Need help reading component value

Dear All

Can someone please tell me what the value of this resistor is supposed to be? There's a photo at this link.

**broken link removed**

My Fluke MM just reads it as a dead short. The component comes from the DC motor controller used in a small Chinese made milling machine which I'm attempting to repair. Unfortunately, apart from removing and testing individual components, my troubleshooting skills are minimal so I'd be most grateful for ant help.

Thnaks

Trevor

Looks like 0.03 Ohms at 5 Watts

This would show as a short on most meters.

Many thanks. That explains it.

rmn_tech said:

Looks like 0.03 Ohms at 5 Watts

This would show as a short on most meters.

Click to expand...

It looks to be a 5 Watt .03 Ω ceramic resistor. Try shorting your DMM leads and note the lead resistance and then measure the resistor. Even then .03 Ohms may not be easily seen depending on the DMM resolution.

Resistors of that type and value are commonly found in DC Motor speed controllers.

Is the motor a permanent magnet motor or a motor with 4 wires having a field winding and armature?

<EDIT> Man you guys are quick! </EDIT>

Ron

Hi Ron

Many thanks for the reply. My MM is only a 2 1/2 digit so it can't read 1/100ths of an Ohm. Now I know the resistor is supposed to be 0.03 Ohms, it's obvious why I can't see any readable value.

I wonder why such a circuits need resistors whose resistance is little more than a length of wire.

The motor is a permanent magnet. I measured each armature windings and they are all between 24 and 26 Ohms so I assume they are all ok.

The only other problem I've come across is a 3 Watt carbon film with a marked value of 150 Ohms (brown, green, brown) but reads 14K so I'll be replacing that. Funny, I've seen resistors burn and go open circuit but not jump to a higher value. Does that happen often?

Thanks again for the help.

Trevor

Reloadron said:

It looks to be a 5 Watt .03 Ω ceramic resistor. Try shorting your DMM leads and note the lead resistance and then measure the resistor. Even then .03 Ohms may not be easily seen depending on the DMM resolution.

Resistors of that type and value are commonly found in DC Motor speed controllers.

Is the motor a permanent magnet motor or a motor with 4 wires having a field winding and armature?

<EDIT> Man you guys are quick! </EDIT>

Ron

Click to expand...

At work I have used a large number of Bodine fractional horsepower motors and controllers for permanent magnet and older field / armature motors. Sitting here I can't remember exactly why they use the low ohms (may as well be a wire) resistors but know they are quite common. Maybe tomorrow once at work I can dig a schematic out. I just remember it is common to see those puppies in there. Maybe someone else here has the answer?

I have seen many resistors over the years that physically look fine but have increased in value as you seem to see. Generally a result of heat from excessive current. They can look normal but... pretty much are toast.

Ron

Just guessing, but some powerful, low armature resistance, DC motor may use low ohm resistors to limit max current, or maybe it's a fuse link. I don't know, just guessing. Need to do something similar my self.
Kinarfi

It's probably a current sense resistor.

The good thing about using low values is it keeps the voltage loss and power dissipation down.

If the current was 10A, the voltage across the resistor would be 300mV and the power loss would be 3W. A differential amplfier with a gain of 33 1/3 could be placed across the resistor so the reading will be 1V per amp.

Hero999 said:

It's probably a current sense resistor.

The good thing about using low values is it keeps the voltage loss and power dissipation down.

If the current was 10A, the voltage across the resistor would be 300mV and the power loss would be 3W. A differential amplfier with a gain of 33 1/3 could be placed across the resistor so the reading will be 1V per amp.

Click to expand...

I think Hero is into something. I tend to want to believe on the Bodine manufacture the older motor controllers use them for current sense. I also think that on some controllers you chose a resistor from several based on motor size and ratings. I just am not sure. Tomorrow at work if I have the time I'll dig around for an old Bodine manual.

All the newer units we build for test use permanent magnet speed controllers. Not much to them at all.

Ron

Low Ohm resistor

Ron and all

Thanks again for the helpful replies. I was told it was a current sensing resistor as you rightly suspect. I guess I can put it back in and move on from there. I'll be replacing the 150 Ohm carbon with a new one today and with a bit of luck, may find this fixes the problem.

Shot gun troubleshooting has worked for me on a number of occasions. This particular circuit has been a bit more of a challenge. It would be nice to have the electronics background that would allow a more logical approach to faultfinding but I'm too long in the tooth to go back to school now!

Thanks again.

Trevor

Reloadron said:

I think Hero is into something. I tend to want to believe on the Bodine manufacture the older motor controllers use them for current sense. I also think that on some controllers you chose a resistor from several based on motor size and ratings. I just am not sure. Tomorrow at work if I have the time I'll dig around for an old Bodine manual.

All the newer units we build for test use permanent magnet speed controllers. Not much to them at all.

Ron

Click to expand...

Can you elaborate on this or point me in a direction to read about this, I have been working with LEM a hall effect current device, but always looking to learn about other methods. Or am I asking about two different things, speed control and current sensing.
Kinarfi