I am collecting voltage spikes (not constant current) in a 50v 40,000 microfarad capacitor, and I want to discharge this cap into a 24v battery when the voltage in the cap rises to 32v. Once discharged I want to start the cap charge-discharge cycle again, and loop this process indefinitely.

I have done something similar with an SCR triggered by a diode. However this was discharging at approx 13v into a 12v battery, and only 10 microfarads - and I didn't really understand how that worked.

How do I specify the correct SCR and how do I trigger it for this voltage (32v)?

If there is a more efficient way than the SCR please feel free to explain. (I am not very experienced in electronics.)

 

I don't quite understand what you are trying to do?
An SCR (Silicon Controlled Rectifier) is a controllable medium- to high-power self-latching solid-state DC power switch. This article explains its basic operation and shows some practical ways of using it. Once we gate on and SCR in a DC circuit as you describe it will act as a closed switch. The SCR will not open until power is removed.
The only applications where I have seen a capacitor charged to well above a battery voltage and discharged into a battery were to try and remove internal shorts in a NICAD type battery. So what exactly is the objective here? I do not see an SCR behaving the way I see you wanting it to.

Ron

 

As I understand, you are trying to transfer the energy from the capacitor into the battery.
This can indeed be done with a SCR between the battery and capacitor, with a comparator to trigger the SCR when the battery voltage reaches 32V.
When the transfer is complete (capacitor drops to 24V) and the current drops to zero, the SCR will reset for another cycle.

For best transfer efficiency you should put an inductor in series with the SCR to perform inductive charging which will give close to 100% efficiency, otherwise the transfer efficiency will always be less then 50%.

How often will you be transferring the energy i.e. how much energy to you think you will be capturing?

 

I'm charging lead-acid batteries Ron. I believe they quite enjoy a higher voltage pulse from a cap. Helps them de-sulphate as well.\

I think an SCR might work. I suppose it would need to handle 40,000 microfarads of discharge. I'd have to figure out how to trigger the gate at 32v. (You can tell by now I'm new to all this.) Thanks for the article link.

 

A suggestion: the diacs used to trigger light dimmers are about 32V to 34V at their break over voltage and do not require a power supply to get them to make a decision. They just stand there until they suddenly short the current into the triac gate.

 

Crutschow

...put an inductor in series with the SCR to perform inductive charging which will give close to 100% efficiency, otherwise the transfer efficiency will always be less then 50%.

How often will you be transferring the energy i.e. how much energy to you think you will be capturing?

Hey Crutschow - thanks for the reply. Any tips on how to spec an inductor for this task? Sorry for being such a noob - I've never heard of that technique. I believe the cap will charge up every 5 seconds or so - so that's the rate of capture.

 

A suggestion: the diacs used to trigger light dimmers are about 32V to 34V at their break over voltage and do not require a power supply to get them to make a decision. They just stand there until they suddenly short the current into the triac gate.

Ah - that sounds like it's worth pursuing further. Thanks.

 

I'm charging lead-acid batteries Ron. I believe they quite enjoy a higher voltage pulse from a cap. Helps them de-sulphate as well.

OK and looking at Carl's post it does make sense. The current will get to zero and the SCR will shut off till again. Per Carl's suggestion in post #3 the best method would likely be a comparator circuit for detecting a voltage level at which to trigger the SCR.

Ron

 

Here ya' go. 21 cents. No power supply, no comparator, no constant use of power to make them run.
http://www.mouser.com/Semiconductor...istors/Diacs/_/N-ax23w?P=1z0w7a0&Ns=Pricing|0

or you can just steal one out of a dead light dimmer.

 

Ah - that sounds like it's worth pursuing further. Thanks.

Have you thought about charging your battery directly with the voltage spikes? It might be more efficient.

 

What is charging the capacitor?
If you aren't concerned about efficiency, then you don't need the inductor.

 

Below is the LTspice sim of my take on a circuit with an SCR and an inductor.
V1 and R1 simulate the charge source for the capacitor.
The 7.5V Zener diode causes the SCR to trigger when the capacitor voltage is about 8.8V above the battery voltage.
Diode D2 prevents reverse gate current which showed up in the simulation.
Due to the inductive resonant charging effect from the 1mH inductor, C1 discharges to <18V, about 6V below the battery voltage.
This improves the energy transfer efficiency and also insures that the SCR will be reverse biased and turn off after each charge pulse.



Here's a short article I wrote on inductive charging of a capacitor, if interested.

 

Here ya' go. 21 cents. No power supply, no comparator, no constant use of power to make them run.
http://www.mouser.com/Semiconductor...istors/Diacs/_/N-ax23w?P=1z0w7a0&Ns=Pricing|0

or you can just steal one out of a dead light dimmer.

Thanks Now just got to figure out how to use it. So if the breakover voltage is 32v it will output 5v. Before that it does not output anything. So it's giving a 5v "signal" that can be used to trigger the SCR... is that what you mean?

 

Below is the LTspice sim of my take on a circuit with an SCR and an inductor.
V1 and R1 simulate the charge source for the capacitor....

Thank you for this. To be honest it's way over my simple knowledge at the moment. I hope I'll progress to where I can get these things but I'm taking baby steps right now.

 

Have you thought about charging your battery directly with the voltage spikes? It might be more efficient.

I think the frequency - which is in the megahertz range - is a big high.

 

Thank you for this. To be honest it's way over my simple knowledge at the moment. I hope I'll progress to where I can get these things but I'm taking baby steps right now.

It's fairly simple.
The capacitor voltage increases until it's voltage is greater than the battery voltage by the sum of the zener voltage (7.5V), the diode voltage (0.7V) and the SCR gate-anode voltage (0.7V) in series (about 8.8V in the simulation).

The current through them then increases until it's high enough to trigger the SCR (about 25mA for the one I used) which then conducts until the capacitor has discharged sufficiently that the current through the SCR stops.
The resets the SCR to non-conducting for the next cycle.

 

So if the breakover voltage is 32v it will output 5v.

No. If the breakover voltage is achieved, the diac will allow current to flow through it until the capacitor is down to 5 volts...which won't happen because the SCR is dumping onto a 24 volt battery. The only thing the diac does is notify the SCR that 32 volts are available in the capacitor. The Diac slaps a start current pulse into the SCR and the SCR takes over from there. It doesn't matter what the minimum voltage of the diac is because your needs never get down as low as 5 volts.

Just go with the constantly power consuming comparator route.

 

I am collecting voltage spikes (not constant current) in a 50v 40,000 microfarad capacitor, and I want to discharge this cap into a 24v battery when the voltage in the cap rises to 32v. Once discharged I want to start the cap charge-discharge cycle again, and loop this process indefinitely.

I have done something similar with an SCR triggered by a diode. However this was discharging at approx 13v into a 12v battery, and only 10 microfarads - and I didn't really understand how that worked.

How do I specify the correct SCR and how do I trigger it for this voltage (32v)?

If there is a more efficient way than the SCR please feel free to explain. (I am not very experienced in electronics.)

The SCR is probably as good as it gets - there may be a case for very low RDSon MOSFETs.

Your trip voltage is around the same ball park as a diac - but most can only handle about 2A peak pulse.

An SCR will boost the current handling, just hook the diac from anode to gate - I'd suggest at least 10R in series with the diac to limit the current until the SCR takes up the load.

diacs are getting harder to come by - many CFLs have one in the start up circuit.

 

I think the frequency - which is in the megahertz range - is a big high.

Tell us some more about the source of the voltage to charge the cap. It's still not clear to me why you need to store it first unless you just want to pulse charge the batteries.

 

The SCR is probably as good as it gets - there may be a case for very low RDSon MOSFETs.

Your trip voltage is around the same ball park as a diac - but most can only handle about 2A peak pulse.

An SCR will boost the current handling, just hook the diac from anode to gate - I'd suggest at least 10R in series with the diac to limit the current until the SCR takes up the load.

diacs are getting harder to come by - many CFLs have one in the start up circuit.