20 amp fire siren 12 volt to 6 volt circuit.

Thread Starter

fireftrmike

Joined Sep 15, 2011
24
Call me crazy but building something is way more satisfying then buying it.
I've had this 6 volt siren for a few years now and would like to add it to our groups convention carts. Using a 6 volt battery would be TO easy. I would like to build a circuit to to do this so that any member could move it from cart to cart for parades and such and just hook it to existing 12 volt battery. The siren specs claim 20 amps. It would have intermittent use so a circuit could have less then that for current supply with a peak now and then if heat sinked well. Does anybody have a parts list and or circuit diagram that would aid me in building this item? Thanks in advance!
 

wayneh

Joined Sep 9, 2010
16,389
There are two basic approaches: 1) Place some device in series with the load to burn off half the voltage (and power) or 2) Use a DC-DC converter to buck the voltage down.

Option (1) could be as simple as putting the right light bulb in series, or another similar horn. A voltage regulating "linear" circuit is the higher-tech approach but would ultimately do much the same thing. Option (2) is far more efficient but also more complicated to build for a DIYer to build, especially at 20A.
 

Thread Starter

fireftrmike

Joined Sep 15, 2011
24
Option 1 with double siren in series...... never thought of that. More noise but harder to move from cart to cart. I like to try option 2. Years ago I used to etch my own boards for projects and would enjoy something like that again. I work now as a traveling heavy equipment mechanic and deal with a lot of electronic issues on a daily basis but it's all part/board changes or software updates. Ebay has some prebuilt 10 and 20 amp 6 volt supply units but I question the quality. I have a local radio supply business (not radio shack) that I should be able to pick up my components.
If your group has a circuit schematic and component list that would be great.
Thank you
 

Thread Starter

fireftrmike

Joined Sep 15, 2011
24
I think your talking about a fan speed control? (pulse width mod?) These old sirens are louder in pitch the faster they spin. They start out low in pitch and gain until top rpm then slowly drop as switch is released. I'm sure they pull top amp as they rev to top rpm then back off somewhat. New electronic sirens today just don't give you the same sound.
 

iONic

Joined Nov 16, 2007
1,650
I think your talking about a fan speed control? (pulse width mod?) These old sirens are louder in pitch the faster they spin. They start out low in pitch and gain until top rpm then slowly drop as switch is released. I'm sure they pull top amp as they rev to top rpm then back off somewhat. New electronic sirens today just don't give you the same sound.
With PWM you would not change the RPM or pitch of the siren as the effective voltage would then be 6V as required.
 

Thread Starter

fireftrmike

Joined Sep 15, 2011
24
OK. Where/how do I get one of these. Is there a part#? Do you set it up and adjust it under load to 6 volt. I guess I don't understand. Are they good to 20 amp?
 

gerty

Joined Aug 30, 2007
1,281
I can bring 12 volts in and output 6 out at 20 amps with this and still maintain full motor RPM?
Yes you can use it, the motor rpm will be at whatever you set it to. You can meter the voltage to be near the rated 6 volts (effective voltage).
Because the 12 volts is switched (?frequency) the effective voltage output is lower
 

wayneh

Joined Sep 9, 2010
16,389
I'll give it a try.
Thank you for the information.
It should do the trick. It's how the efficient converters work, by time-slicing the power rather than the linear approach of burning it off.
Some applications don't like the square wave but things like a lightbulb or a motor will be fine.
 

iONic

Joined Nov 16, 2007
1,650
The duty cycle, which will get you your "effective" 6V out will com when the pot is at about midpoint. Turning it higher will increase the effective voltage to something you may want to consider avoiding. You could replace the Pot with a fixed resistor that is half the value of the pot. For example if the pot is 500K, replace it with a 250K fixed resistor and the duty cycle should be at 50% or an effective 6V output.
 

Thread Starter

fireftrmike

Joined Sep 15, 2011
24
Got it a couple of days ago and tried to get it to work tonight. Hooked it to 12v battery and set output to 6v. Left the freq. jumper to "fixed" It ran a couple of times and went dead. I have no output anymore. Only thing I noticed is that C1, C2 and U1 seem to be hot. The magic smoke has not left the circuit yet............but I'm working on it.
Any help appreciated.
 

wallaby

Joined Jul 26, 2011
34
Have you tried the siren with a full 12 volts? In my experience with 6 volt systems, all the wiring is of sufficent gauge because 20 amps at 6 volts is only 10 amps at 12 volts. The 6 volt wiring is like twice what is needed for a 12 volt system. The motor may spin up more quickly with 12 volts, and probably reach a higher speed and pitch, but I assume you are going to use a momentary switch to control it? Maybe it just needs some operator restraint.
 

Thread Starter

fireftrmike

Joined Sep 15, 2011
24
Your right......... it spins way FAST and the pitch hurts...BUT I was afraid that it may burn out the motor. Restraint at a fire convention is sometimes hard to find :) I put stuff away for the night and will look at it tomorrow. Thanks for reply.
 
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