MTFCA Forum

What is the difference in capacitors with the same rating?
On the computer I put in .47K 400V and came up with different capacitors with different prices and the same rating?
I am not an electronics person so plain language works best.

It would be .47 Mfd at 400 volts. Some caps have different tolerances and temp coefficients along with lead configurations and material composition that can effect the price. But a .47 mfd at 400 volts is a pretty basic value used in many applications. I would say go with one that looks like the one You are replacing with the same tolerance that is the best price from a reliable suppler like Mouser, Antique radio supply just to mention a few. Prices are all across the board for everything these days. You can pay $9.95 dollars for a package of toilet paper or $24.50 for the same item and quantity from another supplier. Its buyer beware. But I don't think its critical s long as new cap value and operating voltage are the same as the old one.

I think the speed of the capacitor is an important factor as well. Why not simply buy from one of the model T parts suppliers. The prices are reasonable and you will get what you need for coils

What happened to the supply of capacitors that are slightly flattened rhather than round. I bought a dozen of them 6 years ago. They fit into the coil without removing the glass. They are still checking ok, but I could use some more of them.

Art Mirtes

This topic belongs in the general discussion section.

The difference how quick that capacitor can be charged and discharged, a term called dV over dT or vice versa, I have dislexi...

Dennis... The suppliers have already selected the capacitors that will work and last in our coils. Yes, there are other capacitors with different ratings but the selection has already been made. Contact one of our suppliers for capacitors and you will be satisfied.

As others have mentioned, if we talk about the capacitors in the model T coils, an important parameter is the dV/dT parameter and it should be in the 500+V/s. Most capacitors you can by with .47mFd 400V do not have that high dV/dT value and they won't last in the coils environment.

What is the difference in capacitors with the same rating?

Capacitors are electronic components with the basic function being the the ability to store and release energy. They also have the ability to pass Alternating Current (AC) while blocking Direct Current (DC). These characteristics of a capacitor have a wide variety of applications in electronics.

A typical application is passing audio (AC) signal from a radio to a speaker, for example, where by AC audio component (voice/music) of the signal is permitted to pass to the speaker while blocking the DC component (The amplifier's DC voltage). The audio (AC) signal current is typically small and does not result in significant internal heating of the capacitor so this is considered light duty application. Simple, low cost capacitors can be manufactured for this light duty application with voltage rating sufficient to block the amplifier's DC component.

Capacitors don't like abrupt changes in voltage so another common application of a capacitor is in AC to DC power adapters commonly used for consumer electronics (aka wall warts). The purpose of the capacitor is to filter out the AC component that is present on the DC power supply output. A radio powered by faulty AC to DC power supply with poor or bad filter capacitor would have a noticeable low frequency Hum due to 60Hz or 120Hz AC power leaking through to the DC output. AC filter capacitors in DC power supplies are typically light to medium duty applications where the residual AC ripple current through the capacitor can become more significant resulting in greater internal capacitor heating. This places restrictions on manufacturing options that can increase material and manufacturing costs.

Capacitors used in Model T coils are a special use application of capacitors that fall into a category of Pulse applications. The capacitor is repeatedly subjected to very abrupt changes in voltage (0V to 250V) each and every time the coil point open. As stated earlier, capacitors do not like abrupt changes in voltage and attempt to absorb those changes generating considerable heat in the process. This internal heating must be effectively dissipated or the internal capacitor materials can be damaged and degrade capacitor function which in turn leads to further degradation in performance as operation continues until the capacitor fails to function as a capacitor. Capacitors design for pulse applications are typically designed with more costly materials and manufacturing process that are typically larger in size compared with capacitors of the same value and size designed for other light duty applications.

Coils are also electronic components with the basic function being the the ability to store and release energy so some may be wondering how they are different. Capacitors store energy in an electric field, between two parallel conductive plates of opposite charge and typically rolled up to form a cylinder with 2 leads; one for each plate. A coil stores energy in a magnetic field, typically created by a current lowing in a coil of wire wrapped around an iron core to intensify the magnetic field. While capacitors oppose abrupt changes in voltage across its parallel plates, coils oppose abrupt changes in the current flowing in the coil of wire. Interesting characteristics that are used in many different applications in electronics. Hope answers your question and this intrigues your interest to learn more.