# How do I calculated a Fuse and Switch for a DC Circuit using a Car Battery ( isolated ) safely.

#### RandyFL

Joined Aug 28, 2014
148
Just bought a new Leeson DC 12 volt .75 Horse Power 60 Amp single phase ( I assume its single phase ) motor. I want to protect it from the DC 12 300 CCA ( cold cranking Amps ) battery. Out of my books Make Electronics and/or Electronics For Dummies ...it states:
page 48 " sparking "
" For this reason, you must use a switch that is appropriate for the voltage and amperage that you are dealing with. Electronic circuits generally are low- current low-voltage, so you can use almost any switch, but if you are switching a motor, it will tend to suck an initial surge of current that is at least double the rating of the motor when it is running constantly. You should probably use a 4-amp switch to turn a 2 amp motor on and off. "

My questions are:
1. what is the initial surge called ( B emf ? ) or I read somewhere that it is the opposing emf...
2. the auto battery cables should be able to carry the correct amount of current to the motor ( or is it over kill )...
3. I intend to use a blade switch ( spst ) to connect to the battery... which side neg or pos as it will be a battery that is not connected to a car ( cars are grounded on the neg side ( which is convention electricity - meaning pos is where the emf side and not electron theory that states it comes the neg side )
4. conversations ( from AAC ) stated Fuses don't really protect semiconductors... they protect from fires....so what size Fuse ( Fast acting or slow )...

Have recovered from the surgeries of 2015 and ready to work on circuit s again...

#### Dodgydave

Joined Jun 22, 2012
11,304
Its not a Single phase motor, its. a 12V DC motor, you said it takes 60amps, so you're going to need a circuit breaker for that value or at least 80A, as for a switch, use a relay or contactor.

#### RandyFL

Joined Aug 28, 2014
148
9:13 AM
I found a Autocraft Battery cut-off switch 300 AMP
Do they have circuit breakers for DC...?
what size wire ? 4 gauge...

R

#### Alec_t

Joined Sep 17, 2013
14,330

#### dl324

Joined Mar 30, 2015
16,941
My questions are:
1. what is the initial surge called ( B emf ? ) or I read somewhere that it is the opposing emf...
It's called inrush. It can be limited with a NTC thermistor or a resistor that is shorted after the inrush event.
2. the auto battery cables should be able to carry the correct amount of current to the motor ( or is it over kill )...
Should always be done, and with a safety margin.
3. I intend to use a blade switch ( spst ) to connect to the battery... which side neg or pos as it will be a battery that is not connected to a car ( cars are grounded on the neg side ( which is convention electricity - meaning pos is where the emf side and not electron theory that states it comes the neg side )
It's a matter of preference and sometimes safety. Given an option, I'd prefer to minimize the exposed 12V.
4. conversations ( from AAC ) stated Fuses don't really protect semiconductors... they protect from fires....so what size Fuse ( Fast acting or slow )...
The current rating doesn't agree with the horsepower. I've never really paid attention to motor ratings, but it seems the rating is for surge current.

Make sure you use a switch rated for switching inductive loads or add a snubber circuit to prevent arcing.
Have recovered from the surgeries of 2015 and ready to work on circuit s again...
Good to hear. How's the Wife doing? Did you ever get around to building a power supply?

#### RandyFL

Joined Aug 28, 2014
148

#### RandyFL

Joined Aug 28, 2014
148
It's called inrush. It can be limited with a NTC thermistor or a resistor that is shorted after the inrush event.
Should always be done, and with a safety margin.
It's a matter of preference and sometimes safety. Given an option, I'd prefer to minimize the exposed 12V.
The current rating doesn't agree with the horsepower. I've never really paid attention to motor ratings, but it seems the rating is for surge current.

Make sure you use a switch rated for switching inductive loads or add a snubber circuit to prevent arcing.
Good to hear. How's the Wife doing? Did you ever get around to building a power supply?
I have to look thru and study what you wrote...^

It was a tough year for me and the wifey... my wife is slowly getting back to square one-she still has to deal with a large clot the went from her groin to the bottom of her leg ( her leg swelled up 3 times the size of her normal leg )...

its still in the planning stage ( the power supply )

#### RandyFL

Joined Aug 28, 2014
148
Its not a Single phase motor, its. a 12V DC motor, you said it takes 60amps, so you're going to need a circuit breaker for that value or at least 80A, as for a switch, use a relay or contactor.
dave...
whenst trying to put everything into the question I forgot about dc and ac ( and phases )...
whilest were talking about phases... the pulse coming from the number 3 pin on a 555 chip is it AC or pulsing DC ... and I don't know the answer to that before hand......

#### RandyFL

Joined Aug 28, 2014
148
A contactor is an electrically controlled switch used for switching an electrical power circuit, similar to a relay except with higher current ratings.[1] A contactor is controlled by a circuit which has a much lower power level than the switched circuit.

Contactors come in many forms with varying capacities and features. Unlike a circuit breaker, a contactor is not intended to interrupt a short circuit current. Contactors range from those having a breaking current of several amperes to thousands of amperes and 24 V DC to many kilovolts. The physical size of contactors ranges from a device small enough to pick up with one hand, to large devices approximately a meter (yard) on a side.

At this point... after reading what a contactor does ( and I'm not opposed to using one down the line ) I would go for a Fuse or a circuit breaker... an 80 amp one.
R

#### Dodgydave

Joined Jun 22, 2012
11,304
The 555 timer is pulsed DC square wave,

As for the contactors, all they are is a relay with more contacts on them, and some have a motor current overload detector as well, you can use a 12V relay for a car with a set of 40- 60 amp contacts.

#### RandyFL

Joined Aug 28, 2014
148
Make sure you use a switch rated for switching inductive loads or add a snubber circuit to prevent arcing.
I have a Battery cut-off switch rated for 300 amps...

Could you show me a switch rated for inductive loads...
and
And a snubber circuit ...

All the best
R

#### Dodgydave

Joined Jun 22, 2012
11,304
I just did in the previous post#12

#### RandyFL

Joined Aug 28, 2014
148
ok...Got it.
12 V 60/80 Amp Bosch Style S Relay with Harness Socket Automotive
Could you show me the circuit to use it in...?

And the snubber circuit?

sorry for the simple questions and the redundancy

All the best
R

#### dl324

Joined Mar 30, 2015
16,941
And the snubber circuit?
In it's simplest form, a diode that can handle the peak motor current and reverse voltage of the power supply.

#### RandyFL

Joined Aug 28, 2014
148
Dave,
And the snubber circuit ( Is it two diodes - protecting forward and back ) ...
And of course ( Maybe I didn't mention it ) this motor isn't ( not ) inside of a car ... so 85 & 86 ?

Hence...
1. connections
2. fuse
3. relay
4. snubber circuit ( diodes )
Can you finish the schematic...

thanx for the assistance
R

#### Dodgydave

Joined Jun 22, 2012
11,304
here is the circuit with a switch, relay, overload fuse...