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| Linear Power Supply based on HP/Agilent E3610A |
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| timsu:
Thank you for your help so far. I would prefer to have only one transistor which needs active cooling (even if the total heat stays the same), it makes mounting and cabling easier. The solution from my previous post should still work, right? I just need a comparator to do the switching. For the constant current sink I think will settle for the solution which makes the output to go to 0V if it is switched off, even if it justs works down to 0.5V. The LF412 looks good so far for the error loops. For the readout maybe OP07, does something comparable exists in a dual package? |
| Kleinstein:
There are a few precision OPs in a dual package, e.g. LT1013, OPA2177, MCP6V12 and similar. The really high precision ones can be quite expensive though. There are many that are better than LF412 / LM358, but still relatively cheap. When doing range switching with a relay, one should not make the filter cap so large as this adds quite some load to the contacts. It might also be worth using two extra diodes and keep the lower voltage always on and only add the path for the higher voltage. With this low power there still is the option of not switching at all. |
| timsu:
I meant switching with one transistor as shown in post #8, not a relay. To summarize my tap switching options: * no switching: (+) simple, small pcb (-) large heatsink * relay switching: (+) filter caps only one required once (-) relay wear, dynamic loads, comparator required * switching as in post #8: (+) no wear, only one power transistor (-) comparator and two filter caps required * switching as in post #9: (+) no wear, few components (-) two filter caps and two power transistors I still have not deciced yet, but maybe I will use two channels. In this case tap switching would be nice. Most heatsinks I looked at go down to 0,8K/W. For one channel this would not be a problem, with two it could get a bit hot. |
| Kleinstein:
For the circuit from post #8, there is still the question on how to power the comparator / driver. If powered from the auxiliary (5V) supply the drive current will flow through the shunt and an extra diode again to negative a base voltage is needed. So for switching the taps I would prefer a MOSFET over a NPN. At the rather low voltage there should be no big problem in protection against too much gate voltage. However using an N-MOSFET at the high side like drawn would need an extra supply, as the +5 V auxiliary supply for the regulator is no enough. So a slightly different circuit is needed. There is also one more difference between relay and transistor use for tap switching: the transistor solution can be fast enough to even include the ripple on the raw voltage. So the transition could be set lower and with less (no) hysteresis. For a dual channel version, the two channels will normally be fully separate, except the digital control signal going from one part to the other. It might get difficult to find a transformer with two spilit windings. So this might make tap switching more difficult. |
| timsu:
Is it even possible to do the switching without another floating supply circuit? It seems kind of difficult because GND has no static potential to the tap switching. I tried using a P-MOSFET with a pullup. But how to pull the gate down with a ground-referenced circuit? I know for two channels the transformer becomes a bit of a problem. I have to see what a custom made one costs. |
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