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kfrench
12-12-2009, 10:29 PM
Situation: Breadboard, DPDT switch, and a 10Vdc battery that became too hot to touch.

Thanks to those responding.

===

On my breadboard, I accidentally ran a 7.5Vdc current from a partially charged 10Vdc battery through a DPDT switch rated for 6Vdc. I used the switch's A and B terminals for negative and positive currents. I did not use terminals C through F.

Everything on the breadboard worked as intended when I slid the switch to the connection position. After a few minutes (about 5) I touched the battery by happenstance and discovered that the 10Vdc battery was extremely hot -- it was too hot to touch. I disconnected the battery (with my shirt) and took the battery outside. After the battery cooled, I determined: that a) the battery's voltage is still 7.5, and b) the center bottom of the battery is very slightly bowed outward and the battery wobbles a little when stood up. I will store the battery in an enclosed container until I take it and its siblings to eco-friendly battery hell (no heaven for this evil battery).

Question 1 (safety!!) :eek:
If the closed dc circuit continued (i.e., exceeding the switch's rated Vdc indefinitely), was there any danger of the battery leaking or exploding?

Question 2 (why the heat) :confused:
What the heck happened? I thought the switch would fail if a current exceeded its rated voltage. I guess, unlike a diode, a switch when overloaded will result in the excess current feedbacking to the battery causing the battery to heat up while the switch continues to do its job up to the its rated Vdc. Correct?

Side note: Nothing else on the board caused this. Everything was fine until I switched (pun intended) from a SPDT to the 6Vdc rated DPDT. Also, going back to the SPDT did not result in any warming of a replacement 10Vdc battery.

Thanks.

("hot tub, much too hot!" - someone will get this SNL skit reference)

jes1510
12-13-2009, 12:00 AM
Check the resistance between the positive and negative terminals going into your circuit. I'd guess that you have a short somewhere. Ideally you would connect power to the circuit with a power supply while measuring the current.

The contacts inside the switch will normally weld themselves closed if the switch fails. I seriously doubt this is a switch failure though unless you were running both power and GND through the same switch.

Adrenalynn
12-13-2009, 04:14 AM
Yeah, agree with Jes, and going one step further - I'd guess the switch was miswired and you were getting a short in the circuit.

MikeG
12-13-2009, 06:17 AM
You connected the positive and negative battery terminals together through the switch. I have to ask, why in the world are you using a DPDT (double pole double throw) switch in your circuit? Are you powering a motor and want to reverse the poles? What the deal with the SPDT? Your circuit has two modes?

Why not use a regular old on/off switch?

kfrench
12-13-2009, 10:30 AM
Mike G
Excellent general point and question about choosing the DPDT.
DPDT used because:
- Waiting to receive my electronics parts order including, my breadboard SPST.
- I used the DPDT for experience - I am new to robotics and going through David Cook's beginner book. I am very strong on programming, weak on electronics and have no engineering experience.

Jes1510 and Adrenalynn
Thanks, a short is the logical explanation. The DPDT looks fine and did not heat up, but Adren may be right about incorrect use of the DPDT. A DPDT seems simple - A is connected to B and D to E, or A is connected to C and D to F. So, I ran directly from the battery the positive current to A and negative to B. The switch and corresponding circuit appeared to work fine - the hot battery appears to be the only anomily.

I will put my Sherlock hat, fire up the multimeter, use a different battery and run some tests.

K French

MikeG
12-13-2009, 11:00 AM
The hot battery is not an anomaly. As stated above A is connected to B and you connected the positive battery terminal to A and the negative terminal to B. We call that a short. It's just like taking a wire and connecting + to -. Your lucky there was no fire or burnt fingers.

Try connecting the negative terminal to your circuit's ground (common) rail and the positive terminal to switch label A. Connect B (or whatever pole) to the positive rail of your circuit. When A and B are connected electrons will flow through the circuit; positive rail to negative rail (I guess really negative to positive - whatever). When A is not connected to B the circuit is open and no current will flow. This is called an on/off switch or SPST.

kfrench
12-13-2009, 08:17 PM
Mike G, this is my understanding from a DPDT data sheet I used. I incorrectly told you A to positive and B to negative.

Using the correct references from the a slide switch data sheet, I connected the positive current to 2 and the negative current to 3:
=======================================
DPDT Slide Switch (through hole mount) Rated 6Vdc, .3A
Contact configuration:
pcb layout
|3 2 1 |
|6 5 4 |

Two possible configurations:
Config. #1(switch right)= [ x] (the one I used for off)
ON=1-2
ON=4-5
OFF=2-3
OFF=5-6

Config. #2(switch left= [x ] (the one I used for on)
OFF= 1-2
OFF= 4-5
ON= 2-3 {this the the connection I used for on, causing the battery to heat up/a short circuit}
ON= 5-6

Data sheet referenced: http://www.ctscorp.co.uk/components/Datasheets/065.pdf (top actuated DPDT, generic - not the exact part I used - Radio Shack 275-0007).

MikeG
12-13-2009, 08:40 PM
Oh, I'm sorry your using the new electronic theory.

lnxfergy
12-13-2009, 08:55 PM
A full schematic, and a picture of your circuit would probably help here....

-Fergs

MikeG
12-13-2009, 09:05 PM
Come on guys you can NOT wire a battery directly to a switch.

kfrench is doing this
http://share.agaverobotics.com/images/forums/DPDT_shortcircuit.gif
When he should be doing this
http://share.agaverobotics.com/images/forums/DPDT_circuit.gif

lnxfergy
12-13-2009, 09:13 PM
MikeG,

Ya, I just re-read his post in more detail, missed the part about shorting the battery with the switch -- regardless, something is missing here: what the heck the circuit is supposed to be.

-Fergs

MikeG
12-13-2009, 09:28 PM
@kfrench; Electricity will take the path of least resistance. Your circuit must have little resistance else you would have seen an arch and some smoke.

MikeG
12-13-2009, 09:43 PM
@lnxfergy; who knows but it feels like an LED and a 220.

Adrenalynn
12-13-2009, 11:32 PM
There's no such thing as "negative current". Current is how hard you're pushing.

Don't hook the ground side (negative, -) into a switch. Ground is ground. Its potential remains. The switch is doing nothing but effectively cutting the red wire, just like a pair of wire cutters, interrupting the flow of electrons.

MikeG
12-14-2009, 09:26 AM
You're right what we call conventional current (I) the rate of positive to negative flow. I mentioned electrons and electrons flow the from negative to positive.

Adrenalynn
12-14-2009, 03:01 PM
Yup - I wasn't correcting/jumping on you, Mike, I was trying to help the OP understand that his terminology was inaccurate and leading [probably] to some of his confusion.

The upshot remains: The power switch goes off the + lead. Leave the ground alone.

kfrench
12-14-2009, 06:40 PM
Thanks, everyone made valuable contributions and it is clear what I did wrong. In addition, I need to be more careful about descriptions and terminology, sorry for any confusion - the clarity of communication is the burden of the person making the post.

I greatly appreciate the consideration and willingness to lend a hand.

kfrench