Looking for possible solutions to switching B+ for delay purposes. Need to switch 470-500vdc supplies in several amps, all CT secondaries.
Has anyone come up with a simple HV switching scheme that has proven robust and reliable? MOSFET, IGBT, one of the new MOS optorelays? I'm not adverse to mechanical relays, but I don't know any compact ones with contacts rated for 500vdc.
Has anyone come up with a simple HV switching scheme that has proven robust and reliable? MOSFET, IGBT, one of the new MOS optorelays? I'm not adverse to mechanical relays, but I don't know any compact ones with contacts rated for 500vdc.
If you go for mechanical type relays then you need to switch the AC ahead of the rectifier(s). Either switch both sides of the HV winding or simply switch the CT to 0V.
Switching the DC should be petty simple using a MOSFET and an Opto-coupler to switch the gate to the drain via a suitable (say 220K) resistor. This is a good solution as multiple HV supplies can switched at the same time by simply running a (say 5mA) low voltage current loop through all of the opto-coupler input LEDS. Dont forget to add a gate to source protection zener and a gate stopper resistor to the MOSFETs. That current loop can be driven directly from the output of the delay on timer circuit (555 timer or similar).
Cheers,
Ian
Switching the DC should be petty simple using a MOSFET and an Opto-coupler to switch the gate to the drain via a suitable (say 220K) resistor. This is a good solution as multiple HV supplies can switched at the same time by simply running a (say 5mA) low voltage current loop through all of the opto-coupler input LEDS. Dont forget to add a gate to source protection zener and a gate stopper resistor to the MOSFETs. That current loop can be driven directly from the output of the delay on timer circuit (555 timer or similar).
Cheers,
Ian
Last edited:
Looking for possible solutions to switching B+ for delay purposes. Need to switch 470-500vdc supplies in several amps, all CT secondaries.
Has anyone come up with a simple HV switching scheme that has proven robust and reliable? MOSFET, IGBT, one of the new MOS optorelays? I'm not adverse to mechanical relays, but I don't know any compact ones with contacts rated for 500vdc.
Has anyone come up with a simple HV switching scheme that has proven robust and reliable? MOSFET, IGBT, one of the new MOS optorelays? I'm not adverse to mechanical relays, but I don't know any compact ones with contacts rated for 500vdc.
Your going to get some serious arcing if you use a relay at that power.
You can get MOSFETs with S-D voltages up to 1500 volts.
You can get MOSFETs with S-D voltages up to 1500 volts.
How about using one transformer for the HV and one for everything else? You could then switch with a relay on the primary.
As pointed out above, you can get MOSFETs that can handle the voltage (across source-drain), but bear in mind that the gate oxide can typically only handle about 20 V. I.e. Vds may be 1 kV, but Vgs is only 20 V. So if you use a high-side switch you need a fancier (floating) gate driver. I'm not a big fan of low-side switches.
You might be able to use a resistive divider and another MOSFET as the gate drive for the main switch.
~Tom
As pointed out above, you can get MOSFETs that can handle the voltage (across source-drain), but bear in mind that the gate oxide can typically only handle about 20 V. I.e. Vds may be 1 kV, but Vgs is only 20 V. So if you use a high-side switch you need a fancier (floating) gate driver. I'm not a big fan of low-side switches.
You might be able to use a resistive divider and another MOSFET as the gate drive for the main switch.
~Tom
Last edited:
Ooops, sorry about the duplicate thread. Not sure why it double posted, I musta goofed.
Anyway, thanks guys, I understand all the considerations involved. My question was a little more practical. I don't want to design and test a switch right now, so I wanted to see if anyone has either 1) a commercially available switch/circuit I can buy or 2) a diy circuit that has been tested and proven reliable in use. I don't want to reinvent the wheel if I don't have to.
And no, an additional transformer for filaments won't fly in any of the amps I have (lack of space and aesthetics) and neither will an Amperite thermal delay.
Anyway, thanks guys, I understand all the considerations involved. My question was a little more practical. I don't want to design and test a switch right now, so I wanted to see if anyone has either 1) a commercially available switch/circuit I can buy or 2) a diy circuit that has been tested and proven reliable in use. I don't want to reinvent the wheel if I don't have to.
And no, an additional transformer for filaments won't fly in any of the amps I have (lack of space and aesthetics) and neither will an Amperite thermal delay.
One of my 5 minute design slow turn-on circuits using an upside down mosfet in the rectifier AC neg leg and the negative bias supply to actuate a small relay as a self protect B+ shutdown in fixed bias systems. Mosfet selected for max voltage.
This circuit works well but haven't tried it with a power IGBT, but the concept should work for other voltages with a bit more workin' out. Those round circles in mosfet legs are ferrite beads. I love'm.
richy
This circuit works well but haven't tried it with a power IGBT, but the concept should work for other voltages with a bit more workin' out. Those round circles in mosfet legs are ferrite beads. I love'm.
richy
Attachments
richy - have you checked what time duration and peak current level pass through the fet during soft turn-on, and how that compares with SOA? Ciao, Tim
High-voltage thyristor will be optimal solution, will cost just few bucks. Google for T-Reg power supply, or Rubli board, they all provide B+ delay circuits.
I would have thought that you could switch on with a thyristor, however if the latch current is exceeded then the gate will not effect power off unless you use GTO or a contact. Remember that the gap of the contact in a switch or relay is the important factor so you can series contacts for voltage "break" and parallel for current. You could also bypass the HV contacts with a polypropylene capacitor, however this should not be done if its failure could be a safety problem in which case a HV mov can be used across the load side of the contacts.
Regards
M. Gregg
Last edited:
I like that Rubli board. You need one per channel, but still seems like a pretty robust idea. If it fails either short or open, no damage to anything else. That's good.
Anyone actually using that Rubli circuit? Or one like it?
The vacuum relays also a good idea. I shoulda thought of that.
Anyone actually using that Rubli circuit? Or one like it?
The vacuum relays also a good idea. I shoulda thought of that.
There is a difficulty with putting contacts in series to increase voltage widthstanding. Generally the contacts will not open at exactly the same moment, so you end up with the first contact that opens having to cope with the full voltage.
jan didden
I used solid-state relays for switching HV in my tube amp. In general, it is better to switch AC than DC because of arcing. Advantage of the SSR is that it switches at the zero crossing of the cycle. There is an internal optocoupler, the control is an internal LED. I connected it at the secondary of the mains transformer just before the rectifier diodes. It worked fine.
The end of the story: I reverted to a mechanical relay, with 4 contacts parallel. No arcing whatsoever. One silicon device less 🙂
The end of the story: I reverted to a mechanical relay, with 4 contacts parallel. No arcing whatsoever. One silicon device less 🙂
Last edited:
Jan,
I agree that one contact will open a fraction of a second before the others, however this is used in industry with much higher currents. Also if a capacitor is used to absorb the inital opening this is not a major problem. A Mov can also be used.
The gap / cap will quench the arc. The Rubli circuit looks interesting, I cannot comment (never used it as yet!)
Also you can switch off the B+. Scr will latch at switch on and maintain until power off.
Regards
M. Gregg
Last edited:
- Status
- Not open for further replies.