Voltage, Current, Resistance, and Continuity in Plain English

Published August 12, 2026 · Revised September 6, 2026

The electrical basics a biomed actually needs for troubleshooting

Electrical theory can get complicated very quickly.

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What This Page Explains

This page covers:

The goal is not to turn you into an electrical engineer.

The goal is to give you enough electrical understanding to troubleshoot logically.

The Simple Version

Voltage is the electrical potential difference between two points, current is the rate of charge flowing through a path, and resistance describes how strongly that path opposes current. They are related, but they are not interchangeable: a supply can show the correct voltage with no load and still collapse when the circuit draws current.

Continuity is a quick indication that a sufficiently low-resistance path exists under the meter's small test signal. It can help locate an open fuse, conductor, or switch on a de-energized circuit, but it does not prove insulation, current capacity, connector integrity, or performance under load. Choose the measurement from the troubleshooting question and never use resistance or continuity mode on an energized circuit.

Voltage

Voltage is electrical potential difference.

That definition is technically correct.

It is not always very helpful.

A practical way to think about voltage is:

Voltage provides the electrical push that can cause current to flow.

You may have heard the water-pressure analogy.

Voltage is similar to pressure in a pipe.

Higher pressure creates more potential to move water.

Higher voltage creates more potential to move electrical current.

The analogy is not perfect, but it works well for basic troubleshooting.

Common Voltages in Medical Equipment

You may encounter:

The exact voltage depends on the device.

The service manual tells you what should be present.

Your meter tells you what actually is present.

Then you compare the two.

Voltage Is Always Between Two Points

You do not really measure:

Voltage at this wire.

You measure the difference in electrical potential between two locations.

For example:

12 V rail to circuit common

might measure:

12.0 VDC.

If both probes are placed on points at the same electrical potential, the meter reads approximately:

0 V.

This is why you need to understand your reference point.

Current

Current is the flow of electrical charge.

Current is measured in:

Amperes, usually shortened to:

amps

You may also see:

A practical way to think about current is:

How much electrical flow is moving through the circuit?

Using the water analogy:

Voltage is pressure.

Current is flow.

Current Only Flows Through a Complete Path

You can have voltage present without current flowing.

Imagine a battery sitting disconnected on your bench.

Voltage exists between its terminals.

But nothing is connected.

There is no load.

Very little useful current flows.

Connect the battery to a circuit and now current can flow through the load.

That distinction matters during troubleshooting.

The Load

A load is anything using electrical power.

Examples:

The power supply provides electrical energy.

The load uses it.

If the load fails, you may still measure perfectly normal voltage at the power supply.

That is why:

Voltage is present.

does not automatically mean:

The device is good.

Resistance

Resistance is opposition to electrical current.

It is measured in:

ohms

represented by:

Ω

Higher resistance makes it more difficult for current to flow.

Lower resistance allows current to flow more easily.

Everything Has Some Resistance

Wires have resistance.

Connectors have resistance.

Switches have resistance.

Even good electrical connections are not mathematically perfect.

But in many troubleshooting situations, the resistance is low enough that we treat it as essentially zero.

For example:

A good fuse may measure:

0.1 Ω

while an open fuse may read:

OL.

That difference is very useful.

Continuity

Continuity means there is a continuous electrical path between two points.

A multimeter in continuity mode may beep when resistance is sufficiently low.

For example:

One probe on one side of a good fuse.

Other probe on the other side.

Beep.

There is continuity.

If the fuse is blown:

No beep.

Open circuit.

Continuity Is Basically a Yes-or-No Question

Continuity asks:

Can electricity travel through this path?

That is useful for checking:

But sometimes you need more than yes or no.

You may need to know the actual resistance.

A Cable Can Have Continuity and Still Be Bad

Suppose a cable should have almost no resistance.

It measures:

25 Ω.

The meter may still beep.

Technically, there is continuity.

But that connection may have far too much resistance for proper operation.

So:

Continuity does not automatically mean good.

Sometimes resistance matters.

Open Circuit

An open circuit means the electrical path is broken.

Examples:

If the circuit is open, current cannot flow through that path.

This often produces:

Short Circuit

A short circuit is an unintended low-resistance path.

Current takes a path it should not take.

This can create:

Short circuits can be destructive.

That is why repeatedly replacing a fuse without investigating why it blew can be a bad idea.

Resistance Controls Current

This relationship is described by Ohm's law.

The basic formula is:

V = I × R

Voltage equals current multiplied by resistance.

You can rearrange it:

I = V ÷ R

or:

R = V ÷ I

You do not need to solve equations every day.

But understanding the relationship is useful.

A Simple Example

You have:

12 volts

across:

6 ohms

Using:

I = V ÷ R

Current is:

2 amps.

Now imagine resistance rises to:

12 ohms.

With the same 12 volts:

Current drops to:

1 amp.

Higher resistance reduced the current.

Why Bad Connections Matter

Suppose a connector becomes corroded.

That corrosion adds resistance.

The circuit may technically still be connected.

But under load, the resistance causes:

This is why a connector can:

Look connected.

and still cause problems.

Voltage Drop

Voltage drop is the reduction in voltage as current passes through resistance.

Every real conductor has some resistance.

Normally the drop is small.

But if resistance becomes excessive, the voltage drop can become significant.

For example:

Power supply output:

24 VDC.

Voltage at board input while running:

18 VDC.

Something between the power supply and board is dropping:

6 volts.

Possible causes:

That is a powerful troubleshooting clue.

Voltage Drop Often Appears Under Load

This is important.

A bad connection may measure normally when almost no current is flowing.

Then the device starts.

Current increases.

Voltage collapses.

Example:

Battery disconnected from load:

12.5 V.

Device starts:

Battery terminal voltage drops to 8 V.

That tells you much more than the unloaded reading.

Series Circuits

In a series circuit, components are arranged in one path.

Current flows through each component.

A simple example:

Battery → Switch → Fuse → Motor → Battery

If any component opens, the entire path stops.

That is why one blown fuse can make the whole device appear dead.

Parallel Circuits

Parallel circuits provide multiple paths.

Medical devices use parallel circuits extensively.

For example:

One power supply may feed:

A failure in one branch may not stop the others.

This can explain partial failures.

Example:

Screen works.

Fan works.

Printer does not.

The entire power supply may not be dead.

Focus on the printer branch.

Power

Electrical power is measured in watts.

The basic relationship is:

Power = Voltage × Current

or:

P = V × I

You may see a power supply rated:

24 VDC, 5 A.

That means it can provide up to approximately:

120 watts.

Understanding this can help when selecting or evaluating power supplies.

Why a Higher-Amperage Power Supply Is Not Automatically Dangerous

This sometimes confuses people.

Suppose the original power supply is:

12 VDC, 2 A.

A replacement is:

12 VDC, 4 A.

If everything else is compatible, the device does not automatically receive 4 amps.

The power supply rating describes how much current it can provide.

The load determines how much current it draws.

Voltage must still be correct.

Polarity and connector configuration must also be correct.

And medical equipment should use manufacturer-approved components.

Voltage Must Match

Supplying a device designed for:

12 VDC

with:

24 VDC

can damage it.

Current capacity and voltage are different concepts.

Do not confuse them.

Resistance Can Create Heat

Electrical resistance converts some energy into heat.

That is useful in heaters.

It is not useful in a loose power connector.

A high-resistance connection may become hot.

Signs include:

Heat damage is often evidence of excessive resistance or current.

Real-World Example: Device Will Not Power On

Power supply output should be:

24 VDC.

You measure:

24.2 VDC.

At the main board while powered:

0 V.

You now know the supply itself is producing voltage.

Something in the path between the supply and board deserves investigation.

Maybe:

You narrowed the problem.

Real-World Example: Fuse Keeps Blowing

You replace the fuse.

It blows immediately.

That is telling you something.

The circuit may be drawing excessive current.

Possible causes include:

Do not keep feeding it fuses.

Investigate why the current is too high.

Real-World Example: Motor Runs Slowly

A motor should receive:

24 VDC.

At idle:

24 VDC.

When motor runs:

Voltage falls to 15 VDC.

Possible causes include:

That under-load measurement is much more useful than the idle reading.

Real-World Example: Intermittent Cable

Cable conductor measures:

0.2 Ω.

Flex cable.

Reading jumps to:

50 Ω.

Flex again.

OL.

You found an intermittent connection.

The cable was never completely reliable even though it initially passed continuity.

Common Mistakes

Treating Voltage and Current as the Same Thing

They are related but different.

Assuming Voltage Means the Circuit Works

The load may still be bad.

Assuming Continuity Means Good

Resistance may still be excessive.

Replacing Blown Fuses Repeatedly

Find out why the fuse opened.

Measuring Resistance on an Energized Circuit

Power should generally be removed for resistance measurements.

Ignoring Voltage Drop

Bad connections often show themselves under load.

A Useful Way to Think

When troubleshooting an electrical problem, ask:

Is the correct voltage present?

Then:

Can current actually flow through the required path?

Then:

Is something adding unexpected resistance?

That framework works surprisingly often.

What Did You Actually Prove?

A reading means something specific.

If you measure:

24.0 VDC

you proved:

24 volts existed between those two test points under those conditions.

You did not automatically prove:

Every measurement has limits.

Final Thoughts for Biomeds

Electrical troubleshooting becomes easier when you stop thinking about electricity as something invisible and mysterious.

Think about the path.

Voltage provides the push.

Current flows through the circuit.

Resistance opposes that flow.

Continuity tells you whether the path exists.

Then ask:

Where should the voltage be?

Where should current be able to flow?

Where could the path be open?

Where could resistance be too high?

You do not need to solve advanced circuit equations to answer many medical equipment problems.

You need to understand what your measurement means.

Once you do, your meter stops being something that gives you numbers.

It becomes something that helps you prove where the problem is.

— Jake

Important Note

This page is a basic educational overview. Electrical troubleshooting can expose technicians to hazardous voltage, current, stored energy, and other risks. Follow manufacturer service procedures, facility safety practices, appropriate test-equipment ratings, and your authorized service scope.

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