Voltage level switch

[JDeV]

I eventually got hold of a small 500VA variac and is bussy to build a bench PSU. I will use the variac as priamary to a 45-0-45 1000VA toroidal. This combo will give me up to + and - 60 Vdc @ 8 amp. I got 14 x 10,000uF , 40 v caps and 2 x 15,000uF 63 v caps.

What I want to do , is to keep the 40v caps in circuit up to about 34v and then switch them out and only use the 63v caps further on.

Will the following circuit do this job, or is there a better/easier way to do it.

Thanx for all help.

[Sch3mat1c]

Humm... Should be good to 20 amps at full voltage.. connect some of the 40V caps in series and switch out the variac instead

Tim

[peranders]

In general when you have comparator functions you should also have hystereses so you won't get any "nervous" switching.

Just place a resistor from output to + input.

Your reference voltage should also be connected to the supply voltage not the "input" voltage. Otherwise you can get undefined states.

[Jennice]

With this many 10.00uF's you better be sure to ALWAYS start the variac from zero. Otherwise you rectifier bridges will not last long.

Also, I would use the 63V caps at all times.
If you really feel a need to use all those caps, I would (like someone else suggested) connect the 40V caps in series (and add a low-current voltage devider with resistors to ensure equal voltage sharing over the caps).
Each series connection will give 5000 uF, 80V, and two such sets will bring back the 10000 uF at 80V. This uses 4 caps, but as I recall you have a lot of them at hand.

Jens

[janneman]

May I also respectfully remark that the limiting factor would be the variac, not the toroid, so with a 500W variac you're back to +/- 60V at 4 amps (about).

Jan Didden

[JDeV]

Thanx for all advice. I am taking note of all suggestions Surely don't want to fry my precious little variac.