Ignorant question about DC resistance

[voodoochild]

Hello all,

I am a newbie here and want to understand how everyone relays the DC resistance of their pickups to their customer. I ask this simply because when I measure say a P-90 coil with no magnets or lead wires attached, I get a different reading than when I wax pot, install leads and magnets. It seems like the other pickup makers use the DC resistance before the wires are attached and wax potted. Because they are way off from the stated DC resistance. Is that why? Thanks.

[bbsailor]

DC Resistance is only one measurement

Pickup vendors choose to describe their pickup products in ohms because it is a covenient and relative (to other brands or models) measurement that can be easily done with a common, inexpensive Volt Ohm Meter (VOM).

The DC resistance should stay constant whether the metal slugs are in or out of the pickup. The only change, when slug cores are added, is the coil impedance which is also called Z. Z is the AC impedance and changes with fequency, unlike resistors which have a constant Z at all frequencies.

Inductor Z increases with increasing frequency while capaitor Z decreases with increasing frequency. All coils also have capacitance due to the thousands of turns of wire laying close to each other with very thin insulation. In addition to pickup inductance usually measures in Henrys (H), coils also have a phantom capacitance characteristic measures in pf that when combined with the inductance, causes a pickup to have a certain resonant point. This resonant point occurs when the Z of the inductance component matched the Z of the phantom capacitance component and forms a parallel resonant circuit. This is what gives each pickup a characteristic sound.

DC rsistance tells you something about the pickup coil output and the typical sound characteristic like bright, dark, hot, weak, muddy, etc. DC resistance also gives you a clue about the value of the volume pot to use. Rules of thumb state that the pot value should be 35 to 40 times the coils DC resistance value. Therefore, a 6.2K ohm pickup coil would use a 250K pot while a 12.4K humbucker would use a 500K volume pot. Pickups with very high resistance in the 25K range would dictate the use of a 1Meg volume pot to follow this rule of thumb.

What is more imortant is the pickup impedance at resonance. Go to the following web site and enter the published pickup inductance in Henrys and the resonant frequency to see the phantom capacitance that causes that resonance. http://www.pronine.ca/lcf.htm. Look at the Z or impedance at resonance and use a volume pot value no lower than this value to maintain the unchoked flavor of your pickup's sound.

The guitar cable typically adds about 380pf for a 10 ft length, so add this to the phantom capacitance result to see the actual pickup resonance (lower) using a typical guitar-to-amp connecting cable.

If the DC resistance changes when tinkering with the pickup, you may have some problems with the pickup coil.

Joe Rogowski

[voodoochild]

Thank you my friend, that is some great information. I was really wanting to know what reading the manufacturers use when stating the DC resistance. What I mean is when I wind a P-90 coil and read the DC resistance before I install the baseplate, Lead wire, and wax pot it, I get an approx. reading of 8.1K. Then after I install the baseplate, lead wire, and wax pot it, I get a reading of approx 8.5K. I know this is normal, but which reading do I tell the customer? I see other manufactures who state a certain DC resistance but I usually get higher readings than what is stated no matter what meter I use. I am shooting for an 8.5K bridge pickup but does that mean before or after I install the wire, baseplate, and wax?

[bbsailor]

DC Resistance is Temperature Sensitive

Cool the pickup in the refrig. for about 15 minutes first. Then wax it and cool again for 30 minutes to reach the same temp. The DC readings should be the same. Resistance readings are temperature sensitive: warmer is higher.

Joe Rogowski

[madialex]

I would say use the reading that the end user will see. That is not always the case though, Just holding the coil in your sweaty little hand can change the resistance. As the coil heats up the reading gets higher. Test a coil at room temperature then take it outside tonight if its cold where you are, let it sit for 1/2 an hour then check it, should read lower.

or as Sailor said, the fridge or freezer works too. I was typing when he replied or else I wouldnt have posted almost the same answer.

[voodoochild]

Thank you. Your right, I measured the DC resistance soon after I removed it from the wax while warm. Duh me. That makes total sense.

[David Schwab]

I am a newbie here and want to understand how everyone relays the DC resistance of their pickups to their customer.

I don't. DC resistance can be meaningless depending on the pickup design.

[Possum]

...........

Let the coil cool overnite after you wind it, then measure the pickup without handling it, your hands will heat it up if you handle it and you'll get a higher reading, room temperature will change the reading, don't stick it in the fridge you'll get a whacked out reading that will change an hour later. It has to stabilize at room temperature and this takes an overnite sit. You shouldn't be getting a different reading after potting. After potting it has to sit for a long time before you'll get a stable reading.

[Zhangliqun]

When you pot the pickup, when you solder the leads on, or even when you handle the pickup a lot in your bare hands, it heats the coil some and will raise the DCR reading. I give the customer the final DCR spec after it has sat a few hours at room temperature.