brih 58s1624a00360p2a
1、首先是從現(xiàn)場應(yīng)用的安全考慮
安全重點(diǎn)是以防爆安全火花型儀表來考慮的，并以控制儀表能量為前提，把維持儀表正常工作的靜態(tài)和動(dòng)態(tài)功耗降低到限度。輸出4~20mA.DC標(biāo)準(zhǔn)信號(hào)的變送器，其電源電壓通常采用24V.DC，采用直流電壓的主要原因是可以不用大容量的電容器及電感器，就只需考慮變送器與控制室儀表連接導(dǎo)線的分布電容及電感，如2mm2 的導(dǎo)線其分布電容為0.05μ/km左右；對(duì)于單線的電感為0.4mH/km左右；大大低于引爆氫氣的數(shù)值，顯然這對(duì)防爆是非常有利的。
2、傳送信號(hào)用電流源優(yōu)于電壓源
因?yàn)楝F(xiàn)場與控制室之間的距離較遠(yuǎn)，連接電線的電阻較大時(shí)，如果用電壓源信號(hào)遠(yuǎn)傳，由于電線電阻與接收儀表輸入電阻的分壓，將產(chǎn)生較大的誤差，如果用電流源信號(hào)作為遠(yuǎn)傳，只要傳送回路不出現(xiàn)分支，回路中的電流就不會(huì)隨電線長短而改變，從而保證了傳送的精度。
3、信號(hào)電流選擇20mA的原因
電流20mA的選擇是基于安全、實(shí)用、功耗、成本的考慮。安全火花儀表只能采用低電壓、低電流，4~20mA電流和24V.DC對(duì)易燃?xì)錃庖彩前踩模瑢?duì)于24V.DC氫氣的引爆電流為200mA，遠(yuǎn)在20mA以上，此外還要綜合考慮生產(chǎn)現(xiàn)場儀表之間的連接距離，所帶負(fù)載等因素；還有功耗及成本問題，對(duì)電子元件的要求，供電功率的要求等因素。
4、信號(hào)起點(diǎn)電流選擇4mA的原因
輸出為4~20mA的變送器以兩線制的居多，兩線制即電源、負(fù)載串聯(lián)在一起，有一公共點(diǎn)，而現(xiàn)場變送器與控制室儀表之間的信號(hào)聯(lián)絡(luò)及供電僅用兩根電線。為什么起點(diǎn)信號(hào)不是0mA？這是基于兩點(diǎn)：一是變送器電路沒有靜態(tài)工作電流將無法工作，信號(hào)起點(diǎn)電流4mA.DC，不與機(jī)械零點(diǎn)重合，這種“活零點(diǎn)”有利于識(shí)別斷電和斷線等故障。
brih 58s1624a00360p2a
6ES7971-0BA00
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6ES7964-2AA01-0AB0
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6ES7421-7DH00-0AB0
S7-400 16DO 24VDC 2A optic. isolated
6ES7422-1BH11-0AA0
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6ES7421-1BL00-0AA0
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6ES7432-1HF00-0AB0
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6ES7470-0AA00-0AA0
S7-400 Cable Duct
6ES7408-0TA00-0AA0
S7-400 CP441-1 for point to point
6ES7441-1AA04-0AE0
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6ES7441-1AA00-0AE0
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6ES7441-2AA04-0AE0
6ES7441-2AA03-0AE0
6ES7441-2AA02-0AE0
6ES7441-2AA01-0AE0
S7-400 CP443-1
6GK7443-1EX20-0XE0
6GK7443-1EX11-0XE0
6GK7443-1EX10-0XE0
S7-400 CP443-1
6GK7443-1EX02-0XE0
6GK7443-1EX01-0XE0
6GK7443-1EX00-0XE0
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6GK7443-1EX41-0XE0
S7-400 CP443-5
6GK7443-5DX04-0XE0
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6GK7443-5DX00-0XE0
S7-400 CPU 412-1
6ES7412-1XJ05-0AB0
S7-400 CPU 412-1
6ES7412-1XF00-0AB0
S7-400 CPU 412-2
6ES7412-2XG00-0AB0
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