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I have purchased a horn which is rated 48 V, 1.5 A. I hooked it up through a horn switch directly to a battery pack which is 48 V (54.6 V fully charged). When the battery pack is fully charged it's outside the specification range of the horn and all it does is make a "click" sound when the button is pressed for the horn. The horn indeed works when the voltage of the battery pack drops.

I don't know at what voltage it actually starts working, but it is at least a few volts lower than that of a fully charged battery. Having a fully charged battery pack is a must for the road I take, but it makes horn completely useless.

Any suggestions? I am not open to adding more or separate batteries; it's dangerous enough to have one on a bike, let alone additional batteries which add weight and fire hazard. Yes, I have checked online; the only solution according to most posts is to connect the ground terminal to the horn but since it's on a bike there are only positive and negative, no ground. Any other suggestions or solutions?

there was no specification sheet listed where it was purchased (aliexpress) this is the link to the item. https://www.aliexpress.us/item/3256803312568915.html?spm=a2g0o.order_list.order_list_main.11.2f36180251WVi5&gatewayAdapt=glo2usa&_randl_shipto=US The only information listed on the horn is 48v 1.5A (nothing else)

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  • \$\begingroup\$ What is the model of the horn? Is there a link to its specifications, datasheet? It is actually surprising that it wouldn't work with a slightly higher voltage. \$\endgroup\$ Apr 28, 2023 at 6:10
  • \$\begingroup\$ What is the highest and the lowest voltage at which it will work? \$\endgroup\$ Apr 28, 2023 at 6:17
  • \$\begingroup\$ Use two resistors as a voltage divider, fed by the switch powering the horn. \$\endgroup\$
    – Solar Mike
    Apr 28, 2023 at 7:12
  • \$\begingroup\$ There is no specification sheet it was ordered from aliexpress this is the item that was ordered the 48v version. aliexpress.us/item/… \$\endgroup\$ Apr 28, 2023 at 8:07

3 Answers 3

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One advantage you've got with this problem is that the horn doesn't have to be efficient. Even at 50% efficiency you'll probably be deaf before battery usage becomes an issue.

I would try a power resistor in series and see if that works. Not elegant but it's very simple. 6.6V/1.5A = 4.4 ohms. So maybe 5 ohm power resistor?

The (-) terminal on a vehicle is often referred to as "ground", even though it is not actually connected to the earth. In fact, (-) and ground are generally interchangeable terms when it comes to amateur electronics. When the ground is actually connected to the earth, it's usually referred to as "earth ground".

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This could be done with a linear 48V voltage regulator, but because the 48V battery can be as high as 54 V, it would dissipate 6 V at 1.5 A, or 9 watts. It may be hard to find such a beast, but it could be constructed as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

Simple 48V regulator

That will probably work just fine to drop voltage for the horn, which will probably be activated for only a few seconds. Otherwise, a buck converter would be more efficient, and here is one for about $20.

(edit) Here is a simulation of a simple LDO regulator. The resistor values are a bit "fiddly", and I had to play with them to avoid oscillation at higher input voltages. But it exhibits less than 200 mV dropout voltage.

LDO regulator

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  • \$\begingroup\$ If a buck convertor was used, would it be better to get the 24 V version of the horn, so that if the battery voltage drops below 48 V the horn would still sound? \$\endgroup\$ Apr 28, 2023 at 16:08
  • \$\begingroup\$ The horn does not need exactly 48 V, but I don't know its range of operating current. I'm surprised that it wouldn't work on 54 V. Most such devices will be designed to work on the nominal voltage +/- 10% or so, which would be 43 to 53 VDC. I don't know how the buck converter responds to an input voltage equal to or less than its output setting. Ideally it would just run at 100% duty cycle and pass the input to the output. The 24 volt unit should work, as long as the converter can deliver 3 amps. \$\endgroup\$
    – PStechPaul
    Apr 28, 2023 at 21:47
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The venerable TL431 could be used with two external transistors to make a low dropout voltage regulator. The circuit below would be a starting point. I haven't tried it, but at least at DC it seems to be doing its job. It's probably overkill for this purpose, too :)

schematic

simulate this circuit – Schematic created using CircuitLab

Note: The simulation above uses a trimmed Zener diode in place of TL491, for simplicity's sake, as CircuitLab doesn't have a native TL491 model. It's enough to demonstrate the static principle of operation.

R3 may need adjustment so that there's about 3V..20V at the base of Q2. The base voltage is roughly proportional to R3. If the output voltage at the load is below 48V, and Q1's base is around 24V, then R3 has to be decreased until the base voltage is 15V or so. Conversely, if the voltage on Q1's base is below 5V, increase R3 until the base voltage is 10-15V.

Q1 can be any general-purpose small-signal NPN transistor. Q2 doesn't need to be TIP32 specifically - it should be a power transistor rated to dissipate 10W at least, and should be heat-sunk. Since the horn is used only occasionally, it shouldn't be a problem.

C2 is optional, based on how stable the circuit is with the load.

C3 and C4 increase the AC gain, and correspondingly decrease the lag in the feedback loop for stability.

TL431 is a shunt regulator that will conduct more as the reference voltage from the R4-R5 divider raises past 2.5V. This will deprive the base drive of Q1, lowering the conductance of Q2, and bringing the output voltage lower, thus closing the negative feedback loop.

D1 limits the voltage across U1, as it can only take up to 36V.

The 2A automotive fuse F1 is essential for short-circuit protection, and should be an in-line type placed as close to the battery as possible.

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  • \$\begingroup\$ CircuitLab would not allow me to run the simulation, but it would be good to use a variable input voltage from 56 V down to 44 V to see its response. A time domain graph would be helpful. I don't think an LDO is really needed for this application, and your circuit is probably too complex for the OP to build and test. The emitter follower circuit I posted uses only three (3) parts, and achieves a dropout voltage of only about 2 V. \$\endgroup\$
    – PStechPaul
    Apr 28, 2023 at 22:03

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