Capacitor Characteristics

High value capacitors usually have a 'bleed resistor' to discharge them if they are charged to a high (say over 100v) voltage for safety reasons.

If the cap's charge to this voltage in normal use, then re-applying the power should not do any harm... in fact it may reduce the switch on surge as they don't need re-charging.

All this assumes I have understood your question correctly

Del

Does it mean that for nominal voltage of 35v it is not necessary to provide bleed resistor for discharging the filter capacitor?

*in fact it may reduce the switch on surge as they don't need re-charging.* That is well known, why, Cap acts like a short till charging, charge Cap is not.

Yes...but some of the switch on surge will be the magnetization current of the transformer.. that's why I used the word 'may'.

No, you won't get 70V. Your power supply and the capacitor are effectively in parallel so the voltage will be the same. I am tempted to say that it's alright. However, safety compells me to say otherwise.

The 35V that is "left" on the capacitor will, as Del mentioned, prevent the current surge during the switching on of power.

However, it also means that there is 35V of electricity in the device that can bite the hand that reaches in and touches it. It may just give you an unpleasant jolt; it may cause your hand to jump and impale or slice open the skin on something sharp; it may give you a slight burn injury; or it may even stop your heart.

Ok !

Thank you very much for such a quick response.

Thiis issue i need to know because in that stepper control project the program of Micrcontroller 89v51rd2 get corrupted offently upon swtich on/ off sequence.

Let me describe the whole circuitary.

There are two power supplies :

1) Unregulated +35 v for stepper H-bridge.

2) regulated +5v for microcontroller.

Both share common GND. and derived from transformer with two different (12v ac for 5v regulated and 24v ac for +35v unregulated supply)tappings at output terminal of the transformer.

Hmm, I encountered a similar problem with a medical transfusion machine once. I don't know what it's proper name is, actually, but it's a microprocessor controlled pump for injecting fluids into patients at controlled flow-rates.

Sometimes it started up normally and sometimes it didn't. I didn't have any medical equipment background but the supplier's technician asked me to take a look.

The microprocessor, fortunately, was a Z80 (this was decades ago, obviously) which I was familiar with.

I discovered that the reset pin was floating! The thing was, the circuit was designed that way! There was simply no provision to start the microprocessor from zero. I guess what happened was that when power was switched on, the microprocessor started right off before all the other circuits had a chance to enter their initial state.

I took a chance and bought a resistor and capacitor and soldered them onto the reset pin to create a RESET signal when power was switched on. Essentially, the RESET signal keeps the microprocessor in standby mode for a second or two before letting it loose. By then, all the other circuits had powered up and are waiting for a command. It worked.

The supplier's technician fired off a report to the manufacturer. Unfortunately, I never heard what happened next. I charged what I considered a hefty sum at that time but when I think back, I just solved a hardware glitch in their design and all I got was a repair fee and a thank you. I comfort myself on the thought that a couple hundred or so patients are safely using that machine.

Now, after such a long story, I'm not sure if you're problem has the same solution. It's worth a look-see though.

Now, about that fee...

I don't think 35v will give you much trouble.... 60v maybe.

Nuffin' to do wiv me mate....

Years ago when I was teaching physics, I commonly had students hold (dry hand) on to two wires in such a position that they could not see how fast I was turning up the voltage, and tell me when they first felt the voltage. It's been sufficiently long ago that I'm not sure whether I used AC or DC... In any case virtually no one could feel 35 Volts, and most could still feel nothing at 50V. I myself could not feel anything 'till around 60V, but I am an oily skinned person; sweaty skinned people will presumably feel an electric current at lower voltages.

On the other hand, I did once get shocked, I think by 5 VDC, When I unknowingly had a metallic sliver penetrating my skin.

I certainly can't argue with the possibility that reacting to feeling the voltage could make one jerk and hit some nearby sharp object - I've done that more than once!

I definitely agree that safety should be a significant concern.

But if you get a burn from 35V, it's because you connected something that was a lot better conductor than your skin, and that something got hot!

I've been shocked a lot of times under many different circumstances, and once even thrown across a small room by an electric shock. Now I'm not a doctor, but I believe it would take an extremely rare set of conditions, probably involving a deep impalement of a good conductor, for 35V to stop a heart.

I've felt 24Vdc coursing through my hand and it was enough to make me jerk my hand away. Now, the circuit was only 1A maximum so my reaction was more out of instinct (teacher says, "beware the power of electricity") than because of the current.

I've heard that a 12V car battery can pack enough juice to give you a bad jolt though I've never tried it, obviously.

My premise that a shock can stop your heart was more in thinking of people with heart problems. A shock might trigger a heart attack. I guess that would count as a "rare" condition.

Hi,

you can easily kill yourself or others by voltages as small as 12V or may be lower!

The resistance to the current is only at the skin (as stated above by the mentioning of metallic sliver!), so if some unknown or unexpected couplin is existing the current can enter and leave our body without much resistance.

This will rapidly let more than 3 to 5 mA pass, a current estimated to be safe.

Much higher currents may be survivable if not passing the clock region that is the impulse generator for our heart.

The skin may be not integral by any wound or small scratch, by wet allergic lesions, by ourselves partially in water especially if it is saltwater or if one of the conductors is connected to any of the wet and well conducting surfaces of our body openings.

So be a little bit careful.

There have been multiple deadly accidents with 12 V batteries: the babtist preacher whose microfone fell into the big pool and at retrieving it he suffered a deadly shock, some swim and boat lovers at sea when after swimming reentering their boat and touching the bare wires at the starter battery.

Enjoy your life and sharpen your conscience!

RHABE

the babtist preacher whose microfone fell into the big pool and at retrieving it he suffered a deadly shock,

I know I shouldn't laugh ...but maybe he's gone to a better place?

low votages such as this are generally not dangerous with regards to schocks, but if you short a metal watch strap for example, the heat generated in some cases (especially car batteries) can be considerable and burns can be expected.

Never treat any voltage with disdain, as someone else wrote, it could bite the hand that switched it on.

I would recommend a bleed resistor of say 1 Meg Ohm, to take the voltage down within say 10 seconds or so......then there will be no nasty surprises. It will not affect (usually) the functions of the circuit.

The capacitor has to be in parallel with the 35V supply. It is there to filter the supply or to provide high switching current to the motor or for both the functions. If one doesn't want the voltage to remain on the capacitor after switching the supply off, a 10k ohm 0.25W resistor can be connected permanently in parallel with the capacitor. This will discharge the capacitor in about 40 seconds.

A capacitor can be charged only upto the peak value of the source voltage. A 35 V DC source can charge the capacitor only to 35V. To charge to 70V, a source of 70VDC is needed.