1. ## Blown Fuses

I just finished my 3 axis mill and making a few cuts. I have an older CCNC 2 amp control board with 100 oz stepper motors. The machine runs fine, but when I let the machine sit for about 5 minutes, the Z axis slow blow 2A fuse blows. I am not sure if I have too much weight on the z axis to make it blow or what. The control board has current limiting on it and I have it set for a little under 2 amps. Any help would be appreciated.

2. Why not install a current meter (found on many VOM"s anymore) and find out how much current you're drawing???.

Once you find out how much is drawing (more accurately than by waiting for fuses to blow), you can start removing motors to see what is drawing current and why???

Find out where current is going and then why it is going there. The sollution should then be almost self apparent.

3. Normally if you have current limiting you can use time-delay fusing, which if you are using fast blow or one-time fuse, it could be the problem of nusance blowing.
Al.

4. Two 1 amp slow blows mounted in parallel will resist surges a bit better than one single 2 amp slow blow yet still provide essentially a 2 amp current limit, albeit via a cascading trip effect.

• A well designed fuse protection will protect the equipment precisely against surges and overloads, the intention is not to protect the fuse, so if the fuse blows it is doing what it is supposed to do. When a fault occurs, fuses will open within 1/4 and 1/2 cycles (0.00833 seconds) That fast response is what protects your electronic equipment.

In general fuse values should be selected so that the normal continuous circuit current is 75% of the fuse's value. So a 1 amp fuse should be selected for a circuit with a normal continuous current of 750mA. Fuses are rated for a ambient temperature rating of 25 Degrees Celsius [ambient temperature around the fuse, not the room]. So under normal operating conditions a fuse should be de-rated 25% [normal current / .75] while operating at 25 Degrees Celsius. Another rule of thumb to protect a circuit from excessive current is to select the fuse value for 125% [normal current x 1.25] over the normal circuit load. To protect against a direct short, select the fuse value as 150% above the normal circuit current. Fuse current is de-rated based on the fuse type and ambient temperature of the air. In general de-rate the current rating 5 to 10% per 10 degrees Celsius of temperature increase, check the manufacturer for derating information.

Circuit designs having high in-rush or turn-on current may require the use of a 'slow-blow' fuse to allow the momentary current spike to pass with out blowing the fuse. Power supplies and motors are two devices that have large in-rush currents.

By the way, the voltage rating of the fuse does not indicate the operational voltage of the fuse. The voltage rating determines the maximum voltage that will not jump the gap (arcing) between the elements after the fuse has already blown. So the fuse will operate at the rated voltage and any voltage below the rating.

Connecting fuses in parallel is not recommended at all.

UL listed or recognized time delay fuses typically open in 2 minutes maximum when subjected to 200% to 250% of rated current. IEC has two
categories of time delay (Sow Blow) fuses:
• T = time lag, opens 10x rated current within 0.01
seconds to 0.1 seconds
• TT = long time lag, opens 10x rated current within
0.1 seconds to 1 second

Measure the current consumption of you Z-axis controller when not moving (braking => peak current on both coils in case of bipolar motor, unless you have a controller capable of reducing the braking current), then chose a "Time delay" fuse with a rate 125- 130% bigger. (Type T)

• If Z axis has automating balancing ( usual hydraulic one ), check with clampamp in stop position ( input voltage for Z drive is zero ). The amps must be under 1A.

• I'm with NC Cams, put a meter in there and see exactly what is going on, a setting that says a little under 2 amps might actually be a little over 2 amps, just remember that when you are finished reading amps not to leave the meter in that position/setup, depending of course on the type of meter, next time you use it may be for a voltage reading, and its not a good idea to test voltages with an ammeter...

Russell.