# Help me figure out this Magnetic Drill Base Control Circuit

#### sheltonfilms

Joined Feb 11, 2020
12
I'm a collector of old tools and I just picked up an early 1970s Milwaukee Electric mag drill for $50 which is quite cheap. Only issue is it is missing the control panel which has been discontinued for some time now. I was able to find the control board on ebay but the seller wants$250 for it which to me is insane. Good thing is he took great pictures of the board and I found some other pictures online to where I can just about replicate the circuit. I've attached all the pictures I have. Ignore the DPDT switch on the right of the board as that is for motor forward and reverse, which is already figured out.

The 120V AC signal goes into a VH247 rectifier (6A 200V rating) and the two outputs go into nodes 1 and 2 (middle terminals of DPDT switch).

When the switch is turned to the ON position it connects 1 and 2 to A and B which go to the magnetic coil (measured resistance is 106.8 ohms). In parallel is a 250V 100uF capacitor for ripple smoothing. 27K resistor is also in parallel.
Also in ON, it seems current flows from A to D via a 2.2k ohm resistor and then from D to one of the AC inputs through a 56k ohm resistor and an lamp.

In LTspice, I modeled a 170 PP 60 Hz signal. I also used 4 voltage controlled switches to act like the DPDT switch. They are controlled by a pulse signal that essentially simulates the switch being flipped to ON for 2 seconds, then OFF for 1 second, and then to DEMAG for 2 seconds. I also modeled the coil as a resistor at the point of 106 ohms (don't know the inductance as I don't have an LCR to measure it with).

What I've gathered:
When ON the coil gets a voltage that ripples from 167V to 104V at 60Hz.
When OFF the coil/capacitor drop to 0V in about 125ms.
When DEMAG is selected the coil sees a negative voltage that ripples from -4.5 to -5.2

My biggest concern/interest is in the indicator lamp. It gets it's current from terminal A through a 2.2k ohm resistor then to a 56k ohm resistor, through the lamp and then connects to one of the AC input lines. The 58.2k series resistance severely limits the current flow to this lamp. How can this lamp light up? Even at 0 ohms resistance, it can only flow about 3ma. What am I missing here?

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#### sheltonfilms

Joined Feb 11, 2020
12
Here are some simulations and the schematic in case you don't have LTSPICE

#### sheltonfilms

Joined Feb 11, 2020
12

#### AlbertHall

Joined Jun 4, 2014
10,397
No idea but they are all pretty much the same given that form factor.

#### BillB3857

Joined Feb 28, 2009
2,536

#### sheltonfilms

Joined Feb 11, 2020
12
Thanks for the suggestion guys. I think I maybe down to one last question:

After you are done with the drill you flip the switch to demag to reverse any residual magnetization. I ran a power simulation and the 2.2k resistor during this time shows 12 watts peak down to 0 W at 60Hz, average power is around 6W. Now the resistor shown on the board doesn't appear to be a 6W resistor but much less. Is it just a 1W resistor and because of the short duration (couple of seconds max) that it's ok to use an under rated resistor?

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#### BillB3857

Joined Feb 28, 2009
2,536
Is the switch an ON-OFF-MOM (on, off, momentary) type with the demag position being the momentary position? If so, the short time it is in the DEMAG position would not cause the resistor to burn up. Resistor wattage rating is based upon continuous dissipation.

#### sheltonfilms

Joined Feb 11, 2020
12
Is the switch an ON-OFF-MOM (on, off, momentary) type with the demag position being the momentary position? If so, the short time it is in the DEMAG position would not cause the resistor to burn up. Resistor wattage rating is based upon continuous dissipation.
Based on the usage, I believe you are correct on the switch. So is there a rule of thumb for non-continuous use? Do you have an idea of the wattage based off the size in the pictures?

#### BillB3857

Joined Feb 28, 2009
2,536
There is a concept called "duty cycle". For example if power is on only for 10 seconds and off for 90 seconds, it is a 10% duty cycle. For your application, you wouldn't be holding the "demag" position for more than a second or two. That time, compared to the long time of either no use or magnetized use would be a very small duty cycle so the resistor, even though it may get slightly warm, would not burn to the point of damage. From the pictures, I would think that the resistors are 2 Watt rated.
Also, when the switch is moved to the "demag" position, does the coil tend to 'buzz"? It is not uncommon to apply an AC current to a coil to demag a magnet.

#### sheltonfilms

Joined Feb 11, 2020
12
No A/C signal being used when demag is activated only DC. Only rectified voltage/current is feed into the DPDT switch.

I'm not able to hear anything because I haven't built anything yet. Just trying to model the circuit before I do to better understand what I'm building.

#### MisterBill2

Joined Jan 23, 2018
7,025
Rather than re-engineer the board, are you able to get service information from the manufacturer? That should give you a schematic diagram, a parts list, and probably information on how it works and what voltages to expect.

#### shortbus

Joined Sep 30, 2009
7,940
Rather than re-engineer the board, are you able to get service information from the manufacturer? That should give you a schematic diagram, a parts list, and probably information on how it works and what voltages to expect.
Did you see in the first post where he said -
Only issue is it is missing the control panel which has been discontinued for some time now. I was able to find the control board on ebay but the seller wants \$250 for it which to me is insane.

#### MisterBill2

Joined Jan 23, 2018
7,025
Did you see in the first post where he said -
Yes, in the first post the TS tells us that the product has been discontinued for some time. ALL decent companies are able and willing to provide service information on products that they no longer produce. And Milwaukee is certainly a decent company. Thus they should be able to download a PDF of the service manual, or at least some details. And with the good collection of photos to show both the circuit and the components a simple circuit drawing is really all that would be required. The only really tricky part is that demagnetizing section.

#### shortbus

Joined Sep 30, 2009
7,940
The only really tricky part is that demagnetizing section.
Why do you say that? It is just a reversal of the poles in the electromagnet. done by the DPDT switch. Or at least that's how they do it in my old Black and Decker, and the magnetic chucks on surface grinders.

Making a blanket statement about "decent companies" sharing schematics for their obsolete product is outrageous, can tell you've never tried to go down that route much.

#### MisterBill2

Joined Jan 23, 2018
7,025
Why do you say that? It is just a reversal of the poles in the electromagnet. done by the DPDT switch. Or at least that's how they do it in my old Black and Decker, and the magnetic chucks on surface grinders.

Making a blanket statement about "decent companies" sharing schematics for their obsolete product is outrageous, can tell you've never tried to go down that route much.
It seems that I HAVE been able to get service information from some companies, while others are blunt in stating that if they no longer sell a product then your only option is to replace it. And I stand by my assertion! Of course in searching for information there are a lot of sites that immediately pop up and claim to have "whatever it is" in stock, even if they have no clue what it is. So is SB telling us that some decent companies make products not worth repairing?? Or have I struck a very sensitive part of some organizations policy of NEVER sharing any useful product service information???

#### AlbertHall

Joined Jun 4, 2014
10,397
I was once tasked with making a replacement of a twenty year old piece of equipment as all the parts were now obsolete. Now, as some of you may know, there are companies that specialise in stocking obsolete parts so all the parts were available except two specially wound signal transformers. The company that made them had been bought out so I phoned the new company. They didn't make the transformers, they had no stock, BUT they would send me the drawings so I could get them made by some other company.

So some companies are good like that.

#### shortbus

Joined Sep 30, 2009
7,940
This was all that Milwaukee provides at least without calling them up:
Your luck is like most of mine when looking for information from what mrbill calls reputable companies. He says he has better results so maybe he will get in touch with them for you?