Voltage Drop Across Load

What did you find when measuring between the two ends of the black wire (or, for that matter, any wire)?

Why did you stop measuring voltage after measuring across the lamp?

"Should a voltage drop reading on BLACK measure 6 volts ?"

It must, unless your story is incomplete.

"What is the most likely cause of the voltage drop"

Resistance.

Use a thicker(more cross section area) wire.

Gajanan Phadte

Agreed.

Why the original assumption that the battery is 24 V when loaded!

Apart from the checking battery volt drop under load- You talk a simple DC cct but is it physically - do you know the complete run? how good are your connections? How clean are terminals? just my (Au) two cents worth.

If the battery is in full charge and good condition, it should read 25-26 volts with no load(depending on the type of cells). With the circuit closed, check voltage across the battery. I suspect it will read 18 or 19v and if you leave the circuit closed the bulb will gradually dim more. If you are convinced the battery is good then the other 6v will ultimately end up as heat. Trace the source. It is either the battery, fuse, switch or wire! Funnily enough this is one of the questions I gave my Grade 11 students.

All switches, fuses and other connections will have some resistance and therefore a voltage drop across them.

You haven't mentioned the current in the circuit, the length of the circuit or the size of the wiring, so it's not possible to estimate the voltage drop in the rest of the circuit, but 6 volts would appear to be excessive.

Look for a poor connection somewhere in the circuit, a poor connection to the chassis (if you are using it for the return) is a likely contender, a battery that suffers voltage depression under load is another one.

You measured 24v in but was that with the light on?

If you are saying that you have 6v drop across the black wire then that's a good place to start looking for problems.

Cheers

Every battery has its inner small resistance (R), and the halogen bulb must consume a high current (I), so 6V = I * R.

With light on, check voltage at battery.If it is 24 volts,and the voltage at the light is substantially less, then the problem is a loose or bad connection,(or too small of a conductor) or switch contact.Measure across the switch contacts with light on.There should be zero volts.If there is any voltage across switch with the contacts closed,the contacts are dirty or bad.

Check voltage across fuse.Some fuses have a built in resistor,but sometimes not visible.Also check the fuse holder.There should be no voltage across the fuse with fuse installed and light on.I have seen dirty or bad fuse holders.

I am making the following assumptions:-

• Your measurements are referred to the negative battery terminal
• Your fuse and switch are in the +ve feed
• You read 24v at the +ve feed connection to the bulb
• The 6v reading is between the black wire connection and battery -ve

Based on the above the only explanation is a bad return connection to the battery as there must be a resistance in that path for the voltage to be shared in the given proportions. With power switched off the resistance between the black wire and the battery -ve terminal should be checked with a good quality ohm meter. Assuming there is only a wire in the return path then the final battery connection is the most likely candidate causing the problem. Bulbs are quite low resistance at start up and it does not take much contact resistance to cause a voltage drop.

Excellent answer. I have forgotten what an actual load is defined as & you reminded me that without a load there is no voltage drop. I believe that there is unwanted resistance in the ground circuit somewhere since all voltages on power equal 24 V.

- Thank you

there was a lot of causes for voltage drop ...cause no.1 - due to loose connection ,you hav to check each point ,cause no.2 - due to the low cross-sectional area ..check every point cable terminals that may cutting conductor only a few no's of strands may in contact. cause no.3 - the fuse and switch are good condition or not check that, you hav to mention the capacity of each bulb,switch,fuse and dc source ..wheather they are correct or not. in series circuit we hav design for constant current but variable voltage level ...all things you hav to check.

Is the bulb rated for 24V?

Whatever the voltage rating of the bulb, if all else is correct, 24 volts should be read across the bulb. If the rated voltage of the bulb is low, the bulb will fuse in short duration compared to its expected life and the life expectancy will increase if opposite.

Gajanan Phadte

Model it as a Thevinen circuit. You have 24 volts source driving battery internal resistance, in series with fuse & switch resistance, in series with the bulb's resistance. The unexpected drop is due to those three external resistances not present in the bulb.

I made a quick check on over 2 month ago charged and then used 12VDC battery

55/60W bulb the upper thread broken so i gess it should be 55W

27cm Ø.6mm (.12Ωmm²/m x .27m / (π x .3²mm²) = 115mΩ) ""steel"" wire at 1 terminal

98cm x Ø.4mm (.018Ωmm²/m x .98m / (π x .2²mm²) = 140mΩ) Cu wire on another

(+)<-(V)->(-)
noLoad = 12.36V
Load = 5A x 11.95V = 60W
r.Battery = (12.36 - 11.95) x 1V / 5A = 82mΩ

i gess the first thing to check is your switch amperes rating at low voltages

Measure the voltage drop across the fuse. If you have a slow blow fuse you may have some resistance. Measure if any voltage drop across the switch when it is closed, you should have none. Measure voltage at battery when the switch is on and off.