".

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Making high-voltage probes? This is unwise.
The probes shipped with the meter are somewhat safe at hundreds of volts on this meter. They should not be used at higher voltages.
You may think that a high-voltage probe merely requires a larger plastic shell to increase path length between your hand and high-voltage probe-end. The surface quality of that path needs attention too, as well as its cleanliness.
I have seen a 5kV probe break down in a user's hand. He barely survived with injuries only because a buddy ran to the breaker panel to shut down power.