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Old 25th September 2006, 11:11 PM   #1
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Default Relay for power on

I just built a 22wpch chipamp for my car using parts I had pulled from my parts box.


it uses a TDA7375 in dual bridge.


I had a question on how to switch/fuse it properly before hooking it to the car.

ihad originally designed a relay power board that used a small switch to apply 12v from the battery to the coil, and then it switched the main power from the battery to the amp.

In the supply I opted to use a 5 amp fuse ( since the absolute peak power output is 44 watts, and it draws a max of 3.5 amps per channel , I thought 5 amp should be safe for first turn on ...please correct me if I am wrong!! )


Also , I noticed that when using my SLA battery I had inside, the relay got very hot

I found out I had it wired backwards , so I have yet to install it the right way. I was wondering though, are relays supposed to be warm or not? I got it out of a tv ... It says on the side 10 amp 24vdc/240vac for the coil, and 1/3 hp 120v/240vac for the switch.


will I burn the relay if I connect it directly to the car battery and do I need something to drive it or not?


as an alternative....is there anything wrong with just a 10 amp switch inline with the power from the battery instead.....
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Old 25th September 2006, 11:21 PM   #2
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Why not just get a regular Bosch-style automotive relay? They should only be about $5.
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Old 25th September 2006, 11:24 PM   #3
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Quote:
Also , I noticed that when using my SLA battery I had inside, the relay got very hot
Hmmmm? What voltage is the battery?

Your specs for the relay coil " 10A 24VDC/240VAC" doesn't make sense to me. Are you sure that's not the contact rating?

Could you post the DC coil resistance and the SLA battery voltage?

Normally, relays shouldn't get hot. ....at least not the relays I deal with. Automotive stuff might be heavier duty. Big contactors might have large power dissipations due to requiring larger hold currents, holding larger contacts. I just don't know.

To minimize dissipation in a relay you can, figure what the pull in and hold currents are. You can feed a relay through a resistor that is set for hold current. You can bypass this resistor with an electrolytic cap that supplies pull-in current for some time constant.
It's just a way to make them run cooler.
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Old 26th September 2006, 01:13 AM   #4
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I might run to the shack and get a cheap relay. The one I have says 24v on the side, but on the other side it says 12v...


anyways DC resistance is 195 ohms, my SLA is just 12v. should be about 60ma right?


shouldnt be getting hot I dont think... ALthough I do remember measuring when I ran it off my plug in supply... which is supposed to be 12v 4 amp, but under light load its 18v.


will a resistor/capacitor give me a little soft start? Although with this amp I don't think its needed because its BTL , so no opc's to charge.

The relay is made by fijitsu/takamisawa (weird name)
VS12MB306 is the model... cant seem to find a datasheet on it online



Edit: btw that is the contact rating... I believe the coil is 12v.


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Old 26th September 2006, 02:38 AM   #5
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Yeah, that shouldn't get that hot. It's about 0.7 W. I looked for some similar NAiS relays. Their 12 coil is 360 ohms @ 33mA. Yours sounds like NAiS's 9V relay, @ 202 ohm 44.4mA (@ 9V).

....So, the other possible heat source. The current load is too great for the relay contacts. The IR drop on the contacts is large enough to create large heating. If you're switching a big auto power amp on, you may need to greatly beef up your switch contacts!! 10 A for such a load sounds wee.
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Old 27th September 2006, 01:04 AM   #6
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as I said in my first post, my amp is only 22 watts per channel. It draws a max of 6 amps at full nasty distorted output power.


The one I have says 10 amp on it, by the way the top of it is removable to see the inner workings, and its not the contacts getting hot it is definatly the coil. I went out and bought a 6 dollar car relay though at the shack today.... will post as soon as its working to see what my results are
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Old 28th September 2006, 11:52 PM   #7
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alright I got it together and working! The coil becomes warm , but never hot. seems fine. I have it on a 4 amp fuse running from the ciggarette lighter wire.


Something I don't get... its not as loud on my 4 ohm speakers in my car as my 8 ohm speakers inside. My car speakers are 92db so their not total junk.

I don't think my cd player has enough output to drive it to full volume.. Also, I'm only measuring a max of 7v on the ouputs at full volume ( full undistorted volume, my cd player cant push it to distortion yet. )

What does this translate into in watts? Its capable of 10 watts per channel RMS @ .08% thd, which should be plenty as my onkyo I was using was 15 watts per channel @ .5% thd.
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Old 29th September 2006, 12:24 AM   #8
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http://www.installer.com/tech/ohmslaw.html

Watts = volts^2 / ohms

49 / 8 = 6.1 watts
49 / 4 = 12.3 watts
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Old 29th September 2006, 03:01 AM   #9
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whats the 49 from ?
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Old 29th September 2006, 03:09 AM   #10
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(7 volts)^2
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