### Author Topic: Bleed resistor - how?  (Read 4467 times)

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#### amateur_25

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##### Bleed resistor - how?
« on: March 22, 2012, 01:26:31 pm »
I've used this website to calculate my bleed resistor but I would like to understand the formulas behind it ?
http://www.welwyn-tt.com/products/resistors/calculation-tools.asp

#### alm

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##### Re: Bleed resistor - how?
« Reply #1 on: March 22, 2012, 04:45:42 pm »

#### NiHaoMike

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##### Re: Bleed resistor - how?
« Reply #2 on: March 23, 2012, 05:18:17 am »
Bleed resistors for safety reasons are rarely used nowadays since they waste power and even one that is assumed to be good cannot be trusted.
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#### Kilroy

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##### Re: Bleed resistor - how?
« Reply #3 on: March 23, 2012, 02:36:36 pm »
Well, in some cases, the use of bleeder resistors is absolutely prudent...like for when you want to throw away leakage current or from semiconductor devices like SCRs, triacs, diodes etc. Even in the "off" state these devices can leak enough current to potentially ruin your day, so it can be cheap and effective insurance to install appropriate leakage current bleeders.

For example, when working with Programmable Logic Controllers, it is extremely wise to use bleeder resistors to pull down leaky triac field device switches, and such. Proximity switches are pretty bad for leakage current, too. You can see the signal present indicator flicker on the input modules and sometimes leakage current can actually be enough to switch the input module into the conducting state, with potentially scary results...CNC machines and industrial robots move *very* quickly.

Similarly, you can have issues with outrush currents when triacs and SCRs are connected to inductive loads. When power is removed, the power is stored in the inductive load will try to make its way to ground any way it can, like taking a convenient path across a device such as a triac in the "off" state. This is a bigger problem if the current to the inductive load has been cut off at the voltage peak...it will not likely be noticed until the power is switched back to the "on" state. Installing suitable capacitance across the secondary side of the disconnect that supplies power to the inductive loads soaks up the outrush.

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